Models of climate change can predict and explain shifting rainfall patterns globally, says a new study. In a study set to come out in Nature tomorrow, an international group of scientists reports that they simulated atmospheric behavior using several different models and used them to forecast anthropogenically driven changes in average annual rainfall at different latitudes from 1925 to 1999. The predictions matched actual rainfall measurements during the 75-year period, both in the magnitude (amount) and the trend (increase or decrease) of precipitation. The move comes just five months after the release of an Intergovernmental Panel on Climate Change (IPCC) report, which contained accurate predictions of temperature variations due to global warming using the same models.
Between 1925 and 1999 precipitation between 40 and 70 degrees north latitudes increased at the rate of 62 millimeters (2.44 inches) per century. The northern tropics and subtropics, between 0 and 30 degrees, became drier at 98 millimeters (3.86 inches) a century, while it got wetter in the corresponding zone between the equator and 30 degrees south at a rate of 82 millimeters (3.23 inches) per century. The models, which factor in natural effects such as solar winds and volcanic eruptions, along with anthropogenic forcings like greenhouse gases and aerosols, match these precipitation variations accurately in trend and reasonably well in magnitude.
These trends would further desiccate many of the world's great deserts like the Sahara and the Arabian (both in the northern subtropics), whereas tropical rain forests like those in Amazonia and Africa straddling the equator and the southern tropic zone would get wetter. Most of Europe, and Canada, lying above 40 degrees north as well as southern Greenland are expected to get more drenched. This sets up a competition of sorts between higher snowfall, which increases the Arctic ice cover, and the higher temperatures that melt it. "Overall, I expect warming to win in the long run," says study co-author Gabriele Hegerl of Duke University.
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"A warmer globe means more water vapor in the atmosphere, which increases the potential for rainfall," she says, explaining the increase in total global rainfall over the past several decades. "The way [the moisture] turns into rain is more complex, however," she adds, which causes both increments and decrements in local rainfall. The step from moisture to clouds involves cooling, seed particles (including pollutant aerosols) and global wind patterns that blow the moisture from its place of origin to its place of condensation. There are even factors, like change in forest cover, that are known to influence local rainfall but are not very well represented in any of the models. All these complications have traditionally rendered attempts at modeling rainfall—which is much harder than modeling temperature changes—futile. "We were surprised by how well the results matched [real-life data]," Hegerl says.
This, however, is not the end-all of climate modeling. Almost all of the rainfall data available today are over land, whereas oceans cover 70 percent of Earth's surface. Difficulty in measuring rainfall over the oceans has precluded any analysis of this immense area. Furthermore, for reasons still unknown, of the 10 or so models used, different ones make accurate predictions at different latitudes; no single model works over all latitudes, and the mean of all of them is closest to observed data. And lastly, although the models get the precipitation trends spot-on, they "significantly underestimate the magnitude of change [in rainfall]," Hegerl admits, explaining that better modeling is near the top of the agenda for the researchers.
So what's next? Now that the link between shifting rain trends and increasing greenhouse gas and aerosol emissions has been confirmed, scientists are looking to explore connections between climate change and other atmospheric metrics such as cloud cover.
