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Trinity Lake in northern California, one of the state's largest water reservoirs, was at 30 percent capacity in August. This photo shows the lake in February. Credit: California Water Science Center

California's severe and ongoing drought is just a taste of the dry years to come, thanks to global warming, a new study finds.

"California's warming trend is driving an increase in the risk of drought, " said study co-author Daniel Swain, a doctoral student in climate science at Stanford University in California.

A dry year does not always trigger a drought, even in the arid West. But the new report finds that dry years are now more likely to hit during long heat waves — and human-caused climate change is to blame, the researchers said. [The 5 Worst Droughts in US History]

"Warming in California has made it more probable that when a low precipitation year occurs, it occurs in warm conditions and is more likely to produce severe drought, " said lead study author Noah Diffenbaugh, an associate professor in the School of Earth Sciences at Stanford. "That warming would not have occurred without greenhouse gas emissions."

Heat and low precipitation are a deadly combination in California, which relies on winter rain and snow to fill its rivers and reservoirs. The state is now entering its fourth year of severe drought conditions, with an estimated economic impact approaching $2 billion, according to a University of California, Davis report. And 2015 won't see much relief: The snowpack is trending at record low levels because of high temperatures and low snowfall. Warm temperatures also cause more water loss and evaporation from soils and reservoirs, Swain said.

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