6.1 Introduction to the atmosphere

Learning objectives

• Outline the composition of the atmosphere and how its gases and processes support life on Earth.

• Describe the layers of the atmosphere and explain how temperature changes with altitude in each.

• Construct a systems diagram of the atmosphere, labelling inputs, outputs, storages, transfers and transformations.

• Explain how differential heating of the Earth leads to the redistribution of heat from the equator towards the poles.

• Identify the main greenhouse gases and aerosols and explain how they absorb and re-emit infrared radiation.

• Explain the natural greenhouse effect and distinguish it from the enhanced greenhouse effect, global warming and climate change.

• Evaluate the role of aerosols, using the eruption of Mount Pinatubo as an example.

• Explain why the atmosphere is a dynamic system and why it thins with altitude, and apply the lapse rate. (HL)

• Explain how Milankovitch cycles and positive feedback drive glacial–interglacial cycles, and why they do not explain current warming. (HL)

• Discuss how present warming is moving Earth away from the glacial–interglacial pattern of the Quaternary. (HL)

• Outline how the evolution of life changed the composition of the atmosphere, and how that change shaped later evolution. (HL)

Introduction

The atmosphere is the layer of gases held around the Earth by gravity. It supplies the oxygen that animals breathe and the carbon dioxide that plants use, shields the surface from harmful ultraviolet radiation, carries water from the oceans to the land, and keeps the planet warm enough for liquid water and life.

Viewed as a system, the atmosphere has inputs of energy from the Sun and gases from the surface, storages of gases and heat, and outputs of radiation to space. Human activities have changed the size of some of these storages, especially carbon dioxide and methane, and people with different perspectives disagree about how fast and by what means to respond. The atmosphere also links every other part of the course: it is part of the carbon cycle and the water cycle, it shapes the distribution of biomes, and it is shared by every country, which makes it a global commons.

The atmosphere: Earth’s boundary with space

The atmosphere forms the boundary between the Earth and space. It is also the outer limit of the biosphere, the part of the Earth where life exists: birds, insects, pollen, spores and bacteria are carried upwards by air currents, but very few living things are found more than a few kilometres above the ground.

There is no sharp upper edge. The air simply becomes thinner with height until it merges into space. By convention, the edge of space is often placed at the Kármán line, 100 km above sea level, but most of the air is far lower. About half of the mass of the atmosphere lies below 5.6 km, and about 99% lies below 30 km. Compared with the Earth’s radius of about 6,400 km, the atmosphere is a very thin film.

What the air is made of

The atmosphere is a mixture of gases, not a chemical compound. Two gases, nitrogen and oxygen, make up about 99% of dry air. The remaining 1% is mainly argon, with small amounts of carbon dioxide, methane, nitrous oxide, ozone and other trace gases. Water vapour is not included in the figures for dry air because its amount varies from almost nothing over deserts and polar regions to about 3% in warm, humid air.

Sources: Wikipedia, “Atmosphere of Earth” (2026); NOAA Global Monitoring Laboratory (2025, 2026).

Very small concentrations are given in parts per million (ppm) or parts per billion (ppb) by volume. A CO₂ concentration of 425 ppm means that 425 of every million molecules of air are carbon dioxide. Small as these numbers are, the trace gases control much of the Earth’s energy balance, so a change of a few tens of parts per million matters.

How the atmosphere supports life

The composition of the atmosphere and the processes that take place in it make the Earth habitable. Each of the following depends on the atmosphere:

• Gases for life processes: oxygen for respiration, carbon dioxide for photosynthesis, and nitrogen that enters food webs once it has been fixed into nitrates and ammonium.

• A shield against radiation: ozone in the stratosphere absorbs most of the Sun’s harmful ultraviolet radiation before it reaches the surface.

• A suitable temperature: greenhouse gases keep the average surface temperature at about 15 °C rather than about −18 °C, so most water stays liquid.

• Moving water and heat: winds carry water vapour from the oceans over the continents, where it falls as precipitation, and carry heat from the tropics towards the poles.

• Protection from small objects from space: most meteoroids burn up in the upper atmosphere, heated as they rush into the air at very high speed.

The layers of the atmosphere

The atmosphere is divided into layers according to how temperature changes with height. In some layers temperature falls as altitude increases; in others it rises. The boundary between two layers, where the trend reverses, is called a “pause”: the tropopause, the stratopause and the mesopause.

