How do we know how long carbon dioxide remains in the atmosphere?

Scientists use models of the atmosphere to estimate the lifetime of carbon dioxide, informed by measurements of the ocean, plants, and other “carbon sinks” that absorb CO₂ from the air.

January 17, 2023

When scientists give estimates for how long carbon dioxide (CO₂) lasts in the atmosphere, those estimates are often intentionally vague, ranging anywhere from hundreds to thousands of years.¹ The reason for the murky timeline is that CO₂ molecules, once they enter the air, follow different paths and can last for radically different amounts of time.

Unlike some other atmospheric gases, CO₂ mostly does not break down into smaller molecules while in the atmosphere. Methane, for instance—another important greenhouse gas—reacts with oxygen to turn into CO₂ and water within a matter of years, and that process can be observed and measured. But CO₂ molecules typically linger until something absorbs them from the air.² Some are quickly taken up by the ocean, plants, and soil.³ Other atmospheric carbon sticks around for generations.⁴

“The first 10% goes quickly, but it's not very much of it. The second part goes on a scale of centuries to millennia, but that only gets 80% of it,” says Ed Boyle, a professor of ocean geochemistry at MIT. He says the last of the carbon dioxide that enters the atmosphere takes tens of thousands of years to leave. 

Of course, scientists can’t track individual carbon dioxide molecules for thousands of years. But since the 1950s, we have had the tools to precisely measure the concentration of CO₂ in the atmosphere. From there, scientists use a combination of measurement methods and models to determine how much CO₂ is taken up by the Earth’s plants, soil, and water, a process called the “carbon cycle.” Scientists must also calculate how much carbon is released on the same time-scale, taking into account human activities that release carbon, like burning fossil fuels, as well as natural ones, like plant decay and wildfires.

That gives us a strong basis for understanding how quickly the extra CO₂ being emitted today can build up. 

How do we get these real-world measurements of carbon cycling? One answer is atmospheric sampling: measuring the chemical composition of the atmosphere can sometimes tell us when and how CO₂ was emitted, because different activities release different types of carbon molecules (or “isotopes”).

Scientists can also directly measure how much CO₂ is being absorbed by the natural world, as in samples of ocean water. At its surface, the ocean chemically reacts with air in the atmosphere, absorbing CO₂. Ocean circulation draws that water down, allowing surface water to absorb more carbon.⁵ Global sampling of CO₂ at the ocean surface confirms what scientists would expect, says Boyle: as the amount of carbon dioxide in the air has grown, the amount of carbon dissolved in ocean water has increased alongside it, at a constant ratio.⁶ “That is determined by simple laws of equilibrium and physical chemistry,” he says. 

Plants also absorb carbon as they grow, and scientists have various ways of measuring that absorption. CO₂ monitors or infrared analysis can catch CO₂ absorption in action, or analysis of total plant mass can reveal how much carbon has built up in plant tissues. Land can also take up carbon in soil and through “rock weathering,” where chemical reactions between minerals and water store carbon inside new rock. Methods to measure those reactions are still being studied.

All these measurements help us see how quickly CO₂ is leaving the atmosphere today. But we also want to know how long the excess CO₂ will linger. For that, scientists plug the real-world data into climate models, helping us better predict how the Earth system will absorb and release carbon far into the future.

And this brings up an important finding about CO₂’s lifetime: it’s getting longer as humans emit more of it. The capacity of oceans and plants to take up carbon is simply not keeping pace with our emissions. As it stands, says Boyle, human-generated carbon dioxide is expected to continue warming the planet for tens of thousands of years.

Thank you to Jacqueline Brook for the question.

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