Atmospheric processes and global climate change

The atmosphere: composition and vertical structure

The atmosphere is a thin shell of gas held to the Earth by gravity, and the syllabus asks only for an outline of what it contains. Nitrogen makes up about 78% of dry air and oxygen about 21%; argon accounts for nearly 1%, and carbon dioxide for only about 0.04%, which is a little over 420 parts per million. The remaining gases - neon, helium, methane, ozone and the CFCs - are present in tiny quantities, but several of them matter far more than their share of the mass suggests, because they absorb longwave radiation.

Water vapour is the one component that varies enormously, from almost nothing over a cold desert to about 4% of the air in the humid tropics, and it is the gas that carries latent heat around the planet. The atmosphere also holds aerosols: dust, sea salt, smoke, pollen and pollution particles. Their mass is negligible, but they act as condensation nuclei - the surfaces on which water vapour condenses into cloud droplets - so without them there would be very little cloud and very little rain.

The atmosphere is layered by temperature, and the direction in which temperature changes with height is the quickest way to identify each layer. In the troposphere temperature falls with height at roughly 6.5°C per km, because the air is heated from below by the Earth's surface rather than by the Sun directly.

In the stratosphere temperature rises with height, because ozone there absorbs ultraviolet radiation from the Sun; that warming makes the layer stable, which is why large commercial jets cruise in its lower part and why very little weather reaches it. Above that, temperature falls again through the mesosphere and then rises sharply in the thermosphere, where the gas is so thin that temperature means very little.

The boundary between the troposphere and the stratosphere is the tropopause, and it matters for two reasons. It is much higher over the equator, at about 16 to 18 km, than over the poles, at about 8 km, because tropical air is warm and rises further before it cools to the surrounding temperature; and it acts as a lid on weather, because the change in temperature trend stops air rising through it, so almost all cloud, rain and storm systems are trapped below it. The jet streams flow just beneath the tropopause, which is why they sit at roughly the height and latitude described later in this topic.

• Nitrogen about 78% and oxygen about 21% of dry air: the bulk of the atmosphere, but neither absorbs much longwave radiation.
• Carbon dioxide about 0.04% (a little over 420 ppm), argon about 0.9%: small shares, but carbon dioxide is the reference gas for the greenhouse effect.
• Water vapour varies from almost zero to about 4%: the main carrier of latent heat and the most abundant greenhouse gas.
• Aerosols act as condensation nuclei, so they are essential to cloud formation even though they are invisible.
• Troposphere: temperature falls with height, contains most of the atmosphere's mass and water vapour, and holds almost all weather.
• Stratosphere: temperature rises with height because ozone absorbs ultraviolet radiation; stable air, few clouds, used by jet aircraft.
• Mesosphere and thermosphere: temperature falls then rises again, but the air is so thin that these layers have little effect on the surface.
• Tropopause: the boundary at about 8 km over the poles and 16 to 18 km over the equator; it caps vertical motion and marks the level of the jet streams.

Energy budget systems: inputs, outputs and transfers

An energy budget is a balance sheet: the energy coming in, the energy going out, and what happens to it in between. The input is shortwave solar radiation, or insolation, with a peak wavelength of about 0.5 micrometres because the Sun's surface is extremely hot.

The output is longwave terrestrial radiation, with a peak wavelength of about 10 micrometres, emitted by the Earth's surface and atmosphere because they are far cooler. Shortwave in, longwave out is the first thing to establish in any answer about the budget.

Not all of the incoming radiation reaches the ground. Some is absorbed by gases, aerosols and cloud droplets; some is reflected straight back to space, and the proportion reflected is the albedo; and some is scattered sideways by gas molecules and particles, which is why the sky is blue and why you can be sunburnt on a cloudy day from diffuse radiation.

Of every 100 units of solar radiation arriving at the top of the atmosphere, roughly 30 are reflected back to space, about 19 are absorbed by the atmosphere and clouds, and about 51 are absorbed at the Earth's surface. The global average albedo of the Earth is therefore about 30%.

