Every time extreme weather hits, the question inevitably asked by the media is: Did climate change do that? The answer is that the atmosphere is a complex place, and many factors play into a weather event. Attributing a hurricane to climate change alone is like blaming your divorce on your distant and inattentive parents. The causes are likely more complicated.
Take the California drought. It’s disappointing how many news outlets ask, “Did Climate Change Cause the California Drought?” without adequately answering the question. The simplistic answer is, not really. The most direct cause seems to be a high-pressure system hovering just off the Pacific Coast, which re-routed storms originally headed to California, depriving the state of much-needed rain.
If you ask a smarter question, though, you’ll get a smarter answer. Did climate change contribute to the California drought? The answer is, almost certainly.
"Warming changes the baseline amount of water that's available to us because it sends water back into the sky."
A study recently published in the journal Geophysical Research Letters adds to an already substantial body of research detailing the ways a warming climate has exacerbated California’s water emergency. The authors examined 114 years of weather data up to last year and found a steady temperature increase of 2.5 degrees Fahrenheit over that period. Since warming increases the air’s capacity to hold water, this temperature increase accelerated evaporation from the ground and from plants. The parched plants and surface soil, in turn, sucked water from deeper layers of Earth, robbing California of its groundwater resources.
“A lot of people think that the amount of rain that falls out the sky is the only thing that matters,” says A. Park Williams, the Columbia University bioclimatologist who led the study. “But warming changes the baseline amount of water that's available to us because it sends water back into the sky.”
How much of our previously stored-up groundwater is now floating around in the sky because of global warming? You can’t really count evaporating water molecules, so we’re limited to the ranges produced by mathematical models. Williams estimates that the warming associated with climate change probably made the California drought between 15 percent and 20 percent worse, although the number could be as low as eight percent or as high as 27 percent.
Many scientists have forwarded similar theories. John P. Holdren, one of President Obama’s top science advisors, argued in February 2014 that evaporation driven by climate change was drying out the soil and intensifying the California drought. Princeton climatologist Michael Oppenheimer told the New York Times in April of this year, “The drought is made of two components: not enough rain and too much heat.” The Williams study supports those hypotheses with data.
Climate change may be contributing to the California drought in other ways as well. Way back in 2004, Lisa Sloan of the University of California–Santa Cruz argued that melting Arctic ice would change the path of storms, depriving the West of rainfall. Sloan’s theory is still controversial, but the drought has made her look like a climatological Cassandra.
Another important finding of the recent study is that climate change will make California droughts more common and severe over the next century. Williams’ data suggest that accelerated evaporation will continue to diminish the state’s groundwater resources, and that even if normal rainfall patterns were to return, they’d unlikely be able to offset the losses. That point confirms previous research as well. Earlier this year, a NASA-led study warned that California’s midcentury droughts could be worse than the megadroughts of the Medieval Warm Period (900–1300 CE).
These studies underline the severity of the California drought in an additional, unintended way. Most extreme weather events end long before we have time to say, with any kind of certainty, what factors contributed to them. Now in its fourth year, this drought has been around so long that it has spawned its own field of research. A graduate student who began her program as the drought got going could have published her thesis before it was over.
Hopefully, she would not have chosen the title, “Did Climate Change Cause the California Drought?”