Clouds are Moving Higher, Subtropical Dry Zones Expanding

Study confirms computerized climate simulations projecting effects of global warming

Satellite image of world cloud cover

Satellite image of cloud cover.Photo Credit: NASA Goddard Space Flight Center Image by Reto Stöckli

RIVERSIDE, Calif. (www.ucr.edu) — A University of California, Riverside assistant professor and a team of researchers have found that the cloudy storm tracks on Earth are moving toward the poles and subtropical dry zones are expanding. Cloud tops are also moving higher in the atmosphere.

The findings, based on satellite cloud records, confirm computer climate models that have predicted these changes to have taken place during the past several decades as a consequence of the accumulation of societally generated greenhouse gases in the atmosphere.

“This work increases our confidence in climate model projections,” said Robert Allen, a UC Riverside assistant professor of earth sciences. “This work is particularly important, because clouds have remained one of the biggest uncertainties in climate model simulations, and in future climate projections.”

The findings were outlined in a paper published July 11 in Nature. Researchers from Scripps Institution of Oceanography at the University of California San Diego, Lawrence Livermore National Laboratory and Colorado State University are co-authors.

With the simultaneous roles clouds play in cooling and heating the planet – reflecting solar radiation back to space but also trapping solar energy in their structures – clouds are among the most important variables in climate. Their complex behavior has been one of the biggest areas of uncertainty for scientists attempting to understand current climate and forecast future trends.

Inconsistent satellite imaging of clouds over the decades has been a hindrance to improving scientists’ understanding. 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. These cloud changes enhance absorption of solar radiation by the earth and reduce emission of thermal radiation to space. This exacerbates global warming caused by increasing greenhouse gas concentrations.

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.

The Nature paper is called “Evidence for Climate Change in the Satellite Cloud Record.” Co-authors are Joel Norris and Amato T. Evan (Scripps), Mark Zelinka and Stephen Klein (Lawrence Livermore), Christopher O’Dell (Colorado State).

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