If you follow climate science news, you know that one of the hotter topics is “climate sensitivity”—the precise amount of warming you get for a given increase of greenhouse gases. A few years ago, a couple papers caused a stir by trying to estimate this sensitivity based on simple equations for the recent past, coming up with a lower warming sensitivity than numerous other studies based on climate models or paleoclimate records. The last IPCC report even widened its estimated range slightly to encompass these studies, which proved controversial.
Researchers have already found reasons to think those low sensitivity estimates were problematic, including the fact that the simplistic, global representations of warming and cooling factors missed some important spatial patterns that change things. A new study from the University of Washington’s Kyle Armour comes at the question from a different angle.
There are several different timeframes we can use to describe climate sensitivity. The one you most commonly hear about is called “equilibrium climate sensitivity." Technically, this is the amount of warming you’d get if you instantaneously doubled atmospheric CO2 and waited a couple centuries or so for the climate to adjust. But there is also something called the “transient climate response," defined as the warming you get at the time a gradually increasing level of CO2 reaches the doubling point. This is a smaller number, because the climate doesn’t have time to fully reach equilibrium.