# Climate Sensitivity Climate sensitivity is the central quantitative question of the field: how much will Earth's average surface temperature rise for a doubling of atmospheric CO2? Equilibrium climate sensitivity (ECS) is the long-term warming once the system fully adjusts to doubled CO2 from the pre-industrial 280 parts per million to 560 ppm. Almost every dispute about how bad climate change will be reduces, at bottom, to a dispute about this number. ## Why the Number Is Above the Bare Physics The radiative forcing from doubled CO2 — about 3.7 W/m², per Radiative Forcing and Earth's Energy Balance — produces only about 1°C of warming on its own. The reason sensitivity is higher is the Climate Feedbacks and Tipping Points: water vapor, ice-albedo, and cloud responses amplify that initial push. The water vapor feedback roughly doubles it; ice-albedo and other feedbacks add more. The largest remaining uncertainty is in cloud feedbacks, which is why the estimate has a range rather than a single value. ## The Range, and Why It Has Narrowed For decades the estimated range was frustratingly wide — the 1979 Charney Report put ECS at 1.5–4.5°C, and that range barely moved for forty years. A major 2020 assessment (Sherwood and colleagues), combining the physical understanding of feedbacks, the historical temperature record, and the Paleoclimatology and Ice Cores evidence, narrowed the *likely* range to about 2.6–3.9°C, with a best estimate near 3°C. The IPCC's Sixth Assessment Report adopted a similar narrowed range. This convergence is one of the quieter but most important advances in the field, and it matters for the politics: the low end of the old range, around 1.5°C, was the basis for "lukewarmer" arguments that warming would be mild and manageable. The narrowing has largely ruled out the comforting low values. ## The Asymmetry of Uncertainty A favorite delay argument holds that uncertainty in sensitivity justifies waiting for better numbers. The structure of the risk runs the other way. The probability distribution of sensitivity has a long upper tail — there is a real, if smaller, chance that sensitivity is well above 4°C, which would be catastrophic — while the lower tail has been largely eliminated. Uncertainty is therefore a reason for more urgency, not less, because the unresolved possibilities are skewed toward worse outcomes, not better ones. This is the same logic that governs insurance against low-probability, high-severity events. ## Connections Climate sensitivity is set by Climate Feedbacks and Tipping Points acting on the forcing from Radiative Forcing and Earth's Energy Balance, and it is the dominant source of spread among [[Climate Models]]. For the field overview, see [[Climate Science]].