by Liz Kalaugher, editor of environmentalresearchweb, March 30, 2009
Aerosols cause a sea change
Temperatures in the tropical North Atlantic Ocean have been rising faster than in other tropical basins, despite projections that it should be warming more slowly than other areas. Now scientists in the US reckon the anomaly may be due to a decrease over recent years in the amount of atmospheric aerosols in the region from the Sahara desert and volcanoes, and a consequent fall in their cooling effect.
"To understand future climate change, or at least future changes in the temperature of the northern tropical Atlantic, we really need to be able to anticipate how dust storms will also be changing," Amato Evan of the University of Wisconsin Madison told environmentalresearchweb. "This is an effect that is not included in a dynamic way in, for example, the model runs of the IPCC report."
Some studies estimate that Atlantic dust cover could decrease by 40-60% if carbon dioxide levels were to double, which would lead to an additional 0.3-0.4 °C of warming in the northern tropical Atlantic.
Evan and colleagues from the University of Wisconsin Madison, NOAA, National Environmnetal Satellite, Data and Information Service (NESDIS), Center for Satellite Applications and Research, National Climatic Data Center, used 26 years' worth of satellite records, which included information about aerosol coverage, to drive a simple physical model of the temperature response of the ocean mixed layer to aerosol loading. The data ranged from 1982 to 2007; the maximum dust activity occurred in 1985 and the minimum in 2005.
"The findings suggest that on long time periods volcanoes and dust from Africa play a dominant role in shaping the variability of tropical Atlantic Ocean temperatures," said Evan. "For example, we find that 70% of the upward trend in temperatures over this period is due to month to month changes in volcanic and dust aerosols, with 25% of the trend attributed to dust and 45% attributed to the eruptions of El Chichon (Mexico 1982) and Mt Pinatubo (Philippines 1991)."
Temperatures in the tropical North Atlantic Ocean have been rising at nearly 0.25 °C per decade since 1980. Studies have proposed global warming, mean northern hemisphere temperature variations, and/or changes in the thermohaline circulation as the explanation for this trend. But Evan and colleagues' findings indicate that aerosol coverage that scatters sunlight and reduces the surface irradiation are the key factor.
The tropical North Atlantic is downwind of West Africa, the world’s largest source of dust, making it the only tropical ocean basin that often experiences heavy and extensive aerosol cover. Temperatures there are rising faster than in other tropical ocean basins despite the fact that projections indicate the tropical North Atlantic should be warming more slowly than the other tropical basins.
"We suggest this apparent disconnect between observations and models may be due to the influence of Atlantic dust cover," write the researchers in their paper in Sciencexpress.
According to Evan, upward trends in summertime northern tropical Atlantic Ocean temperature, in the region thought to be important to hurricane activity, appear to be totally forced by aerosol variability. "So, there may be some important ramifications for attributing causality to year-to-year changes in Atlantic hurricane activity," he said. Dusty years tend to have low hurricane activity, but the drivers for dust variability from year to year are not yet well understood.
Now, Evan says he is trying to improve his treatment of the satellite data. He would also like to combine the satellite observations with more sophisticated climate models to understand more completely how aerosols are shaping climate and climate change.
Link to article: http://environmentalresearchweb.org/cws/article/research/38427