The Earth's atmosphere, past and present – GCSE Combined Science Chemistry

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The air around us is a mixture of gases. It has not always been the same. The Earth is about 4.6 billion years old, and the air has changed a great deal in that time.

This matters because the story explains why we can breathe, why there is limestone in the ground, and where coal, oil and gas came from. It also helps you understand how human activity can change the air again.

By the end you will know what is in the air today, what the first atmosphere was like, and the main events that changed it.

The air today

For the last 200 million years the proportions of gases in the air have been much the same as today. About four-fifths (approximately 80%) is nitrogen. About one-fifth (approximately 20%) is oxygen.

The rest is small amounts of other gases. These include carbon dioxide, water vapour and noble gases such as argon. More exact figures are 78% nitrogen, 21% oxygen, about 1% argon and about 0.04% carbon dioxide. The amount of water vapour changes with the weather.

The early atmosphere

During the first billion years of the Earth there was intense volcanic activity. Volcanoes released gases that formed the early atmosphere. This was mainly carbon dioxide, with little or no oxygen. At the start the air may have been like the atmospheres of Mars and Venus today.

Volcanoes also released water vapour and nitrogen. Nitrogen slowly built up in the air. There may also have been small amounts of methane and ammonia.

We cannot be sure. The Earth is 4.6 billion years old, so the evidence for the early atmosphere is limited. Scientists use rocks, models and other planets as clues, and theories have changed over time.

Oceans and falling carbon dioxide

As the Earth cooled, the water vapour in the air condensed. This made liquid water that collected to form the oceans.

Carbon dioxide dissolved in the oceans. Carbonates were then precipitated, which means they formed solids that settled out. They built up as sediments, and the carbon dioxide in the air went down.

So two things reduced carbon dioxide early on: it dissolved in the oceans, and carbon was locked up in sediments such as limestone.

Oxygen rises

Algae first produced oxygen about 2.7 billion years ago. Soon after that, oxygen appeared in the air. The oxygen came from photosynthesis: carbon dioxide + water → glucose + oxygen.

Over the next billion years plants evolved. The percentage of oxygen gradually increased until there was enough for animals to evolve.

Photosynthesis also removed carbon dioxide from the air, so as oxygen went up carbon dioxide went down.

Carbon locked away in rocks and fuels

Carbon dioxide was also reduced by the formation of sedimentary rocks and fossil fuels, which contain carbon. Limestone formed from the shells and skeletons of sea creatures that built up on the sea floor.

Coal formed from dead plants buried and squashed over millions of years. Crude oil and natural gas formed from the remains of tiny sea creatures buried under mud and squashed by heat and pressure.

The carbon in these rocks and fuels came from carbon dioxide that was once in the air. When we burn fossil fuels we put it back.

Theories and evidence

Nobody was there to measure the early atmosphere, so scientists build theories from evidence. The theory in this lesson is the volcano theory. Theories can change when new evidence is found.

If you are given information, you can be asked to interpret the evidence and judge how good a theory is. Ask whether the evidence is direct or only a clue, how old it is, and whether other planets really match the Earth.

Keep your answer to what the information shows. For example, ice cores and rocks give clues, but they cannot show exactly how much of each gas there was 4 billion years ago.

Recap

Today: about 80% nitrogen, about 20% oxygen, and small amounts of carbon dioxide, water vapour and noble gases.

Early atmosphere: mainly carbon dioxide from volcanoes, with water vapour and nitrogen, and little or no oxygen.

Carbon dioxide fell as it dissolved in oceans, was used in photosynthesis, and was locked in sedimentary rocks and fossil fuels. Oxygen rose because of algae and plants.