According to CSIRO's Dr Leon Rotstayn, tiny atmospheric particles, known as sulfate aerosol, have contributed to a global climate shift.
The particles, says Dr Rotstayn, which are concentrated mainly in the northern hemisphere, make cloud droplets smaller. This makes the clouds brighter and longer lasting, so they reflect more sunlight into space, cooling the Earth's surface below. As a result, the tropical rain belt, which migrates northwards and southwards with the seasonal movement of the sun, is weakened in the northern hemisphere and does not move as far north.
The main impact of the weaker rain belt is in the Sahel. Since the 1960s, this region of northern Africa, which borders the fringe of the Sahara, has experienced a devastating drought. Rainfall was twenty to forty-nine per cent lower than in the first half of the 20th century, causing widespread famine and death.
Scientists also believe that air pollution over China has affected their summer monsoon rainfall belt. Northern China had successive droughts in the summers of 1997, 1998 and 1999.
Dr Rotstayn has investigated the impact of sulfate aerosol by running sophisticated global climate simulations on a supercomputer. Simulations without aerosol pollution show no drought in the Sahel; the inclusion of aerosol leads to simulated rainfall declines similar to those suffered over northern Africa and elsewhere.
Tropical and eastern Australia have experienced an increase in rainfall over the 20th century, and this may be related to the same effect.
"The tendency of tropical rainfall to move southwards could be affecting Australia, but we need further studies with more detailed models before we can answer this question," says Dr Rotstayn.
Further support for Dr Rotstayn's findings comes from a reduction in the severity of the Sahelian drought during the 1990s. Emission controls in Europe and North America lowered atmospheric aerosol concentrations during that decade.
"The Sahelian drought may be due to a combination of natural variability and atmospheric aerosol," says Dr Rotstayn. "Cleaner air in future will mean greater rainfall in this region.
"However, we are not yet seeing reductions in aerosol emissions in Asia,' says Dr Rotstayn.
'It is possible that other forms of aerosol in the air, such as black soot emitted from South-East Asia, could affect Australia's climate, he says.
The majority of sulfate aerosol comes from the burning of fossil fuels and metal smelting. Smaller amounts come from the burning of vegetation in the tropics, and natural sources such as marine plankton. Atmospheric aerosol concentrations are far greater in the northern hemisphere, cooling the atmosphere there more than in the southern hemisphere. It is this imbalance that affects the tropical rain belt.
"Global climate change is not solely being caused by rising levels of greenhouse gases. Atmospheric pollution is also having an effect," says Dr Rotstayn.
CSIRO's research into aerosol and climate is in part supported by the Australian Greenhouse Office and involves collaboration with the University of Michigan in the USA and Dalhousie University in Canada. The findings on air pollution and the tropical rain belt have just been published in the international Journal of Climate.