California drought worst in at least 1,200 years

The ongoing California drought is the driest period in the state’s history since before Charlemagne ruled the Holy Roman Empire, a new study concludes.

Despite this week’s rainstorm, more than half of California remains in “exceptional” drought conditions, the most severe drought designation. The three-year period since 2011 is the driest in 120 years of recorded history, and a new survey of tree rings in blue oaks now provides a larger historical context. The current drought is the most extreme since the year 800, researchers report in a paper in press in Geophysical Research Letters.

They find that California’s current lack of precipitation, while abnormal, isn’t unprecedented even though the drought severity is. Severity is a product of both reduced rainfall and hot temperatures and the researchers estimate that record high temperatures in recent years probably exacerbated the dry spell’s severity by about 36 percent. Rising temperatures from climate change will worsen future droughts in the state, warns study coauthor Kevin Anchukaitis, a geochronologist at the Woods Hole Oceanographic Institution in Massachusetts.

“Almost always when we look back in time, it’s been drier at some point,” he says. “Not only did this current drought rank among the handful of driest years, it appears to have been the worst.”

In the United States, monthly weather measurements date back only to 1895. To glean information about earlier times, scientists look at geological markers in the environment such as sediments in lake beds and stalagmites in caves. These measurements can’t provide year-to-year comparisons, however, so Anchukaitis and climate scientist Daniel Griffin of the University of Minnesota, Twin Cities studied the climate data locked inside centuries-old trees.

The growth of blue oaks (Quercus douglasii) is especially sensitive to droughts, Anchukaitis says. Over the course of a year, each oak grows a new band around its trunk. The ring’s width depends on the amount of water available in the soil, with damp years resulting in rings a few millimeters thick and parched years leading to no visible ring at all.

Earlier this year, Anchukaitis and Griffin measured tree rings in 63 blue oaks throughout Southern and Central California; in 2013, Griffin and other researchers had studied 273 living and fallen oaks. Anchukaitis and Griffin then paired the two years’ results with the North American Drought Atlas, a larger tree-ring data bank, to create a comprehensive history of California drought conditions from around 1,200 years ago to 2014.

California’s current drought stands out in the last millennium: It is more extreme than the 1580 drought, previously considered the most severe in recent centuries, and it even edges out droughts lasting four to nine years. Despite the drought’s severity, the pair found 11 years in the last few centuries with less rainfall than 2014.

This result demonstrates the importance of temperature in shaping California droughts, Anchukaitis says. The state is on track for its hottest yearon record, according to the National Oceanic and Atmospheric Administration.

“Rainfall will recover,” he says. “It will get wet again in California, but the future will hold more hot droughts like this one.”

Temperature’s role in worsening the California drought probably applies to other Southwestern states as well, says climate scientist Benjamin Cook at NASA's Goddard Institute for Space Studies in New York City. “Warming’s effect on droughts in the West is already starting to manifest,” he says. “Climate change is not something far off in the future; it’s something that’s happening right now.”