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The unprecedented drought currently afflicting California is 'very likely' linked to human-caused climate change, researchers have found.

They say a persistent region of high atmospheric pressure hovering over the Pacific Ocean that diverted storms away from California was much more likely to form in the presence of modern greenhouse gas concentrations.

It is one of the most comprehensive studies to investigate the link between climate change and California's ongoing drought.

An irrigation canal that is drying up near Yuba City, Calif. California's third year of drought ends Tuesday Sept. 30 and California Department of Water Resource Director Mark Cowin said it is essential to use water sparingly as the state faces the possibility of a fourth dry winter. The unprecedented drought currently afflicting California is 'very likely' linked to human-caused climate change, researchers have found.

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.

House boats are docked at Lake Shasta's Bay Bridge resort near Redding, Calif. After three years of drought the water level at the lake has dramatically receded. 

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.'