One of the most important methods of reconstructing past climate is by looking at cores taken from the glaciers and ice caps found in some of the most remote regions of our planet. Although the last major glacial event of the Pleistocene Ice Ages tapered off 10,000 years ago, the poles and high mountains have remained ice covered for tens of thousands of years. Ice cores taken from these locations contain an enormous amount of information embedded within them. Ice Cores Ice sheets are built as snow falls in winter but doesn't completely melt in summer. In a core drilled through an ice sheet, each year appears as a layer containing ice, dust, pollen, and volcanic ash.
Cores hold clues to the environmental conditions at the time the ice was deposited.
The oxygen and hydrogen isotopic composition of rain and snow is controlled by air temperature at precipitation. In both cases, the colder the temperature, the lower the amount of the heavier isotope and the lower the isotope ratio. Because 16 O is lighter, more of it changes to gas when water evaporates. This has been quantified for Antarctica, where a cooling of 10C is reflected as a decrease in D of 9 per mil. Scientists can also determine the composition of the atmosphere at the time the ice was deposited because atmospheric gasses get trapped in the ice.