: Zhang et al., Nature 448 (26 July 2007)

Smoking gun?
Rainfall across various latitudes over a 75-year period, with green or gray bands showing wetter weather, yellow suggesting less rainfall, and white indicating insufficient data.

The Power to Influence Showers

A new study has shown for the first time that humans have been altering rainfall patterns across various parts of the globe for the past century. The research could help scientists predict future rainfall patterns within geographic regions and allow nations to prepare better for changing weather.

For more than 2 decades, the world's climate scientists have been building a case that the byproducts of human activity--particularly greenhouse gases--are warming the planet. At the same time, scientists have been investigating whether those greenhouse gases are affecting global precipitation, perhaps causing the severe droughts that have afflicted the U.S. Southwest and Africa's Sahel region for years (ScienceNOW, 10 October 2003). So far, however, computer models have only suggested that rainfall patterns have changed due to the influence of Homo sapiens.

Now, an international group of scientists has provided the first empirical evidence that human activity really is having an effect on precipitation. The researchers cross checked detailed rainfall observations going back over 80 years against a new and extensive array of climate change simulations that take into account two types of emissions related to human activity: greenhouse gases and sulfate aerosols. They then broke down those data into broad latitudinal bands circling the planet and restricted the analyses to rainfall over land. After comparing observational data with the results of 92 separate simulations, the team has concluded that humans are indeed altering rain patterns in three latitudinal regions. Specifically, the computer models show that human activity has added up to two-thirds of the extra rain observed in the northern temperate regions, including Canada, the United States, Europe, and Russia; removed up to a third of the rain that has disappeared from the northern tropics and subtropics, including Mexico and Africa's Sahara and Sahel regions; and added nearly all of the additional precipitation in the southern tropics and subtropics, including Brazil, southern Africa, and Indonesia.

The amounts are not trivial. In the case of the southern tropics and subtropics, up to 82 millimeters more rain has fallen per year over the past century, the team reports in this week's issue of Nature. So, in addition to changing temperatures, changing precipitation will present environmental and economic challenges to humans living in the affected areas, the researchers say. The new models also give scientists the resources to ask better questions about those changes, says climate scientist and co-author Francis Zwiers of Environment Canada in Toronto. The next task, he notes, will be to tease out the influences of greenhouse gases and aerosols to see which are affecting precipitation patterns more strongly, perhaps leading to new strategies to counter their effects.

But other researchers wonder whether the team is placing too much blame on humans. One potential problem is that no one knows what long-term natural patterns of drought and recovery look like, says climate scientist David Battisti of the University of Washington, Seattle, so linking the patterns to human activity may be premature. Another difficulty could arise from including extremely arid regions like the Sahel in the latitudinal bands, says researcher Michael Glantz of the National Center for Atmospheric Research in Boulder, Colorado, because rainfall in such areas can be so erratic that "it's difficult to separate the signal from the noise."

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