The likelihood of California experiencing more warm, dry years leading to severe drought is increasing, according to research by Stanford University scientists.
The study, which was published March 2 in the Proceedings of the National Academy of Sciences, found that rising temperatures combined with dry years are likely compounding severe drought more than dry, cool years.
The research team, led by Stanford Professor Noah Diffenbaugh of the School of Earth, Energy and Environmental Sciences, looked at California's weather data as far back as 120 years and the role of temperature and drought. The researchers found that the state's worst droughts historically occurred during the combination of warm and dry conditions.
"We've seen the effects of record heat on snow and soil moisture this year in California, and we know from this new research that climate change is increasing the probability of those warm and dry conditions occurring together," he told the Stanford Report last month.
In the past, California's temperatures came up dry and hot one quarter of the time, but in half of the past 20 decades, conditions have been warm and dry together, he noted.
Analyzing computer simulations of the state's precipitation and temperature levels throughout the 20th century with and without greenhouse gasses, human-created emissions were clearly implicated in the statewide warming and increased dry years. The probability of the two factors coinciding also increased significantly by the mid-21st century, according to their calculations.
"We found that essentially all years are likely to be warm -- or extremely warm -- in California by the middle of the 21st century," Daniel Swain, a graduate researcher and co-author, told Stanford Report.
The current four-year drought is also one of the longest periods in the historical record when the two conditions of dryness and heat collided, the researchers said.
Diffenbaugh noted the effects of such drought. He pointed to the unprecedented low snowpack this spring in an April 2 Q&A in the Stanford Report.
"This is the kind of extreme event that falls outside our historical experience. This is not only a low April 1 snowpack, but it is much lower than the previous record low," he said.
The record high temperatures currently being experienced this winter and spring combined with previous years of drought and the low snowpack mean the drought is likely to worsen this summer, Diffenbaugh noted.