The troposphere is the lowest layer, from the surface to about 12 km on average (about 9 km at the poles and 17 km at the equator). It holds about 80% of the mass of the atmosphere and almost all of its water vapour, so nearly all clouds and weather occur here. The troposphere is heated mainly from below, by the warm ground and sea, so its temperature falls with height, to about −56 °C at the tropopause.

The stratosphere extends from the tropopause to about 50 km. Its temperature rises with height, from about −56 °C to just below 0 °C, because ozone molecules absorb ultraviolet radiation from the Sun and release the energy as heat. Most of the atmosphere’s ozone lies between about 15 km and 35 km, in the ozone layer. Warm air lying above cooler air is stable, so there is little vertical mixing in the stratosphere and substances that reach it, such as volcanic aerosols, can stay there for months to years.

Above the stratopause, temperature falls again through the mesosphere (about 50–80 km) to about −85 °C, the coldest part of the atmosphere. In the thermosphere, above about 80 km, the very thin gas absorbs high-energy solar radiation and its temperature rises steeply with height.

The troposphere and stratosphere matter most in environmental systems and societies. Global warming is mainly a change in the troposphere and at the surface, while ozone depletion is a change in the stratosphere. Ozone is found in both layers but has opposite effects: in the stratosphere it protects life, while near the ground tropospheric ozone is a harmful pollutant in photochemical smog.

Mixing of gases by wind

Winds and convection currents continually stir the lower atmosphere, so its main gases are well mixed and the proportions of nitrogen, oxygen and argon are almost the same everywhere. Gases released in one place are spread around the world: carbon dioxide from a power station in one country raises the concentration measured on a remote Pacific island within months, and chlorofluorocarbons released mostly in the Northern Hemisphere reached the stratosphere above Antarctica.

This redistribution by physical processes has two important consequences. First, the atmosphere behaves as a single global storage, so emissions from anywhere affect everyone. Second, the atmosphere is a shared resource that no country owns, which is why managing it needs international cooperation and why it is often described in terms of the tragedy of the commons.

The atmosphere as a system

Systems thinking treats the atmosphere as a set of interacting parts with inputs, outputs, storages and flows. The same drawing conventions are used as for any systems diagram: storages are boxes, flows are labelled arrows, and the system boundary is a dashed line. Each flow is either a transfer, in which energy or matter changes location, or a transformation, in which it changes its chemical nature, state or form of energy.

The atmosphere exchanges both energy and matter with its surroundings, so it is an open system. The Earth as a whole is close to a closed system: it exchanges large amounts of energy with space but almost no matter, apart from small inputs of meteoritic dust and small losses of hydrogen and helium from the top of the atmosphere.

Differential heating and the redistribution of heat

The Earth is heated unevenly by the Sun. Near the equator the Sun is high in the sky for much of the year, so its rays strike the surface almost at right angles. At high latitudes the Sun is low, so the same amount of sunlight is spread over a larger area of ground and passes through a longer path of atmosphere, where more of it is scattered and reflected. Snow and ice near the poles also reflect much of the sunlight that does arrive, because they have a high albedo.

The Earth loses energy to space as infrared radiation at every latitude. Outgoing infrared varies much less with latitude than incoming sunlight does, because warm air and water are carried towards the poles. The result is an energy surplus between about 35° N and 35° S and an energy deficit nearer the poles.

Heat is moved from the surplus to the deficit regions by moving air and moving water. In the atmosphere, warm air rises near the equator, flows towards the poles at high altitude, cools and sinks, and returns towards the equator near the surface. The tricellular model describes this circulation as three cells in each hemisphere: the Hadley cell in the tropics, the Ferrel cell in the mid-latitudes and the polar cell. The same circulation explains the belt of heavy rainfall near the equator and the deserts near 30° N and S, which shape the distribution of biomes.

Much of the heat is carried in a hidden form. When water evaporates from warm tropical seas, it absorbs energy (latent heat). When the vapour later condenses to form clouds and rain at higher latitudes, it releases that energy into the surrounding air. Ocean currents such as the Gulf Stream also carry warm water polewards and cold water towards the equator.

This redistribution reduces the heat at the equator and raises temperatures at higher latitudes. Western Europe, for example, has much milder winters than places at the same latitude in eastern Canada partly because of the heat carried north by the atmosphere and the North Atlantic. In systems terms, the circulation is a set of transfers of energy that keeps the temperature difference between the equator and the poles far smaller than radiation alone would produce.

