California's ongoing drought is due primarily to natural variability in the Pacific Ocean, not human-induced global warming, a government report released Monday argues.

But whether that variation in ocean conditions is truly "natural" or is driven by a changing climate remains a major matter of debate among scientists.

The new study, conducted by New York's Columbia University for the National Oceanic and Atmospheric Administration (NOAA), concludes that "natural oceanic and atmospheric patterns are the primary drivers behind California's ongoing drought," according to a NOAA press release. Human-induced climate change, on the other hand, was not found to be a significant factor, concludes the report.

The worst drought in the state's recorded weather history is actually not a complete outlier when compared with older events preserved in the records of tree rings, says Richard Seager, the new report's lead author and a professor at Columbia University's Lamont-Doherty Earth Observatory. "Events like this have happened before, although this is the most severe drought since records began in the 1850s."

Another unrelated new study, published in Geophysical Research Letters, used tree-ring data to conclude that the current drought is the most severe to affect the state in at least 1,200 years.

Ocean-Climate Nexus

According to the new NOAA report, the drought's primary cause is a high-pressure ridge that has remained parked over the Pacific off California for much of the past three years. That ridge has diverted storms that would normally bring precipitation to the state. (See "Hard California Rains Provide Little Relief From Drought.")

The question vexing scientists has not been the existence of that ridge, says Noah Diffenbaugh, a climate scientist at California's Stanford University not involved in the NOAA report, but how conditions have come together to make such ridges more likely and to make them stay around so long.

"We are trying to understand whether the global warming that has already happened is increasing the probability of such extremes," says Diffenbaugh.

In September, Diffenbaugh published a study in the Bulletin of the American Meteorological Society concluding that such a high-pressure ridge off California is indeed more likely to develop in a warming atmosphere.

Seager says the problem is that the leading computer models of future climate predict that low pressure will dominate off California, leading to more precipitation, not less.

"According to the models," he adds, "increases in greenhouse gases should lead to relatively uniform increases in temperature in the Pacific Ocean." But over the past three years of the drought, the Pacific has seen unusual patterns of heating and cooling.

Those anomalous patterns are best explained by natural variability in the ocean and in atmospheric cycles, not by human-induced global warming, says Seager. Still, the next step will be to "try to work out if there is some subtlety behind the [ocean heating] spatial pattern that we don't understand," he says. (See how the drought is driving a groundwater-drilling boom.)

Probing Uncertainty

Diffenbaugh thinks it is possible that the changes brought by human-induced global warming are in fact behind the unusual oceanic and atmospheric changes, which are in turn ultimately behind the drought. "The causes of the drought are very complex, with multiple components," he says.

Perhaps underscoring that uncertainty, Martin Hoerling, a co-author of the report and a researcher with NOAA's Earth System Research Laboratory, says, "This report is by no means the final word on the California drought." Other researchers will have to build on the work, he notes.

The fact that this past year has been the worst yet for drought in California is particularly perplexing, notes Hoerling, because ocean conditions have been more favorable for precipitation that largely has not come.

"We also need to better understand what role was played by the warm temperatures," says Hoerling, who notes that 2014 has shaped up to be the hottest year in the state's recorded history.