In August, Columbia University’s Earth Institute found evidence that global warming has contributed to California’s drought. And in September, NASA and Columbia teamed up to produce a study showing that five centuries have passed since the Golden State has been as dry as it currently is. Each of the studies drew on research that goes back decades.
None of the studies could explain exactly why the West is baking today. Temperatures reached 120 degrees Fahrenheit in parts of California this week. Since October, 26 million trees have died in six counties across 760,000 acres in the Sierra Nevada mountains that run along California’s spine.
That brings the number of dead trees to 66 million over four years of drought, the service said. A combination of heat, dryness and a greedy little beetle, according to the latest estimate by the U.S. Forest Service.
In Arizona, where temperatures reached 118 degrees in Yuma on Tuesday, a pair of tourists who went for a hike died on the trail under the scorching heat. A third member of their party informed authorities because he was the only one strong enough to straggle back for help.
California, Nevada, New Mexico and Arizona are experiencing large wildfires fairly early in the season. Two large fires burning near Santa Barbara are threatening to combine, and another is burning north of San Francisco, stretching thin the personnel and material needed to fight them.
When a monster wildfire struck Colorado in 2012, Sherman Harris, then-undersecretary at the Agriculture Department, which oversees the Forest Service, acknowledged that big fires driven by climate were here to stay. The wildfire season that ran from June to September expanded to include May and October.
Since then, it has gotten even worse. The season starts in March and ends in December. Once, it was rare to see 5 million cumulative acres burn in a year; recent seasons have recorded twice that. Last year’s wildfire season set a record with more than 10 million acres burned — more land than Maryland, the District and Delaware combined, the Forest Service said.
More than half the fires were in Alaska, where dryness due to historically low mountain snowpack and a freak lightning storm created perfect conditions for a huge blaze. But there were also mammoth fires in Washington and Oregon, where drought had left forests dry and ready to burn.
“The climate is changing, and these fires are a very strong indicator of that,” Sherman said in 2012, predicting what was to come.
The Earth Institute study was silent on wildfires, but it said increased warming is creating a pattern in which the small amount of moisture stored in plants and the soil evaporates into the drier atmosphere.
“A lot of people think that the amount of rain that falls out the sky is the only thing that matters,” said Park Williams, a bio-climatologist at Columbia University’s Lamont-Doherty Earth Observatory and the study’s lead author. “But warming changes the baseline amount of water that’s available to us, because it sends water back into the sky.”
The Earth Institute study analyzed month-to-month climate data between 1901 and 2014 to find fluctuations in precipitation, wind, temperature and humidity. It found that the average temperature in California increased by 2.5 degrees Fahrenheit over 113 years.
And, starting in the 1960s, heat increased with the introduction of more greenhouse gases from automobiles and other sources. Warming, the study said, increased the impact of natural conditions by as much as 25 percent.
Over the past 15 years, temperatures have risen in California, resulting in annual periods of extreme heat. At the same time, low and moderate precipitation cycles in the state haven’t changed since 1977. That means it’s far more likely that extreme-heat years will mingle with dry years.
Stanford University professor Noah Diffenbaugh and two graduate students at the School of Earth, Energy and Environmental Sciences studied climate records dating back 120 years to determine how temperature changes are playing a role in California’s drought.
Between 1896 and 1994, climate patterns in the state created a 50 percent chance that a year of extremely warm temperatures would merge with a year of moderately dry conditions. But between 1995 and 2014, extreme-temperature years were so common that their chance of combining with dry years increased to 80 percent.
Diffenbaugh said California needed to manage its risks with smart water policies that use the little precipitation it gets to bank groundwater so that farms, which use 77 percent of the state’s water, can survive.