Den stora tråden om global uppvärmning/växthuseffekten

Low‐level clouds have a net cooling effect. In a warming scenario, the reported decrease in low level clouds suggests a positive feedback and may contribute to enhanced global warming.

Abstract

The Greenland Ice Sheet (GrIS) has been losing mass at an accelerating rate since the mid-1990s. This has been due to both increased ice discharge into the ocean and melting at the surface, with the latter being the dominant contribution. This change in state has been attributed to rising temperatures and a decrease in surface albedo. We show, using satellite data and climate model output, that the abrupt reduction in surface mass balance since about 1995 can be attributed largely to a coincident trend of decreasing summer cloud cover enhancing the melt-albedo feedback. Satellite observations show that, from 1995 to 2009, summer cloud cover decreased by 0.9 ± 0.3% per year. Model output indicates that the GrIS summer melt increases by 27 ± 13 gigatons (Gt) per percent reduction in summer cloud cover, principally because of the impact of increased shortwave radiation over the low albedo ablation zone. The observed reduction in cloud cover is strongly correlated with a state shift in the North Atlantic Oscillation promoting anticyclonic conditions in summer and suggests that the enhanced surface mass loss from the GrIS is driven by synoptic-scale changes in Arctic-wide atmospheric circulation.

The melting of Greenland’s glaciers in the summer of 2012 was as extensive as it has been in more than a century, and U.S. scientists now conclude that a very particular type of cloud formation made the difference.

“To understand the region’s future, you’ll need to understand its clouds,” said Matthew Shupe, a research meteorologist involved in the study backed by the National Oceanic and Atmospheric Administration (NOAA).

The research team found that July 2012 cloud conditions over Greenland’s ice sheet caused just enough surface warming and retention of planetary heat to cause an abnormally high level of warming on the world’s largest island. Published April 3 in Nature, the research reveals that thin, low-lying clouds allowed the sun’s energy to warm the surface of the ice, and they were also just thick enough to trap the heat on the ice and create conditions for melting.