Evidence From Ice Cores
Of the many types of evidence that paleoclimatologists study, ice cores had by 2000 proved to be the most valuable. Drilling deep into the ice is like reaching back in time—the farther down you go, the farther back in time you can see. Ice cores offer a detailed record of climate change over long periods, and so they serve as excellent “yardsticks” for comparison with other evidence. The oldest ice on Earth is found near the North and South poles in the icecaps of Greenland and Antarctica, where drilling has reached ice that formed almost half a million years ago.
It was Antarctic ice that led Petit's team to their conclusion about the variability of Earth's ancient climate. The researchers studied a 3,600-meter (11,800-foot) cylinder of ice drilled near Vostok Station, a Russian scientific research base in Antarctica, 1,500 kilometers (930 miles) from the South Pole. The Vostok ice core spans 420,000 years and contains a continuous record of climate changes through four ice ages and the interglacial periods between them.
Antarctic ice is glacial ice, which begins as snow. As one snowfall after another piles up on top of the last, the increasing weight of the overlying snow eventually changes the buried snow into solid ice. This continues from one year to the next, forming a series of annual layers. The ice layers, which are sometimes so distinct that scientists can identify individual years, become a record of climatic changes that are literally "frozen in time." By measuring differences between one layer and the next, scientists can identify climatic shifts and gather clues about what could have caused them. The most common sources of evidence in an ice core, besides the composition of the ice itself, are air bubbles and dust.