Earth's climate

Class 12 Physics, unit 24, book pages 183–200

Earth's climate is the average weather of a place over about 30 years. This unit treats the climate as a physical system of five interacting parts (air, water, ice, rock and living things) powered by energy from the Sun. It explains how the uneven heating of the equator and the poles, the Earth's rotation (the Coriolis force) and differences in sea-water density set up the winds and ocean currents that carry heat around the planet. It ends with the Earth's energy budget, the greenhouse effect, climate inertia and why the climate is a chaotic system that is hard to predict.

Learning outcomes (10)

Describe Earth's climate system as a complex system having five interacting components: the atmosphere (air), the hydrosphere (water), the cryosphere (ice and permafrost), the lithosphere (Earth's upper rocky layer) and the biosphere (living things).
Relate ocean currents and wind patterns to the climate system. Climate is the statistical description of the climate system, representing the average weather, typically over a period of 30 years, and it is determined by processes in the climate system such as ocean currents and wind patterns.
Explain climate inertia as the phenomenon by which climate systems show resistance or slowness to changes in significant factors; for example, a stabilization of greenhouse gas emissions might show a slow response due to the action of complex feedback systems.
Explain that climate change can be categorized into internal variations and external forcing.
Explain how global climate is determined by energy transfer from the Sun; state and use the term Earth energy budget.
Explain how the energy imbalance between the poles and the equator can affect atmospheric circulation.
Explain that, due to the conservation of angular momentum, the Earth's rotation diverts air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, thus forming distinct atmospheric cells.
Explain that ocean water with more salt has a higher density, and that differences in density play an important role in ocean circulation.
Explain how the thermohaline circulation transports heat from the tropics to the polar regions.
Explain how climate science is an example of a chaotic system, using the metaphor that a butterfly's wing flaps may cause a hurricane in another part of the world: very complex systems are very sensitive to initial conditions, so their outcomes are very difficult to predict (the mathematics of chaos theory is not required).

Weather, climate and the climate system

Weather is the state of the atmosphere at a particular place and time: its temperature, humidity, cloud cover, rainfall, wind and air pressure. It changes from hour to hour and from day to day. Climate is the average weather of a region over a long period, usually 30 years, together with the usual range of variation about that average (its seasons and extremes). A cold, rainy morning in Islamabad is weather. The fact that Islamabad has hot summers, a wet monsoon and cool winters, year after year, is its climate.

Weather: The state of the atmosphere (temperature, humidity, wind, pressure, cloud, precipitation) at a given place and time.
Climate: The statistical description of the weather of a region, its average and its variation, usually taken over 30 years. It is set by processes in the climate system such as ocean currents and wind patterns.
Climate system: The five interacting parts of the Earth (atmosphere, hydrosphere, cryosphere, lithosphere and biosphere) that together decide the climate.

Weather and climate compared.
Feature	Weather	Climate
Meaning	State of the atmosphere at a place at a given time	Average weather of a region and its usual range
Time scale	Hours to days	Usually 30 years or more
How it changes	Quickly, from hour to hour	Slowly, over decades and centuries
Example	It is raining in Lahore today.	Lahore has a hot, wet monsoon season every summer.
Predicted by	Weather forecasts (a few days ahead)	Climate models and long records (averages)

The Sun drives the climate. One of the key ingredients of the climate system is the radiant energy received from the Sun. Solar energy powers the atmospheric circulation (winds), the ocean currents and the hydrological (water) cycle. These carry heat and moisture from one part of the planet to another.

Surfaces matter. Oceans, ice caps, forests, deserts and cities absorb, reflect and emit radiation differently, so they warm by different amounts. This produces the complex patterns of temperature and rainfall across the globe. Living things take part too, through photosynthesis, respiration and the release of greenhouse gases.

Composition of the air. The atmosphere is mainly nitrogen and oxygen, with small amounts of greenhouse gases such as carbon dioxide and methane. These gases act like a blanket that keeps the planet warm.

Scientists study the climate system with satellite observations, computer models, field experiments and reconstructions of past climates (for example from ice cores and tree rings).

24.1Earth's climate system: five interacting componentsOften examined

Earth's climate system is a complex system with five interacting components: the atmosphere (air), the hydrosphere (water), the cryosphere (ice), the lithosphere (rock) and the biosphere (living things). Each component stores energy, water or carbon, and each exchanges them with the others. So a change in one part changes the others: melting ice raises sea level, reflects less sunlight and changes ocean currents.

The five components of the climate system. Each one exchanges energy, water and carbon with the others, and the Sun powers the whole system.

The five components of the climate system.
Component	What it contains	Main role in climate
Atmosphere (air)	The layer of gases around the Earth, mainly nitrogen and oxygen	Shields life from UV, keeps Earth warm (greenhouse gases), moves heat and moisture by winds
Hydrosphere (water)	All water: oceans, rivers, lakes, groundwater, water vapour	Stores a huge amount of heat, ocean currents carry heat, the water cycle brings rain
Cryosphere (ice)	All frozen water: snow, glaciers, sea ice, ice sheets, permafrost	Reflects sunlight (cools the Earth), stores fresh water, affects sea level
Lithosphere (rock)	The crust and the uppermost part of the mantle	Surface on which climate acts; mountains steer winds; volcanoes and rocks exchange carbon
Biosphere (life)	All living things and their surroundings	Photosynthesis removes $C O_{2}$ ; respiration and decay return it; plants add water vapour

Factors that influence global climate. The climate depends on (i) the Sun, which supplies almost all the energy; (ii) Earth's position in space relative to the Sun: its distance, the shape of its orbit and the tilt of its axis; and (iii) human-made factors such as greenhouse gas emissions, aerosols (smoke and dust) and cutting down forests.

Examples of interaction between the components

Water cycle: water evaporates from the oceans (hydrosphere) into the air (atmosphere), falls as snow on mountains (cryosphere), is taken up by plants (biosphere) and flows over rocks and soil (lithosphere) back to the sea.
Carbon cycle: plants take $C O_{2}$ from the air; oceans dissolve it; limestone rocks store it for millions of years; volcanoes and burning fossil fuels return it to the air.
Ice and sunlight: when ice melts, the darker ground or sea below absorbs more sunlight, which warms the air and melts more ice.
Volcanoes: an eruption in the lithosphere puts ash and gases into the atmosphere, which can cool the whole planet for a year or two.

24.1 (1)The atmosphere (air)Often examined

The atmosphere is the layer of air that envelops the Earth. It extends hundreds of kilometres above the surface but becomes thinner with height, so most of its mass lies in the lowest few kilometres. Gravity holds it to the Earth.

Composition of the atmosphere.
Gas	Share of dry air by volume
Nitrogen, $N_{2}$	78%
Oxygen, $O_{2}$	21%
Argon, $Ar$	0.9%
Trace gases: carbon dioxide ( $C O_{2}$ , about 0.04%), methane ( $C H_{4}$ ), ozone ( $O_{3}$ )	less than 0.1% together
Water vapour, $H_{2} O$	varies, from almost 0 to about 4%

The trace gases are small in amount but very important: water vapour, carbon dioxide, methane and ozone decide how much heat the atmosphere traps and how much ultraviolet radiation reaches the ground.

Roles of the atmosphere in the climate

Shielding. The ozone layer absorbs most of the harmful ultraviolet (UV) radiation from the Sun, and most meteors burn up before they reach the ground. This protects life on Earth.
Insulation. The atmosphere acts as an insulating layer that keeps the planet warm and prevents extreme temperature differences between day and night. The Moon, which has no atmosphere, heats to about $12 0^{\circ} C$ by day and cools to about $- 17 0^{\circ} C$ at night.
Convection and weather. The Sun heats the ground, the ground heats the air above it, and the warm air rises (convection). Rising and sinking air drives the winds, forms clouds and rain, and creates the weather patterns of the whole globe.
Greenhouse effect. Carbon dioxide and water vapour let visible sunlight pass to the surface but absorb part of the infrared radiation emitted by the warm surface. This traps heat and keeps the average surface temperature near $1 5^{\circ} C$ instead of about $- 1 8^{\circ} C$ .
Transport of heat and moisture. Winds carry heat from the tropics towards the poles and carry water vapour from the oceans over the land, where it falls as rain.

Layers of the atmosphere

The atmosphere is divided into layers according to height and to the way the temperature changes with height. The book gives these heights:

Layers of the atmosphere.
Layer	Height (book)	Temperature with height	Key facts
Troposphere	0 to about 16 km	Falls, by about $6. 5^{\circ} C$ per km	Weather occurs here; contains most of the air and nearly all the water vapour and clouds
Stratosphere	16 km to 50 km	Rises, because ozone absorbs UV	Contains the ozone layer; calm and stable; long-distance jets cruise near its base
Mesosphere	50 km to 136 km	Falls; its top is the coldest part of the atmosphere	Meteors burn up here
Thermosphere	136 km to about 600 km	Rises steeply (absorbs X-rays and far UV)	Auroras occur here; contains most of the ionosphere, which reflects radio waves
Exosphere	above 600 km	Gas so thin that temperature has little meaning	Outermost layer, merging into space; satellites orbit here

Layers of the atmosphere with the heights used in the book (not to scale). The curve on the right shows how the temperature changes with height: it falls, rises, falls and rises again.

Why the stratosphere is calm and stable

In the troposphere the air near the ground is the warmest, so warm air keeps rising and mixing the layer. This vertical movement (convection) produces clouds, storms and weather. In the stratosphere the opposite happens. The ozone layer absorbs ultraviolet radiation, so the temperature increases with height (a temperature inversion). Warm, light air sits on top of cooler, denser air, so there is almost no vertical mixing. There is also almost no water vapour, so there are no clouds or rain. The stratosphere is therefore calm and stable, which is why long-distance aircraft prefer to fly near its lower boundary.

The ionosphere

From roughly 60 km upwards, and mainly in the thermosphere, solar X-rays and ultraviolet radiation knock electrons out of gas atoms and produce ions. This ionised region is called the ionosphere. It reflects radio waves back to the Earth, which makes long-distance wireless communication possible. Auroras (the northern and southern lights) glow in the same region when charged particles from the Sun strike the gas atoms.

24.1 (2)The hydrosphere and the water balance equationOften examined

The hydrosphere is all the water on the Earth, in any form: liquid water (oceans, seas, rivers, lakes and groundwater), solid ice and gaseous water vapour. About 70% of the Earth's surface is covered by water, and most of it (about 97%) is salty ocean water. The oceans make the Earth look like a blue globe from space and set it apart from all the other planets in the solar system.

Characteristics of the hydrosphere

It plays a critical role in regulating the Earth's climate. Water has a high specific heat capacity, so the oceans store a large amount of heat and release it slowly.
It influences precipitation patterns and moves energy through the climate system. Evaporation takes latent heat from the surface; when the vapour condenses into clouds, that heat is released into the air.
Ocean currents, evaporation and condensation are all parts of the hydrological (water) cycle, which continuously moves water around the planet.

The hydrological cycle. Water evaporates from the sea and from plants, condenses into clouds, falls as precipitation, and returns to the sea as runoff and groundwater.

Water balance equation

The amount of water stored in a region, such as a lake, a river basin or a watershed, can change only because water comes in or goes out. The water balance equation is the basic tool for tracking this. It relates the input, the outputs and the change in storage over a period of time:

Δ S = P - E - Q

(24.1)

$Δ S$ : change in storage of water in the region (lakes, soil water, groundwater).
$P$ : precipitation, the water falling from the atmosphere as rain, snow or hail (input).
$E$ : evapotranspiration, the water lost by evaporation from the Earth's surface plus transpiration from plants (output).
$Q$ : runoff, the water that flows away over the surface into rivers and lakes, together with the water that infiltrates into the groundwater (output).

$P$ , $E$ , $Q$ and $Δ S$ are usually measured as a depth of water in millimetres, just like the reading of a rain gauge. To turn a depth into a volume, multiply the depth (in metres) by the area $A$ of the region (in $m^{2}$ ).

Derivation Water balance in terms of volume

Aim. Write the water balance equation for volumes, as used in Example 24.1 and in numerical problem 1.

Assumptions.

Each quantity is spread uniformly over the same area $A$ (the watershed).
Depths are converted to metres and the area to square metres before multiplying.

The volume of a uniform layer of water is its depth multiplied by the area it covers. Apply this to every term.
$P_{V} = P A, E_{V} = E A, Q_{V} = Q A, Δ S_{V} = Δ S A$
Multiply both sides of the water balance equation $Δ S = P - E - Q$ by the area $A$ .
$Δ S A = P A - E A - Q A$
Replace each product by the matching volume.
$Δ S_{V} = P_{V} - E_{V} - Q_{V}$
To get a depth back from a volume, divide by the area.
$P = \frac{P _{V}}{A}$

Result

Δ S_{V} = P_{V} - E_{V} - Q_{V} = (P - E - Q) A

If more water comes in than goes out ( $P > E + Q$ ), the storage increases ( $Δ S > 0$ ) and lakes and groundwater fill up. If $P < E + Q$ , the region dries out.

Example 24.1 (book)

Find the change in storage of hydrosphere in terms of volume whose area of the watershed is

120 k m^{2}

, with precipitation of

110 mm

while evapotranspiration and runoff are

60 mm

and

30 mm

respectively.

Given

$A = 120 k m^{2} = 120 \times 1 0^{6} m^{2} = 1.2 \times 1 0^{8} m^{2}$ area of the watershed
$P = 110 mm = 0.110 m$ precipitation
$E = 60 mm = 0.060 m$ evapotranspiration
$Q = 30 mm = 0.030 m$ runoff

To find

change in storage by volume, $Δ S_{V}$

Convert each depth into a volume using volume = depth $\times$ area.
$P_{V} E_{V} Q_{V} = 0.110 \times 1.2 \times 1 0^{8} = 1.32 \times 1 0^{7} m^{3} = 0.060 \times 1.2 \times 1 0^{8} = 7.2 \times 1 0^{6} m^{3} = 0.030 \times 1.2 \times 1 0^{8} = 3.6 \times 1 0^{6} m^{3}$
Apply the water balance equation for volumes.
$Δ S_{V} = P_{V} - E_{V} - Q_{V} = 13.2 \times 1 0^{6} - 7.2 \times 1 0^{6} - 3.6 \times 1 0^{6}$
Evaluate.
$Δ S_{V} = 2.4 \times 1 0^{6} m^{3}$
Quick check: find the net depth first and multiply by the area only once.
$Δ S Δ S_{V} = 110 - 60 - 30 = 20 mm = 0.020 \times 1.2 \times 1 0^{8} = 2.4 \times 1 0^{6} m^{3}$

Δ S_{V} = 2.4 \times 1 0^{6} m^{3} (2, 400, 000 m^{3})

Assignment 24.1 (solved)

Find the precipitation in mm if the change in storage of water is

20 mm

, while evapotranspiration is

50 mm

and runoff is

30 mm

Make $P$ the subject of $Δ S = P - E - Q$ by adding $E$ and $Q$ to both sides.
$P = Δ S + E + Q$
All three quantities are depths in mm, so no conversion is needed.
$P = 20 + 50 + 30 = 100 mm$

P = 100 mm

24.1 (3)The cryosphere (ice)

The cryosphere is every part of the Earth where water is in its solid form, where low temperatures freeze water into ice or snow. It exists in the polar regions and wherever there is snow, sea ice, glaciers, permafrost, ice sheets, ice shelves and icebergs. In Pakistan, examples of the cryosphere are the snow-capped mountains and glaciers of the Himalaya, Karakorum and Hindu Kush ranges in the north, whose meltwater feeds the Indus river system.

Many of these places are remote, so direct measurements are difficult. Satellites monitor how the snow and ice cover shrinks and expands, and this is one of the clearest signals of regional and global climate change.

