Earth's Atmosphere: Past and Present
The Atmosphere Today
The air around us has a remarkably stable composition. Today’s atmosphere is approximately 78% nitrogen and 21% oxygen, with about 1% of other gases including argon (roughly 0.9%), carbon dioxide (about 0.04%) and water vapour. This composition has been relatively stable for approximately 200 million years. Yet Earth’s early atmosphere was completely different — understanding how it changed, and what evidence tells us this, is a key part of chemistry’s story about our planet.
The current atmosphere is approximately 78% nitrogen, 21% oxygen, and 1% other gases (mainly argon with a small amount of carbon dioxide). This composition has been relatively stable for around 200 million years.
The Early Atmosphere
About 4.6 billion years ago, when Earth first formed, there was no free oxygen in the atmosphere. Scientists believe intense volcanic activity during the first billion years produced a very different atmosphere: mainly carbon dioxide, with some water vapour, nitrogen, methane and ammonia — similar to the current atmospheres of Mars and Venus. As Earth gradually cooled, the water vapour condensed to form the oceans. Some carbon dioxide dissolved in this new ocean water, removing it from the atmosphere.
No direct sample of Earth’s early atmosphere survives, so scientists piece together evidence from multiple sources. Ancient rocks provide clues — the chemical composition of very old rocks shows what gases were available in the environment when they formed. The absence of rust-like oxidised iron minerals in very ancient rocks, for example, suggests there was little or no free oxygen.
Ice cores drilled from Antarctic and Greenland ice sheets preserve trapped air bubbles from up to around 800,000 years ago. That is far too recent to sample the early atmosphere, so ice cores record recent change rather than the original composition. Volcanic emissions today still contain carbon dioxide, water vapour and sulfur dioxide, supporting the idea that early volcanoes produced a similar mixture. Comparison with other planets — Mars and Venus both have atmospheres that are mostly carbon dioxide — provides further circumstantial support that Earth’s early atmosphere was similar before photosynthesis changed it.
This is an important example of how science works: where we cannot directly observe or measure the past, we construct explanations from multiple independent sources of evidence. The more independent sources that converge on the same picture, the greater our confidence in the explanation.
How Oxygen Increased
The dramatic increase in atmospheric oxygen is attributed to the evolution of organisms capable of photosynthesis. Early photosynthetic organisms — particularly cyanobacteria — evolved in the oceans around 2.7 billion years ago. They absorbed carbon dioxide and released oxygen as a waste product. Over hundreds of millions of years, this gradually increased the oxygen level in the atmosphere and decreased the carbon dioxide level.
As oxygen levels rose, it accumulated in the upper atmosphere and formed the ozone layer, which absorbs much of the Sun’s harmful ultraviolet radiation. The ozone layer’s formation allowed life to move out of the oceans onto land. Meanwhile, carbon dioxide levels continued to fall as photosynthesis removed it, as some dissolved in the expanding oceans, and as marine organisms used dissolved carbon dioxide to build calcium carbonate shells. Over geological time, these shells became compressed into sedimentary rocks such as limestone, locking carbon away. The organic carbon in dead organisms eventually formed fossil fuels — coal, oil and natural gas.
Carbon dioxide decreased by three routes: dissolving in the oceans; being used by marine organisms to build calcium carbonate shells that became sedimentary rocks (limestone); and organic carbon in dead organisms becoming fossil fuels over millions of years.
Unlike carbon dioxide, nitrogen is relatively unreactive under normal atmospheric conditions. Small amounts were present in the early volcanic atmosphere and also produced by the action of bacteria on nitrogen-containing compounds. Because nitrogen does not participate in photosynthesis or the formation of sedimentary rocks, it was not ‘locked away’ like carbon dioxide. Instead, as oxygen increased and carbon dioxide decreased, nitrogen became the dominant gas by default — its proportion rose simply because the other gases decreased.
The timeline of atmospheric change spans billions of years: heavy volcanic outgassing in the first billion years produced a CO\(_2\)-rich atmosphere; photosynthesis from roughly 2.7 billion years ago gradually increased oxygen and reduced CO\(_2\); the atmosphere has been roughly its current composition for the last 200 million years, though small changes continue as a result of human activities.
Monitoring Atmospheric Change Today
Although the atmosphere has been relatively stable for about 200 million years, its composition is not fixed. Monitoring stations around the world continuously track the concentration of carbon dioxide and other gases. Data from these stations shows that carbon dioxide levels have risen significantly since industrialisation — a direct result of burning fossil fuels that had locked carbon away for millions of years. This modern change is happening far faster than the natural changes driven by photosynthesis and geological processes over billions of years. Understanding the history of the atmosphere — how it formed, how it changed, and why it became stable — gives us the scientific context to evaluate what is happening to it now. The evidence for how the atmosphere evolved is the same kind of evidence that now tells us it is changing again: ice cores, atmospheric measurements, and comparisons across time and place all point to the same conclusion.