Historical cloud coverage has moved significantly around the world in the last 30 years due to global warming - but volcanic eruptions can reverse the trend, according to a new study by researchers at Scripps Institution of Oceanography at University of California San Diego.
Researchers analyzed satellite cloud records and found that the cloudy storm tracks on Earth are moving toward the poles while subtropical dry zones are expanding. Cloud tops are also moving higher in the atmosphere.
Inconsistent satellite imaging of clouds over the decades has been a hindrance to improving scientists' understanding of historical cloud coverage around the globe. Records of cloudiness from satellites originally designed to monitor weather are prone to spurious trends related to changes in satellite orbit, instrument calibration, degradation of sensors over time, and other factors.
When the researchers removed such artifacts from the record, the data exhibited large-scale patterns of cloud change between the 1980s and 2000s that are consistent with climate model predictions for that time period, including poleward retreat of mid-latitude storm tracks, expansion of subtropical dry zones, and increasing height of the highest cloud tops.
|Change in observed and simulated cloud amount and albedo between the 1980's and 2000's. Credit: Joel Norris|
The researchers drew from several independent corrected satellite records in their analysis. They concluded that the behavior of clouds they observed is consistent with a human-caused increase in greenhouse gas concentrations and a planet-wide recovery from two major volcanic eruptions, the 1982 El Chichón eruption in Mexico and the 1991 eruption of Mt. Pinatubo in the Philippines. Aerosols ejected from those eruptions had a net cooling effect on the planet for several years after they took place.
Barring another volcanic event of this sort, the scientists expect the cloud trends to continue in the future as the planet continues to warm due to increasing greenhouse gas concentrations.
"What this paper brings to the table is the first credible demonstration that the cloud changes we expect from climate models and theory are currently happening," said study lead author Joel Norris, a climate researcher at Scripps.
The study, "Evidence for Climate Change in the Satellite Cloud Record," appeared July 11 in the journal Nature.
Global cloud photo credit: NASA Goddard Space Flight Center