In March 1958, a young geochemist named Charles David Keeling began taking daily carbon dioxide measurements at an observatory perched 3,397 meters up the side of Mauna Loa, a volcano in Hawaii, far from any city or factory that could contaminate the reading.
His first measurement read 313 parts per million (ppm): 313 molecules of CO2 for every million molecules of air. He kept measuring, month after month, and the scientists who took over after him still are. The result, now known as the Keeling Curve, is the longest continuous record of direct atmospheric measurement in the world.
What would you expect that record to show, nearly 70 years on?
The rise is accelerating. The annual average climbed from 316 ppm in 1959 to 427 ppm in 2025, and the rate of increase is itself increasing: the 2010s added CO2 at roughly 2.4 ppm a year, and the 2020s are running at roughly 2.6 ppm a year, faster than any decade on record.
Ice cores drilled at Vostok and EPICA Dome C in Antarctica extend the picture much further back. Tiny bubbles of ancient air trapped in the ice give a direct CO2 record stretching 800,000 years into the past. Across eight full glacial cycles in that record, CO2 has never naturally exceeded about 300 ppm. The chart below puts both records on one timeline.
The x-axis is deliberately compressed and not to scale: the ice-core portion spans 800,000 years, the Keeling Curve portion spans under 70. Squeezed onto a true linear timescale, the recent rise would be a vertical line at the very edge of the chart.
The current level is already more than 100 ppm above the entire natural range the ice cores capture, reached within a few centuries rather than tens of thousands of years. This subtopic builds the atmospheric concepts, composition, circulation, and the greenhouse effect, needed to explain why that rise matters.
- Keeling Curve: continuous direct CO2 measurement at Mauna Loa Observatory, Hawaii (3,397 m elevation), since March 1958.
- First reading: 313 ppm (1958). 2025 annual average: 427 ppm. Rate of increase: about 2.4 ppm/year in the 2010s, about 2.6 ppm/year in the 2020s.
- Ice-core record: Vostok and EPICA Dome C, Antarctica; EPICA spans roughly 800,000 years of trapped-air samples.
- Natural range across 8 glacial cycles: roughly 180 ppm (depths of an ice age) to 280-300 ppm (interglacial warm periods, including the one Earth is in now).
Figures rounded from NOAA Global Monitoring Laboratory / Scripps Institution of Oceanography (Keeling Curve) and the EPICA Dome C ice-core CO2 record.
Earth's outer boundary
The atmosphere is the layer of gases surrounding Earth. It forms the boundary between the planet and space, and it marks the outer limit of the biosphere, the zone where life exists. Its composition and physical processes support life directly: supplying oxygen and carbon dioxide for respiration and photosynthesis, filtering out harmful radiation, and keeping surface temperatures within a survivable range.
The atmosphere is a mixture of gases, not a single substance, and physical processes constantly redistribute that mixture. Wind is the clearest example: air moves horizontally and vertically in response to pressure and temperature differences, mixing gases and carrying heat, moisture, dust and pollutants across the planet.
The "Other, ~1%" segment is drawn wider than its true share so it stays legible. It breaks down as: argon (~0.93%), water vapor (variable), carbon dioxide (~0.04%), and traces of methane, nitrous oxide and other gases.
CO2 is a tiny fraction of the atmosphere by volume, but its effect on climate is completely out of proportion to how little of it there is.
Why the atmosphere circulates: the tricellular model
The Sun heats Earth's surface unevenly. The equator receives strong, direct sunlight year-round, while high latitudes receive weak, glancing sunlight. This differential heating drives the atmosphere into motion. The resulting pattern, three linked circulation cells in each hemisphere, is called the tricellular model: the Hadley cell (roughly 0-30°), the Ferrel cell (roughly 30-60°) and the Polar cell (roughly 60-90°).
Warm air rises where heating is strongest, moves toward the pole at altitude, cools and sinks, then returns along the surface, one closed loop per cell. Together the three cells pass heat from the equator toward the poles, reducing the temperature difference between them and keeping most of the planet within a livable range. Click each cell below for its detail.
