Starting in the 1950s, scientists began drilling deep into the ice sheets of Antarctica and Greenland to extract tubes of ice called ice cores. Like the rings of a tree, those ice cores contain distinct layers that each represent a year of snowfall. As snow accumulates, it slowly applies pressure to the older snow underneath. Eventually, that snow becomes ice, and the air pockets between snowflakes become isolated bubbles, shut off from other pockets of air.
Those bubbles contain “small amounts of ancient air,” says David McGee, an associate professor in the MIT Department of Earth, Atmospheric and Planetary Sciences. Each bubble can be a tenth of a millimeter to one millimeter in diameter, and there are hundreds of them in every cubic centimeter of ice.
By crushing the ice to extract the air in those tiny pockets, and then testing them for the concentration of gases like carbon dioxide (CO2)—the main driver of global warming—scientists can understand what the atmosphere was like a very long time ago. Among other things, that information has helped scientists verify the cause of today’s climate change. Just as expected, ice core samples show that CO2 levels have risen swiftly since the early 1800s, just as humans began burning large amounts of carbon-rich fossil fuels.