Greenhouse gases and aerosols

The Sun emits most of its energy as visible light and short-wavelength infrared, often called short-wave radiation. The Earth is much cooler than the Sun, so the radiation it emits has much longer wavelengths, mostly in the infrared between about 4 and 100 micrometres. Most gases in the atmosphere let both kinds of radiation pass straight through. A small number of gases, the greenhouse gases, let most of the incoming short-wave radiation through but absorb part of the outgoing long-wave infrared.

How a greenhouse gas absorbs infrared

A molecule absorbs infrared radiation when the radiation makes its atoms vibrate or its bonds bend. Molecules made of three or more atoms, such as H₂O, CO₂, CH₄ and N₂O, can vibrate in ways that absorb infrared at particular wavelengths. Molecules made of two identical atoms, such as N₂ and O₂, cannot. This is why the two gases that make up 99% of dry air have almost no greenhouse effect, while trace gases have a large one.

After absorbing infrared energy, a greenhouse gas molecule passes most of it to neighbouring air molecules through collisions, which warms the air. The warmed air, in turn, emits infrared radiation in all directions. Some travels upwards and eventually escapes to space; much travels downwards and is absorbed by the surface, which warms it further. The overall effect is that energy leaving the Earth is slowed down on its way to space.

The main greenhouse gases

Carbon dioxide and water vapour are the most abundant greenhouse gases. Methane is far less abundant, but each molecule absorbs infrared much more strongly, so it has a significant warming effect. Nitrous oxide and a range of human-made gases containing fluorine and chlorine also contribute. The strength of each gas is compared using its global warming potential.

Sources: IPCC AR6 WG1 Table 7.15, via Wikipedia, “Global warming potential” (2026); Wikipedia, “Atmospheric methane” (2026).

Methane and nitrous oxide come mostly from agriculture, and methane also from the fossil fuel industry. Methane has a short lifetime, about 10 years, because it reacts with hydroxyl radicals (OH) in the air and is converted into carbon dioxide and water; a sudden pulse of extra methane takes somewhat longer, about 12 years, to decay, because extra methane slows its own removal. A cut in methane emissions therefore lowers its concentration within a decade or two, which makes methane an attractive target for slowing warming quickly.

Emissions of different gases are added together as carbon dioxide equivalents (CO₂e). The mass of each gas is multiplied by its GWP, which converts it into the mass of carbon dioxide that would cause the same warming over 100 years.

Water vapour: a greenhouse gas that is not targeted

Water vapour is the most abundant greenhouse gas and has the largest single effect on the natural greenhouse effect. It is nevertheless left out of emission targets and is usually treated in climate models as a response to warming rather than as a cause of it. The amount of water vapour that air can hold rises with temperature, so warmer air holds more of it, and water vapour stays in the atmosphere for only about nine to ten days before it falls as rain or snow.

This means that people cannot control the amount of water vapour in the atmosphere directly. If extra water vapour were added, it would rain out within days; if the air warms because of carbon dioxide, more water evaporates and stays in the air. Water vapour therefore acts as a positive feedback that amplifies warming caused by other gases. It is also dynamic, essential for life and part of the water cycle, so it cannot be “mitigated” in the way that CO₂ or methane emissions can.

Radiative forcing

Scientists compare the effect of different gases and particles using radiative forcing: the change in the balance between energy arriving at the Earth and energy leaving it, caused by a change in some factor since a reference time (usually about 1750). It is measured in watts per square metre (W m⁻²). A positive forcing means more energy is retained and the climate warms; a negative forcing means less energy is retained and the climate cools.

The forcing from long-lived greenhouse gases has grown steadily. Between 1990 and 2024 it rose by 54%, and carbon dioxide caused about 81% of that increase. The forcing from CFCs has started to fall because their production was phased out under the Montreal Protocol, while the forcing from HFCs, which replaced them, is rising.

Aerosols and black carbon

An aerosol is a suspension of tiny solid or liquid particles in the air: dust, sea salt, volcanic ash, soot, sulfate droplets and smoke. Aerosols are not gases, but they also affect the Earth’s energy balance. Most aerosols scatter and reflect incoming sunlight back to space, which cools the surface. They also act as nuclei on which water vapour condenses, producing clouds with more, smaller droplets that reflect more sunlight. Sulfate aerosols from burning coal and from volcanic eruptions have this cooling effect.

