Earlier this year, reports showed the several consecutive dry years combined with record supercharged California droughts. In 2014, temperatures were by far the highest – fueled mainly by a warmer than normal winter – in the 120 years of record-keeping. On average it was 1.8 degrees Fahrenheit warmer than the previous standard, set in 1934.
By 2014, even though precipitation was near average, the state got roughly as much usable water as in 1977, one of the lowest years on record, said Michael Dettinger of the U.S. Geological Survey.
The problem was that the combination of heat and lack of precipitation created the worst drought conditions in the state in 1200 years. While precipitation was low, it was not unprecedented. It was the heat that pushed the drought to the next level.
Now the LA Times is reporting that climate change is likely to increase the risk of severe drought by overlapping warm and dry periods with increased frequency.
“The key for drought stress is not just how much precipitation there is,” said Noah Diffenbaugh, the lead author of a critical new study and an associate professor at Stanford University’s School of Earth, Energy and Environmental Sciences. “Temperature is an important influence on the water available in California.”
The impact of temperatures leads to increased evaporation and a reduction in the amount of moisture in the soil – the leads to an intensification of California’s dry season.
When they analyzed historical climate data, “they found that warm-dry years have occurred more than twice as often in the last two decades than they did in the preceding century.”
Worse yet, “It appears that the situation is set to get worse. A continuing rise in global temperatures — fueled in part by human activity — will greatly increase the chances that dry periods are accompanied by warm conditions, the team predicted. That’s what has happened during the state’s current drought, now entering its fourth year and by some measures the worst on record. “
“Our results highlight the fact that efforts to understand drought without examining the role of temperature miss a critical contributor to drought risk,” wrote the authors, whose work was published Monday in the Proceedings of the National Academy of Sciences.
At the same time, the question of climate change on overall impact on the California drought is a subject of debate. A published report from last year “concluded that there is no definitive link.”
However, a paper published by Noah Diffenbaugh and Stanford graduate student Daniel Swain, a co-author of the PNAS (Proceedings of the National Academy of Sciences) study, “contributed a paper that said the type of stubborn high-pressure system that persistently pushed storms north of the state is more likely to occur with climate change, suggesting a link to global warming.”
Others disagreed and attributed the drought to natural variability. They write that there was “no appreciable long-term change in the risk for dry climate extremes over California since the late 19th century.”
But the question is not merely one of lack of precipitation, but of the combined effects.
“In the PNAS study, Diffenbaugh, Swain and Stanford graduate student Danielle Touma note that California’s average precipitation has not appreciably declined over the last century. Indeed, climate models suggest that winter precipitation in much of the state could modestly increase this century,” the Times writes. Rather, it is rising temperatures that contribute to the increasing drought risk.
“There’s no question that low precipitation is a prerequisite for severe drought in California but it’s not sufficient,” said Mr. Diffenbaugh. “The efficiency of low precipitation turning into severe drought is much higher if there are warm conditions.”
Prior to 1995, nearly half of the moderately dry years overlapped with warmer temperatures, but since then, every dry year has coincided with a warm year. That’s because, overall, 80 percent of the years since 1995 have been warmer than what was normal for California.
“It used to be flipping two coins independently and getting two tails one-quarter of time. Now we’re getting tails on the temperature coin much more often,” Mr. Diffenbaugh said.
“The emergence of a condition in which there is ~100% probability of an extremely warm year substantially increases the risk of prolonged drought conditions in the region,” they concluded. “Our results strongly suggest that global warming is already increasing the probability of conditions that have historically created high-impact drought in California.”
The problem is that their models suggest that, by 2030, the temperatures that drive this drought condition could become a yearly occurrence.
“We’re not arguing every year will be a drought year. But even without any change in precipitation, when low precipitation years occur, there’s 100 percent risk they’ll be occurring in warm or even extremely warm conditions,” Mr. Diffenbaugh said.
—David M. Greenwald reporting