The computer model studies show less wind, more moisture, and enhanced vegetation in desert areas as carbon dioxide increases over the next century, keeping more of the world's dust on the ground. Coauthor Natalie Mahowald of the National Center for Atmospheric Research presented the results this week at the American Geophysical Union's annual meeting in San Francisco.

"Reductions in global dust levels could have a profound impact on future climate predictions," says Mahowald. Dust helps to lower global temperature by reflecting sunlight, as well as by depositing iron in the ocean and thus fertilizing marine organisms that remove carbon dioxide from the atmosphere.

Mahowald and Chao Luo (University of California, Santa Barbara) combined NCAR's global Climate System Model with other software specifically tailored to simulate dust under a variety of climate regimes. The climate changes are driven primarily by an increase in atmospheric carbon dioxide from 280 parts per million in 1890 (preindustrial) to 500 ppm by 2090--a scenario considered reasonable by the Intergovernmental Panel on Climate Change.

The NCAR simulation shows decreasing winds and increasing moisture across arid, low-lying regions such as the Sahara, which produce much of the world's dust. It also includes the process through which a gradual increase in atmospheric carbon dioxide may stimulate photosynthesis of plants in arid regions, which in turn would reduce the extent of unvegetated areas and the dust they produce.

Mahowald and Luo examined six different scenarios for the interaction of plants and climate across each of three decades: 1880�1889 (preindustrial), 1990-1999, and 2090�2099. For the six scenarios, the decrease in extent of desert dust sources in 2090�2099 compared to 1990�1999 ranges from 0 to 39 percent. The decrease in how much dust gets entrained into the atmosphere is even more dramatic: from 20 to 63 percent, depending on the scenario.

The wide variation among scenarios highlights the uncertainty in this new area of research, says Mahowald. She believes that climate assessments such as those from the Intergovernmental Panel on Climate Change may be underestimating both the magnitude and the uncertainty of dust's global impact on climate.

"There is substantial spread in the model projections for climate close to large arid regions such as the Sahel," says Mahowald. "It is very difficult to predict whether particular regions will get wetter or drier."

University Corporation for Atmospheric Research
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