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Empirical Estimates of Global Climate Sensitivity: An Assessment of Strategies Using a Coupled GCM

【摘要】:正An analysis of climate sensitivity in the NCAR coupled atmosphere-ocean Climate System Model was undertaken. A control integration and an integration with the solar constant increased by 2.5% were conducted, and the change in global-mean surface temperature computed. Then a series of calculations were undertaken to see how well the actual realized global warming could be predicted just by analysis of the control results. This is a test, within a model context, of proposals that have been advanced to use a knowledge of the present day climate to make "empirical" estimates of global climate sensitivity. The scaling of the top of-the-atmosphere IR flux and the planetary albedo as functions of surface temperature were inferred by examining different temporal and geographical variations of the control simulations. Four separate approaches exploited (i) the variations of the annual-mean over latitude bands, (ii) the seasonal cycle of zonal-mean values at a single latitude, (iii) interannual variations of the global-mean, and (iv) seasonal variations of the global-mean. Each of these approaches greatly overestimates the climate sensitivity of the model, largely because of the behavior of the cloud albedo. In each approach the control results suggest that cloudiness and albedo decrease with increasing surface temperature. However, the experiment with the increased solar constant actually has higher albedo and more cloudiness at most latitudes. The increased albedo is a strong negative feedback, and this helps account for the rather weak sensitivity of the climate in the NCAR model. To the extent that these model results apply to the real world, they suggest that empirical evaluation of the scaling of global-mean radiative properties with surface temperature in present day climate provides little useful guidance for estimates of the actual climate sensitivity to global changes.

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