Energy leaves the surface by four routes. It is emitted as longwave radiation; it is transferred by conduction into the thin layer of air touching the ground; it is carried upward by latent heat transfer, in which evaporation at the surface uses energy to turn water into vapour and condensation higher in the atmosphere releases that energy again; and it is carried upward by sensible heat transfer, the direct warming of air by a warm surface and its removal by convection. Back radiation is the part of the longwave radiation emitted downwards by greenhouse gases in the atmosphere, which returns energy to the surface and is the mechanism of the greenhouse effect.

Sensible heat is heat you can feel: a warm surface heats the air in contact with it, the air expands, becomes less dense and rises, carrying the heat with it. Latent heat is hidden heat: it is the energy absorbed when water evaporates and released when it condenses, so it moves energy from the surface into the atmosphere without any change in temperature at the point of evaporation. About 2 260 joules are needed to evaporate one gram of water and the same amount is released when that gram condenses, which is why a rising, condensing air mass warms itself and keeps rising - the latent heat released inside a cumulonimbus is what drives a thunderstorm.

The global energy budget: surplus, deficit and the natural greenhouse effect

The Earth as a whole receives and loses almost exactly the same amount of energy, so the global budget is in balance. That balance is not evenly distributed.

Between roughly 35° north and 35° south the surface absorbs more shortwave radiation than it loses as longwave radiation, so the tropics and subtropics run an energy surplus; polewards of that, the loss exceeds the gain, so the high latitudes run a deficit. The surplus zone is sometimes drawn as extending to about 35° of latitude in each hemisphere, which is why the boundary is described as roughly the latitude of southern Spain or of Sydney.

Three things explain the surplus at low latitudes. The Sun's rays strike the surface more directly, so the same beam is spread over a smaller area and delivers more energy per square metre.

The rays also travel through a shorter path of atmosphere, so less is reflected, scattered and absorbed on the way down. And the tropics have little or no snow or ice, so their albedo is low - a dark ocean absorbs most of what reaches it, while the ice and snow of high latitudes reflect a large share of the little energy they receive, which deepens the deficit there.

If the surplus were not moved away, the tropics would get steadily hotter and the poles steadily colder, and the temperature difference would keep growing. It does not, because the atmosphere and the oceans transfer the excess energy polewards, which is the subject of the next block. This transfer is the reason the global budget is described as a system of excesses and deficits: the pattern of heating creates the temperature gradient, and the gradient drives the winds and currents that remove it.

The natural greenhouse effect is the process that keeps the planet habitable, and it existed long before humans did. Shortwave radiation from the Sun passes through the atmosphere almost unchanged and is absorbed by the surface. The surface re-emits that energy as longwave radiation, and gases such as water vapour, carbon dioxide, methane, nitrous oxide and ozone absorb specific wavelengths of it.

Those gases then re-emit radiation in all directions, including downwards, so some of the energy that would have escaped to space is returned to the surface as back radiation. Without it the average surface temperature would be about -18°C instead of about 15°C, and liquid water - and life as we know it - would be impossible over most of the planet.

• Surplus: between about 35°N and 35°S, absorption of shortwave radiation exceeds loss of longwave radiation.
• Deficit: polewards of about 35°, loss exceeds gain, and the deficit is largest over the polar ice sheets.
• Why the tropics gain more: a higher angle of the Sun, a shorter path through the atmosphere, and a low albedo because there is little snow or ice.
• Why the poles lose more: a low angle of the Sun, a long atmospheric path, and a high albedo from snow and ice, plus months of winter darkness.
• Why it matters: without transfer the gradient would grow without limit; winds and currents remove the surplus polewards.
• Natural greenhouse effect: greenhouse gases absorb outgoing longwave radiation and re-emit some of it downwards as back radiation.
• Without the natural greenhouse effect the average surface temperature would be about -18°C rather than about 15°C.

