The equilibrium sensitivity of the Earth's temperature to radiation changes

The Earth's climate is changing rapidly as a result of anthropogenic carbon emissions, and damaging impacts are expected to increase with warming. To prevent these and limit long-term global surface warming to, for example, 2 degrees C, a level of stabilization or of peak atmospheric CO2 concentrations needs to be set. Climate sensitivity, the global equilibrium surface warming after a doubling of atmospheric CO2 concentration, can help with the translation of atmospheric CO2 levels to warming. Various observations favour a climate sensitivity value of about 3 degrees C, with a likely range of about 2-4.5 degrees C. However, the physics of the response and uncertainties in forcing lead to fundamental difficulties in ruling out higher values. The quest to determine climate sensitivity has now been going on for decades, with disturbingly little progress in narrowing the large uncertainty range. However, in the process, fascinating new insights into the climate system and into policy aspects regarding mitigation have been gained. The well-constrained lower limit of climate sensitivity and the transient rate of warming already provide useful information for policy makers. But the upper limit of climate sensitivity will be more difficult to quantify.