

A section of Lake Oroville is seen nearly dry on August 19, 2014 in Oroville, California. (Photo by Justin Sullivan/Getty Images)

The build-up of heat-trapping gases in the atmosphere may worsen western droughts in the future, but it is not the principal driver of historic drought afflicting the entire state of California right now, says a major NOAA report released today.

“Natural oceanic and atmospheric patterns” are to blame for the drought, the 42-page report says. Specifically, it highlights a persistent area of high pressure off the West Coast that has blocked rain-bearing storms from coming ashore – which some have dubbed the “ridiculously resilient ridge.”

The pattern of sea surface temperatures also contributed to the drought, the study says.

The NOAA-sponsored study was developed by eight authors from NOAA, NASA, Columbia University, and the International Research Institute for Climate and Society. It comprehensively examines the drivers of rainy season (November-April) precipitation during the previous three years (2011-2012 to 2013-14). The study compares observations with a large set of model simulations to detect the relevant natural and manmade contributions to the drought.

The drought was likely set off by La Nina – a pattern of cooler than normal ocean temperatures in the tropical Pacific – which has historically been linked to reduced precipitation in the West, the study says. But even after the La Nina broke down, a pattern of warm ocean temperatures in the western Pacific set up a storm track that arched northeastward through the Pacific, away from the Golden State.

This series of atmospheric and ocean events led to three straight very dry years, though the study stressed the lack of rain and snow was not uncommon.

“Multi-year droughts appear regularly in the state’s climate record, and it’s a safe bet that a similar event will happen again,” said Richard Seager, report lead author and a professor at Columbia.

The report comes on the heels of an independent study published last week by the American Geophysical Union (AGU) that found the California drought was unprecedented in the last 1,200 years. Despite an apparent contradiction between this study and the NOAA report – which concludes the drought is not outside the range of recent climate variability – the authors of the AGU study insist the results are complementary.

Related: Study: California drought is the most severe in at least 1,200 years

The AGU study authors, Daniel Griffin (of the University of Minnesota) and Kevin Anchukaitis (of Woods Hole Oceanographic Institute), explained the apparent difference in the study’s conclusions about how unusual the drought was reflects differences in how drought was defined and in the areas analyzed (the NOAA study examined all of California, whereas the AGU study examined just central and southern California).

“I want to emphasize that I think the [NOAA report] is really important and is moving our understanding forward,” said Anchukaitis.

Kevin Trenberth, a researcher at the National Center for Atmospheric Research, praised some of the modeling techniques employed in the NOAA report but found the results unsurprising and incomplete.

“I would contend that all droughts are largely natural in the sense that they arise from internal variability in the atmosphere-ocean systems,” Trenberth said. “But this study completely fails to consider what climate change is doing to water in California.”

He continued: “[The report] completely misses any discussion of evapotranspiration and the increased drying associated with global warming. In a drought, where there is an absence of precipitation, it is easily demonstrated that the extra heat from global warming – the increasing heating from increased greenhouse gases, including carbon dioxide – enhances drying, increases risk of heat waves, and greatly increases risk of wild fire to a significant degree.”

Supporting Trenberth’s claim, the previously-mentioned AGU study found record warm temperatures intensified the California drought substantially.

“One simple modeling exercise in our study indicates that the record high temperatures could have exacerbated the 2012-2014 drought by approximately 36 percent,” said Griffin. But Griffin’s co-author Anchukaitis emphasized their study did not evaluate whether the record warmth was linked to the build-up of heat-trapping gases in the atmosphere.

While dismissing much of a role for global warming in the present, NOAA’s report does indicate rising temperatures may worsen future California droughts.

“Future California hydroclimate may … experience a reduction in surface moisture as a projected increase in evapotranspiration is larger than the projected increase in precipitation,” the report says.