Humans are responsible for the global changes in rainfall observed over the past few decades, most notably decreases in rainfall in sub-Saharan Africa and southern Asia, according to scientists.
The research was published last week in the journal Nature (26 July).
These changes in precipitation will be much more difficult to deal with than any future changes in temperature, says Francis Zwiers, director of the Climate Research Division from Environment Canada, and an author of the research.
Researchers from Environment Canada split the globe into latitudinal (horizontal) bands and compared the changes in precipitation in these areas with changes predicted by various climate models over the past 75 years.
They found that the observed rainfall patterns couldn't be accounted for by natural variation in precipitation — those caused by natural changes in climate, or volcanic eruptions and changes in solar radiation, which can also cause changes in rainfall.
The model that did match the observed changes was the one that included response to human activities, such as increases in greenhouse gas emissions and sulphate and carbon aerosols — released as a waste-product of burning coal.
"The thing that provides the best explanation, and explains most of the changes, is the human influence on the climate system," Zwiers told SciDev.Net.
Many developing countries have experienced rainfall changes. Between the equator and 30 degrees north — including the northern part of sub-Saharan Africa and Latin America, South Asia, South-East Asia, and southern China — there has been an average 6–7 per cent decrease in rainfall over the last 75 years.
Countries in the band between the equator and 30 degrees south — including the central part of sub-Saharan Africa and Latin America — have experienced an increase in rainfall because a band of heavy precipitation, called the intertropical convergence zone, has shifted further south.
These changes in rainfall have occurred because the warming climate causes the water cycle to run more quickly, says Zwiers. "If you think of the planet as an engine that moves water vapour around from one place to another, it does so a bit more vigorously in a warmer world."
Zwiers says that cutting emissions is essential, because changes in rainfall will impact many people negatively, especially in developing countries just north of the equator.
"If you live in a society that has, over the centuries, developed a means of sustaining itself that is very dependent on local water resources, where agriculture is just marginal, then losing some of that water in the future will be very difficult to deal with," he adds.
Bwango Apuuli, deputy director of the Intergovernmental Authority on Development's Climate Prediction and Applications Centre in Nairobi, Kenya says more extreme climate events in Africa, like drought, will "threaten livelihoods and increase vulnerabilities of the communities who are mostly poor". A changing climate will threaten food security, and put more people at risk of malnutrition and disease, he says.
Apuuli says that there are currently several projects helping communities adapt to extreme climate events by disseminating information on drought and rain harvesting, irrigation, drought resistant and quick maturing crops, and replanting forests.
Reference: Nature 448, 461 (2007)