Current drought is worst in 1,200 years in California and the American West, new study shows

The drought that has patched California and the American West for much of the past two decades ranks as the driest 22-year period in at least 1,200 years, according to a new study published Monday.

Measuring historical moisture patterns by looking at thousands of tree rings, scientists concluded that the West is in a “megadrought,” the likes of which have not been seen in the region since at least 800 AD, when Vikings sailed the North Atlantic and Mayans built temples in Mexico and Central America.

Climate change, which is leading to increasing temperatures, is making the current dry period more severe than it otherwise would have been, the researchers concluded.

“Here we are 22 years into a bad drought, and because of climate change we are now surpassing the severity of megadroughts that have always been thought of as the worst-case scenarios,” said Park Williams, an associate professor of geography at UCLA and lead author of the study.

“Scientists are the eyes for society,” Williams added. “It’s like when your eyes tell you that your head is about to hit a tree branch. You have to do something to avoid that. We see a danger now, right here in front of our eyes.”

Over the past 20 years, California has had three stretches of short-term drought: 2000-2003, 2007-2009 and 2012 to 2016. Each drought ended with a soaking year, most recently in 2017, when heavy rains caused flooding that damaged Oroville Dam and submerged parts of downtown San Jose.

The past two years have seen another sharp period of drought, which is still underway in California and much of the West. Despite rains in December, reservoirs across the state remain at lower than average levels, wildfire danger is rising, and the Bay Area hasn’t seen any rain in five weeks.

Taken together, the last 22 years now exceed the previous worst 22-year period, 1571 to 1593, in the historical record, measured by soil moisture. And with only two more months left in California’s winter rainy season, odds are this drought will continue, Williams said.

“It’s extremely unlikely that this drought can be ended in one wet year,” he said.

As of last week, 66% of California and 64% of the American West were experiencing “severe drought,” according to the US Drought Monitor, a weekly update.

Last summer, two of the largest reservoirs in North America — Lake Mead and Lake Powell, both on the Colorado River — reached their lowest recorded levels. California’s largest reservoir, Shasta Lake, near Redding, was 36% full on Monday, and its second largest, Lake Oroville, in Butte County, was 47% full.

Experts said Monday that the study, published in the journal Nature Climate Change, is the latest example of how California and the West are entering a new era that is already posing significant challenges for water supplies, wildfire risk and extreme heat events.

“Our approach to Western water allocation for the past century-and-a-half has been based on the unjustified optimism that good years and good runs of years are typical,” said biologist Chris Field, director of Stanford University’s Woods Institute for the Environment “This new paper drives home the point that profound droughts are part of the history and are getting worse.”

“We need to find a way to live within the real-world, increasingly limiting constraints,” said Field, who was not part of the research.

Global temperatures have risen 2.1 degrees Fahrenheit since 1880, mostly due to the burning of fossil fuels, which traps heat in the atmosphere. The 10 warmest years since 1880 on Earth have all occurred since 2005, according to NASA and NOAA.

Williams and his colleagues studied records of tree rings from roughly 30,000 trees in nearly 1,600 locations around the West to measure the amount of rainfall and soil moisture over the centuries. In a paper published two years ago, they found that the first 19 years of this century were the second-driest back to 800 AD, which is about as far back as the tree-ring estimates in their research are reliable.

After the most recent two dry years, the current period became the driest in the West in at least 1,200 years. Human-caused climate change is responsible for about 42% of the soil moisture deficit since 2000, the paper concluded.

There have been at least seven major “megadroughts” in the American West back to 800 AD, Williams, his colleagues and other researchers have found. Some lasted decades. A megadrought in the 1100s and 1200s is believed to have contributed to the collapse of the Anasazi civilization in Arizona and New Mexico.

Today, modern technology, including reservoirs, groundwater pumps, desalination plants and other infrastructure has improved the resilience of people living in the arid West. If the current drought went on for many years, society would change, but not collapse, said Jay Lund, a professor of civil and environmental engineering at UC Davis.

“Los Angeles, San Jose and the other big cities wouldn’t dry up and blow away,” he said. “There would be lots of water conservation, and we’d build big desalination plants like Saudi Arabia and Qatar. Agriculture would really suffer.”