Black carbon (soot) is an exception. It is produced by the incomplete combustion of fuels such as diesel, coal, wood, dung and crop waste, and by open burning of forests and grasslands. Being dark, it absorbs sunlight and warms the air around it. When it settles on snow and ice, it darkens the surface, lowers its albedo and speeds up melting. Black carbon stays in the atmosphere for only days to weeks, so cutting its emissions would reduce its warming effect almost immediately.

Black carbon from South and East Asia is carried by winds to the Himalayas and the Tibetan Plateau, where it settles on glaciers that supply water to rivers used by hundreds of millions of people. Studies have estimated that black carbon has caused about 0.6 °C of warming over the Himalayas and that, in some parts of the range, its effect on melting may equal that of carbon dioxide. Smoke from traditional cooking stoves that burn wood and dung is a major source in South Asia, so cleaner stoves would reduce both melting and the indoor air pollution that harms millions of people.

Sources: Wikipedia, “Black carbon” (2026), citing Ramanathan and Carmichael (2008) and later studies.

Cleaning up air pollution therefore has mixed effects on temperature. Removing sulfate aerosols improves health and reduces acid deposition but removes part of their cooling effect, which has been hiding some of the warming caused by greenhouse gases. Removing black carbon improves health and reduces warming at the same time.

The greenhouse effect

The greenhouse effect is the warming of the Earth’s surface and lower atmosphere that results from greenhouse gases absorbing and re-emitting infrared radiation. It is a natural process that has operated for as long as the Earth has had an atmosphere containing water vapour and carbon dioxide.

How the greenhouse effect works, step by step

1. The Sun emits a broad spectrum of radiation, mostly visible light with ultraviolet and short-wave infrared. On average about 340 W of solar energy arrives for every square metre at the top of the atmosphere.

2. About 30% of it (about 100 W m⁻²) is reflected back to space by clouds, air molecules, aerosols and bright surfaces such as ice.

3. The rest (about 240 W m⁻²) is absorbed, mostly by the land and oceans. Most of this short-wave radiation passes through the atmosphere because greenhouse gases absorb little of it.

4. The warmed surface emits long-wave infrared radiation, about 398 W m⁻² on average.

5. Greenhouse gases and clouds absorb most of this infrared and re-emit it in all directions. Much of it returns to the surface, which is warmed further.

6. Energy finally escapes to space mainly from the cold upper troposphere. At equilibrium, about 240 W m⁻² leaves, balancing the energy absorbed, but the surface stays much warmer than it would without greenhouse gases.

Sources: Wikipedia, “Earth’s energy budget” and “Greenhouse effect” (2026).

Without any greenhouse gases, the Earth’s average surface temperature would be about −18 °C, and most water would be frozen. The actual average is about 15 °C, so the natural greenhouse effect warms the surface by about 33 °C. The temperature of the Earth therefore depends on the concentration of greenhouse gases: more greenhouse gases absorb more of the outgoing infrared, so the surface and lower atmosphere must warm until the energy leaving again balances the energy absorbed.

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The name is an analogy, and an imperfect one. A glass greenhouse stays warm mainly because it stops warm air from rising and mixing with cooler air outside. The atmospheric greenhouse effect works by absorbing and re-emitting radiation, not by blocking air movement.

The enhanced greenhouse effect, global warming and climate change

Since the Industrial Revolution, human activities have added large amounts of carbon dioxide, methane, nitrous oxide and fluorinated gases to the atmosphere. The CO₂ concentration has risen from about 280 ppm before industrialisation to about 425 ppm, and the methane concentration from about 729 ppb to about 1,935 ppb. These additional gases strengthen the natural greenhouse effect.

The enhanced greenhouse effect raises the amount of energy kept in the lower atmosphere and at the surface, and the most direct measure of this is temperature.

Global warming is one part of a wider set of changes. Higher temperatures change the amount and pattern of rainfall, the frequency of heatwaves and droughts, the extent of sea ice and glaciers, sea level and the chemistry of the oceans. Climate change is the term for this whole range of changes.

Perspectives on the atmosphere and the enhanced greenhouse effect

How people judge the problem and its solutions depends on their environmental value system. The atmosphere is a shared resource, so the costs of emissions are spread across all countries and future generations while the benefits of burning fossil fuels go mainly to those who burn them.

Climate change is linked to SDG 13 (climate action), and climate change is one of the nine planetary boundaries. The boundary for climate change is set at an atmospheric CO₂ concentration of 350 ppm; the present level of about 425 ppm is well beyond it, as the evidence for global warming shows.

