Study: Human-caused global warming behind Calif. drought

Doyle Rice | USA TODAY

Human-caused climate change helped fuel the current California drought, says a Stanford University study released Monday.

Climate change has increased the chance that the two main weather conditions that led to the drought — higher than average temperatures and little rain or snow — will occur at the same time, the study shows.

Almost 98% of California is now enduring a drought, which is entering its fourth year and shows no signs of relenting, according to the U.S. Drought Monitor.

The study, led by Stanford scientist Noah Diffenbaugh, found that the worst droughts in California's history occurred when conditions were both dry and warm, and that global warming is increasing the probability those two weather patterns will coincide.

Diffenbaugh said having very dry years that are also very warm would not have happened without human influence.

Burning of fossil fuels such as oil, gas and coal increases the amount of carbon dioxide and other "greenhouse" gases in the atmosphere and cause the planet to warm to levels that cannot be explained by natural causes.

The study also projected that the trend will continue of dry and warm years happening together.

Diffenbaugh and his team looked at historical weather records and computer model simulations to reach their conclusions.

Other scientists not involved in the Stanford study questioned some of its methodology and findings.

Scientist Martin Hoerling with the National Oceanic and Atmospheric Administration said the current lack of rain or snow in California is clearly not part of a long-term trend, or part of human-induced climate change, as many studies have already shown.

He also said a drought measurement tool used by the Stanford team — the Palmer Drought Severity Index — is "flawed and unreliable" to assess the impact of climate change.

"The warming trend can only account for a small fraction of the actual warmth in California the past two winters," Columbia University scientist Richard Seager said.

He said most of the warm temperatures were caused by the same ridge of high pressure in the atmosphere that also minimized any rain or snow.

Hoerling and Seager were co-authors of a NOAA report in December that said natural weather patterns, not man-made global warming, were the primary cause of the drought.

The Stanford study appeared in the Proceedings of the National Academy of Sciences.