To measure fluctuations in atmospheric conditions over the past hundreds of thousands of years, scientists have analyzed samples of ice from locations in Greenland and the Antarctic, such as the scientific research site of Vostok. Ice can provide clues to the composition of Earth's atmosphere in the distant past, because bubbles of air are sometimes trapped in ice for many millennia. By analyzing air bubbles trapped in ancient ice, scientists can determine the composition of Earth's atmosphere at the time the air was trapped.
Cross sections of ice core samples. The gradually increasing weight of overlying layers of snow compresses deeper snow into ice, but annual bands, like age rings on a tree stump, can still be seen. Relatively new snow becomes packed into coarse and granular crystals called firn (top: 53 meters deep). Older and deeper snow is compacted further (middle: 1,836 meters). At the bottom of an ice core, rocks, sand, and silt discolor the ice (bottom: 3,050 meters). Photographs courtesy U.S. National Ice Core Laboratory
Ice core samples gathered at Vostok and other research stations indicate great fluctuations of CO2 over time. However, within the past century, these levels have increased faster and further than ever before. This sudden and dramatic increase in atmospheric CO2 parallels recent increases in human production of CO2 due to industrialization, which leads many scientists to conclude that human activity is the cause.