An abandoned ship is stuck in the solidified salts of the Oroumieh Lake, Iran. Environmentalists and activists have been raising alarms for years that the lake is threatened by drought and aggressive agriculture policies.  (Vahid Salemi/AP)

Winter droughts have become increasingly common in the Mediterranean region, particularly over the past 20 years, and a new study finds that global warming has driven at least half of the change.

Drought conditions in this politically explosive region are expected to grow more severe over the course of the century unless countries begin to significantly reduce their emissions of heat-trapping greenhouse gases, particularly carbon dioxide, many researchers say.

Those emissions come from burning fossil fuels, as well as from land-use changes.

Winter storms historically have delivered most of the annual rain and snowfall to the already arid Mediterranean region.

Yet precipitation measurements from the region and modeling studies point to a relatively rapid shift in the winter rain and snowfall trends that began in the 1970s, according to the study.

That change could signal that the region "has moved into a new climate regime," says Martin Hoerling, a scientist at the National Oceanic and Atmospheric Administration's Earth Systems Research Laboratory in Boulder, Colo., and the study's lead author.

The shift is not the result of temperature trends in the region itself, Dr. Hoerling notes.

Instead, he and his colleagues trace drier Mediterranean winters to changes in long-range atmospheric circulation patterns.

These changes, the study suggests, are triggered by rising ocean temperatures in the tropical Indian Ocean, a trend scientists have previously attributed to climate change.

When this area of the world's oceans is warmer than other tropical seas, the temperature difference appears to set up conditions over the North Atlantic that steer a higher proportion of Atlantic winter storms across northern Europe.

The results have been accepted for publication in the Journal of Climate.

The study reaches "a very important conclusion, for a number of reasons," says Peter Gleick, president of the Pacific Institute, a environmental-policy research group in Oakland, Calif., that focuses much of its effort on water-resource issues.

From 60 to 80 percent of the region's water irrigates crops, researchers say.

And existing sources of fresh water already are oversubscribed.

"Water is critical for this region and has been for a long time," Dr. Gleick says.

"The fact that climate change now appears to be making things worse or more severe is just more bad news for people who care about water conflicts and water scarcity in the Middle East."

In addition, he says, the study adds to a growing body of work that is finding global warming responsible — at least in part — for long-term changes in precipitation and temperature patterns at continental and even regional scales.

"We know that climate is changing. We know that humans are an important factor," he says.

"But unless you can say, 'this or that event is due to climate change or partly due to climate change,' there's a lack of a sense of urgency."

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