Components of the cryosphere

Snow: Precipitation that forms when water vapour freezes into ice crystals. It forms when the air is humid and cold. Snow reflects most of the sunlight that falls on it back into space, so it cools the planet.
Ice: Liquid water that has become solid below its freezing point. Ice makes up sea ice, glaciers, ice shelves, icebergs and frozen ground. It covers about 10% of the Earth's land surface and is disappearing rapidly.
Permafrost: Permanently frozen ground: soil, rock and ice that stay frozen for long periods (at least two years in a row). It is found mainly in high-latitude regions such as Siberia, Alaska and northern Canada, and also high in mountains.
Glacier: A large mass of ice on land, formed from compacted snow, that moves slowly downhill under its own weight.
Ice sheet: A glacier that covers a very large land area, such as Antarctica or Greenland.
Ice shelf: The floating edge of an ice sheet or glacier that extends over the sea.
Iceberg: A large block of freshwater ice that has broken off a glacier or ice shelf and floats in the sea, with about 90% of it under water.
Sea ice: Frozen sea water floating on the ocean surface.

Components of the cryosphere: snow, ice cap, glacier, permafrost, sea ice, iceberg, ice shelf and ice sheet.

Effects of the cryosphere on the climate

Reflection of sunlight. Snow and ice reflect much of the incoming sunlight back into space (high albedo). This keeps the poles cold and cools the whole planet.
Salinity of sea water. When sea water freezes, most of the salt stays in the water below. The sea ice layer therefore increases the salinity (and density) of the surrounding sea water, which helps to drive deep ocean currents (section 24.2).
Protection of coasts. Sea ice restricts wind and wave action near coastlines.
Sea level and fresh water. Glaciers and ice sheets store fresh water. When they melt, sea level rises and river flows change.

24.1 (4)The lithosphere (Earth's upper rocky layer)

The lithosphere is the outermost solid shell of the Earth. It consists of the crust and the uppermost part of the mantle, and it is made of various rocks and minerals. It is broken into large pieces called tectonic plates, which move slowly over a softer, partly molten layer of the upper mantle called the asthenosphere. The lithosphere interacts with the atmosphere, the hydrosphere and the biosphere.

Features of the lithosphere that affect the climate

Land surface. The lithosphere provides the surface on which climate processes take place. Different land surfaces affect the climate differently: forests moderate the temperature, while deserts have extreme temperature changes between day and night.
Carbon cycle. Rocks store carbon and exchange it with the atmosphere over very long times. Volcanoes and the weathering and erosion of some rocks release carbon dioxide into the atmosphere, while rocks such as limestone are carbon sinks that lock carbon away over geological time.
Tectonic activity. Tectonic plates move a few centimetres a year. Over millions of years their collisions build mountain ranges; for example, the Himalaya formed where the Indian plate pushes into the Eurasian plate. Mountains block and steer the winds and force moist air to rise and give rain, so they change the atmospheric circulation and the regional climate.
Volcanic activity. Volcanic eruptions, which are linked to tectonic activity, release large amounts of gases and fine particles into the atmosphere. These particles block sunlight and can cool the climate temporarily, causing short cooling events called volcanic winters.

Movement of tectonic plates and volcano formation. An oceanic plate sinks under a continental plate, forming a trench; the rock melts at depth and the magma rises to form volcanoes. The lithosphere (crust plus uppermost mantle) rides on the soft asthenosphere.

24.1 (5)The biosphere (living things)

The biosphere is the relatively thin, life-supporting layer of the Earth's surface. It extends from a few kilometres up into the atmosphere down to the deep-sea vents on the ocean floor. It consists of all living organisms (plants, animals, fungi and microorganisms) together with the non-living factors (air, water, soil and sunlight) from which they get energy and nutrients. It deals with living things and their effect on the climate system.

Role of the biosphere in the climate system

Climate regulation. The biosphere plays a key role in regulating the climate. Changes in the biosphere, such as cutting down forests, directly change local temperature, rainfall and the amount of $C O_{2}$ in the air.
Carbon cycle. The biosphere is a major reservoir in the carbon cycle. In photosynthesis, living things absorb carbon dioxide from the atmosphere and turn it into organic matter; respiration and decay return it to the air.
Energy flow. Plants capture solar energy during photosynthesis. This energy flows along food chains as organisms eat one another, and this energy transfer affects climate dynamics.
Water cycle. Living things take part in evaporation, condensation, precipitation and especially transpiration (water released from plant leaves).
Nutrient cycles. Elements such as carbon ( $C$ ), nitrogen ( $N_{2}$ ) and phosphorus ( $P$ ) cycle through the biosphere, affecting both living things and the environment.

6 C O_{2} + 6 H_{2} O sunlight C_{6} H_{12} O_{6} + 6 O_{2}

Photosynthesis removes carbon dioxide from the air and stores the carbon in plant material.

The carbon cycle. Photosynthesis takes

C O_{2}

out of the air; respiration, decay and the burning of fossil fuels put it back.

The biosphere is essential for maintaining the variety of species (biodiversity), for regulating the climate and for supporting life on the Earth. It overlaps with all the other spheres: it takes gases from the atmosphere, water from the hydrosphere and minerals from the lithosphere.

24.2Ocean currents and surface currentsOften examined

Ocean currents and wind patterns are connected processes that play a crucial role in regulating the Earth's climate. Winds drive the surface currents, and both carry heat from the warm tropics towards the cold poles.

Ocean water is always moving. Its movement is broadly of three types: waves, tides and currents. Water moves both horizontally and vertically: horizontal movements are called currents, while vertical movements are called upwelling (deep water rising to the surface) and downwelling (surface water sinking).

Ocean currents: The streams of water that flow constantly on the ocean surface in definite directions. They are continuous, predictable and directional movements of sea water driven by gravity, wind and differences in water density.

Ocean currents distribute heat around the globe and so shape climate patterns. They also carry nutrients and food to marine organisms and support sea life. Ocean currents are of two kinds: surface ocean currents and deep ocean currents.

Surface ocean currents

Surface ocean currents are the currents produced in the shallower water near the surface (roughly the top few hundred metres). They are produced by:

Wind. The major surface currents are set in motion by the wind. As the wind blows across the ocean it drags the surface water along by friction, creating a current.
Subtropical gyres. Large rotating currents that start near the equator are called subtropical gyres. They carry warm water from the equator towards the poles, which helps to moderate the climate. They also collect floating plastic rubbish in certain areas of the ocean, such as the Great Pacific Garbage Patch.
The Coriolis effect. The Earth's rotation deflects moving water to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, and so bends the currents into loops.

How the Coriolis effect shapes a subtropical gyre

Just north of the equator the trade winds blow from east to west and pull the surface water westwards.
The Coriolis effect deflects the moving water to the right, so near the western side of the ocean the current turns and heads north (for example the Gulf Stream in the Atlantic).
At about $3 0^{\circ}$ north latitude and beyond, the westerlies push the water back towards the east.
Along the eastern side of the ocean the water turns south and returns towards the equator. The result is a closed clockwise loop.
In the Southern Hemisphere the deflection is to the left, so the gyres turn anticlockwise (counter-clockwise).

Trade winds and westerlies drive the surface water; the Coriolis effect bends the currents into gyres, clockwise in the Northern Hemisphere and anticlockwise in the Southern Hemisphere.

24.2 (2)Deep ocean currents and the density of sea waterOften examined

Ocean currents deeper than about 500 m are called deep ocean currents. The wind cannot reach them. They are driven by differences in the temperature and salinity (salt content), and therefore in the density, of the water in different parts of the ocean.

ρ = \frac{m}{V}

Density $ρ$ is mass $m$ per unit volume $V$ , in $kg m^{- 3}$ . Typical sea water: about $1025 kg m^{- 3}$ ; fresh water: about $1000 kg m^{- 3}$ .

What controls the density of sea water.
Change	Effect on density	Reason
More salt (higher salinity)	Density increases	More dissolved salt is packed into the same volume, so each cubic metre has more mass
Lower temperature	Density increases	Cold water contracts, so the same mass takes up less volume
Higher temperature	Density decreases	Warm water expands
Greater depth (pressure)	Density increases slightly	The weight of the water above squeezes the deep water molecules closer together

So the densest water is cold and salty. Dense water sinks beneath less dense water, in the same way that cold air sinks beneath warm air. Wherever surface water becomes denser than the water below it, it sinks, and this vertical motion drives the deep currents.

How deep currents form

Warm water moves north. Surface currents carry warm water north from the equator. As this water moves into higher northern latitudes it cools down.
Its density increases. Cooling increases the water's density. In the polar seas, when sea ice forms it leaves most of its salt behind, so the water below becomes saltier and denser still. Evaporation also raises the salinity.
Dense water sinks. Near the surface, the combination of high salinity and low temperature makes the water dense enough to sink into the deep ocean.
Deep flow. The dense water flows back towards the equator at deep levels, forming deep ocean currents.

Water movement due to density difference. In the polar seas, cooling and the salt left behind by freezing sea ice make cold, salty, dense water that sinks and flows along the sea floor towards the equator.

Deep circulation affects the entire ocean, including the abyssal zone: the deepest region of the ocean, with no light, very little oxygen, very high pressure and very few living things. Wind-driven currents have no effect there; only the slow, density-driven flow renews its water.

24.2Thermohaline circulationOften examined

The deep ocean currents caused by differences in water density make up the thermohaline circulation. The name has two parts: thermo refers to temperature and haline to saltiness (salinity), the two quantities that set the density of sea water.

Thermohaline circulation: A large-scale ocean circulation driven by density differences that are created by surface heat and freshwater exchanges (heating, cooling, evaporation, rainfall, freezing and melting).

How it transports heat from the tropics to the polar regions

Heating in the tropics. Strong sunlight warms the surface water near the equator. Warm water is less dense, so it stays at the surface.
Surface flow towards the poles. Wind-driven surface currents carry this warm water from the equatorial region towards the poles, for example the Gulf Stream and the North Atlantic Drift.
Heat is given out at high latitudes. On the way, the water gives heat to the cold air above it. This keeps places such as north-west Europe much warmer than other places at the same latitude.
Sinking. The cooled water becomes denser, and sea-ice formation and evaporation make it saltier. The cold, salty, dense water sinks at high latitudes, near Greenland and Antarctica.
Deep spreading. The dense water spreads slowly back towards the equator along the ocean floor.
Upwelling. Far away, the deep water slowly rises to the surface again, bringing cold, nutrient-rich water upwards, and it is warmed once more. The cycle continues, transporting energy (heat) and mass around the globe.

Thermohaline circulation in cross-section: heating in the tropics, warm surface flow towards the pole, cooling and sinking at high latitudes, deep spreading of cold, dense water and upwelling.

Surface and deep ocean currents compared.
Feature	Surface ocean currents	Deep ocean currents
Depth	Upper ocean, roughly the top 400 to 500 m	Below about 500 m
Main cause	Wind, bent by the Coriolis effect	Density differences (temperature and salinity)
Speed	Fast, up to about $2 m s^{- 1}$ in the Gulf Stream	Very slow, a few centimetres per second or less
Water	Warm or cool, less dense	Cold, salty and dense
Pattern	Gyres: clockwise (north), anticlockwise (south)	Global conveyor belt linking all the oceans
Share of ocean water	About 10%	About 90%
Role in climate	Moves heat quickly from the equator towards the poles; sets coastal climates	Moves heat, nutrients and dissolved gases over centuries; gives the climate a long memory

24.2Earth's angular momentum and the Coriolis forceOften examined

The Earth spins once a day about its axis, and every point on its surface is carried eastwards. The speed is not the same everywhere. A point on the equator is far from the axis and moves at about $465 m s^{- 1}$ ; a point near a pole is close to the axis and hardly moves. Air or water that travels north or south therefore arrives over ground that is moving at a different speed, so it appears to curve. This apparent turning is the Coriolis effect.

v_{ground} = ω r = ω R cos θ

$ω$ : angular velocity of the Earth, $R$ : radius of the Earth ( $6.37 \times 1 0^{6} m$ ), $θ$ : latitude, $r = R cos θ$ : distance from the Earth's axis.

The ground moves more slowly at higher latitudes.
Latitude $θ$	Distance from the axis $r = R cos θ$	Eastward speed of the ground
$0^{\circ}$ (equator)	$6.37 \times 1 0^{6} m$	$465 m s^{- 1}$
$3 0^{\circ}$	$5.52 \times 1 0^{6} m$	$402 m s^{- 1}$
$6 0^{\circ}$	$3.19 \times 1 0^{6} m$	$232 m s^{- 1}$
$9 0^{\circ}$ (pole)	$0$	$0$

Conservation of angular momentum

Angular momentum is a property of rotating objects. It depends on the speed of rotation and on the perpendicular distance from the axis of rotation. For a parcel of air of mass $m$ moving eastwards with speed $v$ at a distance $r$ from the Earth's axis:

L = m v r = m v R cos θ

If no twisting force (torque) acts, $L$ stays constant. So air that moves to a place where $r$ is smaller (towards a pole) must speed up eastwards, and air that moves to a place where $r$ is larger (towards the equator) must slow down. The book describes this with the moment of inertia: near the equator the distance from the axis is large, near the poles it is small.

Air moving towards a pole (for example high up in the Hadley cell) reaches latitudes where the ground is slower, while the air itself speeds up. It runs ahead of the ground and turns eastwards. In the Northern Hemisphere a turn from northwards to eastwards is a turn to the right. This produces the westerly winds and the jet streams.
Air moving towards the equator (for example near the ground from $3 0^{\circ}$ back to the equator) reaches latitudes where the ground moves faster than the air. The air lags behind and turns westwards. For air moving south in the Northern Hemisphere this is again a turn to the right, and it becomes the north-east trade wind.
In the Southern Hemisphere the same reasoning gives a deflection to the left.

Size of the angular momentum effect (illustration)

Air at rest on the equator rises and moves north to latitude

3 0^{\circ}

without friction. Use conservation of angular momentum to find its eastward speed relative to the ground at

3 0^{\circ}

. Take

ω = 7.29 \times 1 0^{- 5} rad s^{- 1}

and

R = 6.37 \times 1 0^{6} m

At the equator the air moves with the ground, so its speed and its distance from the axis are:
$v_{0} = ω R = 7.29 \times 1 0^{- 5} \times 6.37 \times 1 0^{6} = 464 m s^{- 1}, r_{0} = R$
At $3 0^{\circ}$ the distance from the axis is $r = R cos 3 0^{\circ}$ . Conservation of angular momentum, $m v_{0} r_{0} = m v r$ , gives:
$v = \frac{v _{0} R}{R cos 3 0 ^{\circ}} = \frac{464}{0.866} = 536 m s^{- 1}$
The ground at $3 0^{\circ}$ moves eastwards at:
$v_{ground} = ω R cos 3 0^{\circ} = 402 m s^{- 1}$
Relative to the ground, the air now blows towards the east, so it has turned to the right:
$536 - 402 \approx 134 m s^{- 1}$

the air turns east at about 1.3 \times 1 0^{2} m s^{- 1} relative to the ground

The Coriolis force

The turning of winds and currents due to the rotation of the Earth is called the Coriolis effect. To use Newton's laws in the rotating frame of the ground, we say that a sideways force acts on every moving body. This apparent force, which bends the winds because of the change in speeds, is the Coriolis force. Its magnitude is:

F_{c} = 2 m v ω sin θ

(24.2)

$F_{c}$ : Coriolis force, in newtons ( $N$ ).
$m$ : mass of the moving air packet ( $kg$ ).
$v$ : wind speed relative to the ground ( $m s^{- 1}$ ).
$ω$ : angular velocity of the Earth's rotation, approximately $7.292 \times 1 0^{- 5} rad s^{- 1}$ (the book's examples use $7.3 \times 1 0^{- 5} rad s^{- 1}$ ).
$θ$ : latitude, $0^{\circ}$ at the equator and $\pm 9 0^{\circ}$ at the poles.

Direction. The Coriolis force always acts at right angles to the velocity: to the right of the motion in the Northern Hemisphere and to the left in the Southern Hemisphere.

Main features of the Coriolis force

Zero at the equator: $sin 0^{\circ} = 0$ , so $F_{c} = 0$ .
Maximum at the poles: $sin 9 0^{\circ} = 1$ , so $F_{c} = 2 m v ω$ . Winds are deflected more strongly at higher latitudes, closer to the poles.
Proportional to speed: air at rest ( $v = 0$ ) feels no Coriolis force, and faster winds are deflected more strongly.
Changes direction, not speed: it acts at right angles to the velocity, so it does no work. The magnitude of the wind speed is not directly affected; only the direction of the wind changes.
Apparent force: it appears only because we observe from the rotating Earth. Seen from space, the air moves in a straight line while the Earth turns beneath it.