NASA, public domain
Build your own labeled system diagram of the atmosphere's energy flows, inputs, internal processes and outputs, in the Skills tab.
The same heat-redistribution logic, warm fluid rising where heating is strongest, moving toward a region of net heat deficit, then cooling and sinking, reappears in the ocean as thermohaline circulation (Topic 4). There, differences in temperature and salinity rather than differential solar heating alone drive the deep currents, but the underlying job, moving heat from where it is abundant toward where it is scarce, is the same job the tricellular model does for the atmosphere.
Greenhouse gases and aerosols
Greenhouse gases (GHGs) and aerosols in the atmosphere absorb infrared (long-wave) radiation emitted from Earth's surface and re-emit some of it back down, preventing it from escaping directly into space. The main greenhouse gases are water vapor, carbon dioxide, methane and nitrous oxide (N2O). Black carbon, a form of aerosol rather than a gas, has a similar warming effect.
Carbon dioxide and water vapor are the most abundant GHGs in the atmosphere, and methane, though far less abundant, is also a significant contributor because each molecule traps disproportionately more heat. Water vapor is unusual: although it is a powerful GHG, it is usually left out of climate models, because its atmospheric concentration responds dynamically to warming (a warmer atmosphere holds more water vapor) rather than driving warming on its own, and because it is essential for life and constantly cycling through evaporation and precipitation, it cannot be mitigated the way fossil-fuel emissions can.
Sort each substance into the category it belongs to.
The greenhouse effect: natural and enhanced
The Sun's radiation spans a broad spectrum and reaches Earth's surface largely unimpeded as short-wave (visible) light. The warmed surface then re-radiates that energy as long-wave infrared radiation, which greenhouse gases absorb and re-radiate in all directions, including back toward the surface. This is the greenhouse effect: it keeps Earth roughly 33°C warmer than it would be with no atmosphere at all, close to -18°C rather than the actual global average of about 15°C. It is a natural process, essential for life; without any GHGs, Earth's surface would be far below freezing on average.
The temperature of Earth depends on the concentration of GHGs in the atmosphere. Human activity, mainly the combustion of fossil fuels, has added extra GHGs on top of the natural background level. This additional, human-driven warming is often called the enhanced greenhouse effect: an addition on top of a naturally occurring, life-sustaining process, not something separate from it.
The greenhouse effect itself is not the problem. Without any greenhouse effect at all, Earth's average surface temperature would sit around -18°C, far too cold to sustain life as it exists today. The natural greenhouse effect is what keeps the planet habitable in the first place. What drives current warming is the enhanced greenhouse effect: the extra, human-added GHGs sitting on top of that same natural process, not a separate phenomenon invented by human activity.
Global warming refers specifically to the rise in mean global temperature. Climate change is the broader term, encompassing global warming plus the full range of resulting shifts: changing precipitation patterns, sea level rise, more intense storms, and so on. Global warming is one driver within the broader pattern of climate change, not a separate phenomenon.
Which term refers specifically to the rise in mean global temperature, rather than the full range of resulting changes?
Global warming is the temperature rise specifically. Climate change is the broader umbrella term that includes global warming and its knock-on effects.
Apply your knowledge: two other atmospheres
Challenge 1
Transfer test. Venus's atmosphere is 96.5% carbon dioxide with a surface pressure about 92 times Earth's, and its thick cloud cover reflects roughly 80% of incoming sunlight back to space, a higher share than Earth reflects. Despite that, Venus's surface temperature averages around 467°C, hot enough to melt lead. Using the greenhouse effect, explain how Venus can be so much hotter than Earth despite reflecting away most of the sunlight that reaches it.
Your explanation
- Despite reflecting away most incoming sunlight, the small fraction that does reach Venus's surface still has to escape as outgoing infrared radiation, and Venus's atmosphere is so overwhelmingly CO2-rich, and so dense (92 times Earth's surface pressure), that almost none of that outgoing radiation can escape directly to space.
