Title: A best estimate of Earths equilibrium climate sensitivity
Abstract: The foundation of the climate sciences is the idea that the average surface temperature of the Earth responds to radiative forcing in order to restore the planetary radiation balance. It is the basis for understanding Earth's past and projecting climate into the future. It is common to measure the equilibrium climate sensitivity (ECS) as the longterm global surface warming to a doubling of atmospheric CO$_2$. The 1979 Charney report hypothesized a best estimate of 3 K, the first and second IPCC reports suggested it is 2.5 K, and the fourth returned to 3 K. The fifth report refrained from a best estimate and assesses an uncertainty range of 1-6 K [5-90th percentile]. In this talk I present a concise argument for an ECS of 2.7 K – along with a set of conditions under which the best estimate is – true whereby I emphasize a set of recent model-observational constraints on ECS based on global change over other lines of evidence. The argument as such is not unique and the basic method could be applied to other best estimates; an activity which I believe could guide the debate over the best estimate of Earths equilibrium climate sensitivity forward.