Studying Air Bubbles, Dust, and Ocean Debris
Ice cores also preserve another valuable type of evidence: bubbles containing the free gases present in the ancient atmosphere. Two of the most important gases scientists look for are carbon dioxide and methane. Both are so-called greenhouse gases, which keep the Earth's surface warm by trapping heat, which the Earth absorbs from the sun, and preventing it from reradiating out into space. The more of these gases there are in the atmosphere, the stronger the greenhouse effect. Comparing the levels of atmospheric greenhouse gases in ice-core samples with their present levels helps scientists estimate how strong the greenhouse effect was at the time the sample was locked into the ice. They incorporate this value into their temperature calculations.
Bubbles of air found in ice cores from both Greenland and Antarctica indicate that atmospheric concentrations of carbon dioxide and methane were low during cold times and high during warmer times. At first, scientists took this as confirmation that the burning of fossil fuels, which releases additional greenhouse gases and other chemicals into the atmosphere, had indeed caused the recent observed increase in global temperatures. However, more precise dating made possible by ice-core analysis has indicated that temperature changes in the distant past actually occurred prior to changes in greenhouse gas concentrations. Some scientists think that this surprising finding may mean that rising levels of greenhouse gases reinforced past global warming episodes, but were not necessarily responsible for triggering them.