date: Fri Sep 17 08:43:03 1999 from: Mike Hulme subject: Re: HadCM2 to: "Melvin Cannell" Melvin, The simple answer is that the model climate sensitivity in all these experiments is the same, about 2.5degC equilibrium T rise for a CO2 doubling, since the same model was being used. The complex answer is that we do not know! It is becoming clearer - and IPCC TAR will discuss this - that the climate sensitivity is a rather artificial construct, the value of which changes during a model integration and which, ultimately, we can only ever get an approximate answer to. For example, the climate sensitivity of HadCM2 increases as the integration proceeds, apparently because of changes in the heat flux into the oceans. If one examines the climate sensitivity in HadCM2 over several 100 years (rather than the first 100-200 we are interested in) then the value rises from 2.5 to about 3.3degC. The 'true' answer is unknowable because no coupled GCM has ever been run out to equilibrium following a 2xCO2 increase. The time constants in the ocean response do not allow us yet to estimate this. Using atmosphere-only GCMs with slab oceans - typically what has been done in the past to estimate climate sensitivity - is clearly unrealistic. And of course if one followed through the logic, then the 'true' climate sensitivity of the planet (i.e., the eventual T rise of a re-equilibrated climate system following doubling) would require a fully interactive biosphere and human society model component! Humans are part of the climate system given our capacity to alter it. For convenience, I would stick with the simple answer of 2.5degC, but state that this is the model sensitivity over the first 100-200 years of the model integrations. Mike At 16:42 16/09/99 +0100, you wrote: >What was the climate sensitivity in the HadCM2 ensemble and S550 and >S750 experiments? > >Melvin >