- The enormous total quantity of greenhouse gas, not just the percentage composition, drives an extremely strong greenhouse effect: heat is absorbed and re-radiated back toward the surface so many times over that it builds up to a surface temperature of about 467°C, even though the incoming energy supply is smaller than the total amount of sunlight Venus receives.
Challenge 2 · Extension
Mars's atmosphere is also mostly carbon dioxide, about 95% by volume, similar in composition to Venus's. Yet Mars's average surface temperature is only about -60°C, far colder than Earth, with barely any greenhouse warming at all. Explain why a CO2-dominated atmosphere does not produce a strong greenhouse effect on Mars the way it does on Venus.
Your explanation
- Composition alone does not determine how strong a greenhouse effect is; the total quantity of greenhouse gas present matters just as much.
- Mars's atmosphere is extremely thin, with a surface pressure less than 1% of Earth's, so despite being almost entirely CO2, there are far too few greenhouse gas molecules overall to trap much outgoing infrared radiation.
- The greenhouse effect scales with the total amount of GHG present in the atmosphere, not the percentage composition on its own, which is why Mars stays cold even with a CO2-rich atmosphere while Venus, with a similar percentage but a vastly denser atmosphere, is the hottest planet in the solar system.
- NOAA Global Monitoring Laboratory and Scripps Institution of Oceanography: Mauna Loa Observatory CO2 record (Keeling Curve), continuous since March 1958.
- EPICA Dome C ice-core CO2 record, Antarctica, spanning approximately 800,000 years; Vostok ice-core record.
- NASA Venus Fact Sheet and NASA Mars Fact Sheet (NSSDCA): atmospheric composition, surface pressure, surface temperature and albedo data.
- IB Environmental Systems and Societies Guide, first assessment 2026, Topic 6, Subtopic 6.1, statements 6.1.1-6.1.4.
Photographs and figures:
- Keeling Curve chart (Skills tab): Scripps Institution of Oceanography, UC San Diego, Scripps CO2 Program, data through December 2024, CC BY 4.0
- Tricellular circulation globe diagram: NASA, public domain
Build a system diagram of the atmosphere
Application of skills, statement 6.1.2: create system diagrams to represent the atmospheric system.
A system diagram sorts every component of a system into three categories: inputs (matter or energy entering the system), processes (what happens to that matter or energy while inside the system), and outputs (matter or energy leaving the system). Sort the seven components below into the category each belongs to for the atmospheric system.
When a question asks you to draw or label a system diagram, use arrows for inputs and outputs (crossing the system boundary) and boxes or labels for processes (staying inside it). Getting the direction of the arrows right, in versus out, is usually worth its own mark.
Read the Keeling Curve
Real exam data questions expect you to read more out of a graph than just "it goes up." Below is the actual, current Keeling Curve: the full monthly CO2 record from Mauna Loa Observatory, 1958 to the present, published by the Scripps CO2 Program.
Monthly average CO2 concentration, Mauna Loa Observatory, Hawaii, 1958-2024.
Scripps Institution of Oceanography, UC San Diego, Scripps CO2 Program, data through December 2024, CC BY 4.0
What does the small up-down zigzag pattern represent?
CO2 dips each year as Northern Hemisphere plants (most of the world's land vegetation) draw it down through photosynthesis in spring and summer, then rises again as leaves die back in autumn and winter. The real amplitude is roughly 5-6 ppm, peaking in May and bottoming out in September or October.
Ignoring the zigzag, is the steepness (gradient) of the overall rise constant, or changing?
The gradient itself is increasing. The 2010s added CO2 at roughly 2.4 ppm a year; the 2020s are running at roughly 2.6 ppm a year. A changing gradient like this means the rate of change is accelerating, not just the value itself.
Glossary
Every term introduced in this subtopic. Terms marked HL are only required at Higher Level.
- Atmosphere
- The layer of gases surrounding Earth, forming the boundary between the planet and space and the outer limit of the biosphere.