Derivation Where $F_{c} = 2 m v ω sin θ$ comes from (turntable model)

Aim. Show that a body moving with speed $v$ across a surface that rotates with angular velocity $ω$ appears to be pushed sideways with acceleration $2 v ω$ , and extend the result to latitude $θ$ .

Assumptions.

The book states the formula without proof. This simple model is for understanding only.
A flat turntable (like the region around the North Pole seen from above) rotates anticlockwise with angular velocity $ω$ .
No real horizontal force acts on the ball, so seen from above (a non-rotating frame) it moves in a straight line at constant speed $v$ .

Top view of a turntable turning anticlockwise. The ball travels straight from O to B, but the mark A has moved to A

^{'}

, so the rider sees the ball curve to the right by the arc

s

A ball is rolled from the centre O towards the point A on the rim. After a short time $t$ it has travelled in a straight line a distance:
$r = v t$
In the same time the turntable has turned through an angle $ω t$ , so the mark A has moved to A $^{'}$ . A person riding on the turntable expected the ball to be on the line OA $^{'}$ .
$angle turned = ω t$
The ball is therefore off the expected line by an arc of radius $r$ and angle $ω t$ (arc length = radius $\times$ angle).
$s = r (ω t) = (v t) (ω t) = v ω t^{2}$
To the rider this sideways drift starts from zero and grows as $t^{2}$ , exactly like motion from rest with constant acceleration $a$ , for which $s = \frac{1}{2} a t^{2}$ . Compare the two expressions.
$\frac{1}{2} a t^{2} = v ω t^{2} ⟹ a = 2 v ω$
Newton's second law gives the apparent sideways force. The drift is to the right of the motion because the table turns anticlockwise, as the Northern Hemisphere does when seen from above the North Pole.
$F = ma = 2 m v ω$
At latitude $θ$ the ground does not spin fully about the local vertical. Only the part $ω sin θ$ of the Earth's rotation turns the local horizontal plane: all of $ω$ at a pole ( $θ = 9 0^{\circ}$ ) and none at the equator ( $θ = 0^{\circ}$ ). Replace $ω$ by $ω sin θ$ .
$F_{c} = 2 m v ω sin θ$

Result

F_{c} = 2 m v ω sin θ

The Coriolis force grows with speed and with latitude. It vanishes at the equator and is largest at the poles, and it always acts at right angles to the motion, so it bends the path without changing the speed.

Coriolis force and latitude

100

200

300

400

500

600

700

θ

F_{c} (N)

(2 5^{\circ}, 274 N)

maximum at the pole

zero at the equator

F_{c} = 648 sin θ

Coriolis force on the air packet of Example 24.2 (

m = 2 \times 1 0^{5} kg

v = 22.2 m s^{- 1}

) at latitudes

θ

from

0^{\circ}

9 0^{\circ}

. It follows a sine curve: zero at the equator and largest at the pole.

Example 24.2 (book)

Find the magnitude of the Coriolis force which bends the path of air mass

200, 000 kg

up to

2 5^{\circ}

, while the mass of air is moving at the speed of

80 km h^{- 1}

Given

$m = 200, 000 kg = 2.0 \times 1 0^{5} kg$ mass of the air packet
$v = 80 km h^{- 1}$ speed of the air
$θ = 2 5^{\circ}$ latitude
$ω = 7.3 \times 1 0^{- 5} rad s^{- 1}$ angular velocity of the Earth

To find

Coriolis force $F_{c}$

Convert the speed to SI units: $1 km h^{- 1} = \frac{1000}{3600} m s^{- 1}$ .
$v = \frac{80 \times 1000}{3600} = 22.2 m s^{- 1}$
Write the formula for the Coriolis force.
$F_{c} = 2 m v ω sin θ$
Substitute the values.
$F_{c} = 2 (2.0 \times 1 0^{5}) (22.2) (7.3 \times 1 0^{- 5}) sin 2 5^{\circ} = 648 \times 0.423$
Evaluate.
$F_{c} = 274 N$

F_{c} \approx 274 N

Assignment 24.2 (solved)

Find the angle through which a Coriolis force of

200 N

will deflect the air mass of

100, 000 kg

moving at the speed of

60 km h^{- 1}

Given

$F_{c} = 200 N$
$m = 1.0 \times 1 0^{5} kg$
$v = 60 km h^{- 1} = 16.7 m s^{- 1}$
$ω = 7.3 \times 1 0^{- 5} rad s^{- 1}$

To find

the angle (latitude) $θ$

Make $sin θ$ the subject of $F_{c} = 2 m v ω sin θ$ .
$sin θ = \frac{F _{c}}{2 m v ω}$
Substitute the values.
$sin θ = \frac{200}{2 ( 1.0 \times 1 0 ^{5} ) ( 16.7 ) ( 7.3 \times 1 0 ^{- 5} )} = \frac{200}{243} = 0.822$
Take the inverse sine (calculator in degree mode).
$θ = sin^{- 1} (0.822) = 55. 3^{\circ}$

Here $θ$ is the latitude at which the air mass is moving; the book calls it 'the angle'.

θ \approx 5 5^{\circ}

24.2Atmospheric circulation cells and wind beltsOften examined

The equator is heated strongly, so air rises there; near 30^\circ and at the poles, air sinks. If the Earth did not rotate, one giant loop in each hemisphere would carry air from the equator to the pole high up and back again along the ground. The rotation breaks each hemisphere's circulation into three cells, because the Coriolis effect turns the moving air sideways before it can travel far north or south. The deflection of air by the Coriolis effect therefore creates the distinct atmospheric circulation cells.

The three-cell model

Hadley cell (equator to 30^\circ): warm air rises at the equator because of intense heating, moves towards the pole at high altitude and finally descends around 30^\circ latitude. It returns to the equator along the surface as the trade winds, completing the cycle.
Ferrel cell (30^\circ to 60^\circ): air descends around 30^\circ, moves towards the pole along the surface and rises at 60^\circ latitude, then moves back towards the equator at high altitude. Its surface winds are the westerlies.
Polar cell (60^\circ to the pole): air rises at 60^\circ latitude, moves towards the pole at high altitude and finally descends at the pole. It returns along the surface as the polar easterlies.

The three-cell model of one hemisphere (not to scale). Rising air gives low pressure (L) at the surface and sinking air gives high pressure (H). The surface winds blow from high to low pressure and are turned by the Coriolis effect.

Pressure belts.
Latitude	Air motion	Surface pressure	Name and weather
0^\circ	rises	low	Equatorial low, the Intertropical Convergence Zone (ITCZ): light winds, heavy rain
about 30^\circ	sinks	high	Subtropical high: dry, clear skies; most hot deserts lie here
about 60^\circ	rises	low	Subpolar low: cloudy and stormy
90^\circ	sinks	high	Polar high: cold and dry

Surface wind belts. Winds are named after the direction they blow from.
Wind belt	Latitudes	Blows from (Northern Hemisphere)	Blows from (Southern Hemisphere)
Trade winds	0^\circ to 30^\circ	north-east (north-east trades)	south-east (south-east trades)
Westerlies	30^\circ to 60^\circ	south-west	north-west
Polar easterlies	60^\circ to 90^\circ	north-east	south-east

Surface wind belts. Air moving towards the equator is turned to the right in the Northern Hemisphere (north-east trades) and to the left in the Southern Hemisphere (south-east trades); air moving towards the poles becomes the westerlies.

These cells distribute heat and moisture around the globe and so set the pattern of weather and climate: wet equatorial forests under the rising air of the ITCZ, deserts under the sinking air near 30^\circ, and stormy weather along 60^\circ.

24.2Geostrophic wind and the importance of wind patternsOften examined

Wind patterns are an essential part of the climate system and shape both regional and global climate. They result from complex interactions between atmospheric and oceanic processes, and they distribute heat, moisture and energy across the Earth's surface over both short and long time scales.

Forces on moving air

Pressure gradient: The change in pressure measured across a given distance, \Delta p/\Delta x.
Pressure gradient force: The force that drives air from areas of higher pressure to areas of lower pressure. The closer together the isobars, the steeper the gradient, the stronger the force and the faster the wind.
Isobars: Lines on a weather map joining places that have the same air pressure.
Geostrophic wind: A wind that blows parallel to the isobars because the pressure gradient force is balanced by the Coriolis force (friction and changes in wind speed and direction are ignored).

Geostrophic balance. The basic relationship between atmospheric pressure and horizontal wind is described by the geostrophic balance. As an air mass starts to move from high towards low pressure, the Coriolis force deflects it to the right (in the Northern Hemisphere). The deflection increases until the Coriolis force is equal and opposite to the pressure gradient force. At this point the wind blows parallel to the isobars; it is then called the geostrophic wind.

F_{\text{pressure gradient}} = F_c = 2mv\omega\sin\theta \qquad (\text{geostrophic balance})

Geostrophic wind in the Northern Hemisphere. The pressure gradient force (towards low pressure) is balanced by the Coriolis force, so the wind blows parallel to the isobars with low pressure on its left.

Wind speed increases as the distance between isobars decreases, that is, as the pressure gradient increases. Near the ground, friction slows the wind, so surface winds cross the isobars at a small angle towards low pressure; the geostrophic balance describes winds well above the ground.

Importance of wind patterns for the climate

Heat transport: winds carry heat from one region to another. For example, ocean currents driven by winds carry warm water from the equator towards the poles.
Moisture transport: winds carry water vapour from the oceans over the land and so decide precipitation patterns. (Precipitation is any liquid or frozen water that forms in the atmosphere and falls back to the Earth.) The trade winds, westerlies and polar easterlies distribute moisture around the globe.
Pollutant dispersion: winds spread dust, smoke and industrial emissions over large areas.
Pollen distribution: winds carry pollen and help plants to reproduce.
Daily weather patterns: coastal regions have daily changes in wind direction because land and water heat up differently (differential heating).

Land and sea breezes (differential heating)

Land has a lower specific heat capacity than water, so by day it heats up faster. The air above the land rises and cooler air flows in from the sea: a sea breeze (as felt in Karachi on summer afternoons). At night the land cools faster, so the flow reverses and a land breeze blows out to sea.

Over a 30-year period these wind patterns show statistical regularities, and these regularities form part of the climate of a region. Changes in the patterns over long time scales can change regional and even global climate. Monitoring winds helps to predict weather events, map natural resources (such as sites for wind power) and assess the impacts of climate change.

24.3Climate inertiaOften examined

A large pot of water does not boil the moment the stove is switched on, and it stays hot for a while after the stove is switched off. The Earth's climate behaves in a similar way. Climate inertia is the tendency of the Earth's climate to resist, or respond slowly to, changes in external factors such as greenhouse gas concentrations and solar radiation. It arises from complex feedback mechanisms and slow processes within the climate system, above all the huge heat capacity of the oceans.

Climate inertia: The tendency of the Earth's climate to resist or respond slowly to changes in external factors like greenhouse gas concentrations and solar radiation.

Feedback mechanisms

Positive feedback: A process that amplifies an initial change. Example: as Arctic ice melts because of warming, the Earth's reflectivity (albedo) falls; the darker surface absorbs more sunlight, which causes further warming.
Negative feedback: A process that dampens an initial change. Example: higher carbon dioxide levels stimulate plant growth; the plants absorb some of the extra carbon, which slows the rise of \mathrm{CO_2}.

A positive feedback (ice and albedo) amplifies a change; a negative feedback (plant growth) reduces it.

Time dependence: short-term and long-term inertia

Climate inertia depends on the time scale. If greenhouse gas emissions were stabilised immediately, the Earth's climate would continue to warm for some time because of the heat already stored in the oceans. Climate processes operate on time scales from days to centuries:

Short-term inertia: the oceans and land surfaces absorb and release heat relatively slowly. This smooths the changes of temperature from day to night and from season to season. For example, the hottest time of day comes in the afternoon, a few hours after the Sun is highest, and coastal towns such as Karachi have much smaller day and night temperature differences than inland deserts.
Long-term inertia: changes in greenhouse gas concentrations, such as carbon dioxide, take decades to centuries to affect the climate system fully. When greenhouse gas emissions are stabilised, the climate system does not respond immediately; it keeps warming slowly towards a new balance.

Climate inertia (schematic)

Time t in years. After a sudden, lasting increase in greenhouse forcing at t = 0, the temperature rises quickly at first (land and upper ocean) and then keeps creeping up for centuries as the deep ocean warms. This slow tail is climate inertia.

Why the oceans cause most of the inertia

Oceans play the main role in climate inertia. Water has a high specific heat capacity (about 4200\,\mathrm{J\,kg^{-1}\,K^{-1}}, against about 1000\,\mathrm{J\,kg^{-1}\,K^{-1}} for air) and the oceans have an enormous mass. The whole ocean can store about 1000 times more heat than the whole atmosphere for the same rise in temperature. The oceans therefore absorb heat slowly and release it slowly, and changes in ocean temperature, and so in climate patterns, happen gradually over long periods.

Q = mc\Delta T

Heat Q needed to raise the temperature of a mass m by \Delta T; c is the specific heat capacity. A large mc means a large amount of heat for a small change in temperature.

Carbon cycle: carbon dioxide is exchanged slowly between the atmosphere, the oceans, the land and vegetation, and organic matter decomposes slowly, so extra \mathrm{CO_2} stays in the air for a long time.
Ice sheets and glaciers: they melt over centuries, so their contribution to sea level rise continues long after the warming that caused it.

Adaptation and mitigation

Because of climate inertia, the effects of past emissions will continue even if we act immediately. Climate inertia therefore shows why we need both adaptation (adjusting to changes that are already happening, such as flood defences and heat-tolerant crops) and mitigation (reducing emissions, for example by using renewable energy and planting forests).

Changes in the Earth's climate can be divided into two main kinds: internal variations and external forcing (next two sections).

24.3.1Internal variations

Internal variations are natural fluctuations within the climate system itself. They arise from the interactions between the components of the climate system, such as the atmosphere, the oceans, the land and the ice. Examples given in the book:

Atmospheric circulation patterns: changes in circulations such as the jet streams and the monsoons cause the climate to vary from region to region and from year to year.
Solar radiation: variations in solar radiation due to sunspots and solar cycles affect the climate over shorter time scales.
Volcanic activity: eruptions release aerosols into the atmosphere and change global temperatures temporarily.
Ocean oscillations: repeating patterns of ocean currents and sea surface temperatures, such as the El Niño Southern Oscillation (ENSO), the Atlantic Multidecadal Oscillation (AMO) and the Pacific Decadal Oscillation (PDO), change weather patterns around the globe.

Internal variations are often cyclic and occur over periods of years to decades or centuries.

Main ocean oscillations.
Oscillation	Where	Typical period	Effect
ENSO (El Niño and La Niña)	Tropical Pacific Ocean	2 to 7 years	El Niño weakens the trade winds and warms the eastern Pacific; it often brings drought to Australia and South Asia, including a weaker monsoon in Pakistan
PDO	North Pacific Ocean	20 to 30 years	Shifts sea temperatures and storm tracks over the Pacific
AMO	North Atlantic Ocean	60 to 80 years	Affects Atlantic hurricanes and rainfall in Africa, Europe and North America

An ocean oscillation (ENSO). In a normal year strong trade winds push warm water to the western Pacific; in an El Niño year they weaken and the warm water spreads east, moving the rain with it.

24.3.2External forcing

External forcing refers to factors outside the climate system that influence its behaviour. These factors alter the Earth's energy balance (energy coming in against energy going out) and so lead to long-term climate change. External forcing factors operate over long time frames, from decades and centuries to many thousands of years, and produce gradual climate shifts.

Greenhouse gas concentrations: human activities have increased the levels of carbon dioxide, methane and other greenhouse gases. This enhances the greenhouse effect, traps more heat and causes global warming.
Solar variability: changes in the Sun's output over long time scales affect the Earth's climate.
Orbital variations: the Earth's orbit around the Sun and the direction of its axis change periodically. The eccentricity of the orbit, the tilt of the axis and the wobble (precession) of the axis change how solar energy is distributed over latitudes and seasons. These are long-term changes, acting over tens of thousands of years, and they have paced the ice ages.
Aerosols and pollution: an aerosol is a suspension of tiny particles or droplets in the air, such as dust, mist, fumes or smoke. Anthropogenic (human-made) aerosols from industry and combustion reflect sunlight back into space and so cool the climate.

