California’s worst drought in four decades, which has threatened agriculture and exacerbated wildfire risk, has been driven primarily by natural atmospheric patterns, according to a U.S. government-sponsored study.
A recurring high-pressure ridge off the West Coast influenced by varying sea surface temperatures blocked wet-season storms over the past three years, causing the least winter precipitation since 1974-1977 for the world’s eighth-largest economy, according to the report released today.
The study was conducted by researchers at Columbia University’s Lamont Doherty Earth Observatory and International Research Institute for Climate and Society and at NASA and the National Oceanic and Atmospheric Administration, which sponsored the work.
While models project that the southwestern portion of North America as a whole will become more arid because of rising greenhouse gas emissions, determining the impact of human-induced climate change from the observational record is difficult, they concluded.
“Observed trends, even over very long time periods, could arise from natural variability,” the study said.
Examples in the region include serious drought in the 1930s and 1950s and another round of serious drought in the past two decades.
“Precipitation trends computed amidst such a rich record are most likely heavily influenced by natural variability,” the researchers said.
Of three other studies published recently by the American Meteorological Society, only one argued that climate change may have played a role in the drought. Climate models don’t support that conclusion because the models suggest a low-pressure system should have developed off the California coast, rather than the high-pressure system that actually did, Columbia researcher Richard Seager, co-author of the report, said in a conference call with reporters.
“We are saying climate change would have not been a main driver of the precipitation anomalies, which was the fundamental cause of the drought,” Seager said. “California lost essentially one full year of precipitation.”
The researchers are preparing to submit their work to the Journal of Climate, Seager said.
The latest findings are unlikely to temper the push by federal regulators and environmental groups to further curb power-plant carbon emissions to reduce the threat of climate change in California and elsewhere in the country. The Environmental Protection Agency has proposed existing plants cut carbon emissions by 30 percent from 2005 levels by 2030.
Most of the water that California uses in the spring and summer to grow crops, water lawns and hydrate people and livestock comes in the winter. That means it needs to be captured in reservoirs as it falls or it has to come as snow. Without that water being replenished, the impact has been widespread.
According to the report, winter 2013-14 in California was the sixth-driest since records began in 1895, and the three- winter average precipitation from 2011-12 to 2013-14 was the second-lowest, behind 1974-1977.
Forecasters should be on the lookout for similar atmospheric and water temperature patterns in the future and “pay close attention to model predictions when they occur, because the potential for improving seasonal prediction for the West Coast is clearly there,” the researchers said.
California’s power grid has been tested by shrinking hydroelectric supplies and there have been large numbers of wildfires, which have taken down transmission lines in Southern California. The California Energy Commission, the state Public Utilities Commission and grid operator California Independent System Operator Corp. have warned that water curtailments ordered because of the drought could put further stress on the power system.
California’s water shortages have shown no signs of abating, and the state should expect lower hydropower supplies to be “the new normal,” Steve Berberich, chief executive officer of California ISO, said in an interview at the agency’s headquarters in Folsom on Nov. 12.
“We’ll continue to keep our fingers crossed that we’ll get more rain, but we derated the hydro fleet by 15,000 megawatts this year, and I have no reason to believe it won’t be the same next year,” Berberich said.