Climate models

Climate models underestimate global warming by exaggerating cloud brightening

Researchers from Lawrence Livermore National Laboratory and Yale University have found that climate patterns are aggressively making clouds “brighter” as the planet heats up. This can cause models to underestimate the amount of global warming that will occur due to increased carbon dioxide. The research appears in the April 8 edition of Science.

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

But most model clouds contain too much ice that is likely to become liquid with warming, making their stabilizing cloud phase feedback unrealistic. Using a state-of-the-art climate model, the researchers tweaked the parameters to bring the relative amounts of liquid and ice in the clouds in line with clouds observed in nature. The bias correction led to weaker cloud phase feedback and greater warming in response to carbon dioxide.

“We found that climate sensitivity increased from 4 degrees Celsius in the default model to 5-5.3 degrees Celsius in versions modified to bring liquid and ice amounts closer to observations,” the researcher said. Yale Ivy Tan, lead author of the study. paper.

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

“We found a systematic weakening of the cloud phase feedback and an increase in climate sensitivity when we went from model versions that readily convert liquid to ice below zero to model versions that can maintain the liquid at 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 more icy the clouds are initially, the more liquid is gained as the planet heats up; this stabilizing feedback is stronger in models containing less liquid compared to ice at subzero temperatures.

“Most climate models are a bit too eager to drop below freezing, so they’re likely exaggerating the increase in cloud reflectivity as the atmosphere warms,” ​​the co-author said. LLNL, Mark Zelinka. “This means they may systematically underestimate the magnitude of the warming that will occur in response to carbon dioxide.”

These results add to a growing body of evidence that mid-to-high latitude cloud-stabilizing feedback in climate models is overestimated. Additionally, several recent studies have concluded that other significant cloud feedbacks are also likely to exacerbate the warming rather than mitigating it. These include amplifying feedbacks from increases in cloud top height and decreases in subtropical low cloud cover.

“Evidence is mounting against an overall stabilizing cloud feedback,” Zelinka concluded. “Clouds don’t seem to be doing us any favors when it comes to limiting global warming.”

Source of the story:

Materials provided by DOE/Lawrence Livermore National Laboratory. Note: Content may be edited for style and length.