How much warmer will Earth get if we double its carbon dioxide?

If we doubled the carbon dioxide in the air and let the climate fully settle, Earth would most likely warm by about 3 °C. The IPCC's 2021 report puts it between 2.5 and 4 °C, with high confidence. This number is called climate sensitivity. The striking part is how long it took to pin down.

In 1979, a committee of the US National Academy of Sciences, chaired by the meteorologist Jule Charney, made the first famous estimate. It had only a handful of climate models to go on. Syukuro Manabe's said 2 °C. James Hansen's said 4 °C. As Manabe later told it, Charney took half a degree off the low one and added half a degree to the high one. The result was a range of 1.5 to 4.5 °C. That range kept coming back in assessment after assessment, as recently as 2013.

Why was it so hard? Because of feedbacks: knock-on effects that boost or dampen the first push from carbon dioxide. Warmer air holds more water vapour, which is itself a greenhouse gas. That alone roughly doubles the warming from carbon dioxide. Melting ice reflects less sunlight, which adds more. Clouds are the big unknown. Low, bright clouds cool the planet. High, thin clouds warm it. And no one is sure how a warmer world will shift the mix.

Around 2020, a new generation of models with better cloud physics gave answers from 1.8 to 5.6 °C. About a fifth of them ran "hot", warming more than the real record allows. Checking the models against observations reined them in. The IPCC settled on 3 °C, the same centre Charney had picked, but with a range half as wide.