Stanford scientists say greenhouse gases worsen California drought

SAN FRANCISCO (Reuters) - California?s catastrophic drought has most likely been made worse by man-made climate change, according to a report released Monday by Stanford University, but scientists are still hesitant to fully blame the lack of rain on climate change.

The research, published in the Bulletin of the American Meteorological Society as part of a collection of reports on extreme weather events in 2013, is one of the most comprehensive studies linking climate change and California's ongoing drought, which has caused billions of dollars in economic damage.

The report found that high-pressure ridges like the one that stubbornly parked itself over the Pacific Ocean for the past two winters, blocking storms from hitting California, are much more likely to form in the presence of man-made greenhouse gases.

The ridge, dubbed the Ridiculously Resilient Ridge by researchers, or "Triple R," parched the state during the past two rainy seasons.

?You can visualize it as a fairly large boulder in a small stream,? said Daniel Swain, a lead author on the report, which said the phenomenon has caused storms to bypass not only California but also Oregon and Washington, pushing rain as far north as the Arctic Circle.

Using climate model simulations, the researchers found that ?Triple-R? events are three times more likely to occur today than in preindustrial climates.

Scientists also determined that as long as high levels of greenhouse gases remain, severe droughts could become more frequent. ?California is more likely to see these episodes in the near term,? said environmental scientist Noah Diffenbaugh, who led the study.

Despite the findings, Thomas Peterson, principal scientist at NOAA?s National Climactic Data Center and one of the report's editors, said it is still hard to definitively link rainfall to climate change. ?There is so much variability in rainfall,? Peterson said. ?Finding a signal and attributing how much of the signal is climate change is difficult.?

Marty Hoerling, a research meteorologist with the National Oceanic and Atmospheric Administration who edited some of the reports in the climate study, said atmospheric pressure has increased everywhere due to global warming, so the systems need to be studied in that context.

?It?s not the pressure, per se, that determines which way the storms will move,? Hoerling said during a press conference. ?But it?s the difference of how the pressure changes from one location relative to another.?

(Editing by Sharon Bernstein)

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