The impact of cloud feedbacks on Arctic climate change forced by increased CO2

Arctic clouds exert a strong influence on the surface radiation budget, significantly reducing wintertime cooling and summertime heating. Therefore, changes in clouds due to enhanced greenhouse forcing may greatly affect future Arctic climate. To test this hypothesis, I have run a global climate model (GENESIS2) with and without cloud feedbacks under modern radiative forcing and 2 x CO₂. This model was chosen in part because of its relatively successful representation of modern Arctic cloud cover, a trait attributable to the parameterized treatment of mixed-phase microphysics. When forced with increased CO₂, the model generally simulates vertically integrated cloud increases (decreases) in high (low) latitudes. In the corresponding simulation without cloud feedbacks, cloud fraction is fixed at its modern control value at all grid points and all levels while CO₂ is doubled. A comparison of the fixed-cloud and standard experiments reveals that simulated cloud changes enhance greenhouse warming at all latitudes. This positive feedback is most pronounced in the Arctic, where approximately 40% of the warming is due to cloud changes. This strong cloud feedback in the Arctic is caused not only by local processes but also by cloud changes in lower latitudes, where the cloud radiative forcing anomalies are comparable to those in northern polar regions. This result demonstrates the importance of remote impacts from low- and middle-latitudes on Arctic climate change.