I was watching a program on PBS yesterday, a real interesting one about the ice sheets. The ice sheets in Antarctica and Greenland have layers, like rings of a tree, that can be counted. They are formed as snow falls and is compressed under new snow, but summer snow (with the sun on it 24 hours a day) looks different than winter snow, which falls in the dark, so you get annual banded cross sections that look like rings of a tree:
These layers have lots of data in them--there are trapped air bubbles (down to a certain level, before the pressure becomes too great) so atmospheric composition can be measured, and there are different ratios of isotopes that tell us what the temperature was when the snow fell.
All of that is fine and good, the point to all of this is, in Antarctica they have drilled an uninterrupted ice core record going back to 400,000 years, and in Greenland it goes back to 100,000.