Orbital variations: eccentricity of the orbit, obliquity (tilt) of the axis and precession (wobble) of the axis.

Orbital variations (Milankovitch cycles).
Orbital change	What changes	Period (approximate)
Eccentricity	Shape of the orbit, from nearly circular to slightly elliptical	100,000 and 400,000 years
Obliquity (axial tilt)	Tilt of the axis, between about 22.1^\circ and 24.5^\circ (now 23.4^\circ)	41,000 years
Precession	Direction in which the axis points (a wobble like a spinning top)	about 21,000 years

Internal variations and external forcing compared.
Feature	Internal variations	External forcing
Origin	Inside the climate system (interactions of air, ocean, ice and land)	Outside the climate system
Effect on energy	Moves energy around within the system	Changes the energy entering or leaving the system
Time scale	Years to decades, often cyclic	Decades to thousands of years, gradual
Examples	ENSO, AMO, PDO, changes in jet streams and monsoons	Greenhouse gases, solar output, orbital changes, aerosols

24.4Earth's energy budgetOften examined

Almost all the energy that drives the Earth's climate comes from the Sun. The solar radiation arriving at the Earth is called insolation (incoming solar radiation). The Earth also gives out energy to space as infrared terrestrial radiation. The global climate is decided by the balance between the two.

Earth's energy budget: A way of accounting for the balance between the energy that the Earth receives from the Sun and the energy it radiates back into space. It is the balance between the incoming solar radiation (insolation) and the outgoing terrestrial radiation.

Components of the energy budget

Take the sunlight arriving at the top of the atmosphere as 100 units (on average about 342\,\mathrm{W\,m^{-2}} over the whole globe).

Incoming solar radiation: the Sun emits energy in the form of sunlight (mainly visible light, with some ultraviolet and near infrared). About 49% of it is absorbed by the Earth's surface, mainly by land, oceans and vegetation.
Atmospheric absorption: about 20% of the solar radiation is absorbed by the atmosphere itself (by water vapour, dust, ozone and clouds).
Reflection: the remaining 31% is reflected straight back into space by clouds, air molecules, dust and bright surfaces such as ice. This fraction (about 0.3) is the Earth's albedo.
Outgoing terrestrial radiation: the Earth, having absorbed radiation, re-emits the energy as heat in the form of infrared radiation from the surface and the atmosphere. About 69% of the original input leaves the top of the atmosphere in this way.

Global average energy budget (Kiehl and Trenberth values, on which the book's percentages are based).
Energy flow	Share	Approximate value (\mathrm{W\,m^{-2}})
Incoming sunlight at the top of the atmosphere	100%	342
Reflected to space (clouds, air, surface)	31%	107
Absorbed by the atmosphere	20%	67
Absorbed by the surface	49%	168
Emitted to space as infrared (surface and atmosphere)	69%	235

\text{in: }100 = 31_{\,\text{reflected}} + 20_{\,\text{atmosphere}} + 49_{\,\text{surface}}\qquad \text{out: }31 + 69 = 100

Earth's energy budget per 100 units of incoming sunlight. Energy out (31 reflected plus 69 emitted as infrared) equals energy in, so the average temperature stays steady.

When the energy coming in equals the energy going out, the Earth's average temperature stays steady. If more energy comes in than goes out (for example because extra greenhouse gases reduce the outgoing infrared), the Earth gains energy and warms until a new balance is reached. At present the Earth absorbs slightly more than it emits (by roughly 1\,\mathrm{W\,m^{-2}}), which is why it is warming.

24.4The greenhouse effectOften examined

The greenhouse effect is the warming of the Earth's surface and lower atmosphere by gases that let sunlight in but trap part of the outgoing infrared radiation. It plays a key role in distributing energy in the Earth's climate and in regulating the globe's temperature. It involves the interaction of solar radiation with the greenhouse gases in the atmosphere.

Greenhouse gases: Gases that absorb and emit infrared radiation, mainly water vapour (\mathrm{H_2O}), carbon dioxide (\mathrm{CO_2}), methane (\mathrm{CH_4}), nitrous oxide (\mathrm{N_2O}), ozone (\mathrm{O_3}) and chlorofluorocarbons (CFCs).

How it works, in three steps

Sunlight penetrates the atmosphere and warms the surface. Sunlight is mainly visible light of short wavelength. Greenhouse gases hardly absorb it, so most of it passes through and warms the Earth's surface.
The heated surface emits infrared radiation. The surface is much cooler than the Sun, so it radiates at much longer wavelengths, in the infrared.
Greenhouse gases absorb some of the outgoing infrared. This prevents it from escaping directly into space. The gases re-radiate part of this energy back towards the surface, effectively trapping heat in the lower atmosphere.

The greenhouse effect: (1) sunlight passes through the atmosphere, (2) the warm ground emits infrared, (3) greenhouse gases absorb it and send part of it back down, so heat is trapped.

Without the greenhouse effect the Earth's average temperature would be well below freezing, about -18^\circ\mathrm{C}, instead of the actual average of about 15^\circ\mathrm{C}. The natural greenhouse effect makes the Earth habitable. The problem is the enhanced greenhouse effect: burning fossil fuels and cutting forests have raised the carbon dioxide concentration from about 280 parts per million (ppm) before 1750 to more than 420 ppm today, so more heat is trapped and the planet warms (global warming).

Important greenhouse gases. Sulphur dioxide (\mathrm{SO_2}) is not a greenhouse gas: it forms droplets that reflect sunlight and cool the climate.
Gas	Main human sources	Contribution to warming
Carbon dioxide, \mathrm{CO_2}	Burning coal, oil and gas; cutting forests; cement	Largest, about two thirds of the extra greenhouse effect caused by humans, because the amounts are huge and it stays in the air for centuries
Methane, \mathrm{CH_4}	Rice fields, cattle, landfills, gas leaks	About one sixth; much stronger than \mathrm{CO_2} per molecule but present in far smaller amounts
Nitrous oxide, \mathrm{N_2O}	Fertilisers, industry	A few per cent
CFCs and similar gases	Old refrigerators, air conditioners, sprays	Small but very strong per molecule; they also destroy ozone
Water vapour, \mathrm{H_2O}	Mostly natural evaporation	The largest natural greenhouse gas; its amount depends on temperature, so it amplifies warming as a feedback

Temperature of the Earth without the greenhouse effect (for understanding)

The Earth receives S = 1361\,\mathrm{W\,m^{-2}} from the Sun and reflects a fraction \alpha = 0.30. Treating the Earth as a black body with no greenhouse gases, find the temperature at which it would emit as much energy as it absorbs. Take \sigma = 5.67\times10^{-8}\,\mathrm{W\,m^{-2}\,K^{-4}}.

The Earth intercepts sunlight over its cross-section \pi R^{2} and absorbs the fraction 1-\alpha.
P_{\text{in}} = (1-\alpha)S\,\pi R^{2}
It emits infrared from its whole surface 4\pi R^{2} (Stefan-Boltzmann law).
P_{\text{out}} = 4\pi R^{2}\sigma T^{4}
For balance, energy in equals energy out; \pi R^{2} cancels.
(1-\alpha)S = 4\sigma T^{4}\;\Longrightarrow\; T = \left[\dfrac{(1-\alpha)S}{4\sigma}\right]^{1/4}
Substitute the values.
T = \left[\dfrac{0.70\times1361}{4\times5.67\times10^{-8}}\right]^{1/4} = (4.20\times10^{9})^{1/4} = 255\,\mathrm{K}

T \approx 255\,\mathrm{K} \approx -18^\circ\mathrm{C}

24.4.1Solar variability and climate

The Sun is essential for keeping the Earth habitable, but changes in the Sun's output have only a small influence on present-day climate change compared with other factors:

Solar cycles: the Sun goes through an 11-year cycle during which its activity rises and falls. The number of sunspots, the solar flares and the solar radiation all vary with the cycle, but the Sun's total energy output changes by only about 0.1%.
Long-term trends: satellites have measured the Sun's energy output for over 40 years. It has shown no upward trend while the Earth has warmed. Changes in solar activity explain only a tiny part of the warming observed since 1750 (the book says less than 0.1%).
Human-induced warming: human activities, especially the burning of fossil fuels, release greenhouse gases. The warming caused by these gases is over 270 times greater than the extra warming from the Sun itself over the same period.

24.4.2Energy imbalance between the equator and the polesOften examined

The Sun's energy is not spread evenly over the Earth's surface. The equator receives more solar energy because sunlight strikes it directly (nearly at right angles), while the polar regions receive less because sunlight strikes them obliquely (at a low angle). This uneven distribution creates an energy imbalance between the equator and the poles, and it is the main cause of the atmospheric circulation.

Why the equator receives more energy than the poles

Angle of the Sun's rays (the main reason). A beam of sunlight of a given width carries a fixed amount of energy. At the equator it falls almost vertically and covers a small area, so the energy per square metre (intensity) is high. Near the poles the same beam strikes at a low angle and spreads over a much larger area, so the intensity is low.
Path through the atmosphere. Oblique rays travel a longer path through the atmosphere before reaching the ground near the poles, so more of their energy is absorbed, scattered and reflected on the way.
Reflection by ice and snow. Polar ice and snow reflect most of the sunlight that does arrive (high albedo), while the dark oceans and forests near the equator absorb most of it.
Daylight through the year. At the equator the Sun is high at noon on every day of the year, with about 12 hours of daylight. The poles have months of darkness in winter, and even in summer the Sun stays low in the sky.

Equal beams of sunlight. At the equator the beam falls on a small area; near the pole the same energy is spread over a much larger area and passes through more of the atmosphere (dashed circle).

Derivation Intensity of sunlight at latitude \theta

Aim. Show that the intensity of sunlight on level ground falls as \cos\theta with latitude \theta (at noon at an equinox).

Assumptions.

The Sun is overhead at the equator (an equinox) and its rays are parallel.
Absorption in the atmosphere is ignored.
This result is not derived in the book; it explains the book's statement about direct and oblique sunlight.

Let a beam of cross-sectional area A_0 carry power P. Its intensity, measured at right angles to the rays, is:
I_0 = \dfrac{P}{A_0}
At latitude \theta the level ground is tilted at the angle \theta to the beam's cross-section, so the beam spreads over a larger area:
A = \dfrac{A_0}{\cos\theta}
The same power P is now shared over the larger area.
I = \dfrac{P}{A} = \dfrac{P\cos\theta}{A_0}
Replace P/A_0 by I_0.
I = I_0\cos\theta

ResultI = I_0\cos\theta

At latitude 60^\circ, \cos60^\circ = 0.5, so each square metre receives only half the midday intensity at the equator, even before the longer path through the atmosphere is counted. Near the poles \cos\theta approaches zero.

Midday sunlight on level ground

Relative intensity of midday sunlight at an equinox against latitude \theta in degrees. It halves by 60^\circ and falls to zero at the poles.

How the imbalance drives atmospheric circulation

Warm air rises at the equator, creating a belt of low pressure (the ITCZ).
Cooler air sinks at the poles, forming high-pressure zones.
The resulting pressure gradients produce winds such as the trade winds, the westerlies and the polar easterlies, organised into the Hadley, Ferrel and polar cells.
Ocean currents also help to redistribute heat globally, through the gyres and the thermohaline circulation.

Energy surplus and deficit (schematic)

Yearly averages against latitude \theta in degrees (schematic). Between the equator and about 38^\circ the Earth absorbs more solar energy than it emits (surplus); nearer the poles it emits more than it absorbs (deficit). Winds and ocean currents carry the extra heat towards the poles; without them the tropics would keep getting hotter and the poles colder.

Changes in the energy balance can change the atmospheric circulation, and with it weather systems, ocean currents and climate zones. The Sun's energy is vital for the Earth's climate, but today the human-induced increase in greenhouse gases has a much bigger effect on the balance, and on global warming, than changes in the Sun.

24.4.3Climate science as a chaotic system

A chaotic system is one in which there is no equilibrium and no exactly repeating pattern, and which shows extreme sensitivity to initial conditions: even a small change in the starting conditions can lead to a vastly different outcome later. The system still obeys definite physical laws, but its long-term behaviour cannot be predicted in detail.

The climate is a complex, interconnected system, and the atmosphere and oceans behave in non-linear, chaotic ways. We usually think of the climate as having predictable seasons and weather patterns, but it can also change suddenly and rapidly because of chaotic dynamics. Everyday experience shows the limit: sometimes the news forecasts rain for today and it does not happen.

The butterfly effect

Consider a butterfly flapping its wings in one part of the world. According to chaos theory, this seemingly insignificant event could set off a chain of interactions that eventually leads to a hurricane forming in another part of the world. This idea is popularly known as the butterfly effect. It shows how small disturbances (perturbations) can grow and spread through a complex system and finally influence large-scale events. It does not mean that butterflies cause hurricanes; it means that tiny, unmeasurable differences decide which of many possible futures actually happens.

Two computer forecasts of the same weather model (the Lorenz model) whose starting values differ by only 0.1%. They agree closely at first and then become completely different.

Predictability and uncertainty

Climate scientists call the effects of chaos intrinsic or unforced variability: unpredictable changes that arise from the dynamic interactions within the climate system rather than being caused directly by external factors.
Because the atmosphere and oceans are non-linear chaotic systems, large-scale weather features can be predicted with reasonable certainty only a few days in advance. Forecasts for longer periods are less reliable.
Measurements are never perfectly exact and cannot cover every point, so small errors in the starting data always exist, and they grow with time.

Climate science therefore shows a delicate balance between predictability and chaos. Long-term averages, such as 'summers will become hotter', can be projected with confidence, but the exact weather on a particular day months ahead cannot. The inherent complexity and sensitivity to initial conditions mean that some aspects of our climate remain uncertain and difficult to predict.

