2012: Advent Atmosphere: Layer Cake

by on December 7, 2012

How exactly do we know what was going on in the atmosphere millions and millions of years ago?

If we want to know what the temperature is today, we just stick a thermometer out the window and find out. It is rather more difficult to find out what the temperature was hundreds and thousands of years ago. We don’t have a time machine to do that yet – if we ever do, we might find out some interesting things, but we’ll probably be too busy going “Look! Mammoths!” to pay much attention to the annual rainfall. So we have other ways of looking at historical weather patterns.

They all come down to basically the same method. Find something that has yearly layers – mud, rock, ice, trees – and take a slice through as many layers as you can. Then, by comparing the most recent layers to the direct data, figure out how the layers change according to the weather. And then work the other way and figure out how the weather changed according to the layers for the years you don’t have direct data for.

(This also works for things that are laid down continuously. You just have to calculate how fast it accumulates, instead of counting layers, and it’s difficult to adjust for how the rate of accumulation itself changes.)

The changes can be all sorts of things. Tree rings change width and density and composition depending on the local climate and the quality of the soil. Concentrations of carbon dioxide or oxygen or volcanic dust show up directly in ice cores, trapped as the snow fell. Rocks tell you about where they were formed and when, which tells you where the ground you are standing on used to be – under the sea, beside a volcano, on a floodplain – and if they contain fossils, what kind of creature was living when the rock was laid down.

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