Warming temperatures cause of recent California droughts

“Imagine having two coins-one controls temperature, and the other precipitation,” Diffenbaugh said. “In the past, when you flipped the coins, they each came up tails half the time, meaning that a quarter of the time both coins came up tails, representing a warm and dry year.”

In the past two decades, however, nearly all of the years in California have been either warm or hot. “Now the temperature coin is coming up tails most years,” Diffenbaugh said. “So, even though the precipitation coin is still coming up tails only half the time, it means that over the past two decades we have gotten two tails-warm and dry-in half the years, compared with only a quarter of years in the preceding century.”

“When we look at the historical record, not only do we see a doubling of the odds of a warm-dry year, but we also see a doubling of the frequency of drought years,” said Danielle Touma, a graduate student in Diffenbaugh’s Climate and Earth System Dynamics research group and a coauthor on the study. “Warm conditions reduce snowfall, increase snowmelt, and increase water loss from soils and plants.”

The team also used climate models to investigate the role of global warming in driving the observed warming trend, and the associated increase in drought. They analyzed simulations of California’s temperature and precipitation levels over the twentieth century both with and without human greenhouse gases in the atmosphere.

The results were clear: even though climate change to date has not substantially reduced yearly precipitation, human emissions are clearly implicated in California’s statewide warming, and in the increased probability that dry years are also warm.

Assessment of climate model simulations that projected into the future also led the team to conclude that the risk of co-occurring warm and dry years will continue to increase in the coming decades. “We found that essentially all years are likely to be warm-or extremely warm-in California by the middle of the twenty-first century,” said study coauthor Daniel Swain, who is also a graduate student in Diffenbaugh’s lab. “This means that both drought frequency-and the potential intensity of those droughts which do occur-will likely increase as temperatures continue to rise.”

More frequent warm years also increase the likelihood of multi-year droughts in the future. According to team’s analyses, the current California drought, now entering its fourth year, is one of the longest consecutive periods in the historical record during which conditions were both severely dry and severely warm. The climate models also indicate that such conditions will become even more common if global warming continues in the future, as the state enters a regime in which there is nearly 100 percent risk that every year is warmer than conditions experienced during the twentieth century.

The Stanford researchers say their findings could help California water managers and state officials plan for the future. “While our findings don’t provide any particular recommendations,” Diffenbaugh said, “they do provide very strong evidence that global warming is already making it much more likely that California experiences conditions that are similar to what we have experienced during the current severe drought.”

— Read more in Noah S. Diffenbaugh et al., “Anthropogenic warming has increased drought risk in California,” Proceedings of the National Academy of Sciences (30 January 2015) (doi: 10.1073/pnas.1422385112)