image: This image shows unique cloud patterns over the Pacific Ocean of the coast of Baja California, an area of great interest to Amy Clement and Robert Burgman of the University of Miami and Joel Norris of Scripps Oceanography, as they study the role of low-level clouds in climate change. view more
Credit: NASA
VIRGINIA KEY, Fla.—The role of clouds in climate change has been a major question for decades. As the earth warms under increasing greenhouse gases, it is not known whether clouds will dissipate, letting in more of the sun's heat energy and making the earth warm even faster, or whether cloud cover will increase, blocking the Sun's rays and actually slowing down global warming.
In a study published in the July 24 issue of Science, researchers Amy Clement and Robert Burgman from the University of Miami's Rosenstiel School of Marine and Atmospheric Science and Joel Norris from Scripps Institution of Oceanography at UC San Diego begin to unravel this mystery. Using observational data collected over the last 50 years and complex climate models, the team has established that low-level stratiform clouds appear to dissipate as the ocean warms, indicating that changes in these clouds may enhance the warming of the planet.
Because of inconsistencies in historical observations, trends in cloudiness have been difficult to identify. The team broke through this cloud conundrum by removing errors from cloud records and using multiple data sources for the northeast Pacific Ocean, one of the most well-studied areas of low-level stratiform clouds in the world. The result of their analysis was a surprising degree of agreement between two multi-decade datasets that were not only independent of each other, but that employed fundamentally different measurement methods. One set consisted of collected visual observations from ships over the last 50 years, and the other was based on data collected from weather satellites.
"The agreement we found between the surface-based observations and the satellite data was almost shocking," said Clement, a professor of meteorology and physical oceanography at the University of Miami, and winner of the American Geophysical Union's 2007 Macelwane Award for her groundbreaking work on climate change. "These are subtle changes that take place over decades. It is extremely encouraging that a satellite passing miles above the earth would document the same thing as sailors looking up at a cloudy sky from the deck of a ship."
What was not so encouraging, however, was the fact that most of the state-of-the-art climate models from modeling centers around the world do not reproduce this cloud behavior. Only one, the Hadley Centre model from the U.K. Met Office, was able to reproduce the observations. "We have a long way to go in getting the models right, but the Hadley Centre model results can help point us in the right direction," said co-author Burgman, a research scientist at the University of Miami.
Together, the observations and the Hadley Centre model results provide evidence that low-level stratiform clouds, which currently shield the earth from the sun's radiation, may dissipate in warming climates, allowing the oceans to further heat up, which would then cause more cloud dissipation.
"This is somewhat of a vicious cycle potentially exacerbating global warming," said Clement. "But these findings provide a new way of looking at clouds changes. This can help to improve the simulation of clouds in climate models, which will lead to more accurate projections of future climate changes. "
One key finding in the study is that it is not the warming of the ocean alone that reduces cloudiness -- a weakening of the trade winds also appears to play a critical role. All models predict a warming ocean, but if they don't have the correct relationship between clouds and atmospheric circulation, they won't produce a realistic cloud response.
"I am optimistic that there will be major progress in understanding global cloud changes during the next several years," said Norris. "The representation of clouds in models is improving, and observational records are being reprocessed to remove spurious variability associated with satellite changes and other problems."
Both Clement and Norris, who is a professor of atmospheric and climate science at Scripps, have received National Science Foundation Faculty Early Career Development awards for their work on climate change. The National Oceanic and Atmospheric Administration Climate Program Office also provided support for this research.
About Scripps Institution of Oceanography
Located at the University of California, San Diego, the Scripps Institution of Oceanography is one of the oldest, largest and most important centers for global science research and education in the world. The National Research Council has ranked Scripps first in faculty quality among oceanography programs nationwide. Now in its second century of discovery, the scientific scope of the institution has grown to include biological, physical, chemical, geological, geophysical and atmospheric studies of the earth as a system. Hundreds of research programs covering a wide range of scientific areas are under way today in 65 countries. The institution has a staff of about 1,300, and annual expenditures of approximately $155 million from federal, state and private sources. Scripps operates one of the largest U.S. academic fleets with four oceanographic research ships and one research platform for worldwide exploration.
About the University of Miami's Rosenstiel School
The University of Miami is the largest private research institution in the southeastern United States. The University's mission is to provide quality education, attract and retain outstanding students, support the faculty and their research, and build an endowment for University initiatives. Founded in the 1940's, the Rosenstiel School of Marine & Atmospheric Science has grown into one of the world's premier marine and atmospheric research institutions. Offering dynamic interdisciplinary academics, the Rosenstiel School is dedicated to helping communities to better understand the planet, participating in the establishment of environmental policies, and aiding in the improvement of society and quality of life. For more information, please visit www.rsmas.miami.edu
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