Global transfers of energy: the tri-cellular model, winds, jet streams and ocean currents

Because the tropics have a surplus and the poles a deficit, energy has to move polewards, and it does so through the atmosphere and the oceans. In the atmosphere the movement is described by the tri-cellular model, which divides each hemisphere into three circulation cells. In the Hadley cell, intense heating at the equator makes air expand, become less dense and rise, producing low pressure and heavy convectional rain.

The rising air spreads polewards at high altitude, cools, becomes denser and sinks at about 30° of latitude, creating the subtropical high-pressure belts. At the surface it returns towards the equator as the trade winds.

The Ferrel cell lies between 30° and 60° and is driven indirectly by the other two rather than by heating at its base: air rises at about 60° where warm mid-latitude air meets cold polar air, and sinks at about 30°. At the surface, air flows polewards from the subtropical high as the westerlies, which is why mid-latitude weather comes from the west.

The Polar cell has sinking air over the pole, which flows equatorwards as the polar easterlies and rises at about 60°. Each rising zone is a belt of low pressure and each sinking zone a belt of high pressure, so the model explains the world's pressure belts and wind belts in one diagram.

The intertropical convergence zone (ITCZ) is the belt where the trade winds of the two hemispheres meet, near the equator. Air there is forced to rise, so the ITCZ is a zone of low pressure, light winds, towering cumulonimbus and very heavy rain.

It is not fixed: it migrates north and south following the overhead Sun, reaching about 10° to 15° north in July and a similar distance south in January, and that migration is what gives the tropics their wet and dry seasons. Where the ITCZ fails to arrive, as it did repeatedly in the Sahel in the late twentieth century, the result is drought.

Jet streams are narrow, fast ribbons of wind at the top of the troposphere, flowing west to east at 100 to 400 km per hour. The polar front jet stream forms where the steep temperature contrast between tropical and polar air creates a strong pressure gradient, at about 10 to 12 km altitude and 30° to 60° of latitude; it steers the depressions and anticyclones of mid-latitude weather, and its meanders allow cold polar air south and warm tropical air north.

The subtropical jet stream lies further equatorwards, at about 25° to 30°, and is strongest in winter. Both are found just below the tropopause because that is where the temperature gradient and the wind shear are greatest.

The oceans carry a comparable share of the surplus. Surface currents are driven by the prevailing winds and deflected by the Coriolis force, and because the wind belts are roughly zonal they set up large, closed loops called gyres: clockwise in the northern hemisphere and anticlockwise in the southern. The western side of each gyre carries warm water polewards in a narrow, fast current - the Gulf Stream and its continuation the North Atlantic Drift, and the Kuroshio off Japan - while the eastern side carries cold water equatorwards in a broad, slow current, such as the Canary Current off north-west Africa, the Benguela off south-west Africa and the Peru (Humboldt) Current off South America.

Vertical transfer is driven by density, in the thermohaline circulation. In the North Atlantic and around Antarctica, surface water becomes cold and salty enough - salty because sea ice formation leaves salt behind, and because evaporation concentrates it - to sink to the ocean floor.

That dense water flows slowly through all the ocean basins as a deep current, and it rises again, or upwells, in the Indian and Pacific Oceans where surface water is drawn away. The circuit is often called the global conveyor belt and takes on the order of a thousand years to complete, but it is what distributes heat and dissolved carbon dioxide through the deep ocean.

The atmosphere and the oceans are a coupled system, and the links run both ways. Winds drive the surface currents and the gyres, and the position of the pressure belts decides where upwelling occurs.

The currents in turn heat or cool the air above them, changing the temperature and therefore the pressure of that air, which reinforces or shifts the wind belts. The El Nino Southern Oscillation (ENSO) is the clearest example: a weakening of the trade winds allows warm water to spread east across the equatorial Pacific, which shifts the pattern of convection and rainfall and changes weather across the globe.