Formula sheet

Water balance equation

\Delta S = P - E - Q

\Delta S: change in storage, P: precipitation, E: evapotranspiration, Q: runoff (all as depths of water)

Units: \mathrm{mm} (or \mathrm{m}) of water

Water balance in volumes

\Delta S_V = P_V - E_V - Q_V,\qquad X_V = X\,A

X_V: volume of each quantity, X: its depth in metres, A: area of the watershed in \mathrm{m^2}

Unit: \mathrm{m^3}

Coriolis force

F_c = 2mv\omega\sin\theta

m: mass of the moving air packet, v: its speed relative to the ground, \omega: angular velocity of the Earth (7.3\times10^{-5}\,\mathrm{rad\,s^{-1}}), \theta: latitude

Unit: \mathrm{N}

Coriolis acceleration

a_c = \dfrac{F_c}{m} = 2v\omega\sin\theta

symbols as for the Coriolis force; zero at the equator, largest at the poles

Unit: \mathrm{m\,s^{-2}}

Angular velocity of the Earth

\omega = \dfrac{2\pi}{T} \approx 7.29\times10^{-5}\,\mathrm{rad\,s^{-1}}

T: time for one rotation of the Earth, about 86\,164\,\mathrm{s} (23 h 56 min)

Unit: \mathrm{rad\,s^{-1}}

Eastward speed of the ground

v = \omega R\cos\theta

R: radius of the Earth (6.37\times10^{6}\,\mathrm{m}), \theta: latitude; about 465\,\mathrm{m\,s^{-1}} at the equator, zero at the poles

Unit: \mathrm{m\,s^{-1}}

Angular momentum of an air parcel

L = mvr = mvR\cos\theta

r: distance from the Earth's axis; L is conserved when no torque acts

Unit: \mathrm{kg\,m^2\,s^{-1}}

Speed conversion

v\,(\mathrm{m\,s^{-1}}) = v\,(\mathrm{km\,h^{-1}})\times\dfrac{1000}{3600} = \dfrac{v\,(\mathrm{km\,h^{-1}})}{3.6}

for example 80\,\mathrm{km\,h^{-1}} = 22.2\,\mathrm{m\,s^{-1}}

Unit: \mathrm{m\,s^{-1}}

Density

\rho = \dfrac{m}{V}

m: mass, V: volume; sea water about 1025\,\mathrm{kg\,m^{-3}}, denser when colder or saltier

Unit: \mathrm{kg\,m^{-3}}

Heat and heat capacity

Q = mc\Delta T

Q: heat, m: mass, c: specific heat capacity (water about 4200\,\mathrm{J\,kg^{-1}\,K^{-1}}), \Delta T: temperature change

Unit: \mathrm{J}

Energy budget (per 100 units of sunlight)

100 = 31_{\,\text{reflected}} + 20_{\,\text{atmosphere}} + 49_{\,\text{surface}},\qquad 31 + 69_{\,\text{infrared}} = 100

incoming sunlight is about 342\,\mathrm{W\,m^{-2}} averaged over the globe

Unit: per cent of incoming sunlight

Radiative balance temperature (not in the book)

T = \left[\dfrac{(1-\alpha)S}{4\sigma}\right]^{1/4}

S: solar constant (1361\,\mathrm{W\,m^{-2}}), \alpha: albedo (0.30), \sigma: Stefan-Boltzmann constant; gives 255\,\mathrm{K} without greenhouse gases

Unit: \mathrm{K}

Intensity of sunlight at latitude (not in the book)

I = I_0\cos\theta

I_0: intensity at right angles to the rays, \theta: latitude (noon at an equinox)

Unit: \mathrm{W\,m^{-2}}

Key terms

Weather: The state of the atmosphere at a particular place and time: temperature, humidity, wind, pressure, cloud and precipitation.
Climate: The average weather of a region and its variation, usually measured over 30 years.
Climate system: The five interacting components that decide the climate: atmosphere, hydrosphere, cryosphere, lithosphere and biosphere.
Atmosphere: The layer of gases (78% nitrogen, 21% oxygen, 0.9% argon and trace gases) that envelops the Earth.
Troposphere: The lowest layer of the atmosphere, where weather occurs; about 16 km thick in the book (about 8 km over the poles and 18 km over the equator).
Stratosphere: The layer from about 16 km to 50 km; contains the ozone layer; temperature rises with height, so it is calm and stable.
Mesosphere: The layer above the stratosphere (50 km to 136 km in the book) where meteors burn up; its top is the coldest part of the atmosphere.
Thermosphere: The layer from 136 km to about 600 km (book values) where auroras occur; it contains most of the ionosphere.
Exosphere: The outermost layer of the atmosphere, above about 600 km, merging into space.
Ozone layer: A region of the stratosphere rich in ozone (\mathrm{O_3}) that absorbs harmful ultraviolet radiation from the Sun.
Ionosphere: The ionised region of the upper atmosphere, mainly in the thermosphere, that reflects radio waves back to the Earth.
Hydrosphere: All the water on the Earth, whether liquid, solid (ice) or gas (water vapour).
Hydrological cycle: The continuous movement of water by evaporation, condensation, precipitation, runoff and infiltration.
Water balance equation: \Delta S = P - E - Q: the change in water storage equals precipitation minus evapotranspiration minus runoff.
Precipitation: Any liquid or frozen water that forms in the atmosphere and falls back to the Earth (rain, snow, hail).
Evapotranspiration: Water lost from a region by evaporation from the Earth's surface plus transpiration from plants.
Runoff: Water that moves across the surface into rivers and lakes, including water that infiltrates into the groundwater.
Cryosphere: Every part of the Earth where water is in solid form: snow, sea ice, glaciers, ice sheets, ice shelves, icebergs and permafrost.
Permafrost: Permanently frozen ground (soil and ice) that stays frozen for long periods; found mainly in high-latitude and high-altitude regions.
Albedo: The fraction of incoming sunlight that a surface or planet reflects; about 0.3 for the Earth, very high for fresh snow.
Lithosphere: The outermost solid shell of the Earth, made of the crust and the uppermost part of the mantle.
Asthenosphere: The soft, slowly flowing layer of the upper mantle on which the lithospheric plates move.
Carbon sink: A store, such as limestone rocks, forests or the oceans, that takes carbon out of the atmosphere.
Volcanic winter: A short-term cooling of the climate caused by volcanic gases and particles that block sunlight.
Biosphere: The thin life-supporting layer of the Earth's surface, containing all living organisms (plants, animals, fungi and microorganisms).
Carbon cycle: The exchange of carbon between the atmosphere, living things, oceans and rocks by photosynthesis, respiration, decay, burning and weathering.
Ocean currents: Streams of water that flow constantly on the ocean surface in definite directions, driven by wind, gravity and density differences.
Surface ocean currents: Wind-driven currents in the shallow water near the ocean surface.
Deep ocean currents: Density-driven currents below a depth of about 500 m.
Upwelling and downwelling: Vertical movements of ocean water: deep water rising to the surface (upwelling) and surface water sinking (downwelling).
Subtropical gyres: Large rotating loops of surface current that start near the equator; clockwise in the Northern Hemisphere and anticlockwise in the Southern Hemisphere.
Salinity: The amount of dissolved salt in sea water; higher salinity means higher density.
Abyssal zone: The deepest region of the ocean, with no light, very little oxygen, high pressure and very little life.
Thermohaline circulation: The large-scale, density-driven ocean circulation ('thermo' for temperature, 'haline' for salinity) that carries heat from the tropics to the poles like a conveyor belt.
Angular momentum: A property of a rotating object that depends on its speed of rotation and its perpendicular distance from the axis; for an air parcel L = mvr.
Coriolis effect: The turning of winds and ocean currents due to the rotation of the Earth: to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
Coriolis force: The apparent force that bends moving air or water because of the Earth's rotation, F_c = 2mv\omega\sin\theta; zero at the equator and largest at the poles.
Trade winds: Surface winds between the equator and 30^\circ that blow towards the equator from the north-east (north) and south-east (south).
Westerlies: Surface winds between 30^\circ and 60^\circ that blow from the west (south-west in the north, north-west in the south).
Polar easterlies: Cold surface winds that blow from the east between the poles and about 60^\circ.
Intertropical Convergence Zone (ITCZ): The low-pressure belt near the equator where the trade winds meet and warm, moist air rises, giving heavy rain.
Hadley cell: The circulation cell between the equator and 30^\circ: air rises at the equator, moves polewards aloft, sinks near 30^\circ and returns as the trade winds.
Ferrel cell: The circulation cell between 30^\circ and 60^\circ: air sinks at 30^\circ, moves polewards at the surface (westerlies) and rises at 60^\circ.
Polar cell: The circulation cell between 60^\circ and the pole: air rises at 60^\circ, moves polewards aloft and sinks at the pole.
Pressure gradient: The change in pressure measured across a given distance.
Pressure gradient force: The force that pushes air from higher pressure to lower pressure.
Isobars: Lines on a weather map joining places of equal pressure.
Geostrophic wind: A wind that blows parallel to the isobars because the pressure gradient force and the Coriolis force balance.
Differential heating: The difference in how land and water surfaces absorb and lose heat; it causes land and sea breezes.
Jet streams: Fast-moving bands of air in the upper atmosphere, typically 6 km to 15 km above the surface.
Climate inertia: The tendency of the Earth's climate to resist or respond slowly to changes in external factors such as greenhouse gas concentrations and solar radiation.
Positive feedback: A process that amplifies an initial change, such as melting ice reducing reflection and causing more warming.
Negative feedback: A process that dampens an initial change, such as extra plant growth absorbing extra carbon dioxide.
Short-term inertia: The slow absorption and release of heat by the oceans and land that smooths daily and seasonal temperature changes.
Long-term inertia: The decades-to-centuries delay before changes in greenhouse gas concentrations fully affect the climate.
Adaptation: Adjusting to climate changes that are already happening.
Mitigation: Reducing greenhouse gas emissions to limit future climate change.
Internal variations: Natural fluctuations that arise within the climate system from interactions between its components, such as ENSO, AMO and PDO.
External forcing: Factors outside the climate system, such as greenhouse gases, solar output, orbital changes and aerosols, that alter the Earth's energy balance.
El Niño Southern Oscillation (ENSO): A repeating (2 to 7 years) warming and cooling of the tropical Pacific Ocean, with changes in the trade winds, that affects weather worldwide.
Aerosol: A suspension of tiny particles or droplets in the air, such as dust, mist, fumes or smoke.
Anthropogenic: Caused by human activity, for example anthropogenic aerosols from industry and burning.
Orbital variations: Slow periodic changes in the Earth's orbit and axis: eccentricity, obliquity (tilt) and precession.
Earth's energy budget: The balance between incoming solar radiation (insolation) and the energy the Earth radiates back into space.
Insolation: Incoming solar radiation received by the Earth.
Terrestrial radiation: Infrared radiation emitted by the Earth's surface and atmosphere.
Greenhouse effect: The trapping of outgoing infrared radiation by greenhouse gases, which keeps the Earth's surface warmer than it would otherwise be.
Greenhouse gases: Gases that absorb and emit infrared radiation, such as water vapour, carbon dioxide, methane, nitrous oxide, ozone and CFCs.
Solar cycle: The roughly 11-year cycle in the Sun's activity, seen in sunspots, flares and a small change in its output.
Energy imbalance: The uneven distribution of solar energy between the equator (surplus) and the poles (deficit), which drives the atmospheric circulation.
Chaotic system: A system with no equilibrium and no repeating pattern that is extremely sensitive to its initial conditions, so its long-term outcome is hard to predict.
Butterfly effect: The idea that a tiny disturbance, such as a butterfly flapping its wings, can grow through a complex system into a large event such as a hurricane far away.
Intrinsic (unforced) variability: Unpredictable climate changes that arise from dynamic interactions within the climate system rather than from external causes.

Multiple choice questionsp. 199

11 mark

Which of the following gases has the maximum contribution to global warming?

AChlorofluorocarbons (CFCs)
BMethane (\mathrm{CH_4})
CSulfur hexa-fluoride (\mathrm{SF_6})
DCarbon dioxide (\mathrm{CO_2})Answer

Why

Carbon dioxide causes the largest share of the extra greenhouse effect produced by humans (about two thirds) because it is released in enormous amounts by burning fossil fuels and stays in the air for centuries.

CFCs, methane and \mathrm{SF_6} trap more heat per molecule, but their amounts in the air are far smaller, so their total contribution is less.

21 mark

Which of the following gas is not responsible for global warming?

A\mathrm{N_2O}
B\mathrm{H_2O}
C\mathrm{SO_2}Answer
D\mathrm{CO_2}

Why

Sulphur dioxide is not a greenhouse gas. In the air it forms tiny sulphate droplets (aerosols) that reflect sunlight, so it tends to cool the climate; it also causes acid rain.

Nitrous oxide, water vapour and carbon dioxide all absorb infrared radiation and add to the greenhouse effect.

31 mark

In global warming the temperature of ______ increases:

ATroposphereAnswer
BIonosphere
CMesosphere
DStratosphere

Why

Greenhouse gases trap outgoing infrared radiation in the lower atmosphere, so the Earth's surface and the troposphere warm up. (The stratosphere actually cools slightly as \mathrm{CO_2} increases.)

41 mark

Which type of climate is found in our country Pakistan?

ATundra
BEquatorial
CMonsoonAnswer
DWestern

Why

Pakistan's climate is of the monsoon type: most of its rain comes with the summer monsoon (July to September), after a very hot early summer, and the winter is cool and fairly dry. Tundra climates are found near the poles and equatorial climates near the equator.

51 mark

What is the height of troposphere near the poles and the equator in kilometers?

A8 and 18Answer
B6 and 16
C4 and 16
D10 and 20

Why

The troposphere is thinnest over the cold poles (about 8 km) and thickest over the equator (about 18 km), where strong heating and convection lift the air higher. The book's single value of 16 km is a typical tropical figure.

61 mark

What happens in the ionosphere?

AMeteors burns
BAirplanes fly
CRadio waves reflectedAnswer
DSatellites move

Why

The ionosphere contains ions and free electrons produced by solar X-rays and ultraviolet radiation. These charged particles reflect radio waves back to the Earth, allowing long-distance radio communication. Meteors burn in the mesosphere, and airplanes fly in the troposphere and lower stratosphere.

71 mark

Which atmospheric layer contains ions and helps in wireless communications?

ATroposphere
BThermosphereAnswer
CMesosphere
DStratosphere

Why

The ionosphere, the region of ions that reflects radio waves, lies mostly within the thermosphere. So among the four layers the thermosphere is correct.

81 mark

The part of the Earth atmosphere which contains about 70% of the total air in the atmosphere is:

ATroposphereAnswer
BThermosphere
CMesosphere
DStratosphere

Why

Air is compressed by the weight of the air above it, so most of the atmosphere's mass (roughly 75 to 80%) lies in the lowest layer, the troposphere. 'About 70%' therefore points to the troposphere.

91 mark

Hot and dry winds blowing in the northern plains of Punjab and Sindh are:

ATrade winds
BLooAnswer
CWesterlies
DEasterlies

Why

The Loo is the hot, dry, dusty wind that blows from the west over the plains of Punjab and Sindh in the afternoons of May and June, before the monsoon arrives.

101 mark

Pakistan is located in the region of ______ winds:

ATrade
BNorth easterliesAnswer
CWesterlies
DSouth easterlies

Why

Pakistan (about 24^\circ to 37^\circ N) lies largely in the belt of north-easterly winds: air flowing from the subtropical high towards the equator is deflected to the right by the Coriolis effect and blows from the north-east. These winds come over land and are dry, which is one reason why much of Pakistan is arid.

Option A names the same north-east trade winds in general terms, so it is partly right, but the precise answer is B. South-easterlies blow in the Southern Hemisphere.

Short questionsp. 199

Each answer is worth 3 marks: give the key statement first, then the reason, then an example or a small sketch.

13 marks

Where in the atmosphere is water vapor most concentrated and why?

Solution

Water vapour is most concentrated in the troposphere, especially in its lowest 2 to 3 km, close to the Earth's surface. About 99% of all the water vapour in the atmosphere is in the troposphere. The reasons are:

The source is at the surface. Water vapour comes from evaporation from oceans, lakes and soil and from transpiration by plants, all at ground level.
Temperature falls with height. In the troposphere the temperature drops by about 6.5^\circ\mathrm{C} per km, and cold air can hold much less vapour. Rising vapour therefore condenses into clouds and falls back as rain or snow before it can rise higher.
A cold trap at the top. The very cold tropopause (about -55^\circ\mathrm{C}) freezes out almost all the remaining vapour, so the stratosphere above it is extremely dry.

The concentration is highest over warm tropical oceans and lowest over cold polar regions and deserts.

23 marks

What is the difference between the weather and the climate?

Solution

Weather is the state of the atmosphere (temperature, humidity, wind, cloud, rainfall and pressure) at a particular place and time. Climate is the average weather of a region, together with its usual variations, measured over a long period, usually 30 years.

Weather	Climate
Describes the atmosphere at a given time	Describes the long-term average and range of the weather
Time scale of hours to days	Time scale of about 30 years or more
Changes quickly, from hour to hour	Changes slowly, over decades
Example: it is raining in Lahore today	Example: Lahore has a hot, wet monsoon season every summer
Forecast a few days ahead	Described by long records and climate models

In short, weather is the condition we get on a particular day; climate is the pattern we expect from long experience.

33 marks

Warm air has a greater capacity to hold water vapours. Why?

Solution

The maximum amount of water vapour that can exist in air before it becomes saturated depends on the temperature. At a higher temperature the water molecules have more kinetic energy:

More molecules have enough energy to escape from a water surface, so evaporation is faster.
The fast-moving vapour molecules are less likely to stick together and condense into droplets when they collide.
So a larger amount of vapour can stay as gas before condensation (dew, fog or cloud) begins. The saturation vapour pressure rises steeply, by about 7% for every 1^\circ\mathrm{C} rise.

Air temperature	Maximum water vapour in 1\,\mathrm{m^3} of air
10^\circ\mathrm{C}	about 9\,\mathrm{g}
20^\circ\mathrm{C}	about 17\,\mathrm{g}
30^\circ\mathrm{C}	about 30\,\mathrm{g}

This is why warm, moist air forms clouds and rain when it rises and cools, and why a warmer climate brings heavier downpours.

