Learn · Air · 8 minute read
How the air changed
The first air had no oxygen in it. Tiny living things changed that, and now we are changing it again.
Take a breath. About 78% of what you just breathed in is nitrogen and about 21% is oxygen. Almost all the rest is argon, plus a tiny amount of carbon dioxide. None of that is a given. The air has changed completely over the Earth's history, and living things did most of the changing.
The great oxygen change
The early air had no free oxygen to speak of. Then some microbes learned to use sunlight to split water, making food and giving off oxygen as waste. That is photosynthesis, and plants still do it today. For a long time the oxygen they made was mopped up at once, by iron dissolved in the sea and by gases from volcanoes.
About 2,430 million years ago, oxygen began to build up in the air. A study in 2021 found that it rose and fell several times before it stayed for good, about 200 million years later. Oxygen changed everything. It let a layer of ozone form high up, which shields the surface from ultraviolet light, and it made possible the large, energy-hungry bodies of animals.
Snowball Earth
Between about 717 and 660 million years ago, and again from about 650 to 635 million years ago, ice spread from the poles to the tropics. Rocks laid down by glaciers at that time are found in places that were then near the equator. Many geologists think the oceans froze over almost completely. The Cryogenian period, meaning "cold birth", is named after these ice ages.
The Earth escaped because volcanoes kept breathing out carbon dioxide. With the oceans frozen, little of it was taken up, so it built up until the greenhouse warming melted the ice. Not long after the second freeze, the first large animals appear in the fossil record.
What we are adding now
Carbon dioxide is a small part of the air, measured in parts per million (ppm), but it traps heat. Since 1958 it has been measured on Mauna Loa in Hawai'i, far from any city. The yearly average there has gone from about 316 ppm in 1959 to about 427 ppm in 2025.
Before the Industrial Revolution, the level was 280 ppm or less. Air bubbles trapped in ice cores show that through the ice ages of the last million years or so, it never went above 300. It is now about 50% higher than before industry, and it is rising about 100 times faster than at the end of the last ice age. Most of the extra carbon comes from burning coal, oil and gas: carbon that plants took out of the air millions of years ago, and that the rocks have held ever since.