Seasonal variations in temperature, pressure and wind belts

Seasonal change is caused by the tilt of the Earth's axis, about 23.5° from the vertical, combined with its orbit around the Sun. The tilt does not change direction as the Earth orbits, so the Sun appears overhead at 23.5° north in June, over the equator at the equinoxes, and at 23.5° south in December, and the whole system of pressure and wind belts shifts with it. That migration is why the ITCZ moves, why the westerlies and the subtropical highs move, and why the monsoon reverses.

Latitude decides the angle of the Sun and the length of the day, and therefore the size of the seasonal swing. At the equator the Sun is high all year, day length is almost constant at about 12 hours, and the seasonal range of temperature is only a few degrees.

At 50° to 60° of latitude the Sun's noon angle changes hugely between summer and winter, day length ranges from about 8 to about 16 hours, and the seasonal range is large. Polewards of the Arctic and Antarctic Circles there are days in midsummer when the Sun does not set and days in midwinter when it does not rise at all.

Land and sea distribution modifies that pattern, because land and water have very different specific heat capacities. Water takes far more energy to warm than land does, and it mixes and evaporates, so the sea heats up and cools down slowly while land heats and cools quickly. The result is that coastal and island locations have a maritime climate with a small annual range and a lag of about a month between the sunniest period and the warmest month, while continental interiors have a continental climate with a much larger annual range.

In summer the land becomes a thermal low that draws in moist air from the sea; in winter the land becomes a cold high that pushes dry air outwards. That seasonal reversal of pressure and wind is the monsoon, and it is why the Indian subcontinent has a very wet summer and a very dry winter.

Ocean currents add a third control. A warm current raises winter temperatures and keeps ports ice-free far to the north - the North Atlantic Drift keeps the west of Britain and Norway several degrees warmer than the same latitude in Canada.

A cold current lowers summer temperatures and chills the air above it, so condensation occurs and advection fog forms, which is why the Grand Banks off Newfoundland is one of the foggiest places on Earth where the warm Gulf Stream meets the cold Labrador Current. Cold currents also stabilise the air above them and lie under the subtropical highs, which is why the west coasts of continents in the tropics and subtropics - the Atacama, the Namib, the Sahara coast - are among the driest deserts in the world despite being beside an ocean.

Diurnal energy budgets: day and night

The diurnal energy budget is the budget over 24 hours, and it is the same set of flows as the global budget compressed into one day. During the day the surface receives shortwave radiation and absorbs most of it.

Some of that energy is conducted into the air immediately above, some is used to evaporate water as latent heat, and some is stored in the ground and in the air. As the surface warms, the air in contact with it warms by conduction and then rises by convection, so sensible heat is carried upward, and the evaporation adds water vapour to the air, which will release latent heat when it condenses.

The maximum temperature is not at noon. It occurs in the early to mid-afternoon, because the surface gains energy as long as incoming shortwave radiation exceeds outgoing longwave radiation, and that condition lasts until the incoming radiation falls below the outgoing loss later in the afternoon.

The ground therefore keeps warming for a couple of hours after midday, and the air follows it. Minimum temperature occurs just after sunrise, not at midnight, because the surface continues to lose longwave radiation all night and only starts to gain again when the Sun rises.

At night there is no shortwave input, but the surface keeps emitting longwave radiation, so the budget runs at a net loss. The ground cools, it cools the air in contact with it by conduction, and that air becomes denser and drains downslope into valleys as katabatic or drainage winds, which is why valley bottoms are often colder than the slopes above them.

If the air is cooled below its dew point, water vapour condenses on cold surfaces as dew, or on condensation nuclei near the ground as radiation fog, and if the ground cools below 0°C the dew freezes as frost. The cooling can also produce a temperature inversion, in which temperature rises with height for the first few hundred metres, trapping pollution and mist beneath it.

The balance between the flows decides how big the diurnal range is. A clear, dry, still day over a surface with a low specific heat capacity, such as sand or bare soil, gives a very large range: strong daytime heating, rapid night-time radiative loss, no cloud to trap longwave radiation and no wind to mix the air.

