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.

"We found that the climate sensitivity increased from 4 degrees C in the default model to 5-5.3 degrees C in versions that were modified to bring liquid and ice amounts into closer agreement with observations," said Yale researcher Ivy Tan, lead author of the paper.

Climate sensitivity refers to the change in global mean surface temperature due to a doubling of carbon dioxide. Climate models predict between 2.1 and 4.7 degrees C (3.75 to 8.5 degrees F) of warming in response to a doubling of carbon dioxide.

"We saw a systematic weakening of the cloud phase feedback and increase in climate sensitivity as we transitioned from model versions that readily convert liquid to ice below freezing to model versions that can maintain liquid down to colder temperatures, as observed in nature," Tan explained.

In nature, clouds containing both ice crystals and liquid droplets are common at temperatures well below freezing. As the atmosphere warms due to carbon dioxide emissions, the relative amount of liquid in these so-called mixed phase clouds will increase. Since liquid clouds tend to reflect more sunlight back to space than ice clouds, this phase feedback acts to reduce global warming. The icier the clouds to begin with, the more liquid is gained as the planet warms; this stabilizing feedback is stronger in models containing less liquid relative to ice at sub-freezing temperatures.

"Most climate models are a little too eager to glaciate below freezing, so they are likely exaggerating the increase in cloud reflectivity as the atmosphere warms," said LLNL coauthor Mark Zelinka. "This means they may be systematically underestimating how much warming will occur in response to carbon dioxide."

These results add to a growing body of evidence that the stabilizing cloud feedback at mid- to high latitudes in climate models is overstated. Moreover, several recent studies have concluded that other important cloud feedbacks also are likely to exacerbate warming rather than dampen it. These include amplifying feedbacks from increases in cloud top altitude and from decreases in the coverage of subtropical low clouds.

"The evidence is piling up against an overall stabilizing cloud feedback," concluded Zelinka. "Clouds do not seem to want to do us any favors when it comes to limiting global warming."