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Natural conditions, not human-caused emissions of greenhouse gases, are the driving force behind California's three-year dry spell, scientists on a federal task force concluded Monday. But the report came under fire from some experts who said it downplayed other factors that have humanity's fingerprints on them.
The evidence suggests a naturally induced "warm patch" of water in the western Pacific helped to create a high-pressure ridge that blocked precipitation from entering California, the experts said at a news conference to release the report.
"We have been able to identify this as a mode of ocean forcing of atmospheric circulation that causes West Coast drought," said Richard Seager, a climate model specialist at Columbia University's Lamont-Doherty Earth Observatory.
Other studies suggesting a global warming link are off the mark since they hadn't spotted the warm patch's influence, but that's not to say emissions aren't having other impacts, according to the task force assembled by the National Oceanic and Atmospheric Administration.
"The report is not dismissive of global warming at all," said Marty Hoerling, a meteorologist at NOAA's Earth System Research Lab. "At the same time, drought is not a consequence of the warming planet to date."
But critics included Michael Mann, director of Penn State's Earth Science Center. He quickly penned a piece online, calling the report "deeply flawed" because of how it interpreted ocean and Arctic sea ice data, and focused on rainfall while paying "only the slightest lip service" to record warm temperatures in California.
Kevin Trenberth, a climate scientist at the National Center for Atmospheric Research in Boulder, Colo., criticized the study for not including how higher temperatures aggravate a drought regardless of what causes the reduced rainfall. "It completely misses any discussion of evapotranspiration and the increased drying associated with global warming," he said in an email to NBCNews.com.
Noah Diffenbaugh, a Stanford climate scientist who led a team that recently concluded greenhouse gases were likely a factor in California's drought, was more diplomatic but also voiced concern that temperatures were not included in the report.
"When we're talking about drought, we talk about the interaction of temperature and precipitation," he told NBCNews.com.
Seager said focusing on precipitation was warranted, calling it the "main driver" of California's drought, but acknowledged that the role of increasing temperatures was also important. Manmade "climate change would not have been a main driver of the precipitation anomalies, which were the fundamental cause of the drought," he said.
"The precipitation was the essence of this drought," added Hoerling. "Farmers were praying for rain, not cooler temperatures."
In any case, the scientists did agree on this: The science is far from settled.
"This is by no means a final analysis, or final word, about the California drought," said Hoerling.