A cloudy, humid, windy day over the sea gives a very small range, because cloud reflects some incoming radiation by day and absorbs and re-radiates longwave radiation back down at night, water heats slowly, and wind mixes the air so that no shallow layer can become much hotter or colder. That is why deserts have the largest diurnal ranges on Earth and oceanic islands the smallest.

Atmospheric moisture, and the causes and types of precipitation

Water changes state in the atmosphere through five processes, and each one either takes in or releases energy. Evaporation changes liquid water into vapour and absorbs latent heat from the surface, cooling it. Condensation changes vapour back into liquid and releases that latent heat into the air, warming it.

Freezing changes liquid into ice and releases latent heat; melting changes ice into liquid and absorbs it. Sublimation is the direct change from solid ice to vapour, which absorbs latent heat, and deposition, its reverse, goes straight from vapour to ice and releases it, which is how snow crystals and the ice in a freezer form. Every one of these transfers energy without changing temperature at the moment of change, which is why they are called latent - hidden - heat transfers.

Air contains water vapour, and the maximum amount it can hold depends on its temperature: warm air can hold far more than cold air. Relative humidity compares the actual amount with the maximum possible at that temperature, as a percentage. When air is cooled to the temperature at which it becomes saturated, that temperature is the dew point, and further cooling forces water vapour to condense.

Condensation in the free air also needs condensation nuclei - dust, sea salt, smoke or pollution particles - because water vapour will not readily form droplets on nothing. Condensation alone does not make rain: the droplets are microscopic and their weight is not enough to overcome the rising air, so they must grow by collision and coalescence or by the formation of ice crystals before they can fall.

Air rises for four reasons in this syllabus, and each produces a different kind of rainfall. Convectional uplift happens when the surface is strongly heated, the air above it becomes unstable and rises in a towering column; the result is cumulonimbus, thunder, and short, intense, localised downpours, typically in the afternoon in the tropics. Frontal uplift happens where a warm air mass meets a cold one: because warm air is less dense, it is forced up over the cold air along the front, cooling and condensing as it rises, which gives widespread, often prolonged rain.

Orographic uplift happens when moist air is forced over a mountain range; it cools as it rises, rains on the windward slope, and then descends on the leeward side as warm, dry air, creating a rain shadow and often a desert. Radiation cooling happens when the ground loses longwave radiation at night and chills the air above it to below its dew point; the result is dew, frost and radiation fog rather than heavy rain, but in some situations cooling aloft can destabilise the air enough to produce showers.

Clouds are visible masses of water droplets or ice crystals, classified by height and shape: cirrus are high, thin and wispy and made of ice; stratus are low, flat and grey and may cover the whole sky; cumulus are heaped and cauliflower-shaped with flat bases; and nimbus means a cloud from which precipitation is falling, as in cumulonimbus and nimbostratus. Rain is liquid drops larger than about 0.5 mm; smaller drops are drizzle. Hail forms in cumulonimbus, where strong updraughts carry ice pellets up and down repeatedly so that layers of ice freeze onto them until they are too heavy to hold.

Snow falls as ice crystals when the temperature is below freezing throughout the cloud and the air beneath it. Dew is condensation directly onto a cold surface, and fog is simply a cloud at ground level, usually defined as reducing visibility below 1 km; radiation fog forms on clear, still autumn nights, and advection fog forms when warm, moist air moves over a cold surface.

• Evaporation absorbs latent heat and cools the surface; condensation releases it and warms the air.
• Freezing releases latent heat and melting absorbs it; sublimation (ice to vapour) absorbs it and deposition releases it.
• Dew point: the temperature at which air becomes saturated and condensation begins.
• Condensation nuclei are needed for droplets to form; without aerosols there would be almost no cloud.
• Cloud types: cirrus (high, ice, wispy), stratus (low, flat, grey), cumulus (heaped, flat base), nimbus (producing precipitation).
• Rain is drops over about 0.5 mm; drizzle is smaller; snow is ice crystals falling through sub-zero air; hail grows in the strong updraughts of cumulonimbus.
• Dew is condensation on a cold surface; fog is cloud at ground level, with visibility below 1 km.
• Radiation fog forms on clear, still nights; advection fog forms when warm, moist air crosses a cold surface.

