Climate models underestimate global warming by exaggerating cloud brightening

http://www.eurekalert.org/pub_releases/2016-04/dlnl-cmu040716.php

Public Release: 7-Apr-2016
Climate models underestimate global warming by exaggerating cloud brightening
DOE/Lawrence Livermore National Laboratory

Researchers at Lawrence Livermore National Laboratory and Yale University have found that climate models are aggressively making clouds "brighter" as the planet warms. This may be causing models to underestimate how much global warming will occur due to increasing carbon dioxide. The research appears in the April 8 edition of Science.

As the atmosphere warms, clouds become increasingly composed of liquid rather than ice, making them brighter. Because liquid clouds reflect more sunlight back to space than ice clouds, this "cloud phase feedback" acts as a brake on global warming in climate models.

But most models' clouds contain too much ice that is susceptible to becoming liquid with warming, which makes their stabilizing cloud phase feedback unrealistically strong. Using a state-of-the-art climate model, the researchers modified parameters to bring the relative amounts of liquid and ice in clouds into agreement with clouds observed in nature. Correcting the bias led to a weaker cloud phase feedback and greater warming in response to carbon dioxide.

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