How do scientists determine how much CO2 was in the atmosphere thousands of years ago, and to what level of certainty are they able to perform these calculations?
In: Earth Science
What I want you to do is imagine you’re a giraffe.
Now that you’ve done that completely irrelevant thing, think about snow. Specifically, think about what happens when you walk on snow – you sink. This is because snow is a bunch of tiny ice crystals layered gently on top of each other with lots of air between them. When you stand on it, the air gets pushed out and the ice crystals get compacted. The reason this happens is simply because you are heavy – any other heavy thing put on the snow will also cause this to happen, including more snow.
When you put a bunch of snow on top of snow, the snow at the bottom gets compressed. Some of the air gets forced out of it, and the ice fuses into one big sheet of ice. Some air gets trapped inside the sheet when this happens. In some places, like Antarctica, it snows pretty often, so you keep getting new layers of snow deposited on top of older layers, resulting in the snow at the bottom becoming compressed. This process happens regularly over hundreds of years, creating a thick layer of ice called a glacier or ice sheet. The ice at the bottom of this sheet is older than the ice at the top because of the way snow always falls down from the sky, and not up from under the ground.
You could take a segment out of this ice sheet, a long thin one from top to bottom, and if you did the ice at the top would be very recent and the ice at the bottom very old. All of this ice has air trapped in it, which was trapped when the ice turned from snow to ice, so it’s essentially a time capsule of what the atmosphere was like when that ice was formed. If you open up one of those air bubbles, you can measure the CO2 in it, and you can estimate the age of the ice based on various things like how much snow gets deposited in a given length of time.
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