Evidence for global warming and climate change

Nearly all of the climate record predates thermometers, so past climates are reconstructed from proxies - indirect records that vary with temperature or with the composition of the atmosphere. Ice cores are the richest: as snow accumulates on an ice sheet it traps bubbles of air, so a core drilled through the ice contains a dated sample of the atmosphere itself, and the ratio of oxygen isotopes in the ice indicates the temperature at the time the snow fell. Cores from Antarctica, including EPICA Dome C, reach back about 800 000 years and show carbon dioxide fluctuating between roughly 180 and 300 parts per million, moving in step with temperature through eight glacial cycles.

Tree rings give a year-by-year record, because a tree grows a wider ring in a good warm, wet year and a narrower one in a cold or dry year, and the pattern can be cross-dated and extended back thousands of years. Fossils provide a longer and patchier record: pollen grains identify the vegetation of a region and therefore its climate, fossil corals and shells record the temperature of the sea in their chemistry, and plant remains such as leaves with particular shapes indicate how warm and how wet a place was.

For the last hundred million years the picture is one of long, slow change. The Cretaceous, around 100 million years ago, was much warmer than today, with no polar ice caps and high sea levels; the planet then cooled through the Cenozoic, and ice sheets began to grow on Antarctica from about 34 million years ago.

The Pleistocene saw repeated glacial and interglacial cycles, each lasting about 100 000 years, with ice sheets advancing across North America and Europe and then retreating. The last 10 000 years, the Holocene, have been unusually stable, which is the period in which farming and cities developed, and within it there were smaller fluctuations such as the Medieval Warm Period and the Little Ice Age.

The instrumental record begins in about 1850, when enough weather stations existed to calculate a global average, and it shows a clear upward trend. Global average surface temperature has risen by about 1.1°C above the 1850-1900 average, with most of the rise since the 1970s, and each of the last four decades has been warmer than the one before. The rise is not smooth - there are cooler years after large volcanic eruptions and warmer years during El Nino - but the trend is unambiguous, and it is matched by a rise in the temperature of the oceans, which have absorbed the great majority of the extra heat.

The link with carbon dioxide comes from two records read together. Direct measurements of atmospheric carbon dioxide began at Mauna Loa in Hawaii in 1958, when the concentration was about 315 parts per million, and the continuous record - the Keeling curve - has risen every year since, passing 420 parts per million.

The ice cores extend that curve backwards and show that for 800 000 years carbon dioxide stayed below about 300 parts per million until the industrial period. When the two curves are plotted together, temperature and carbon dioxide rise and fall together over the ice-age cycles and both leave their historical range at the same time as industrial emissions began, which is strong evidence that the extra carbon dioxide is causing the warming rather than merely accompanying it.

Other evidence is independent of the thermometer and all of it points the same way. Sea level has risen by about 20 cm since 1900, from the thermal expansion of warming water and the melting of land ice, and the rate has roughly doubled since the 1990s. Arctic sea ice has shrunk in every season, and its September minimum has declined by roughly 13% per decade since satellite records began in 1979.

Ice sheets in Greenland and West Antarctica are losing mass, and glaciers in most mountain ranges have retreated and thinned, with the Alps, the Andes and the Himalayas all showing the loss. Each of these is a physical consequence of warming, so together they corroborate a trend that a single thermometer record could not establish on its own.

Causes of global warming: greenhouse gases, physical factors and human activity

The greenhouse gases named in the syllabus are carbon dioxide, methane, nitrous oxide and the F-gases (CFCs, HFCs, PFCs and sulphur hexafluoride). They differ in three ways that examiners expect you to compare. Their Global Warming Potential (GWP) is how much heat a unit mass of the gas traps over a set period, usually 100 years, compared with the same mass of carbon dioxide, which is fixed at 1: methane is about 25 to 30 times as powerful, nitrous oxide about 270 times, and sulphur hexafluoride about 24 000 times.

