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How do we know how much CO2 was in the atmosphere hundreds of years ago?

Scientists extract tiny air bubbles from ice cores that date back thousands of years, and measure the amount of CO₂ in those bubbles.

April 23, 2024

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.

“We've been able to reconstruct carbon dioxide levels in the atmosphere going back not just hundreds of years, but hundreds of thousands of years, to understand how carbon dioxide levels have changed through time, and how those changes have been related to changes in global temperature,” says McGee.

In the 1950s, scientists began directly measuring the CO₂ in the air. The National Aeronautics and Space Administration also measures CO₂ concentrations from space. Today, scientists can compare estimates from ice bubbles against those direct measurements. “We see very good agreement,” says McGee. “That gives us strong confidence that these reconstructions based upon the ice core air bubbles are accurate.” Scientists can also compare measurements across different ice cores, because, at this point, researchers have gathered so many.

Ice in Antarctica, which is purer and provides more accurate data than Greenland ice cores, can reach three kilometers in depth (almost two miles). Toward the top of this ice record, matching the year or period with the proper ice is simple, because the layers are quite visible, says McGee. But that becomes more complicated the deeper back in time you look, because the buildup of snow has compressed the ice below into extremely thin layers. At the top of an ice core, tens of centimeters may represent just a single year. Kilometers below the surface, a single centimeter may represent decades or even hundreds of years. In those cases, researchers can sometimes extract volcanic ash layers preserved in ice and estimate age based on the elements the ash contains. Scientists also use computer models to simulate how thin ice layers become over time, so they can estimate the years a certain depth may represent.

Today, scientists have analyzed continuous ice layers dating to about 800,000 years old: so long that they can use this data to study not only today’s human-caused climate change, but also earlier, natural cycles of warming and cooling. Now, says McGee, researchers are eager to find even older ice. In 2019, researchers recovered ice that they believe is more than 2 million years old, which could provide previously unknown details about the ancient climate.¹

“That's a very active area of research right now — trying to find the oldest ice that we can, because it's such a unique archive of ancient CO₂,” McGee says.

Thank you to S.V. Doshi of Bengaluru, India, for the question.

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