Today’s Atmosphere
For about the last 200 million years the proportions of gases in the atmosphere have been roughly what they are now. Two gases dominate, and a thin sliver of everything else does the rest of the chemistry — including the small amount of carbon dioxide that matters far more than its share suggests.
Clean dry air is about four-fifths nitrogen and one-fifth oxygen, with roughly 1% other gases — carbon dioxide, water vapour and the noble gases.
- Nitrogen — about 80% (four-fifths).
- Oxygen — about 20% (one-fifth).
- Small proportions of other gases — carbon dioxide, water vapour and the noble gases (Group 0).
For the recent past — the last few hundred thousand years — scientists read the composition from air bubbles trapped in ice cores: each bubble is a sealed sample of the atmosphere from when that ice formed, and the same record underpins the carbon-dioxide graph in section 5. It does not reach back to the early atmosphere — for that, the evidence is far more limited (section 2).
Carbon dioxide is well under 1% of the air, but it is a powerful greenhouse gas (section 4). A tiny proportion can still have a large effect — so “it’s only a small percentage” is never a good argument in this topic.
🧪 Exam-style questions
Approximately what percentage of today’s atmosphere is nitrogen? Tick (✓) one box.
Which gas in the atmosphere is used up when hydrocarbons burn? Tick (✓) one box.
How do scientists know the composition of the atmosphere from thousands of years ago? Tick (✓) one box.
Dry air is about 78% nitrogen and 21% oxygen. Calculate the percentage made up of all the other gases.
Show answer
100% − 78% − 21% = 1%. 1 mark
That ~1% is mostly argon, with small amounts of carbon dioxide, water vapour and other noble gases.
The Early Atmosphere & the Oceans
The Earth is about 4.6 billion years old, and for its first billion years its atmosphere was utterly different from today’s. Because it was so long ago, the evidence is limited — which is why there are several theories and why exam questions ask you to evaluate them rather than state a single fact.
Drag the slider — or press play — to watch the air change over 4.6 billion years. The volcanic theory below names each stage.
- For the Earth’s first billion years, intense volcanic activity released gases that formed the early atmosphere.
- The early atmosphere was mainly carbon dioxide, with water vapour and little or no oxygen — thought to be like the atmospheres of Mars and Venus today.
- Volcanoes also released nitrogen, which gradually built up, and there may have been small amounts of methane and ammonia.
- As the Earth cooled, the water vapour condensed and fell, forming the oceans.
- Carbon dioxide dissolved in the new oceans, and carbonates were precipitated as sediments — reducing the CO2 in the atmosphere.
That is the volcanic theory — how the early atmosphere and oceans formed. What changed the air next is a separate stage (and not really a “theory”): from about 2.7 billion years ago, algae and then plants used photosynthesis to add oxygen and remove carbon dioxide — the rest of the animation above, covered in section 3.
The great swap: carbon dioxide (red) falls as it dissolves in the oceans, forms carbonate sediments and limestone, and is taken up by photosynthesis; oxygen (blue) climbs once algae and plants appear, levelling off at today’s ~20% — the proportions have then stayed about constant for the last 200 million years.
Theories about the early atmosphere have changed over time as evidence and technology improve. Because it was 4.6 billion years ago, direct evidence is scarce, so models rely on clues like the gases trapped in ancient minerals and comparisons with Mars and Venus. In an exam you may be given an alternative theory and asked to evaluate it — weigh what the evidence does and doesn’t support, rather than declaring one theory simply “right”.
- Saying the early atmosphere had oxygen. It had little or no oxygen — oxygen came later, from life.
- Saying volcanoes released oxygen. Volcanoes released CO2, water vapour and nitrogen (and possibly small amounts of methane and ammonia) — not O2.
- Forgetting the oceans’ role. Water vapour condensed to form oceans; CO2 then dissolved in them and formed carbonate sediments — a major way CO2 was removed.
🧪 Exam-style questions
What was the main gas in the Earth’s early atmosphere? Tick (✓) one box.
Where did the gases in the Earth’s early atmosphere mainly come from? Tick (✓) one box.
Billions of years ago there was no liquid water on the Earth’s surface. Suggest why. Tick (✓) one box.