Their longevity in the atmosphere varies from about 12 years for methane to centuries for carbon dioxide and thousands of years for some F-gases. Their relative concentration also varies enormously - carbon dioxide is present at over 420 parts per million, methane at under 2 parts per million, nitrous oxide at about 0.3 parts per million and the F-gases at parts per trillion.

Those three properties are combined in carbon dioxide equivalence (CO2e), which is the concentration of a gas multiplied by its GWP, so that different gases can be added up on one scale. A gas with a GWP of 1 000 present at 0.1 parts per million is equivalent to 100 parts per million of carbon dioxide.

This is why a gas can be scarce and still matter: the F-gases are measured in parts per trillion, but their GWP is so high that they make a significant contribution. Carbon dioxide still dominates the total because its concentration is so much greater than everything else, even though methane traps far more heat per molecule.

Physical factors also change the global energy budget, and the syllabus separates them by the timescale over which they act. Over the longer term, variations in solar output change how much energy arrives at the top of the atmosphere: sunspot activity follows an 11-year cycle and longer variations, such as the Maunder Minimum of the seventeenth century, coincided with cooler periods. The change in total solar irradiance over a solar cycle is small, of the order of 0.1%, which is far too small to explain the warming of the last century on its own.

Over the shorter term, volcanic eruptions affect the budget by injecting sulphur dioxide into the stratosphere, where it forms a haze of sulphuric acid droplets that reflects sunlight back to space and cools the surface for a year or two. The eruption of Mount Pinatubo in 1991 lowered global average temperature by about 0.5°C for about two years before the aerosols settled out, which is why the temperature record shows short dips after large eruptions rather than a permanent change.

El Nino Southern Oscillation (ENSO) is the third physical influence, and it works on a scale of years rather than centuries. During an El Nino the trade winds weaken, warm water spreads east across the equatorial Pacific and releases its heat to the atmosphere, so global average temperature is pushed up by a few tenths of a degree; during a La Nina the trade winds strengthen and the effect reverses.

The strong El Nino of 1997-98 and again of 2015-16 each coincided with a record global temperature the following year. ENSO therefore explains the year-to-year wiggles in the temperature graph but not its upward slope, because it shifts heat between the ocean and the atmosphere without adding any.

Human (anthropogenic) factors act on the budget in three ways. The first is activities that emit greenhouse gases: burning coal, oil and gas for electricity, transport, heating and industry releases carbon dioxide that has been locked underground for millions of years; clearing tropical forest both releases the carbon stored in the trees and removes the vegetation that would have absorbed more; and agriculture and waste produce methane from livestock, rice paddies and landfill, and nitrous oxide from fertiliser.

The second is activities that change how much greenhouse gas is absorbed and stored: deforestation and the drainage of peatland and wetland remove carbon sinks, the warming and acidification of the ocean reduce its capacity to take up carbon dioxide, and the thawing of permafrost releases carbon that was frozen into the soil. The third is impacts on reflection (albedo): replacing dark forest with pale farmland or concrete changes the reflectivity of the surface, and the deposit of black carbon on snow and ice darkens it so that it absorbs more sunlight and melts faster, which is a positive feedback - melting exposes darker ground or ocean, which absorbs more heat, which melts more ice.

The result is the enhanced greenhouse effect: human emissions have raised the concentration of greenhouse gases, so more outgoing longwave radiation is absorbed and re-emitted downwards, and the Earth's energy budget is pushed out of balance by roughly 1 watt per square metre. That small imbalance, sustained for decades, is enough to warm the surface and the ocean, and more than 90% of the extra energy has gone into the sea.

The topic also asks you to recognise that the system is complex and interdependent: water vapour increases as the air warms and amplifies the warming, cloud cover can either amplify or offset it, the ocean and the biosphere absorb carbon at rates that themselves change, and the physical factors above operate at the same time. That is why the size of future warming is a range rather than a single number, and why attributing any single weather event to greenhouse gases is difficult even when the long-term trend is clear.