Uncertainties in future climate arise from the uncertainties in the future emissions of greenhouse gases and from the uncertainties in characteristics of the climate system which define its response to external forcing. These characteristics include climate sensitivity, strength of aerosol forcing and the rate of ocean heat uptake. Evaluation of uncertainties in future climate projections requires large ensembles of simulations. At the present time Earth system models of intermediate complexity seem to be the best tool for conducting studies of this kind, due to their computational efficiency and ability to vary the above mentioned characteristics over wide ranges.

Here we evaluate the uncertainty in climate response to prescribed changes in greenhouse gas concentrations using the MIT Integrated Global System Model (IGSM).We carried out three 250 member ensembles for SRES scenarios B1, A1B and A2. Probability distributions for the climate sensitivity, strength of aerosol forcing and the rate of ocean heat uptake were obtained by comparing 20th century climate as simulated by the IGSM with available observations.