Case study: the eruption of Mount Pinatubo and the global climate

Context

Mount Pinatubo is a stratovolcano on the island of Luzon in the Philippines, about 90 km north-west of Manila. Before 1991 it had been dormant for about 500 years, and its forested slopes were home to Aeta communities, an Indigenous people who farmed and hunted there. The lowlands around it held farming towns and two large United States military bases, Clark Air Base and Subic Bay Naval Station.

The eruption is studied in environmental systems and societies because it acted as a natural experiment on the atmospheric system. A single, well-measured input of sulfur gas was added to the stratosphere, and scientists could follow how the atmosphere spread it, how it changed the Earth’s energy balance and how long the system took to return to its earlier state.

What happened

After months of earthquakes and steam explosions, the volcano produced its first explosive eruption on 12 June 1991. The climactic eruption came on 15 June, on the same day that Typhoon Yunya passed over Luzon. It was one of the largest eruptions of the twentieth century: an ash column rose about 35 km, into the stratosphere, and more than 5 km³ of material was erupted. Heavy rain from the typhoon mixed with the falling ash, so the ash became wet and heavy enough to make thousands of roofs collapse.

The eruption injected nearly 20 million tonnes of sulfur dioxide (SO₂) into the stratosphere. There, the SO₂ reacted with water vapour to form tiny droplets of sulfuric acid, a sulfate aerosol. Stratospheric winds spread the aerosol around the globe within months, forming a thin haze that lasted for about three years. Because the stratosphere has little vertical mixing and no rain, the aerosol stayed aloft far longer than it would have in the troposphere.

Data

The extra reflection of sunlight gave a negative radiative forcing for a few years. The global mean surface temperature fell by about 0.5 °C between 1991 and 1993 before recovering. The temperature record shows the dip clearly, although year-to-year changes are also influenced by the El Niño–Southern Oscillation and other natural variations.

The aerosol also affected the ozone layer. The droplets provided surfaces on which chlorine from human-made chemicals was converted into forms that destroy ozone. The mean area of the Antarctic ozone hole in the spring of 1992, 22.3 million km², was the largest measured up to that time.

Management and responses

Scientists from the Philippine Institute of Volcanology and Seismology (PHIVOLCS) and the United States Geological Survey monitored earthquakes, gas emissions and swelling of the volcano, and issued warnings at a series of alert levels. About 60,000 people had left the area within 30 km of the volcano before the climactic eruption. The forecasts are estimated to have saved at least 5,000 lives and at least US$250 million of property.

The longer-term impacts were harder to manage. Over the following years, monsoon and typhoon rains remobilised the loose volcanic deposits as lahars (volcanic mudflows) that buried towns, farmland and bridges, and in places caused more damage than the eruption itself. About 20,000 Aeta were displaced from their land, and many were moved to resettlement areas with poor conditions, far from the forests on which their livelihoods depended.

Sources: USGS Fact Sheet 113-97, “The Cataclysmic 1991 Eruption of Mount Pinatubo, Philippines” (1997); Wikipedia, “Mount Pinatubo” and “1991 eruption of Mount Pinatubo” (2026); Hansen et al., Geophysical Research Letters (1992); NASA Ozone Watch (2025).

Evaluation

The eruption shows the strengths and limits of the atmosphere as a self-regulating system. The cooling was temporary: once the aerosol settled out, temperatures returned to their earlier trend, much as a system in steady-state equilibrium returns after a disturbance. It also shows that aerosols mask only part of the warming from greenhouse gases, and only while they remain in the air, whereas carbon dioxide persists for centuries.

What it shows

Mount Pinatubo shows that the atmosphere redistributes material around the globe through wind (a transfer), that aerosols change the global energy balance by reflecting short-wave radiation, and that the surface temperature responds within months to a change in radiative forcing. It also shows how the stratosphere and troposphere behave differently, and why the effect of a short-lived aerosol differs from that of a long-lived greenhouse gas.

The atmosphere as a dynamic system (HL only)

The composition and layering of the atmosphere look stable over a human lifetime, but they are the product of processes that never stop. The atmosphere is in a dynamic state: gases and energy are constantly entering, moving, reacting and leaving, and what appears constant is a balance between these flows.