43 marks

How does the atmosphere retain itself?

Solution

The atmosphere is held around the Earth by the Earth's gravity.

Gas molecules move randomly, but their average speed is only about 500\,\mathrm{m\,s^{-1}} (for nitrogen at 15^\circ\mathrm{C}, v_{rms}=\sqrt{3RT/M}\approx 507\,\mathrm{m\,s^{-1}}).
This is far less than the Earth's escape velocity, 11.2\,\mathrm{km\,s^{-1}}, so gravity pulls the molecules back and almost none can escape into space.
Gravity also compresses the air: the weight of the air above makes the pressure and density greatest near the ground, so most of the atmosphere stays within a few kilometres of the surface.
Only the lightest gases (hydrogen and helium) in the hot, thin exosphere sometimes reach escape speed, which is why the air contains almost no hydrogen or helium.

v_{rms}=\sqrt{\dfrac{3RT}{M}}\approx 0.5\,\mathrm{km\,s^{-1}}\;\ll\; v_{esc}=\sqrt{\dfrac{2GM_e}{R_e}}=11.2\,\mathrm{km\,s^{-1}}

The Earth's magnetic field also shields the upper atmosphere from the solar wind, which could otherwise strip gas away.

53 marks

Describe global latitudinal air movements due to uneven heating at different latitudes.

Solution

The equator is heated much more strongly than the poles because sunlight strikes it almost vertically. This uneven heating sets up three circulation cells in each hemisphere:

**Hadley cell (0^\circ to 30^\circ):** hot air rises at the equator (low pressure, the ITCZ), flows towards the pole high up, cools and sinks at about 30^\circ (subtropical high), and returns to the equator along the surface as the trade winds.
**Ferrel cell (30^\circ to 60^\circ):** air from 30^\circ flows towards the pole along the surface as the westerlies and rises at about 60^\circ (subpolar low).
**Polar cell (60^\circ to 90^\circ):** cold, dense air sinks at the poles (polar high) and flows towards 60^\circ along the surface as the polar easterlies.

Because of the Earth's rotation (the Coriolis effect), these winds are deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

The three cells of one hemisphere: rising air at 0^\circ and 60^\circ (low pressure), sinking air at 30^\circ and 90^\circ (high pressure).

63 marks

What would be the consequences if all the polar ice melts?

Solution

Melting all the polar ice (the Greenland and Antarctic ice sheets and the sea ice) would have very serious consequences:

Sea level rise: the ice sheets on land hold enough water to raise the global sea level by about 65 m (about 58 m from Antarctica and 7 m from Greenland). Coastal cities such as Karachi and many low-lying countries would be flooded. Melting of floating sea ice alone hardly changes sea level, because floating ice already displaces its own weight of water.
Faster warming (positive feedback): ice reflects most of the sunlight falling on it, while dark sea and land absorb it. Less ice means more absorption and even faster warming.
Weaker ocean circulation: the fresh meltwater would make the polar seas less salty and less dense, slowing the thermohaline circulation and changing climates and rainfall around the world.
Other effects: loss of habitat for polar animals, thawing of permafrost that releases \mathrm{CO_2} and methane, and more extreme weather.

73 marks

Is the lithosphere a part of mental? Justify your answer.

Solution

Partly, yes. The lithosphere is the rigid outer shell of the Earth made of the crust plus the uppermost solid part of the mantle. So the lower part of the lithosphere belongs to the mantle, while its upper part (the crust) does not. The lithosphere is not part of the mantle as a whole; it includes the top layer of the mantle.

Justification: the crust and the mantle are layers defined by their composition (different kinds of rock), while the lithosphere is defined by its mechanical behaviour: it is cool and rigid and moves as tectonic plates. Below it, the rest of the upper mantle is hot and soft (the asthenosphere), and the plates slide over it. The lithosphere is roughly 100 km thick: thinner under the oceans and thicker under the continents.

The lithosphere (crust plus uppermost mantle) forms rigid plates that move over the soft asthenosphere, which is also part of the upper mantle.

Note. 'Mental' is the book's misprint for mantle.

83 marks

What are the four components of the biosphere?

Solution

The biosphere is the thin life-supporting layer of the Earth. Its living part has four components:

Plants: producers. They capture solar energy and absorb \mathrm{CO_2} in photosynthesis, and release water vapour by transpiration.
Animals: consumers. They eat plants or other animals, pass energy along food chains and release \mathrm{CO_2} by respiration.
Fungi: decomposers. They break down dead matter and return carbon and nutrients to the soil and the air.
Microorganisms: bacteria, algae and other microbes. They cause decay, fix nitrogen, and (as ocean plankton) carry out a large share of the world's photosynthesis.

These organisms depend on non-living factors (air, water, soil and sunlight) for their energy and nutrients.

93 marks

Why stratosphere is called a calm and stable layer?

Solution

The stratosphere (about 16 km to 50 km) is called calm and stable because there is almost no vertical movement of air in it:

Temperature inversion: the ozone layer absorbs ultraviolet radiation from the Sun, so the temperature increases with height. Warm, light air lies above cooler, denser air, so convection (rising and sinking air) cannot occur.
No weather: almost all the water vapour stays in the troposphere below, so there are no clouds, rain or storms in the stratosphere.
Smooth winds: the winds are mainly steady and horizontal, with very little turbulence.

For these reasons long-distance jet aircraft fly near the lower stratosphere for a smooth flight, and volcanic particles that reach the stratosphere stay there for a year or two because no rain washes them out.

Comprehensive questionsp. 199–200

Section C style answers: use the headings as a guide to the marks, and draw the diagram whenever one is shown.

17 marks

Describe Earth's climate system as a complex system having five interacting components.

Solution

Introduction

Climate is the average weather of a region over about 30 years. It is decided by the climate system, a complex system of five interacting components: the atmosphere, the hydrosphere, the cryosphere, the lithosphere and the biosphere. The Sun supplies the energy, and the components continuously exchange energy, water and carbon with one another.

The five interacting components of the climate system.

1. Atmosphere (air)

The layer of gases around the Earth: 78% nitrogen, 21% oxygen, 0.9% argon and trace gases (water vapour, carbon dioxide, ozone and methane). It shields life from ultraviolet radiation (ozone layer), keeps the Earth warm through the greenhouse effect, prevents extreme differences between day and night temperatures, and moves heat and moisture by convection and winds. Its layers are the troposphere (where weather occurs), stratosphere, mesosphere, thermosphere and exosphere.

2. Hydrosphere (water)

All the water on the Earth in liquid, solid and vapour form; about 70% of the surface is covered by water. Water has a high specific heat capacity, so the oceans store huge amounts of heat and release it slowly. Ocean currents carry heat from the equator towards the poles, and the water cycle (evaporation, condensation, precipitation and runoff) redistributes water and latent heat. The water stored in a region obeys \Delta S = P - E - Q.

3. Cryosphere (ice)

All frozen water: snow, sea ice, glaciers, ice sheets, icebergs and permafrost, including the glaciers of the Himalaya, Karakorum and Hindu Kush. Snow and ice reflect sunlight and keep the Earth cool; freezing sea water leaves salt behind and makes the sea water denser; melting land ice raises sea level.

4. Lithosphere (rock)

The crust and the uppermost part of the mantle. It provides the land surface on which climate acts (forests moderate temperatures, deserts have extremes), stores carbon in rocks such as limestone, builds mountains through tectonic activity that steer winds and rainfall, and its volcanoes can cool the climate for a year or two (volcanic winter).

5. Biosphere (living things)

All living organisms: plants, animals, fungi and microorganisms. Photosynthesis removes \mathrm{CO_2} from the air, while respiration and decay return it. Plants add water vapour by transpiration, and changes such as deforestation alter local and global climate.

Interactions: why it is a complex system

Water cycle: water evaporates from the ocean (hydrosphere) into the air (atmosphere), falls as snow on mountains (cryosphere), is used by plants (biosphere) and flows over rocks and soil (lithosphere) back to the sea.
Carbon cycle: plants take in \mathrm{CO_2}, oceans dissolve it, limestone stores it for millions of years, and volcanoes and the burning of fossil fuels return it to the air.
Ice and sunlight: melting ice exposes darker ground and sea, which absorb more sunlight and cause more melting (a positive feedback).
Oceans and air: changes in ocean temperature, such as El Niño, change the winds and rainfall over whole continents.

Because each component affects all the others through such links and feedbacks, a change in one part (for example extra \mathrm{CO_2} in the air) spreads through the whole system in ways that are difficult to predict. This is why the climate system is called a complex system.

22026 Annual I, Set B, Q.2 (vii) OR2026 Annual I, Set D, Q.2 (vii) OR7 marks

What is climate inertia? Explain.

Solution

Definition

Climate inertia is the tendency of the Earth's climate to resist, or respond slowly to, changes in external factors such as greenhouse gas concentrations and solar radiation. For example, if greenhouse gas emissions were stabilised today, the Earth would continue to warm for some time because of the heat already stored in the oceans.

Cause: feedback mechanisms

Climate inertia arises from complex feedback mechanisms and slow processes within the climate system:

Positive feedback amplifies a change. As Arctic ice melts because of warming, the Earth's reflectivity falls; the darker surface absorbs more sunlight, which leads to further warming.
Negative feedback dampens a change. Higher \mathrm{CO_2} levels stimulate plant growth, and the plants absorb some of the extra carbon, which slows the rise of \mathrm{CO_2}.

A positive feedback amplifies a change; a negative feedback reduces it.

Types of climate inertia

Short-term inertia: the oceans and land surfaces absorb and release heat relatively slowly. This smooths the daily and seasonal changes of temperature; for example, the hottest time of day comes in the afternoon, a few hours after the Sun is highest, and coastal towns have much smaller day and night temperature differences than inland deserts.
Long-term inertia: changes in greenhouse gas concentrations, such as carbon dioxide, take decades to centuries to affect the climate fully. When emissions are stabilised, the climate does not respond immediately; it keeps warming slowly towards a new balance.

Schematic, time t in years: after a sudden, lasting increase in forcing, the temperature rises quickly at first and then slowly for centuries.

Main role of the oceans

Oceans play the main role in climate inertia. Water has a high specific heat capacity (4200\,\mathrm{J\,kg^{-1}\,K^{-1}}, about four times that of air) and the oceans have an enormous mass, so from Q = mc\Delta T a very large amount of heat produces only a small change in temperature. The whole ocean stores about 1000 times more heat than the atmosphere for the same temperature rise. It absorbs heat slowly and releases it slowly, so changes in ocean temperature, and therefore in climate patterns, happen gradually over long periods.

Other slow processes

Carbon cycle: \mathrm{CO_2} is exchanged slowly between the atmosphere, oceans, land and vegetation, and organic matter decomposes slowly.
Ice sheets and glaciers: they melt over centuries, so sea level keeps rising long after the warming that caused it.

Importance

Climate inertia means that the effects of past emissions will persist even if we act immediately. It therefore shows why we need both adaptation (adjusting to changes that are already happening) and mitigation (reducing emissions to limit future change).

37 marks

Explain how global climate is determined by energy transfer from the Sun.

Solution

The Sun as the source of energy

Almost all the energy that drives the Earth's climate comes from the Sun as electromagnetic radiation, mainly visible light. This energy warms the land, oceans and air, and it powers the winds, the ocean currents and the water cycle. How this energy is received, stored, moved and finally radiated back to space decides the global climate.

Energy budget: energy in equals energy out

Of every 100 units of sunlight reaching the top of the atmosphere (about 342\,\mathrm{W\,m^{-2}} on average), about 31 are reflected back to space by clouds, air and ice, about 20 are absorbed by the atmosphere, and about 49 are absorbed by the surface. The warmed surface and atmosphere then emit infrared radiation, and about 69 units leave for space. When energy in equals energy out, the average temperature stays steady.

Earth's energy budget per 100 units of incoming sunlight.

Greenhouse effect

Sunlight passes through the atmosphere and warms the surface; the warm surface emits infrared; greenhouse gases (\mathrm{H_2O}, \mathrm{CO_2}, \mathrm{CH_4}) absorb part of this infrared and re-radiate some of it back down. This keeps the average surface temperature at about 15^\circ\mathrm{C} instead of about -18^\circ\mathrm{C}.

Uneven heating of the Earth

Sunlight strikes the equator almost vertically but the poles obliquely, so the same energy is spread over a larger area near the poles (I = I_0\cos\theta). The rays near the poles also pass through more atmosphere, and polar ice reflects much of what arrives. The tropics therefore have an energy surplus and the polar regions an energy deficit.

Transfer of heat by the atmosphere and oceans

Atmospheric circulation: warm air rises at the equator and sinks near 30^\circ and at the poles, forming the Hadley, Ferrel and polar cells and the trade winds, westerlies and polar easterlies, deflected by the Coriolis effect.
Ocean currents: wind-driven gyres and the density-driven thermohaline circulation carry warm water polewards and cold water back towards the equator.
Water cycle: evaporation takes latent heat from the tropical oceans; condensation in clouds releases it elsewhere.

This transfer of heat from the equator towards the poles sets the climate zones: wet tropics, dry subtropical deserts, stormy mid-latitudes and cold polar regions.

Changes in energy transfer change the climate

Anything that alters the energy balance changes the climate: more greenhouse gases (trapping more infrared), changes in solar output, orbital variations that change the distribution of sunlight, aerosols that reflect sunlight, and melting ice that reduces reflection. Today the rise in greenhouse gases is the main cause of global warming.

47 marks

Discuss the Earth energy budget in detail.

Solution

Definition

The Earth's energy budget is a way of accounting for the balance between the energy the Earth receives from the Sun and the energy it radiates back into space. It is the balance between the incoming solar radiation (insolation) and the outgoing terrestrial radiation.

Incoming solar radiation

The Sun emits energy mainly as visible light, with some ultraviolet and near infrared. Averaged over the whole globe, about 342\,\mathrm{W\,m^{-2}} arrives at the top of the atmosphere. Take this as 100 units.

What happens to the incoming radiation

Part	Share	\mathrm{W\,m^{-2}}
Reflected to space by clouds, air and dust	about 22%	77
Reflected to space by the surface (ice, snow, deserts)	about 9%	30
Absorbed by the atmosphere (water vapour, ozone, dust, clouds)	20%	67
Absorbed by the surface (land, oceans, vegetation)	49%	168

The total reflected, 31%, is the Earth's albedo (about 0.3). The remaining 69% is absorbed and warms the Earth.

Outgoing terrestrial radiation

The Earth, having absorbed radiation, re-emits the energy as heat in the form of infrared radiation from the surface and the atmosphere. The surface also passes heat to the air by convection (thermals) and by evaporation (latent heat). In the end about 69 units leave the top of the atmosphere as infrared, so:

\text{energy in} = 100,\qquad \text{energy out} = 31\ (\text{reflected}) + 69\ (\text{infrared}) = 100

Earth's energy budget per 100 units of incoming sunlight.

Role of the greenhouse effect

Sunlight penetrates the atmosphere and warms the surface; the heated surface emits infrared radiation; greenhouse gases absorb part of it and re-radiate a large share back towards the surface (back radiation). The surface therefore receives energy from both the Sun and the atmosphere, and its budget balances at a comfortable temperature:

\underbrace{168}_{\text{solar}}+\underbrace{324}_{\text{back radiation}}=\underbrace{390}_{\text{infrared emitted}}+\underbrace{78}_{\text{evaporation}}+\underbrace{24}_{\text{convection}}=492\,\mathrm{W\,m^{-2}}

Without the greenhouse effect, the Earth's average temperature would be about -18^\circ\mathrm{C} instead of about 15^\circ\mathrm{C}.

Balance and imbalance

When energy in equals energy out, the average temperature stays constant. If the balance is disturbed, the temperature changes until a new balance is reached. Extra greenhouse gases reduce the outgoing infrared, so at present the Earth absorbs roughly 1\,\mathrm{W\,m^{-2}} more than it emits, and it is warming. Melting ice (lower albedo) adds to the imbalance, while aerosols and volcanic particles (higher albedo) reduce it. The budget is also uneven across the globe: the tropics gain more energy than they lose and the poles lose more than they gain, and winds and ocean currents carry the difference polewards.

