What Is the Atmosphere?

The atmosphere is the layer of gases that surrounds the Earth. It is held in place by gravity, and it stretches from the ground all the way up to space, although it gets thinner and thinner the higher you go. The atmosphere is essential for life: it gives us oxygen to breathe, traps just enough of the Sun's heat to keep the planet warm (but not too hot), blocks dangerous radiation from space, and creates all the weather. Without it, Earth would be a freezing airless ball like the Moon.

  • % nitrogenApprox. 78%Mostly unreactive
  • % oxygenApprox. 21%What we breathe
  • % argonApprox. 0.9%Plus tiny amounts of CO2 and others
  • Total massApprox. 5,000 trillion tonnesAbout a millionth of Earth's mass
  • Where most air sitsBottom 12 kmApprox. 75% of the atmosphere by mass
  • Top of atmosphereApprox. 10,000 kmFades gradually into space

What air is made of

Air is a mixture of many gases. By volume, dry air is:

  • Nitrogen (approx. 78%): mostly unreactive. Just floats around.
  • Oxygen (approx. 21%): the gas we and almost every animal breathes.
  • Argon (approx. 0.9%): a noble gas, completely unreactive.
  • Carbon dioxide (approx. 0.04%): tiny but vital; plants use it to make food.
  • Other gases: neon, helium, methane, krypton and many others in trace amounts.

Real air also contains water vapour, which varies enormously: almost nothing in deserts, several percent in tropical rainforests.

What the atmosphere does

  • Provides oxygen for animals to breathe and carbon dioxide for plants.
  • Blocks ultraviolet radiation through the ozone layer.
  • Keeps the planet warm through the greenhouse effect.
  • Burns up most meteors before they hit the ground.
  • Creates weather and the water cycle.
  • Carries sound: without air, sound waves cannot travel.
  • Provides air pressure: roughly 1 kg of air pressing on every square centimetre at sea level.

How thin is the atmosphere really?

The atmosphere seems huge from the ground, but it is actually very thin compared to the size of the Earth. The lowest 12 km contains approx. 75% of all the air. To scale, if Earth were the size of an apple, the entire atmosphere would be thinner than the skin of the apple. Climbing Mount Everest (8,849 m) takes you above approx. 70% of the air; that is why climbers need bottled oxygen near the top.

Fact Almost all the oxygen in the atmosphere was put there by living things, mostly tiny ocean algae called cyanobacteria that started doing photosynthesis around 2.5 billion years ago. Before that, there was almost no free oxygen in the air. Cyanobacteria pumped out so much oxygen over the next billion years that they accidentally changed the whole atmosphere and made all modern life possible.

How we are changing the atmosphere

Humans have been adding extra carbon dioxide and methane to the atmosphere since the start of the Industrial Revolution by burning coal, oil, gas and forests. CO2 levels have risen from about 280 parts per million in 1800 to over 420 ppm today, the highest level in at least 3 million years. The extra greenhouse gases are trapping more heat and causing the climate to warm. This is the main reason climate change is such an urgent global issue.

Did you know? The atmosphere is not all dry. The total amount of water vapour in the air at any given moment is around 13,000 cubic kilometres: enough water to cover the surface of the Earth in a layer about 25 mm deep. It is the engine that drives the entire water cycle.
Deeper dive: how Earth's atmosphere is different from other planets

Almost every planet and moon in the Solar System has some kind of atmosphere, but they are all wildly different from ours. Comparing them tells us a lot about how special Earth's atmosphere is.

  • Mercury: almost no atmosphere at all. Just a thin trace of atoms knocked off the surface by the Sun.
  • Venus: a thick atmosphere of 96% carbon dioxide. So much greenhouse effect that the surface is 460 °C, hot enough to melt lead.
  • Mars: also mostly CO2, but very thin (less than 1% of Earth's pressure). The thin atmosphere is the main reason Mars cannot hold liquid water on its surface.
  • Jupiter and Saturn: enormous atmospheres of hydrogen and helium, hundreds of kilometres deep. There is no proper surface to land on.
  • Titan (moon of Saturn): a thick atmosphere of nitrogen and methane, the only moon in the Solar System with a substantial atmosphere. It even rains liquid methane.

The reason Earth has the atmosphere it does is partly luck (we are in the right zone, the right distance from the Sun) and partly because of life itself. Without 2.5 billion years of photosynthesising microbes, our air would still be like Venus or Mars: mostly CO2, with almost no free oxygen, and no chance of supporting humans.

For more, see layers of the atmosphere and the greenhouse effect.