Revised projection of future greenhouse warming

FOR the Intergovernmental Panel on Climate Change (IPCC) report¹, using a simple climate/ocean model, we made projections of the greenhouse warming to 2100. Projections were made for four greenhouse-gas scenarios, whose radiative effects in 2100, expressed in terms of an equivalent amount of CO₂, ranged from 2 to 5.5 times the pre-industrial CO₂ concentration. The projected global warming in 2100 for these scenarios, relative to 1990, ranged from 0.62-2.31 °C for the minimum assumed CO₂-doubling temperature sensitivity, ΔT_2x = 1.5 °C, to 1.61-5.15 °C for the maximum sensitivity ΔT_2x = 4.5 °C. Here we broaden these projections to include a recently suggested lower sensitivity, ΔT_2x = 0.5 °C. We also revise all projections by prescribing, using the results of our analysis of simulations by a coupled atmosphere-ocean general circulation model, a lower value for a key parameter of the simple ocean model, II, which indicates the warming of the polar ocean relative to the warming of the non-polar ocean. We find that, for any value of ΔT_2x, the atmospheric temperature increases more rapidly with time as a consequence of the reduction in II. We also find that a delay of ten years in initiating a 20-year transition from the IPCC 'business-as-usuaP scenario to any other IPCC scenario has only a small effect on the projected warming in 2100, regardless of the value of ΔT_2x. This indicates that the penalty for a 10-year delay is small.