The atmosphere is a thin shell of gas between us and space, and it is the outer limit of the biosphere. Almost all of it is nitrogen and oxygen — yet the gases that matter most for climate are the ones present in tiny traces. That mismatch is the whole point of this page.
📚 What you need to know
Dry air is about 78% nitrogen and 21% oxygen, with roughly 1% left over for everything else.
That last 1% includes argon (inert), carbon dioxide (about 0.04%) and trace gases such as methane, ozone and nitrous oxide.
Water vapour is variable, so it is usually quoted separately. It drives weather, clouds and precipitation.
Gases are redistributed by wind, convection, diffusion, turbulence and jet streams.
The atmosphere is layered by temperature. The two layers that matter for life are the troposphere (surface to about 10 km) and the stratosphere (about 10–50 km, containing the ozone layer).
The atmosphere is a system with storages, flows, inputs and outputs, and it exchanges gases with the biosphere, hydrosphere and lithosphere.
The composition of dry air
The orange sliver on the right is everything that is not nitrogen or oxygen. Almost the entire greenhouse effect is produced inside that sliver, plus water vapour.
Gas
Roughly how much
Why it matters
Nitrogen
78%
Largely unreactive in air; the reservoir that feeds the nitrogen cycle once fixed
Oxygen
21%
Needed for aerobic respiration and for combustion; the raw material for ozone
Argon
About 0.93%
Inert — takes no part in chemical reactions, but still part of the composition
Carbon dioxide
About 0.04%
Raw material for photosynthesis and a major greenhouse gas
Water vapour
Variable, up to about 4%
Drives clouds, precipitation and weather; the most abundant greenhouse gas
Trace gases
Tiny amounts
Methane, ozone and nitrous oxide have effects on climate and atmospheric chemistry far larger than their concentrations suggest
If a question asks you to name the main gases, give a figure with each one. “Nitrogen about 78 per cent, oxygen about 21 per cent” is a mark. “Mostly nitrogen and oxygen” often is not.
How gases get moved around
The atmosphere is not a still tank of gas. Mixing is what keeps composition roughly the same everywhere near the surface, and it is what carries pollution far from where it was released.
🧩 Five processes that redistribute gases
Wind — the main mover, caused by differences in air pressure.
Convection — warm air rises, cool air sinks, giving vertical movement.
Diffusion — gases spread from high concentration to low concentration.
Turbulence — irregular flow caused by obstacles such as mountains and buildings.
Jet streams — fast, narrow currents high in the atmosphere that move air rapidly around the planet.
The layers
The atmosphere is divided into layers based on how temperature changes with height — not on what the gases are. Two of those layers matter for living systems.
Chemistry in the troposphere involving pollutants, greenhouse gases and particles controls air quality and climate. Chemistry in the stratosphere involving ozone keeps UV radiation off the surface.
The atmosphere as a system
ESS wants you to treat the atmosphere the way you treat any other system: identify the storages, flows, inputs and outputs.
System component
In the atmosphere
Storages
The gases themselves, held at different concentrations. Greenhouse gas storages such as carbon dioxide and methane change over time through natural and human activity
Flows
Constant movement of gases and particles, driven by air currents, weather systems and atmospheric circulation
Inputs
Natural: volcanic gases, gases from plants and other organisms, desert dust. Human: greenhouse gases from fossil fuel combustion and livestock, industrial air pollutants, aerosols from combustion
Outputs
Gases removed by respiration and photosynthesis; pollutants and aerosols removed by precipitation and dry deposition
It also exchanges continuously with the other spheres. Carbon dioxide moves between the atmosphere and the biosphere through photosynthesis and respiration, dissolves into and out of the hydrosphere, and is locked into or released from the lithosphere over geological time.
Why this framing earns marks: if a question asks how human activity changes the atmosphere, you can answer in system language — humans have increased an input (fossil fuel emissions) faster than outputs can remove it, so the storage grows.
EXAM PRACTICE
Carbon dioxide makes up only about 0.04% of the atmosphere. Explain why such a small proportion is still important. [3]
Point 1: it is the raw material for photosynthesis
Producers use it to make organic matter, so it is the entry point of carbon into every food chain.
Point 2: it absorbs infrared radiation
Nitrogen and oxygen barely absorb outgoing thermal energy, but carbon dioxide does — so the effect per molecule is very large.
Point 3: it persists
It stays in the atmosphere a long time, so emissions accumulate rather than clearing quickly.
Small share of the volume, large share of the greenhouse effectThe trap is answering “because plants need it” and stopping. Three marks means three distinct ideas.
💡 Exam tip
Quote the percentages. 78, 21, about 1, and 0.04 for carbon dioxide.
Say layers are defined by temperature change if asked why the atmosphere is stratified.
Remember the two-layer split: weather and air quality in the troposphere, ozone and UV protection in the stratosphere.
Use storages, flows, inputs and outputs whenever the word “system” appears in a question.
Link the atmosphere to the other spheres — examiners reward connections across the course.
⚠️ Common mix-up
Putting the ozone layer in the troposphere. It is in the stratosphere; ozone near the surface is a pollutant instead.
Saying oxygen is the main greenhouse gas. Oxygen and nitrogen do almost none of the absorbing.
Quoting water vapour as a fixed percentage. It varies with temperature and place, which is exactly why it is quoted separately.
Thinking argon must do something. It is inert. Saying so is the correct answer, not a gap in your knowledge.
Confusing composition with layers. The gases are well mixed through the lower atmosphere; the layers are about temperature.
Up next: How the Greenhouse Effect Works — the natural process that keeps Earth habitable, and the enhanced version that does not.
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