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.