- Biosphere
- The zone of Earth, spanning parts of the atmosphere, hydrosphere and lithosphere, where life exists.
- Differential heating
- The unequal heating of Earth's surface by the Sun, strongest at the equator and weakest at the poles, which drives atmospheric circulation.
- Tricellular circulation model
- The pattern of three linked circulation cells, Hadley, Ferrel and Polar, in each hemisphere, that redistributes heat from the equator toward the poles.
- Hadley cell
- The circulation cell between the equator and about 30° latitude, driven by warm air rising at the equator and sinking around 30°.
- Ferrel cell
- The circulation cell between about 30° and 60° latitude, sandwiched between the Hadley and Polar cells.
- Polar cell
- The circulation cell between about 60° latitude and the pole, driven by air rising around 60° and sinking at the pole.
- Greenhouse gas (GHG)
- A gas that absorbs and re-emits infrared radiation, preventing it from escaping directly to space. The main examples are water vapor, carbon dioxide, methane and nitrous oxide.
- Aerosol
- A fine solid or liquid particle suspended in the atmosphere; black carbon is an aerosol with a warming effect similar to a greenhouse gas.
- Black carbon
- A dark, sooty aerosol produced by incomplete combustion, which absorbs solar radiation and contributes to warming.
- Water vapor
- A powerful natural greenhouse gas whose atmospheric concentration responds dynamically to temperature, which is why it is usually excluded from climate models.
- Infrared (long-wave) radiation
- The lower-energy radiation emitted by Earth's warmed surface, as opposed to the higher-energy short-wave radiation arriving from the Sun.
- Greenhouse effect
- The natural process by which greenhouse gases trap outgoing infrared radiation, keeping Earth's surface roughly 33°C warmer than it would be with no atmosphere at all.
- Enhanced greenhouse effect
- The additional warming caused by extra greenhouse gases added to the atmosphere by human activity, on top of the naturally occurring greenhouse effect.
- Global warming
- The rise in Earth's mean global temperature specifically, one component within the broader pattern of climate change.
- Climate change
- The broad range of changes to Earth's climate system, including global warming, shifting precipitation patterns, sea level rise and more intense storms.
- Radiative forcing
- A measure of the change in energy balance in the atmosphere caused by a factor such as a greenhouse gas, indicating whether it pushes the system toward warming or cooling.
- Dynamic system HL
- A system whose components and structure are the continuous result of ongoing physical and chemical processes, rather than being fixed.
- Lapse rate HL
- The rate at which atmospheric temperature falls with increasing altitude; the standard lapse rate is approximately 1°C per 100 m.
- Milankovitch cycles HL
- Long-term, regular variations in Earth's orbit and axial tilt that alter how much solar radiation reaches Earth, driving climate cycles over tens to hundreds of thousands of years.
- Eccentricity HL
- The Milankovitch cycle describing the changing shape of Earth's orbit, from more circular to more elliptical, with a period of roughly 100,000 years.
- Obliquity HL
- The Milankovitch cycle describing the changing tilt of Earth's axis, ranging between about 22.1° and 24.5°, with a period of roughly 41,000 years.
- Precession HL
- The Milankovitch cycle describing the slow wobble of Earth's axis of rotation, with a period of roughly 26,000 years.
- Glacial period HL
- A colder phase of the glacial-interglacial cycle, associated with lower atmospheric CO2 (around 180 ppm in the ice-core record) and expanded ice sheets.
- Interglacial period HL
- A warmer phase of the glacial-interglacial cycle, associated with higher atmospheric CO2 (around 280-300 ppm in the ice-core record); Earth is currently in one.
- Quaternary period HL
- The geological period spanning the last 2.5 million years, characterized by repeated glacial-interglacial cycles.
- Anthropocene epoch HL
- A proposed new geological epoch defined by the scale of human impact on Earth's systems, including atmospheric CO2 levels unprecedented in the Quaternary record.
- Stratospheric ozone HL
- Ozone (O3) formed high in the atmosphere from oxygen, made possible once photosynthesis had raised atmospheric oxygen levels; it blocks harmful ultraviolet radiation.