Physical processes

Differences in temperature create differences in air pressure. Warm air expands, becomes less dense and rises, leaving lower pressure at the surface; cool air is denser and sinks, producing higher pressure. Air moves from high to low pressure as wind, and the large-scale circulation of the atmosphere carries heat, water vapour and pollutants around the planet. Evaporation, condensation and precipitation move water between the oceans, the air and the land. Global warming is itself a physical process in the atmosphere: an increase in the heat stored in the lower atmosphere and at the surface.

Chemical processes

Chemical reactions continually create and destroy gases. In the stratosphere, ultraviolet radiation splits oxygen molecules (O₂) into single oxygen atoms, which combine with other O₂ molecules to form ozone (O₃); other reactions break ozone down again, so the ozone layer is maintained by a balance between formation and destruction. In the troposphere, hydroxyl radicals (OH) oxidise methane into carbon dioxide and water, which is why methane lasts only about a decade. Sulfur dioxide is oxidised into sulfate aerosols, and nitrogen oxides react in sunlight to form tropospheric ozone in photochemical smog.

Because these processes run continuously, the atmosphere responds quickly when an input changes. A rise in emissions of a gas raises its storage until the removal processes, which usually speed up as the concentration rises, balance the new input. This is a negative feedback that tends towards a new steady state, although for carbon dioxide the removal processes are slow, so the new balance takes centuries to reach.

Gravity, the thinning atmosphere and the lapse rate (HL only)

Gravity pulls every molecule of air towards the Earth’s centre, which is why the atmosphere does not escape into space.

The force of gravity becomes weaker with distance from the Earth, but over the depth of the atmosphere this change is small. The main reason the air thins with height is compression: each layer of air presses down on the layers beneath it. Air near the surface carries the weight of all the air above and is squeezed into a small volume, so it is dense and at high pressure. Higher up there is less air above, so the air is less compressed, its pressure is lower and its molecules are farther apart.

Air pressure at sea level averages 101,325 Pa (about 1,013 hPa). It roughly halves with each 5.5 km of height, which is why about half of the atmosphere’s mass lies below 5.6 km. On the summit of Mount Everest, at 8,849 m, a breath of air contains roughly a third of the oxygen molecules of a breath at sea level, which is why climbers struggle to breathe without bottled oxygen.

The lapse rate

In the troposphere, temperature normally falls with height. The rate of this fall is called the lapse rate. Air near the ground is warmed by the surface, and air that rises expands as the pressure on it falls; expanding air does work on its surroundings and cools. Values commonly used are:

• About 1 °C per 100 m (9.8 °C per km): the dry adiabatic lapse rate, the rate at which a parcel of dry, unsaturated air cools as it rises. This is the value often quoted as the standard rule of thumb.

• About 6.5 °C per km (0.65 °C per 100 m): the average environmental lapse rate of the real troposphere, used in the International Standard Atmosphere up to 11 km. It is lower than the dry rate because rising moist air releases latent heat when its water vapour condenses.

The lapse rate explains why mountains carry belts of different vegetation at different heights, why snow lines lie lower nearer the poles, and why highland areas in the tropics, such as the Ethiopian Highlands, have mild climates. It also matters for the greenhouse effect: because the upper troposphere is colder than the surface, the infrared it emits to space is weaker than the infrared emitted by the surface, so more greenhouse gases raise the height from which energy escapes and the surface must warm to restore the balance.

Milankovitch cycles and natural climate change (HL only)

The Earth’s climate has changed naturally many times. Over the last few million years it has swung between cold glacial periods, when ice sheets covered much of North America and northern Europe, and warmer interglacials such as the present one. The pacemaker of these swings is a set of slow, regular changes in the Earth’s orbit and axis, named after the Serbian scientist Milutin Milanković.

Sources: Wikipedia, “Milankovitch cycles” (2026).

How small orbital changes cause large climate swings

The key to a glacial period is summer sunshine at high northern latitudes, around 65° N, where there is a great deal of land. When the orbital cycles combine to give cool northern summers, some of the winter snow survives each summer and builds up into ice sheets. The direct effect of the orbital change on temperature is small; it is amplified by positive feedback loops within the Earth system.

• Ice–albedo feedback: growing ice sheets and sea ice reflect more sunlight, so less energy is absorbed and the cooling increases.

• Carbon dioxide feedback: colder oceans dissolve more carbon dioxide, and changes in ocean circulation and biology store more carbon in the deep ocean, so atmospheric CO₂ falls and the greenhouse effect weakens.

