One question that’s lingered as long as the drought itself is just how much, if any, of the drought is due to human-caused global warming. New research has an answer: from 8 to 27 percent. It also has another important number: climate change has made the odds of severe droughts like this one twice as likely.
The sobering numbers come from researchers at the Lamont-Doherty Earth Observatory, NASA and University of Idaho who analyzed temperature, precipitation, wind speed and other data to tease out the role climate change has played in the drought in a new study published on Thursday in Geophysical Research Letters.
It’s the latest in a string of studies that have examined this big question. The general consensus is that heat has played a role in exacerbating the drought and that at least some of that heat is part of the larger pattern of global warming. Last year was a punctuation mark with record-setting heat baking the Golden State to crispy golden brown.
The “ridiculously resilient ridge” — an atmospheric feature that’s deflected storms every which way but over California since 2012 — also might have some climate change ties, though those are still somewhat murky. But the specific role climate change has played in the drought hasn’t been truly teased out until this new study.
“This study is the first to parse out the relative contribution of anthropogenic warming versus natural climate variability,” Park Williams, a tree ring researcher at Lamont, said. “From a method standpoint, it’s a big advancement. It’s the first time I know of that data has been parsed apart this way for any drought on the planet.”
Williams led the latest round of research that uses temperature, precipitation, wind and other drought data to break the state up into grid of 23,955 boxes. The data paint the sharpest picture yet of a state in severe drought. Averaged across the state, the findings show that this hasn’t been California’s worst drought on record, but it is the worst drought on record in the places that matter the most to people.
The epicenter of the drought is the Central Valley, the heart of California’s $46.4 billion agricultural industry. The Sierra Nevada mountains, where most of the state’s water comes from, and a large swath of the coast from San Diego to San Francisco, where more than half the state’s population lives, have also experienced record-setting drought conditions since 2012.
The lack of rain is what obviously started the drought but heat has ensured its lasting impacts. By comparing different datasets and historical temperature trends, Williams and his colleagues were able to show that up to 27 percent of this drought can be ascribed to climate change-driven warming.
“We needed to answer this question. Just knowing how much climate change has affected this drought is really important as a tool to help Californians understand that while natural climate variability is still important, but over the long term, we already have a trend underway,” Williams said.
That trend also means the odds of severe drought in California have doubled since the early 20th century.
Noah Diffenbaugh, a senior fellow at the Woods Institute for the Environment at Stanford University, said the new analysis represented a "valuable step" in attribution work, a field of climate science that's developed in the past decade in an effort to understand the role of climate change in specific extreme events.
“They do a nice job showing that exceptionally warm temperatures from 2012-2014 amplified drought conditions for California,” Nate Mantua, a climate scientist at the Southwest Fisheries Science Center who has previously written about temperature variations in the region, said.
However, Mantua was critical of the paper’s attribution aspect. He said that while it’s possible human-driven global warming could have played a role in the region, the study's modeling efforts didn't convincingly replicate certain patterns and that he felt more confident that natural circulation patterns played a more likely role in pushing temperatures up across the region since 1900.
Future projections paint a worrisome picture for California and the Southwest as a whole. Rising temperatures mean that California and other parts of the Southwest are basically guaranteed to deal with a serious drought lasting a decade and have an 80 percent chance of experiencing a drought that lasts multiple decades by 2100.
Williams likened the impacts of rising temperatures to a bully after your lunch money.
“If we think of precipitation as an income for California, natural variations — evaporation, human water use — are expenses. Then we can think of global warming as a bully that comes by every year and tells you to give him a certain amount of your money. And every year, the bully asks for more and more,” he said.
Unfortunately, the bully isn’t going away anytime soon since human emissions to date have locked in climate change-fueled warming for decades if not centuries to come. The new findings give California a lunch money calculator to understand how much water the state can expect to give up and adjust its habits accordingly. It can also allow cities, farmers and others to take advantage of the good years and save for a rainy (or not so rainy) day.
The burgeoning El Niño could bring rains, particularly to southern California, that could help the state in drier times, for example.
“This really starts a new conversation,” Williams said. “It should allow us to plan for the future in a way that we weren’t capable of doing before.”
While the results are specific to California’s current drought, Williams said they could replicated for any drought or any bully, anywhere.
Climate Centralis an independent organization that researches and reports on climate change.
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