- Oxidation HL
- A chemical reaction with oxygen; rising atmospheric oxygen from early photosynthesis oxidized dissolved iron and exposed rock, forming deposits such as iron ore.
Test Yourself is coming soon for this subtopic
Practice questions and markschemes for this page are still being written. Check back once they have been added.
A dynamic system
The atmosphere is not a fixed, static layer of gas. It is a dynamic system: its components and layers are the continuous result of ongoing physical and chemical processes. Physical processes include global warming itself and air movements driven by temperature and pressure differences, the tricellular circulation covered earlier in this subtopic is one example. Chemical processes include the production of ozone from oxygen high in the atmosphere.
Sort each process into physical or chemical.
Global warming: extra heat trapped by rising greenhouse gas concentrations.
Wind: air moving from an area of high pressure to an area of low pressure.
Ozone (O3) forming from oxygen (O2) high in the atmosphere.
Convection: warm air rising and cool air sinking due to density differences.
Atmospheric methane gradually breaking down through reaction with other molecules.
Why the atmosphere thins with altitude
Molecules in the atmosphere are pulled toward Earth's surface by gravity. Because gravitational force weakens with distance from the surface, the atmosphere thins as altitude increases, becoming less dense higher up. Within the lower atmosphere this produces the standard lapse rate: temperature falls by approximately 1°C for every 100 m of altitude gained. Quantifying exact gas volumes or pressures at specific altitudes is not required, just this rate of temperature change.
Try it yourself
A base camp at the foot of a mountain records a temperature of 22°C. The summit is 2,800 m higher. Using the standard lapse rate, estimate the temperature at the summit.
Milankovitch cycles
The ice-core record from the hook of this subtopic shows CO2, and temperature, cycling roughly every 100,000 years across eight glacial cycles. Milankovitch cycles are the reason: regular, predictable variations in Earth's orbit and axial tilt that change how much solar radiation reaches different latitudes and seasons, without any change in the Sun's own output.
These cycles operate over tens to hundreds of thousands of years, far too slowly to explain the accelerating rise in CO2 since 1958. That rise is anthropogenic, not orbital.
Leaving the glacial-interglacial cycle
Global warming is moving Earth away from the glacial-interglacial cycle that has characterized the Quaternary period (which began about 2.5 million years ago) toward new, hotter climatic conditions. Climate has changed over geological time without human influence, but current anthropogenic change is unprecedentedly rapid: a shift the ice-core record would normally show unfolding over thousands of years is instead compressed into a couple of centuries. Some scientists argue this marks a new geological epoch, the Anthropocene.
Investigate the impact of albedo and greenhouse gas concentration on the temperature of a closed system using the sliders below.
Closed-system mini-simulator
A mid-range greenhouse gas level combined with fairly low albedo traps more heat than it reflects away, producing net warming of the closed system.
How life reshaped the atmosphere
The evolution of life on Earth changed the atmosphere's composition, which in turn influenced the evolution of life. The pre-biotic Earth's atmosphere had negligible free oxygen and was very different from today's. Photosynthesis by early organisms reversed that balance over time. The exact chronology of this oxygenation is not required, just the causal chain. Click the steps below in the correct order.
- EPICA Dome C and Vostok ice-core records, Antarctica: CO2 and temperature across eight glacial cycles, roughly 800,000 years.
- Milankovitch cycle periods (eccentricity ~100,000 years, obliquity ~41,000 years, precession ~26,000 years): established orbital mechanics literature.
- Standard atmospheric lapse rate (~1°C per 100 m): standard atmospheric physics.
- IB Environmental Systems and Societies Guide, first assessment 2026, Topic 6, Subtopic 6.1, statements 6.1.5-6.1.9.
Photographs and figures:
- Celestial poles, celestial equator and ecliptic plane diagram: Hawesthoughts, CC0
- Gyroscope precession animation: Lucas Vieira (LucasVB), public domain