Science Puzzle
How Do We Know What the Air Was Like a Million Years Ago?
There's more carbon dioxide in the air today, over 420 parts per million, than at any time in at least 800,000 years. Nobody was there to measure the air hundreds of thousands of years ago, and gas doesn't leave fossils.
So how do scientists know what the air was like that long ago?
The Answer
They measure it directly, from tiny bubbles of ancient air sealed inside the ice of Antarctica and Greenland. Drill down through the ice and you're drilling back in time.
In the middle of Antarctica, snow that falls almost never melts. Year after year, new snow buries the old, and its weight slowly squeezes the snow below into solid ice. As that happens, the little pockets of air between the snow grains get sealed off as bubbles: real samples of the atmosphere at that time, kept in the freezer.
Scientists drill out long cylinders of ice, called ice cores, from kilometres down. They take a piece, crush it in a vacuum chamber to free the trapped air and measure the gases directly. The deeper the ice, the older the air. The ice itself records the temperature too, because water containing heavier atoms freezes out of the clouds a little differently when the climate is colder.
A core drilled at Dome C in Antarctica gave a record going back about 800,000 years. It shows carbon dioxide rising and falling between roughly 180 and 300 parts per million through the ice ages, never close to today's level. In January 2025, a European team reached the bottom of a new core, 2.8 kilometres down, where the ice is expected to be more than 1.2 million years old.
The principle: Natural archives. Snow that piles up year after year seals bubbles of air into the ice, so deep ice cores hold real samples of the atmosphere from long ago, oldest at the bottom.