Dust muddies climate change prediction
A new study has highlighted flaws in experts' assumptions about a key influence on global climate change - airborne dust.
The study of dust carried thousands of kilometres from arid regions of Africa to the Caribbean reveals new complexities in the tricky task of climate change prediction, say the scientists, whose work is published in this week's Science.
Mineral dust from soils is known to influence climate by scattering and absorbing solar radiation, and modifying cloud properties, say the authors, Dr Joseph Prospero from the University of Miami and Dr Peter Lamb, from the University of Oklahoma.
The Intergovernmental Panel on Climate Change (IPCC) has previously assumed that all variability in atmospheric dust emissions result from human land-use impacts - such as over-grazing - which, they estimate contribute as much as 20 to 50 per cent of total present-day dust emissions. The IPCC also assumes that natural dust sources have been effectively unchanged over the past several hundred years.
However, the new work in North Africa - believed to be the largest single source of present-day dust in the world - highlights flaws in these assumptions.
"Although human impacts are clearly evident in many regions, they do not appear to play a major role in large-scale dust mobilisation in North Africa," Drs Prospero and Lamb said in their paper. "Indeed the strongest, largest and most persistent dust sources in North Africa are in regions that are essentially uninhabited."
The pair measured the amount of soil carried by trade winds from North Africa to the Caribbean island of Barbados from 1965 to 1998. They found that dust emissions have been unusually intense in response to persistent drought in a region known as the Soudano-Sahel.
Estimates based on long-term rainfall data suggest dust concentrations were sharply lower during much of the 20th century before drought began in the Soudano-Sahel in the 1970s.
While factors other than lack of rainfall may be involved, the research nonetheless demonstrates the sensitivity of dust mobilisation to changes in regional climate, the researchers say.
"It highlights the need to understand how dust, in turn, might affect climate processes on larger scales," they say, adding that it could conceivably lead to more intense or more prolonged droughts.
A spokeswoman for the Australian Greenhouse Office, Ms Jane Harriss, said the IPCC acknowledged its estimates relating to airborne particles were highly uncertain.
"The monitoring and reporting of them is very difficult at the moment. There is an evolving research agenda that acknowledges a lot of work needs to be done in this area."
The new work was an important extra piece in the puzzle, she said.