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 0.38–0.48C with this ocean in simulations driven by an ‘‘alternative scenario’’ climate forcing (1.1 W m22 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 m22 in 1951 and is now out of balance by about 0.75 W m22. 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.