Europe and US may have caused Africa’s droughts

Can air pollution affect rainfall? The latest study, to be published in next month’s Journal of Climate, suggests Africa’s droughts may have been caused in part by sulphur dioxide pollution from North America and Europe.

Using a computer model to simulate global rainfall patterns, Dr Leon Rotstayn and colleagues from the Commonwealth Scientific and Industrial Research Organization in Australia showed that sulphate aerosol clouds could substantially reduce North African rainfall.

The aerosols, formed from sulphur dioxide particles released from industrial sources such as smokestacks in North America, could act as condensation nuclei, creating clouds that would reflect the Sun’s radiation back into space. Changing the Earth’s albedo (reflectivity) would lead to the cooling of the North Atlantic surface waters which would reduce the evaporation rate and alter the atmospheric water cycle.

“It’s absolutely true that pollution can affect rainfall, but it’s harder to say how and by how much. Air pollution ‘redistributes’ the water in the global water cycle, but it’s difficult to predict localised effects,” says Professor Peter Brimblecombe of the University of East Anglia. “The largest effect is the increased reflection of the Sun’s energy by sulphate aerosols leading to a slight decrease in the planet’s surface temperature, which is a good thing. But with sulphur dioxide emissions expected to decline after 2020, we should be seeing less of these effects. There is already some evidence that when pollution decreases rainfall returns.”

Other studies have also provided evidence for the influence of pollution on weather, in some cases predicting an increase in rainfall in contrast to the CSIRO study. Using rainfall-measuring satellites, NASA scientists found that “urban heat islands” create more summer rain over and downwind of major cities like Atlanta and Dallas.

Dr Marshall Shepherd and colleagues at NASA’s Goddard Space Flight Centre showed that urban areas with high concentrations of buildings and roads retain heat and lead to warmer surrounding temperatures, creating urban heat islands. The study, published in this month’s edition of the Journal of Applied Meteorology, found that heat pollution increased rainfall in downwind areas by up to 51%.

“Cities tend to be one to five degrees warmer than surrounding suburbs and rural areas and the added heat can destabilize and change the way air circulates around cities,” said Shepherd. Rising warm air may help produce clouds that result in more rainfall around urban areas. “A recent United Nations study estimates that 80% of the world’s population will live in cities by 2025, so a better understanding of the impact of urban land use change on Earth’s water cycle system is vital,” Shepherd added.

At the Arizona State University last year, Dr Robert Balling and Professor Randy Cerveny discovered a link between air pollution and weather patterns on the Atlantic coast of North America, where car exhaust and factory smoke produced during the week was believed to trigger a pattern of rainy weekends.

Combining data on carbon monoxide and ozone concentrations at a Canadian monitoring station with rainfall data for the Atlantic Ocean, the scientists found that pollution hit its highest levels at the end of the week, while the East coast of the US was typically rainy at weekends and clear in the week. “The dirt and dust, the solid parts of the pollution, tend to absorb heat. That makes the air around those parts warmer. Warm air rises. As warm air rises, it tends to cause clouds and precipitation,” said Cerveny.

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