evidence: 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. [...] Nicolas Gruber, "4 Archer, David, et al., "Atmospheric Lifetime of Fossil Fuel Carbon Dioxide." Annual Review of Earth and Planetary Sciences, Volume 37, 2009, doi:10.1146/annurev.earth.031208.100206. See also, Inman, Mason, "Carbon is forever." Nature Climate Change, Volume 1, 2008, doi:10.1038/climate.2008.122.