• The reverse: when summer sunlight at high latitudes increases, melting ice lowers the albedo and warming oceans release CO₂, so warming is amplified and an interglacial begins.

Ice cores from Antarctica, such as the EPICA Dome C core, which reaches back about 800,000 years, record this pattern. Air bubbles trapped in the ice show that CO₂ concentrations varied between about 180 ppm in glacial periods and about 280–300 ppm in interglacials, rising and falling together with temperature. The detailed record and what it shows about the present are part of the evidence for anthropogenic climate change.

Why Milankovitch cycles do not explain current warming

• Timescale: the orbital cycles act over tens of thousands of years. The present warming of more than 1 °C has happened in about 150 years.

• Size of the forcing: the orbital cycles hardly change the total sunlight reaching the Earth in a year; they shift it slowly between seasons and latitudes over thousands of years. The forcing from long-lived greenhouse gases, by contrast, has reached about 3.5 W m⁻² in about two centuries.

• Direction: the Earth’s tilt is currently decreasing, which slightly weakens northern summers, so the orbital cycles would favour very slow cooling, not rapid warming.

• The pattern of warming: the stratosphere is cooling while the troposphere warms, which is the pattern expected from more greenhouse gases rather than from more sunlight.

Leaving the glacial–interglacial cycle of the Quaternary (HL only)

The Quaternary is the most recent geological period, which began about 2.58 million years ago and continues today. It has been characterised by repeated glacial and interglacial periods. From about 2.6 to 1 million years ago the cycles followed the 41,000-year rhythm of obliquity; over the last million years they have been longer and stronger, with a rhythm of about 100,000 years. The present interglacial, the Holocene, began about 12,000 years ago, and agriculture and civilisations developed during its unusually stable climate.

Throughout at least the last 800,000 years, CO₂ never rose much above about 300 ppm. It now stands at about 425 ppm, and it has risen in less than two centuries by an amount that took thousands of years at the end of each glacial period.

Climate has changed many times without human influence, but the present rate of change is unprecedented in the ice-core record. Many scientists argue that if emissions continue, the Earth will move into a new, hotter state with no modern analogue, and that the next glacial period may be delayed even further. Orbital changes alone would probably not start one for about 50,000 years, and high CO₂ concentrations would make it harder for summers to become cool enough for ice sheets to grow.

The scale of human influence has led many scientists to describe the present as the Anthropocene, an age in which human activity is a dominant force on the Earth system. In March 2024 the proposal to make the Anthropocene a formal epoch of the geological time scale was rejected by the bodies that govern the time scale, but the term remains widely used informally to describe human impact on the planet.

Sources: Wikipedia, “Quaternary”, “Anthropocene” and “Carbon dioxide in Earth’s atmosphere” (2026).

Investigating greenhouse gases and albedo in a closed system

Simple laboratory models show how greenhouse gases and albedo affect temperature. A sealed container exchanges energy but not matter with its surroundings, so it is a closed system and the gases inside cannot escape.

Life and the atmosphere have changed each other (HL only)

The atmosphere of the early Earth was very different from today’s. It contained a great deal of carbon dioxide, released by volcanoes, and practically no free oxygen. Life began in this oxygen-free world, and the first organisms obtained energy without oxygen.

Photosynthesis and the Great Oxidation Event

Cyanobacteria were the first organisms to carry out oxygen-producing photosynthesis, splitting water and releasing oxygen as a by-product. At first the oxygen was used up by reactions with dissolved iron in the oceans and with gases and rocks. Iron dissolved in seawater reacted with oxygen to form insoluble iron oxides, which settled on the sea floor in layers. These layers are now banded iron formations, the source of most of the world’s iron ore, such as the deposits of the Hamersley Range in Western Australia.

Once these oxygen sinks were largely used up, oxygen began to build up in the atmosphere. This change, the Great Oxidation Event, began about 2.4 billion years ago. Oxygen reacted with methane, a powerful greenhouse gas, converting it into carbon dioxide and water; the weaker greenhouse effect that followed may have triggered a long glaciation. Over hundreds of millions of years, photosynthesis, the burial of organic carbon and the formation of carbonate rocks lowered the concentration of carbon dioxide, while oxygen rose towards its present 21%.

How the new atmosphere shaped life

• Extinction and adaptation: oxygen was toxic to many anaerobic organisms, which died out or survived only in oxygen-free places such as deep sediments, while others evolved ways to tolerate it.

