Climate simulations for 1951-2050 with a coupled atmosphere-ocean model

The authors simulate climate change for 1951 - 2050 using the GISS SI2000 atmospheric model coupled to HYCOM, a quasi-isopycnal ocean model ("ocean E''), and contrast the results with those obtained using the same atmosphere coupled to a passive Q-flux ocean model ("ocean B'') and the same atmosphere driven by observed SST ("ocean A''). All of the models give reasonable agreement with observed global temperature change during 1951 - 2000, but the quasi-isopycnal ocean E mixes heat more deeply and hence sequesters heat more effectively on the century timescale. Global surface warming in the next 50 yr is only 0.3degrees - 0.4degreesC with this ocean in simulations driven by an "alternative scenario'' climate forcing (1.1 W m(-2) in the next 50 yr), only half as much as with ocean B. From the different models the authors estimate that the earth was out of radiation balance by about 0.18 W m(-2) in 1951 and is now out of balance by about 0.75 W m(-2). This energy imbalance, or residual climate forcing, a consequence of deep ocean mixing of heat anomalies and the history of climate forcings, is a crucial measure of the state of the climate system that should be precisely monitored with full-ocean temperature measurements.