52026 Annual I, Set B, Q.2 (x)2026 Annual I, Set D, Q.2 (x)7 marks

Explain how the energy imbalance between the poles and the equator can affect atmospheric circulation.

Solution

The energy imbalance

The Sun's energy is not spread evenly over the Earth. The equator receives more solar energy than the poles because:

Direct and oblique rays: at the equator sunlight falls almost vertically on a small area; near the poles the same beam falls obliquely and spreads over a much larger area. On level ground the intensity is I = I_0\cos\theta, so at 60^\circ it is only half its equatorial value.
Longer path through the atmosphere: oblique rays near the poles lose more energy by absorption and scattering.
Ice and snow: polar ice reflects most of the sunlight that arrives.

Equal beams of sunlight heat a small area at the equator but a large area near the pole.

The tropics (up to about 38^\circ) therefore absorb more energy than they emit (a surplus), while the higher latitudes emit more than they absorb (a deficit). Heat must be carried from the equator to the poles, and the atmosphere and oceans do this.

Effect on atmospheric circulation

Rising air at the equator: strong heating makes the air warm and light, so it rises, creating a belt of low pressure (the ITCZ) with clouds and heavy rain.
**Poleward flow aloft and sinking at 30^\circ:** the rising air spreads towards the poles high in the troposphere, cools, and sinks at about 30^\circ, creating the subtropical high-pressure belt (dry deserts).
Sinking air at the poles: cold, dense air sinks at the poles and forms high-pressure zones.
Pressure gradients make winds: air flows along the surface from high to low pressure, giving the trade winds (30^\circ to 0^\circ), the westerlies (30^\circ to 60^\circ) and the polar easterlies (90^\circ to 60^\circ).
The Coriolis effect turns the winds: due to the conservation of angular momentum, the winds are deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, so the circulation breaks into three cells in each hemisphere: the Hadley, Ferrel and polar cells.

The three-cell circulation produced by the energy imbalance.

Effect on climate

This circulation, together with ocean currents, redistributes heat and moisture around the globe. It sets the climate zones: wet equatorial forests under the rising air, deserts under the sinking air near 30^\circ, and stormy weather where air rises near 60^\circ. Changes in the energy balance change the circulation too; for example, if the poles warm faster than the tropics, the temperature difference and the winds that depend on it change, shifting weather systems, ocean currents and climate zones.

67 marks

Explain that due to the conservation of angular momentum, the Earth's rotation diverts the air to the right in the Northern Hemisphere and to the left in the Southern hemisphere, thus forming distinct atmospheric cells.

Solution

Angular momentum of moving air

Air on the Earth is carried eastwards by the Earth's rotation. For a parcel of air of mass m moving eastwards with speed v at a distance r from the Earth's axis, the angular momentum is

L = mvr,\qquad r = R\cos\theta

where R is the Earth's radius and \theta the latitude. If no torque acts, L is conserved. The distance r is largest at the equator and zero at the poles, so the ground's eastward speed \omega R\cos\theta falls from about 465\,\mathrm{m\,s^{-1}} at the equator to zero at the poles.

Air moving towards the poles

When air moves from the equator towards a pole, r decreases, so to keep mvr constant its eastward speed v must increase. The ground below moves more slowly, so the air runs ahead of the ground and turns eastwards. In the Northern Hemisphere, air moving north that turns east has turned to the right. For example, air starting at rest on the equator would reach 30^\circ moving east at about 134\,\mathrm{m\,s^{-1}} relative to the ground (without friction).

Air moving towards the equator

When air moves towards the equator, r increases and its eastward speed falls, while the ground below moves faster. The air lags behind the ground and turns westwards. For air moving south in the Northern Hemisphere, a turn to the west is again a turn to the right. This is why the trade winds blow from the north-east.

Southern Hemisphere

The same reasoning applies south of the equator, but there a turn towards the east for air moving south, or towards the west for air moving north, is a turn to the left. So winds are deflected to the left in the Southern Hemisphere (the south-east trades and north-west westerlies).

Coriolis force

This turning is the Coriolis effect, described by the Coriolis force F_c = 2mv\omega\sin\theta. It is zero at the equator (\sin0^\circ = 0), largest at the poles (\sin90^\circ = 1), acts at right angles to the motion, and changes the direction of the wind but not its speed.

Air moving towards the equator or the poles is turned to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

Formation of the atmospheric cells

Because the air is turned sideways, it cannot travel all the way from the equator to a pole. The circulation of each hemisphere breaks into three cells:

**Hadley cell (0^\circ to 30^\circ):** air rises at the equator, flows polewards aloft, is turned east (forming the subtropical jet stream), piles up and sinks near 30^\circ, then returns to the equator as the trade winds.
**Ferrel cell (30^\circ to 60^\circ):** air sinking at 30^\circ flows polewards along the surface, is turned east (the westerlies), and rises at about 60^\circ.
**Polar cell (60^\circ to 90^\circ):** cold air sinks at the pole, flows towards 60^\circ and is turned west (the polar easterlies), then rises at 60^\circ.

The three distinct atmospheric cells of each hemisphere.

These distinct cells, with their pressure belts (low at 0^\circ and 60^\circ, high at 30^\circ and 90^\circ) and wind belts, distribute heat and moisture around the globe and shape the weather and climate.

77 marks

Discuss that how differences in density of ocean water play an important role in ocean circulation.

Solution

Density of sea water

Density is mass per unit volume, \rho = m/V. Sea water has a density of about 1025\,\mathrm{kg\,m^{-3}}, and its value depends mainly on two factors:

Salinity: water with more dissolved salt has more mass packed into the same volume, so it is denser.
Temperature: cold water contracts and is denser; warm water expands and is less dense.
Pressure: at great depth the weight of the water above squeezes the water slightly, increasing its density.

So cold, salty water is the densest and sinks, while warm, fresh water floats on top.

Where dense water forms

Surface currents carry warm water from the equator towards the poles. At high latitudes (near Greenland and Antarctica) the water loses heat to the cold air and becomes denser. When sea ice forms, most of the salt is left behind in the water below, which makes it even saltier and denser. Evaporation also increases salinity.

How density differences drive the circulation

Sinking: where surface water becomes denser than the water below it, it sinks to the deep ocean.
Deep flow: the dense water spreads along the ocean floor towards the equator, forming deep ocean currents below about 500 m.
Replacement at the surface: warmer surface water flows towards the poles to replace the sinking water.
Upwelling: in other regions the deep water slowly rises back to the surface, bringing cold, nutrient-rich water up.

Cold, salty, dense water sinks in the polar seas and flows along the sea floor towards the equator.

Thermohaline circulation

This density-driven loop is the thermohaline circulation ('thermo' for temperature, 'haline' for salinity). It acts like a giant conveyor belt that links all the oceans, moving warm surface water towards the poles and cold deep water back towards the equator. One full circuit takes about 1000 years.

Importance

It carries heat from the tropics to the polar regions and keeps places such as north-west Europe mild.
It takes oxygen down to the deep ocean and brings nutrients up by upwelling, supporting sea life.
It stores heat and carbon dioxide in the deep ocean for centuries, which adds to climate inertia.
It can weaken if melting ice makes the polar seas fresher and less dense, which would change climates around the world.

87 marks

How the thermohaline circulation transports heat from the, tropics to the polar regions? Explain.

Solution

Meaning

The thermohaline circulation is a large-scale ocean circulation driven by differences in the density of sea water, which are created by surface heat and freshwater exchanges. 'Thermo' refers to temperature and 'haline' to salinity, the two factors that decide density: cold and salty water is dense and sinks; warm and fresh water is light and floats.

Steps of heat transport

Heating in the tropics: strong sunlight near the equator warms the surface water. Warm water is less dense, so it stays at the surface.
Surface flow towards the poles: wind-driven surface currents, such as the Gulf Stream and the North Atlantic Drift, carry this warm water from the equatorial region towards the poles.
Heat released at high latitudes: on its way, the warm water gives heat to the cold air above it. This is how heat from the tropics reaches the polar regions.
Cooling and sinking: the cooled water becomes denser; sea-ice formation (which leaves salt behind) and evaporation make it saltier as well. The cold, salty, dense water sinks at high latitudes near Greenland and Antarctica.
Deep return flow: the dense water spreads slowly back towards the equator along the ocean floor as deep ocean currents.
Upwelling: far away, the deep water gradually rises to the surface, is warmed again in the tropics, and the cycle continues, transporting energy and mass.

Thermohaline circulation: warm water flows polewards at the surface, cools and sinks, and returns along the ocean floor.

Example

The Gulf Stream and the North Atlantic Drift carry an enormous amount of heat from the Caribbean to the North Atlantic. This is why north-west Europe has much milder winters than places at the same latitude in Canada.

Conveyor belt and importance

The thermohaline circulation acts like a giant conveyor belt: warm surface water moves towards the poles and cold, nutrient-rich deep water moves back. One complete circuit takes about 1000 years. It reduces the temperature difference between the equator and the poles, supplies oxygen to the deep ocean and nutrients to the surface, and stores heat and \mathrm{CO_2} in the deep ocean. If melting ice makes the polar seas fresher and lighter, the sinking could slow, weakening this heat transport and changing climates around the world.

97 marks

Show that the climate science is a chaotic system in terms of butterfly effct.

Solution

Chaotic system

A chaotic system is a system in which there is no equilibrium and no exactly repeating pattern, and which shows extreme sensitivity to initial conditions: even a tiny change in the starting conditions can lead to a vastly different outcome later. It follows definite physical laws, yet its long-term behaviour cannot be predicted in detail.

Why the climate is a chaotic system

The climate is a complex system of many interacting parts (atmosphere, oceans, ice, land and living things) linked by feedbacks.
The atmosphere and oceans are non-linear: small causes do not always give small effects; a small disturbance can grow into a large one.
Although we think of the climate as having predictable seasons and weather patterns, it can undergo sudden and rapid changes because of these chaotic dynamics. Even a forecast of rain for today sometimes proves wrong.

The butterfly effect

Consider a butterfly flapping its wings in one part of the world. According to chaos theory, this seemingly insignificant event could set off a chain of interactions that eventually leads to a hurricane forming in another part of the world. This is the butterfly effect: small perturbations can grow and spread through a complex system and finally influence large-scale events. It does not mean that butterflies cause hurricanes; it means that tiny, unmeasurable differences decide which of many possible futures actually happens.

The meteorologist Edward Lorenz discovered this in 1961. He restarted a computer weather simulation using a number rounded from 0.506127 to 0.506, and after a short time the new forecast was completely different from the original.

Two forecasts from the same model whose starting values differ by only 0.1%: identical at first, completely different later.

Consequences for prediction

Climate scientists call the effects of chaos intrinsic or unforced variability: unpredictable changes that arise from interactions within the climate system itself rather than from external causes.
Because the initial state of the atmosphere can never be measured perfectly, small errors always grow. Large-scale weather features can be predicted with reasonable certainty only a few days ahead; forecasts for longer periods are less reliable.

Conclusion

The butterfly effect shows that the climate system is extremely sensitive to its initial conditions, has no fixed equilibrium and never repeats exactly, which are the features of a chaotic system. Climate science therefore balances predictability and chaos: long-term averages can be projected, but some aspects of the climate remain inherently uncertain and difficult to predict.

Note. 'effct' is the book's misprint for effect.

106 marks

Differentiate between surface ocean currents and deep ocean currents.

Solution

Surface ocean currents

Currents produced in the shallower water near the ocean surface (roughly the top 400 to 500 m). They are set in motion mainly by the wind, which drags the surface water along, and they are bent into large rotating loops (subtropical gyres) by the Coriolis effect: clockwise in the Northern Hemisphere and anticlockwise in the Southern Hemisphere. Examples: the Gulf Stream and the Kuroshio Current.

Deep ocean currents

Currents below a depth of about 500 m. They are driven by differences in water density, caused by differences in temperature and salinity. Cold, salty water sinks at high latitudes and flows slowly along the ocean floor towards the equator. Together they form the thermohaline circulation.

Differences

Feature	Surface ocean currents	Deep ocean currents
Depth	Upper ocean, roughly the top 400 to 500 m	Below about 500 m
Main cause	Wind (with the Coriolis effect)	Density differences due to temperature and salinity
Speed	Fast, up to about 2\,\mathrm{m\,s^{-1}}	Very slow, a few centimetres per second or less
Temperature and density	Warm or cool, less dense	Cold, salty and dense
Pattern	Gyres following the wind belts	A global conveyor belt linking all oceans
Share of ocean water	About 10%	About 90%
Time scale	Water crosses an ocean in months to years	One full circuit takes about 1000 years
Main role	Quick transfer of heat towards the poles; controls coastal climates	Slow transfer of heat, oxygen and nutrients; stores heat and carbon, adding to climate inertia

The surface flow (top) and the deep flow (bottom) are linked in one loop by sinking at high latitudes and upwelling elsewhere.

Connection

The two kinds of current are linked. Warm surface currents carry water towards the poles, where it cools and sinks to feed the deep currents; deep water returns to the surface by upwelling. Together they form the global ocean conveyor belt that redistributes heat around the Earth.

117 marks

What are coriolis force and thermohaline circulations?

Solution

(a) Coriolis force

The Coriolis force is the apparent force that bends the path of moving air or water because of the Earth's rotation. The turning it produces is called the Coriolis effect. Its magnitude is:

F_c = 2mv\omega\sin\theta

where m is the mass of the moving air packet, v its speed relative to the ground, \omega the angular velocity of the Earth (7.3\times10^{-5}\,\mathrm{rad\,s^{-1}}) and \theta the latitude.

Cause: conservation of angular momentum. Air moving towards the poles comes closer to the Earth's axis and speeds up eastwards; air moving towards the equator lags behind the faster ground.
Direction: at right angles to the motion, to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
Size: zero at the equator (\sin0^\circ = 0), largest at the poles (\sin90^\circ = 1), and proportional to the speed.
Effect: it changes the direction of winds and currents but not their speed. It turns the surface winds into the trade winds, westerlies and polar easterlies, forms the atmospheric cells, bends ocean currents into gyres, and balances the pressure gradient force in the geostrophic wind.
Example: an air packet of mass 2\times10^{5}\,\mathrm{kg} moving at 22.2\,\mathrm{m\,s^{-1}} at latitude 25^\circ feels F_c = 2(2\times10^{5})(22.2)(7.3\times10^{-5})\sin25^\circ = 274\,\mathrm{N}.

The Coriolis effect turns winds and ocean currents: gyres run clockwise in the Northern Hemisphere and anticlockwise in the Southern Hemisphere.

(b) Thermohaline circulation

The thermohaline circulation is the large-scale ocean circulation driven by density differences in sea water, created by surface heat and freshwater exchanges. 'Thermo' means temperature and 'haline' means salinity; cold, salty water is dense and sinks.

Warm surface water flows from the tropics towards the poles and gives up heat to the air.
At high latitudes it cools, becomes saltier as sea ice forms, becomes dense and sinks.
The dense water spreads back towards the equator along the ocean floor as deep ocean currents.
It rises again elsewhere (upwelling), bringing nutrients to the surface, and the loop continues.

The thermohaline conveyor belt.

It acts like a global conveyor belt that carries heat from the tropics to the polar regions, takes oxygen to the deep ocean, and stores heat and carbon dioxide for centuries.

Numerical problemsp. 200

Convert every quantity to SI units first (\mathrm{km^2}\to\mathrm{m^2}, \mathrm{mm}\to\mathrm{m}, \mathrm{km\,h^{-1}}\to\mathrm{m\,s^{-1}}).

13 marks

If the volume of change in storage of the hydrosphere, whose area is 30\,\mathrm{km^2} is 600{,}000\,\mathrm{m^3}, find net precipitation (P) if the volume of evapotranspiration is 1{,}500{,}000\,\mathrm{m^3} and runoff volume is 600{,}000\,\mathrm{m^3}.

