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