about Gavin Schmidt

Climate sensitivity is a measure of the equilibrium global surface air temperature change for a particular forcing. It is usually given as a °C change per W/m² forcing. A standard experiment to determine this value in a climate model is to look at the doubled CO₂ climate, and so equivalently, the climate sensitivity is sometimes given as the warming for doubled CO₂ (i.e. from 280 ppm to 560 ppm). The forcing from doubled CO₂ is around 4 W/m² and so a sensitivity of 3°C for a doubling, is equivalent to a sensitivity of 0.75 °C/W/m². The principal idea is that if you know the sum of the forcings, you can estimate what the eventual temperature change will be.

We should underscore that the concepts of radiative forcing and climate sensitivity are simply an empirical shorthand that climatologists find useful for estimating how different changes to the planet’s radiative balance will lead to eventual temperature changes. There are however some subtleties which rarely get mentioned. Firstly, there are a number of ways to define the forcings. The easiest is the ‘instantaneous forcing’ – the change is made and the difference in the net radiation at the tropopause is estimated. But it turns out that other definitions such as the ‘adjusted forcing’ actually give a better estimate of the eventual temperature change. These other forcings progressively allow more ‘fast’ feedbacks to operate (stratospheric temperatures are allowed to adjust for instance), but the calculations get progressively more involved.

Secondly, not all forcings are equal. Because of differences in vertical or horizontal distribution of forcings, some changes can have a more than proportional effect on temperatures. This can be described using a relative ‘efficacy’ factor that depends on the individual forcing. For instance, the effect of soot making snow and sea ice darker has a higher efficacy than an equivalent change in CO₂ with the same forcing, mainly because there is a more important ice-albedo feedback in the soot case. The ideal metric of course would be a forcing that can be calculated easily and where every perturbation to the radiative balance had an relative efficacy of 1. Unfortunately, that metric has not yet been found!

Isotopes can be thought of as different ‘flavours’ of a particular element (such as oxygen or carbon), that are distinguished by the number of neutrons in their nucleus (and hence their atomic mass). Carbon for instance most commonly has a mass of 12 (written as ¹²C), but there are also a small fraction of carbon atoms with mass 13 and 14 (¹³C and ¹⁴C), similarly oxygen is normally ¹⁶O, but with small amounts of ¹⁷O and ¹⁸O. All of the isotopes of an element behave in similar way chemically. However, because the mass of each isotope is slightly different there are certain physical processes that will discriminate (or ‘fractionate’) between them. For instance, during evaporation of water, it is slightly easier for the lighter isotopes to escape from the liquid, and so water vapour generally has less ¹⁸O than the liquid water from which it came. Because of these physical effects, looking at the ratio of one isotope to another can often be very useful in tracing where these atoms came from.