Solution

Given

A = 30\,\mathrm{km^2} = 30\times10^{6}\,\mathrm{m^2} = 3.0\times10^{7}\,\mathrm{m^2} area
\Delta S_V = 6.0\times10^{5}\,\mathrm{m^3} change in storage (volume)
E_V = 1.5\times10^{6}\,\mathrm{m^3} evapotranspiration (volume)
Q_V = 6.0\times10^{5}\,\mathrm{m^3} runoff (volume)

To find

precipitation P (as a depth, in mm)

Write the water balance equation for volumes and make P_V the subject.
\Delta S_V = P_V - E_V - Q_V\;\Longrightarrow\; P_V = \Delta S_V + E_V + Q_V
Substitute the volumes.
P_V = 6.0\times10^{5} + 1.5\times10^{6} + 6.0\times10^{5} = 2.7\times10^{6}\,\mathrm{m^3}
Precipitation is measured as a depth, so divide the volume by the area.
P = \dfrac{P_V}{A} = \dfrac{2.7\times10^{6}\,\mathrm{m^3}}{3.0\times10^{7}\,\mathrm{m^2}} = 0.090\,\mathrm{m}
Convert to millimetres (1\,\mathrm{m} = 1000\,\mathrm{mm}).
P = 0.090\times1000 = 90\,\mathrm{mm}

P = 90\,\mathrm{mm}

Book answer. 90\,\mathrm{mm}

23 marks

If a Coriolis force of 30\,\mathrm{N} deflects a mass of air at 30^\circ moving with speed of 40\,\mathrm{km\,h^{-1}}. Find the mass of this air packet (where \omega = 7.3\times10^{-5}\,\mathrm{rad\,s^{-1}}).

Solution

Given

F_c = 30\,\mathrm{N} Coriolis force
\theta = 30^\circ latitude
v = 40\,\mathrm{km\,h^{-1}} = \dfrac{40\times1000}{3600} = 11.1\,\mathrm{m\,s^{-1}} speed
\omega = 7.3\times10^{-5}\,\mathrm{rad\,s^{-1}} angular velocity of the Earth

To find

mass of the air packet m

Make m the subject of the Coriolis force formula.
F_c = 2mv\omega\sin\theta\;\Longrightarrow\; m = \dfrac{F_c}{2v\omega\sin\theta}
Substitute the values, with \sin30^\circ = 0.5.
m = \dfrac{30}{2(11.1)(7.3\times10^{-5})(0.5)} = \dfrac{30}{8.11\times10^{-4}}
Evaluate.
m = 3.70\times10^{4}\,\mathrm{kg}
Check by substituting back into the formula.
F_c = 2(3.70\times10^{4})(11.1)(7.3\times10^{-5})(0.5) = 30\,\mathrm{N}

m \approx 3.7\times10^{4}\,\mathrm{kg}\ (\text{about } 37{,}000\,\mathrm{kg})

Book answer. 369{,}900\,\mathrm{kg}

Note. The book's answer (369,900 kg) is 10 times too large: it is what you get if \omega is taken as 7.3\times10^{-6}\,\mathrm{rad\,s^{-1}}. With the given \omega = 7.3\times10^{-5}\,\mathrm{rad\,s^{-1}} the mass is 3.7\times10^{4}\,\mathrm{kg}, and substituting it back gives exactly 30\,\mathrm{N}.

FBISE 2026 questions

Questions from the FBISE HSSC-II 2026 (1st Annual) papers that belong to this unit, answered the way an examiner expects.

2026 Annual I, Set B, MCQ 151 mark

The magnitude of Coriolis Force (F_c) that bends the air packet (mass =2\times10^{5}\,\mathrm{kg}) (velocity 22.2\,\mathrm{m\,s^{-1}}) at 25^\circ is:

A2.74\,\mathrm{N}
B274\,\mathrm{N}Answer
C2.74\times10^{4}\,\mathrm{N}
D2.74\times10^{6}\,\mathrm{N}

Why

Using F_c = 2mv\omega\sin\theta with \omega = 7.3\times10^{-5}\,\mathrm{rad\,s^{-1}} (printed on the paper): F_c = 2(2\times10^{5})(22.2)(7.3\times10^{-5})\sin25^\circ = 648\times0.423 = 274\,\mathrm{N}.

These are the numbers of the book's Example 24.2. The other options have the same digits with the wrong power of ten.

2026 Annual I, Set B, Q.2 (vi)1x3 = 3 marks

Briefly explain the significance of wind pattern on climate system.

Solution

Marks: one for each of three explained points.

Wind patterns (the trade winds, westerlies and polar easterlies, and local winds) are an essential part of the climate system because they move heat, moisture and energy around the globe. Over a 30-year period their regular patterns form part of the climate of a region.

Heat transport: winds carry heat from the warm tropics towards the cold poles and reduce the temperature difference between them. They also drive the surface ocean currents, which carry warm water towards the poles.
Moisture transport and rainfall: winds carry water vapour from the oceans over the land and so decide where and when rain falls; for example, the monsoon winds bring most of Pakistan's rain.
Dispersion and daily weather: winds spread pollutants (dust, smoke, industrial emissions) and pollen over large areas, and land and sea breezes caused by differential heating change the daily weather of coastal regions.

Changes in wind patterns over long periods therefore change regional and even global climate.

2026 Annual I, Set B, Q.2 (vii) OR1.5+1.5 = 3 marks

Briefly explain the two types of climate inertia.

Solution

Climate inertia is the tendency of the Earth's climate to resist, or respond slowly to, changes in factors such as greenhouse gas concentrations and solar radiation. It is of two types (1.5 marks each):

1. Short-term inertia: the oceans and land surfaces absorb and release heat relatively slowly, so the temperature cannot change instantly. This smooths the daily and seasonal temperature changes: the hottest part of the day comes in the afternoon, a few hours after the Sun is highest, and places near the sea have smaller day and night temperature differences than deserts.

2. Long-term inertia: changes in greenhouse gas concentrations, such as \mathrm{CO_2}, take decades to centuries to affect the climate fully, because the deep ocean warms slowly and the carbon cycle and ice sheets respond slowly. Even if emissions were stabilised today, the Earth would keep warming for many years because of the heat already stored in the oceans.

2026 Annual I, Set B, Q.2 (x)3 marks

Why does the equatorial region receive more energy than polar regions?

Solution

The equatorial region receives more solar energy per square metre than the polar regions because:

Angle of the Sun's rays: at the equator sunlight falls almost vertically (direct sunlight), so a beam covers a small area and the energy per unit area is high. Near the poles the same beam falls at a low angle (oblique sunlight) and spreads over a much larger area, so the energy per unit area is small. On level ground I = I_0\cos\theta, where \theta is the latitude.
Longer path through the atmosphere: oblique rays near the poles pass through more air, so more of their energy is absorbed, scattered and reflected before it reaches the ground.
Ice and daylight: polar ice and snow reflect most of the sunlight that arrives (high albedo), while oceans and forests near the equator absorb it; the poles also have months of darkness in winter.

Equal beams of sunlight: a small area is heated at the equator, a large area near the pole.

2026 Annual I, Set D, MCQ 11 mark

Velocity of a wind packet (mass =2\times10^{5}\,\mathrm{kg}), if a Coriolis Force (F_c) of 274\,\mathrm{N} bends it at an angle of 25^\circ would be:

A32.2\,\mathrm{km\,hr^{-1}}
B42.2\,\mathrm{km\,hr^{-1}}
C12.2\,\mathrm{m\,s^{-1}}
D22.2\,\mathrm{m\,s^{-1}}Answer

Why

From F_c = 2mv\omega\sin\theta: v = \dfrac{F_c}{2m\omega\sin\theta} = \dfrac{274}{2(2\times10^{5})(7.3\times10^{-5})\sin25^\circ} = \dfrac{274}{12.3} = 22.2\,\mathrm{m\,s^{-1}}.

This equals 80\,\mathrm{km\,h^{-1}} (the book's Example 24.2), so the km/h options A and B are also wrong.

2026 Annual I, Set D, MCQ 171 mark

Which layer of the atmosphere contains the ozone layer?

AMesosphere
BStratosphereAnswer
CTroposphere
DThermosphere

Why

The ozone layer lies in the stratosphere (about 16 km to 50 km). By absorbing the Sun's ultraviolet radiation it makes the temperature rise with height in this layer.

2026 Annual I, Set D, Q.2 (vi)3 marks

Explain the role of atmosphere in maintaining climate balance.

Solution

The atmosphere keeps the Earth's climate balanced in these ways:

Keeps the Earth warm (greenhouse effect): greenhouse gases such as water vapour and \mathrm{CO_2} let sunlight in but absorb part of the outgoing infrared radiation and send some of it back to the surface. This keeps the average temperature near 15^\circ\mathrm{C} instead of -18^\circ\mathrm{C} and prevents extreme differences between day and night temperatures.
Shields life: the ozone layer in the stratosphere absorbs harmful ultraviolet radiation, and meteors burn up in the mesosphere.
Redistributes heat and moisture: solar heating causes convection; the resulting winds and circulation cells carry heat from the equator towards the poles and water vapour from the oceans over the land, producing weather and evening out temperatures across the globe.

Clouds and air also reflect part of the sunlight back to space, so the atmosphere helps to keep the energy the Earth absorbs equal to the energy it radiates (the energy budget).

2026 Annual I, Set D, Q.2 (vii) OR3 marks

How does ocean play a main role in climate inertia?

Solution

Climate inertia is the slow response of the climate to changes in factors such as greenhouse gases. The oceans are its main cause because:

High heat capacity: water has a high specific heat capacity (about 4200\,\mathrm{J\,kg^{-1}\,K^{-1}}) and the oceans have an enormous mass. From Q = mc\Delta T, a huge amount of heat produces only a small change in temperature; the oceans can store about 1000 times more heat than the atmosphere.
Slow absorption and release: the oceans absorb heat slowly and release it slowly, and the deep ocean mixes with the surface only over centuries. So changes in ocean temperature, and in the climate patterns they control, happen gradually.
Stored heat keeps the warming going: even if greenhouse gas emissions were stabilised immediately, the heat already stored in the oceans would keep the Earth warming for decades. The oceans also absorb \mathrm{CO_2} and exchange it slowly with the air, which adds to the delay.

2026 Annual I, Set D, Q.2 (x)3 marks

How does energy imbalance between equator and poles affect earth climate?

Solution

Energy imbalance: the equator receives direct sunlight and gains more energy than it loses, while the poles receive oblique sunlight and lose more energy than they gain.

Winds and circulation cells: warm air rises at the equator (low pressure) and cold air sinks at the poles (high pressure). The pressure differences drive the trade winds, westerlies and polar easterlies, which the Coriolis effect organises into the Hadley, Ferrel and polar cells.
Heat transport: these winds, together with ocean currents (gyres and the thermohaline circulation), carry heat from the tropics towards the poles and reduce the temperature difference.
Climate zones: the circulation creates wet equatorial regions (rising air), dry deserts near 30^\circ (sinking air) and stormy mid-latitudes near 60^\circ. Changes in the balance shift weather systems, ocean currents and climate zones.

The circulation cells set up by the energy imbalance between the equator and the poles.

Exam focus

Use F_c = 2mv\omega\sin\theta to find the Coriolis force, the wind speed, the mass or the latitude. (numerical)
Asked as an MCQ in both 2026 sets (Set B MCQ 15, Set D MCQ 1) with the numbers of Example 24.2; also numerical problem 2.
Go to answer
Why does the equator receive more energy than the poles, and how does this imbalance affect atmospheric circulation and climate? (short)
Asked in both 2026 sets (Set B Q.2 (x), Set D Q.2 (x)); the full version is comprehensive question 5.
Go to answer
Define climate inertia, explain its two types and the main role of the oceans. (short)
Asked in both 2026 sets as OR parts (Set B Q.2 (vii), Set D Q.2 (vii)); the full version is comprehensive question 2.
Go to answer
Describe the Earth's climate system and its five interacting components. (long)
The first learning outcome and comprehensive question 1; the atmosphere part was asked in 2026 (Set D Q.2 (vi)).
Go to answer
Discuss the Earth's energy budget and explain the greenhouse effect in three steps. (long)
Comprehensive questions 3 and 4; the percentages 49%, 20% and 31% and the three steps are easy marks.
Go to answer
Explain how conservation of angular momentum deflects winds and forms the Hadley, Ferrel and polar cells. (long)
Comprehensive question 6 and short question 5; a labelled three-cell diagram earns marks.
Go to answer
Explain how density differences drive the thermohaline circulation and how it carries heat to the poles. (long)
Comprehensive questions 7, 8 and 11 all come from this topic.
Go to answer
State the layers of the atmosphere and the role of the atmosphere in keeping the climate balanced. (mcq)
Five exercise MCQs, short questions 1 and 9, and 2026 Set D MCQ 17 (ozone in the stratosphere) and Q.2 (vi).
Go to answer
Solve water balance problems with \Delta S = P - E - Q (depths and volumes). (numerical)
Example 24.1, Assignment 24.1 and numerical problem 1 use the same method.
Go to answer
Explain the significance of wind patterns for the climate system. (short)
Asked in 2026 Set B Q.2 (vi) for three 1-mark points.
Go to answer
Show that climate science is a chaotic system using the butterfly effect. (long)
A learning outcome and comprehensive question 9; definitions of a chaotic system and the butterfly effect are also good short questions.
Go to answer
Differentiate between surface ocean currents and deep ocean currents. (short)
Comprehensive question 10; a comparison table answers it quickly.
Go to answer

Weather, climate and the climate system

24.1Earth's climate system: five interacting componentsOften examined

Examples of interaction between the components

24.1 (1)The atmosphere (air)Often examined

Roles of the atmosphere in the climate

Layers of the atmosphere

Why the stratosphere is calm and stable

The ionosphere

24.1 (2)The hydrosphere and the water balance equationOften examined

Characteristics of the hydrosphere

Water balance equation

Derivation Water balance in terms of volume

24.1 (3)The cryosphere (ice)

Components of the cryosphere

Effects of the cryosphere on the climate

24.1 (4)The lithosphere (Earth's upper rocky layer)

Features of the lithosphere that affect the climate

24.1 (5)The biosphere (living things)

Role of the biosphere in the climate system

24.2Ocean currents and surface currentsOften examined

Surface ocean currents

24.2 (2)Deep ocean currents and the density of sea waterOften examined

How deep currents form

24.2Thermohaline circulationOften examined

How it transports heat from the tropics to the polar regions

24.2Earth's angular momentum and the Coriolis forceOften examined

Conservation of angular momentum

The Coriolis force

Main features of the Coriolis force

Derivation Where Fc​=2mvωsinθ comes from (turntable model)

24.2Atmospheric circulation cells and wind beltsOften examined

The three-cell model

24.2Geostrophic wind and the importance of wind patternsOften examined

Forces on moving air

Importance of wind patterns for the climate

Land and sea breezes (differential heating)

24.3Climate inertiaOften examined

Feedback mechanisms

Time dependence: short-term and long-term inertia

Why the oceans cause most of the inertia

Adaptation and mitigation

24.3.1Internal variations

24.3.2External forcing

24.4Earth's energy budgetOften examined

Components of the energy budget

24.4The greenhouse effectOften examined

How it works, in three steps

24.4.1Solar variability and climate

24.4.2Energy imbalance between the equator and the polesOften examined

Why the equator receives more energy than the poles

Derivation Intensity of sunlight at latitude \theta

How the imbalance drives atmospheric circulation

24.4.3Climate science as a chaotic system

The butterfly effect

Predictability and uncertainty

Formula sheet

Key terms

Multiple choice questionsp. 199

Short questionsp. 199

Comprehensive questionsp. 199–200

Introduction

1. Atmosphere (air)

2. Hydrosphere (water)

3. Cryosphere (ice)

4. Lithosphere (rock)

5. Biosphere (living things)

Interactions: why it is a complex system

Definition

Cause: feedback mechanisms

Types of climate inertia

Main role of the oceans

Other slow processes

Importance

The Sun as the source of energy

Energy budget: energy in equals energy out

Greenhouse effect

Uneven heating of the Earth

Transfer of heat by the atmosphere and oceans

Changes in energy transfer change the climate

Definition

Derivation Where $F_{c} = 2 m v ω sin θ$ comes from (turntable model)