Late in the last century scientists published reams of data about Earth’s climate derived from ice cores taken from Greenland and Antarctic glaciers. By drilling down into the ice with hollow bits (think of using a spinning pipe as a drill) workers were able to pull columns of ice up to the surface. The material brought to light in this way was very special for several reasons.
First, the ice cores show annual layers going back in time. That means scientists can count backwards through time from the surface downward, a bit like school kids can count the rings of a tree, measuring out history year by year.
The ice in the cores had originally been snow at the surface, snow that was buried by later snows and slowly became compacted to form a coarse icy material called “firn” and then glacial ice itself. The ice has tiny bubbles of air in it, air that was trapped in the snow layers in times gone by. This means that scientists can analyze the small air samples that are sealed in the ice and determine the composition of the air that was blowing around the Earth in past millennia.
Chemical analyses of the air in the glacial ice show that our planet’s atmosphere has gone through cycles of changes over thousands of years. In particular, the concentration of greenhouse gases like carbon dioxide and methane (the main ingredient in natural gas) has varied over time. The specific dates of the changes can be determined by counting up the layers in the ice core – a pretty nifty trick.
If the only thing the ice cores told us was how much Earth’s air has changed, that would be interesting in itself. But scientists are also able to analyze the ice and from it infer a couple of things.