Why are there several different theories about the Earth’s early atmosphere? Tick (✓) one box.
Rising Oxygen, Falling Carbon Dioxide
Two changes turned the early CO2 atmosphere into the one we breathe: oxygen appeared, and carbon dioxide was removed. Both come back to the same cause — life, especially photosynthesis.
How oxygen increased
Algae and plants produced the oxygen now in the atmosphere, by photosynthesis. Algae first produced oxygen about 2.7 billion years ago, and soon after, oxygen began to build up in the atmosphere. Over the next billion years plants evolved and spread, the percentage of oxygen gradually increased, and eventually it reached a level that allowed animals to evolve.
6CO2 + 6H2O → C6H12O6 + 6O2
(carbon dioxide + water → glucose + oxygen, using light). This single reaction does both jobs: it removes CO2 and releases O2, and the equation must balance.
How carbon dioxide decreased
Carbon dioxide came out of the atmosphere by several routes — and much of that carbon is still locked away today:
- Dissolving in the oceans. CO2 is soluble; it dissolved in the new oceans and carbonates precipitated as sediments.
- Photosynthesis. Algae and plants absorbed CO2 as they grew.
- Sedimentary rocks. Marine organisms used carbonates to build shells and skeletons; when they died these formed limestone and other carbonate rocks. (This and the dissolving route above are two faces of the same idea — carbon locked into carbonate rock.)
- Fossil fuels (storage). Carbon that photosynthesis had already taken from the air — in the remains of plants and plankton — was buried under sediment and, over millions of years under heat and pressure, turned into coal, crude oil and natural gas, locking that carbon underground.
So the carbon from the early CO2 is now stored in oceans, limestone and fossil fuels. (Burning those fossil fuels today releases it again — section 5.)
- Saying plants produced the first oxygen. It was algae first (~2.7 billion years ago); plants came later.
- An unbalanced photosynthesis equation. Keep the 6s: 6CO2 + 6H2O → C6H12O6 + 6O2.
- Only giving one route for CO2. Higher-mark questions want several: dissolving in oceans, photosynthesis, sedimentary rocks and fossil fuels.
🧪 Exam-style questions
Which process increased the amount of oxygen in the atmosphere? Tick (✓) one box.
Which organisms first produced oxygen, about 2.7 billion years ago? Tick (✓) one box.
Which of these “locked up” carbon from the early atmosphere underground? Tick (✓) one box.
In photosynthesis, carbon dioxide and water react to form oxygen and which other product? Tick (✓) one box.
The percentages of carbon dioxide and oxygen have changed from Earth’s early atmosphere to Earth’s atmosphere today. Explain the processes that led to these changes. This is a levels-of-response question — identify the processes, then link them into a clear account of how CO2 fell and O2 rose. Plan, then compare.
Show a model answer
How it is marked (levels of response):
- Level 3 (5–6): relevant points (reasons / causes) are identified, given in detail and logically linked to form a clear account. AO2
- Level 2 (3–4): relevant points (reasons / causes) are identified, and there are attempts at logical linking; the account is not fully clear. AO1
- Level 1 (1–2): points are identified and stated simply, but their relevance is not clear and there is no attempt at logical linking.
Indicative content — carbon dioxide decreased because:
- it dissolved in the newly-formed oceans;
- it was used by photosynthesising algae and plants, which removed it from the air (and that carbon was then locked away);
- it became locked in sedimentary rocks — carbonates such as limestone formed from the shells and skeletons of marine organisms;
- it became locked in fossil fuels (coal, crude oil and natural gas) formed from buried plant and plankton remains.
Indicative content — oxygen increased because:
- algae first, and later plants, produced oxygen by photosynthesis: 6CO2 + 6H2O → C6H12O6 + 6O2;
- as algae and plants spread, oxygen gradually built up in the atmosphere.
Conclusion (needed for Level 3): the same process — photosynthesis — both removed carbon dioxide and released oxygen; alongside CO2 dissolving in the oceans and being locked into rocks and fossil fuels, this is why CO2 fell from a high level in the early atmosphere to a small proportion today while O2 rose to about one-fifth of the air.
Source: AQA GCSE Chemistry.