As I wrote in my series on climate change and global warming, clouds and changes in clouds in a future, warmer Earth are a bit of a wild card in current global climate models. These models are the critical tools climate researchers and decision makers use to guide predictions and policy in the complicated field of global climate change and global warming.
Now, new research published this week in Science suggests that there may be a positive feedback between warming from increasing greenhouse gases (principally carbon dioxide) and low clouds -- the type of low clouds often seen over oceans. Scientists know that layers of high clouds (cirrus) tend to trap more heat and increase warming. Think of cool dry nights without any high clouds and with some high clouds. Autumn or winter nights that have some high clouds are not as cold as nights with perfectly clear skies. But low clouds (marine stratus or low cumulus) tend to reflect solar energy-heat and tend to have a cooling effect.
By looking at ocean temperatures and observations of clouds from mariners and satellite data, the researchers at the University of Miami and Scripps Institution of Oceanography found that during episodes of warming, the low cloud cover in the North Pacific decreased, and during “cool” episodes, the low cloud cover increased. The researchers concluded that as the ocean warmed during these natural warm-cool cycles, heat was transferred to the lower atmosphere, thereby warming the air and thinning the low clouds, which let more sunlight reach the ocean surface and thus warm the ocean more.
While the study looked at long-term natural Pacific Ocean temperature cycles (called the Pacific decadal oscillation) and the resulting cloud patterns, it has important implications for understanding the future climate in a warming world from increasing concentrations of greenhouse gases. If the study is verified by future research, it means that there a “positive feedback” between greenhouse warming and low clouds. A warmer atmosphere will lead to decreasing amounts of low clouds, allowing more sunlight to reach the surface and leading to further warming -- a positive feedback.
The researchers also looked at a number of the global climate models to see which might best simulate this cloud-climate positive feedback and found that the model developed by the U.K. Met Office Hadley Center showed the most realistic simulation of low cloud positive feedback. This model also is among the most sensitive to increased greenhouse gases and predicts a global warming of 5-9 degrees by 2100 with a “business as usual” approach to current fossil fuel energy use and resultant increase in the concentration of greenhouse gases.
Continued research into the changes in clouds and cloud patterns in a warming world is critical to more accurate and more confident predications of global changes to come in region temperatures, sea levels and precipitation patterns. The new study, “confirms the observations that low clouds are critical to the climate systems response,” said climate researcher Dr. Gerald Meehl, of the National Center for Atmospheric Research.