Planetary albedo in strongly forced climate, as simulated by the CMIP3 models

In an ensemble of general circulation models, the global mean albedo significantly decreases in response to strong CO 2 forcing. In some of the models, the magnitude of this positive feedback is as large as the CO 2 forcing itself. The models agree well on the surface contribution to the trend, due to retreating snow and ice cover, but display large differences when it comes to the contribution from shortwave radiative effects of clouds. The “cloud contribution” defined as the difference between clear-sky and all-sky albedo anomalies and denoted as Δ CC is correlated with equilibrium climate sensitivity in the models (correlation coefficient 0.76), indicating that in high sensitivity models the clouds to a greater extent act to enhance the negative clear-sky albedo trend, whereas in low sensitivity models the clouds rather counteract this trend. As a consequence, the total albedo trend is more negative in more sensitive models (correlation coefficient 0.73). This illustrates in a new way the importance of cloud response to global warming in determining climate sensitivity in models. The cloud contribution to the albedo trend can primarily be ascribed to changes in total cloud fraction, but changes in cloud albedo may also be of importance.