• Aerobic respiration: organisms that used oxygen to release energy from food gained far more energy per molecule of glucose, which later made large, active multicellular life possible.

• The ozone shield: oxygen in the upper atmosphere allowed stratospheric ozone to form, which absorbed harmful ultraviolet radiation and later made it possible for life to colonise the land.

• Feedback on the climate: by removing CO₂ and methane, life changed the strength of the greenhouse effect and so the temperature of the planet.

This two-way relationship, in which life changes the atmosphere and the atmosphere shapes the evolution of life, is the idea behind the Gaia hypothesis, which treats the Earth as a self-regulating system in which living things help to maintain conditions suitable for life.

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Exam practice

Questions on the atmosphere test whether you can use the correct terms precisely (short-wave and long-wave radiation, absorb and re-emit, greenhouse effect and enhanced greenhouse effect), draw and annotate a greenhouse effect or systems diagram, and handle data on greenhouse gases and temperature. Paper 1 often gives an unfamiliar place or dataset and asks you to apply these ideas; Paper 2 essays reward clear distinctions and named examples such as Mount Pinatubo.

Paper 2 Section A-style questions

The model answer uses the key vocabulary: short-wave in, long-wave out, absorb and re-emit. Answers that say greenhouse gases “reflect heat” or “trap the Sun’s rays” lose marks.

Calculation questions on the atmosphere usually involve CO₂ equivalents, percentage change in a gas concentration or a lapse-rate estimate. Write the formula, substitute with units and give the answer with its unit.

Paper 1-style data question

Resource (real data): global mean concentrations and radiative forcing of three greenhouse gases.

Paper 2 Section B-style structured essay

HL practice

Summary

• The atmosphere is a thin layer of gases held by gravity: the boundary between the Earth and space and the outer limit of the biosphere.

• Dry air is about 78% nitrogen, 21% oxygen and 0.93% argon, with trace gases such as CO₂ (about 425 ppm) that matter greatly for the climate.

• The atmosphere is layered by temperature: it falls with height in the troposphere, rises in the stratosphere (ozone absorbs UV), falls in the mesosphere and rises in the thermosphere.

• The atmosphere is an open system with inputs, storages, outputs, transfers (such as wind) and transformations (such as condensation and ozone formation).

• Differential heating gives an energy surplus at low latitudes and a deficit at high latitudes; atmospheric circulation and ocean currents carry heat polewards.

• Greenhouse gases (water vapour, CO₂, CH₄, N₂O) absorb and re-emit infrared; aerosols mostly cool by reflecting sunlight, but black carbon warms.

• The natural greenhouse effect keeps the surface about 33 °C warmer (about 15 °C rather than −18 °C); the enhanced greenhouse effect from human emissions causes global warming, which is one part of climate change.

• The eruption of Mount Pinatubo (1991) added a sulfate aerosol to the stratosphere and cooled the globe by about 0.5 °C for about two years.

• The atmosphere is dynamic: physical and chemical processes continuously maintain its layers and composition, and it thins with altitude, cooling at a lapse rate of about 0.65–1 °C per 100 m. (HL)

• Milankovitch cycles, amplified by ice–albedo and CO₂ feedbacks, drive glacial–interglacial cycles but cannot explain the rapid present warming. (HL)

• Present warming is moving the Earth beyond the glacial–interglacial range of the Quaternary, and life has long shaped the atmosphere, as in the Great Oxidation Event. (HL)

Self-check

• To do:

  I can outline the composition of the atmosphere and how it supports life.

• To do:

  I can describe the four main layers of the atmosphere and explain the temperature change in each.

• To do:

  I can construct a systems diagram of the atmosphere and label transfers and transformations.

• To do:

  I can explain differential heating and how heat is redistributed towards the poles.

• To do:

  I can identify the main greenhouse gases and aerosols and explain how greenhouse gases absorb and re-emit infrared.

• To do:

  I can calculate CO₂ equivalents using global warming potentials.

• To do:

  I can explain the greenhouse effect and distinguish it from the enhanced greenhouse effect, global warming and climate change.

• To do:

  I can use the Mount Pinatubo eruption as evidence for the effect of aerosols on the climate.

• To do:

  I can explain why the atmosphere thins with altitude and use the lapse rate in a calculation. (HL)

• To do:

  I can explain how Milankovitch cycles and positive feedback cause glacial cycles and why they do not explain current warming. (HL)

• To do:

  I can outline how life changed the atmosphere and how that change influenced evolution. (HL)