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Climate models and clouds

Aerial photo of cumulus clouds

Aerial photo of cumulus clouds

One of the challenges for global climate modelers is accurately simulating cloud cover and its changes over time. This is vital for projecting future temperatures, rainfall, and other aspects of global and regional climate change because clouds of varying types play a vital role both in trapping heat in the atmosphere (the greenhouse effect) and reflecting the Sun’s rays back into space.

NCAR scientists Kevin Trenberth and John Fasullo recently analyzed cloud and energy simulations in the NCAR-based Community Climate System Model (CCSM) and other global climate models used in the 2007 assessment of the Intergovernmental Panel on Climate Change. They evaluated the model simulations by studying observations of cloud behavior and the influence of clouds on solar radiation. Then they explored how the models handle cloud changes throughout this coming century.

Their research showed that the model simulations of climate later this century generally show a reduction in cloud cover, although it is unclear whether this will actually occur. By reducing cloud cover, the models warm the planet by allowing more solar radiation to reach Earth’s surface. At the same time, the decreased cloud cover allows more heat in the atmosphere to escape into space, which has a cooling effect.

Kevin and John point out that the conventional view of global warming being driven by greenhouse gases trapping more heat in the atmosphere is significantly changed by this added role of clouds, and that the basis for these changes is not well established.

Kevin explains that the results don’t necessarily mean that global temperature increases will be much different this century than indicated by models, even if the models are wrong. Instead, climate change could happen in very different ways than shown by models, especially on a regional level where a difference in the amount of sunshine can substantially affect the climate.

Kevin and John recently published their results in Geophysical Research Letters.