Defining equilibrium climate sensitivity
Climate sensitivity is the global average surface warming that results from a doubling of atmospheric CO2 from pre-industrial levels (280 ppm to 560 ppm). It is an equilibrium measure, meaning the climate has fully adjusted to the new forcing; in reality, the climate reaches this state only after centuries because oceans take a long time to warm. Current best estimates range from 2 to 5 degrees Celsius, with a most likely value around 3 degrees. The wide range reflects uncertainty in cloud feedback, ocean circulation, and other amplifying and dampening mechanisms.
Feedbacks that amplify or dampen warming
Positive feedbacks amplify initial warming. As ice melts, the surface becomes darker and absorbs more sunlight (albedo feedback). As the atmosphere warms, it holds more water vapor, a greenhouse gas (water vapor feedback). Negative feedbacks (like increased cloud cover) can dampen warming but are less certain. The net effect of all feedbacks determines the sensitivity. A sensitivity of 3 degrees assumes modest positive feedback overall. Paleoclimate records from ice cores and climate models both suggest 2-4 degrees as the likely range, though estimates remain uncertain.
Implications for future warming
At the current rate of emissions, CO2 could double by 2100. If climate sensitivity is 3 degrees, doubling CO2 implies roughly 3 degrees of warming on top of the 1.1 degrees already seen since pre-industrial times. This would mean total warming of 4+ degrees by 2100. If sensitivity is at the high end (5 degrees), warming could exceed 6 degrees. Even at the low end (2 degrees), total warming would reach 3 degrees. Understanding and narrowing the uncertainty in climate sensitivity is critical for predicting impacts and designing mitigation strategies.