Over at Climate Audit, Nic Lewis reports on the publication of a very important paper in Journal of Climate.
Bjorn Stevens has created a new estimate of the cooling effects of pollution (“aerosols”) on the climate. Readers will no doubt recall that to the extent that aerosol cooling is small the warming effect of carbon dioxide must also be small so that the two cancel out to match the observed temperature record. Only if aerosol cooling is large can the effect of carbon dioxide be large.
Stevens’ results suggest that the aerosol effect is even lower than the IPCC’s best estimates in AR5, which were themselves much lower than the numbers that were coming out of the climate models. He also suggests that the number is less uncertain than previously thought. This is therefore pretty important stuff.
Stevens chose not to calculate the effect on climate sensitivity but, being a helpful chap, Nic Lewis has done so for us, plugging the new numbers into the equations he recently used to calculate a decidedly low estimate of climate sensitivity and transient climate response based on the AR5 estimates. The effects, particularly on the upper bounds, are startling:
Compared with using the AR5 aerosol forcing estimates, the preferred ECS best estimate using an 1859–1882 base period reduces by 0.2°C to 1.45°C, with the TCR best estimate falling by 0.1°C to 1.21°C. More importantly, the upper 83% ECS bound comes down to 1.8°C and the 95% bound reduces dramatically – from 4.05°C to 2.2°C, below the ECS of all CMIP5 climate models except GISS-E2-R and inmcm4. Similarly, the upper 83% TCR bound falls to 1.45°C and the 95% bound is cut from 2.5°C to 1.65°C. Only a handful of CMIP5 models have TCRs below 1.65°C.
Remember folks, the IPCC’s official upper bound is 4.5°C, but Stevens’ results suggest that ECS can’t be above 1.8°C.