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Rainfall Articles from Scientific American

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From Bountiful to Barren: Rainfall Decrease Left the Sahara Out to Dry

How a once-wet landscape became one of the world's great deserts

By Adam Hadhazy   

 

SAHARA SAVANNA?: The sediment deposits in northern Chad's Lake Yoa, pictured here, reveal how the once-green Sahara became a dusty desert.
STEFAN KROPELIN

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In a finding that may help scientists better predict the pace of climate change, research published in Science shows how the Sahara Desert, a region as big as the U.S. that stretches from the Atlantic Ocean to the Red Sea across northern Africa, went from bountiful to bone-dry over a period of several thousand years.

Scientists peered into the Sahara's verdant past by analyzing sediment samples drilled out of the bottom of one of the desert's last living lakes. The samples revealed long-held secrets of how desert-friendly species replaced tropical plants and animals as monsoon rains retreated farther south into the continent.

Finding such a detailed archive in a place as desolate as the Sahara was quite unexpected. "It's the only record of its kind," says study co-author James Russell, an assistant professor of geological sciences at Brown University.

The researchers found bits of pollen, algae, insects, sand and minerals in the samples from Lake Yoa in northern Chad, which they determined had accumulated into thin layers over the past 6,000 years.

"It's like looking at tree rings," says lead study author Stefan Kröpelin, a geoarcheologist at the Institute of Prehistoric Archaeology at the University of Cologne in Germany. "These layers give us detailed seasonal information, and it is even possible to determine which year a species was introduced."

The initial greening of the Sahara occurred about 10,500 years ago and it stayed lush for several millennia. (Cave paintings of crocodiles and giraffes left by prehistoric humans attest to the once savannalike climate.)

Like slides in a presentation, the sediment layers reveal how these humid conditions changed and reduced Yoa to an isolated oasis. Tropical plants and evergreen shrubs, still plentiful about 5,500 years ago, began to decline as the area dried out over the next 1,000 years. The desiccation continued, and, by 700 B.C., mostly desert flora like the hardy acacia tree dotted the now-parched landscape.

This gradual shift in the Sahara's overall climate contradicts a common theory that the region dried rapidly over a few hundred years, and provides clues about a potential re-greening triggered by global warming, Kröpelin says.

The data also confirms that a drop in rainfall was the major reason the area turned into a dusty badland. Based on the sediment samples, the researchers determined that Yoa's waters suddenly grew quite salty about 4,000 years ago. They speculate the salinity spiked because the streams that had previously drained salt out of the lake vanished as rainfall lessened. This abrupt event shows how a relatively small occurrence—such as a slight slide in rainfall—may have a tremendous impact.

"It's like turning a dial for the rain, but flipping a switch for the lake," says Richard Alley, a professor of geosciences at Pennsylvania State University who was not involved in the study.

This crossing of such a threshold, which could result in rapid changes in a region's climate through a series of local events, has stoked fears of spreading deserts in the Sahel, a semiarid region just south of the Sahara. Scientists worry that a runaway "positive feedback" loop, in which one event reinforces or strengthens the next, is already taking place, with fewer plants leading to less rain, leading to still fewer plants, and so on.

Kröpelin says that the new findings will help climatologists fine-tune their computer models—which he says were wrong about what happened to the Sahara—to more accurately predict the effect of global warming.

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  • The Rain in Spain Stays Mainly in the Plain. Or Does It?

    Climate change explains shifting rainfall patterns: wet places getting wetter and dry places drier

    By Sourish Basu

    SWEET SUCCESS Actual change in rainfall (solid blue line) and model prediction (solid red line) at different latitudes from 1925 to 1999. The shaded region is the error estimate of the models, while the dotted red line is the model scaled up to match the data.
    XUEBIN ZHANG ET AL

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    Models of climate change can predict and explain shifting rainfall patterns globally, says a new study. In a study set to come out inNature 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.

    "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.


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