DROUGHT, famine and war have made life hard enough for the people of northern Africa. Yet things are likely to get worse.
For the first time we have proof that greenhouse gas emissions have already begun to alter how much rain falls around the world, and the effect will become more extreme over the coming decades. Tropical regions north of the equator, including Africa’s Sahel, have already begun to get even drier. Far-north regions, including Canada, northern Europe and Russia, will get wetter, as will the southern tropics.
Detecting the effects of climate change on rainfall patterns has been difficult, as precipitation levels naturally vary much more than temperature, for example. To do so, researchers took results from 92 simulations that rely on 14 different global circulation models, some which included the forcing affects of human greenhouse gas emissions, and some which did not, to arrive at the average predicted impact of climate change on rainfall during the 20th century across wide bands of latitude around the world. They compared these with the observed trends in rainfall. The two were similar ().
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“Over the 20th century, we now detect the signal [in rainfall changes] that is predicted by climate models,” says Francis Zwiers, one of the lead authors of the new study. “If you’re able to reproduce the past, you also have greater confidence for predictions of the future.”
Zwiers, of Environment Canada based in Toronto, says the pattern shows a substantial drying of the region from the equator up to 30° north. This band encompasses north Africa, as well as India, south-east Asia, Mexico and northern South America. It already contains some of the world’s driest areas, such as Africa’s Sahara desert. Regions further to the south, including the rainforests of central Africa and South America, have begun to get increased rainfall and will get wetter.
The findings are important, Zwiers says, because changes in precipitation will have a much bigger impact on people than changes in temperature: “Our activities are much more constrained by the limits of water than temperature. In places where agriculture is marginal, it will become more so in the future.”
“Changes in precipitation will have a much bigger impact on people than changes in temperature”
Richard Seager, a climatologist at the Lamont-Doherty Earth Observatory in New York, says this detection of 20th-century rainfall changes seems “barely discernible from the noise right now”. He agrees, though, that the projected trends for the coming century shown by the combined climate models are highly convincing. Seager’s own research has shown that, in addition to the trends shown by Zwiers’s team, there will also be a of areas in the northern subtropics, including the US south-west and the Mediterranean.
Aside from the overall trends, Zwiers says an important message from the combined models is that there will be a significant increase in both floods and droughts in all regions. Even desert areas that will undergo serious drying could also, seemingly paradoxically, be at a greater risk of flash floods.
