The best estimate of Earth’s equilibrium climate sensitivity for a doubling of CO2 is 3°C.
Equilibrium climate sensitivity (ECS) describes the eventual global-average surface warming after atmospheric carbon dioxide doubles and the climate system reaches a new balance. It includes fast feedbacks such as water-vapor increases, snow and ice loss, and changes in clouds.
The IPCC’s Sixth Assessment Report gives 3°C as its best estimate, with a likely range of 2.5°C to 4°C and a very likely range of 2°C to 5°C. The estimate combines evidence from climate models, modern observations, paleoclimate records, and studies of climate feedbacks.
A common mix-up is confusing ECS with the near-term warming expected while CO2 is rising. Oceans absorb heat slowly, so the eventual equilibrium response is larger than the warming seen immediately. Climate sensitivity is also not the same as the temperature effect of one year’s emissions: it refers to a sustained concentration change, specifically a doubling of CO2.