The increasingly frequent and severe droughts that have punished California over the past two decades—including the current record-breaking one—are primarily the result of human-caused climate change and will likely grow even worse, scientists at Stanford University warn.

Published in Monday's issue of the journal Proceedings of the National Academy of Sciences, the new research analyzes historical records, as well as computer simulations of global warming, to investigate the role of changing temperatures during California droughts over the last 120 years.

The researchers concluded that human-driven global warming is exacerbating and increasing the confluent warm and dry conditions that have produced the state's most severe droughts.

"Of course low precipitation is a prerequisite for drought, but less rain and snowfall alone don't ensure a drought will happen," explained Stanford professor and lead author Noah Diffenbaugh. "It really matters if the lack of precipitation happens during a warm or cool year."

"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," Diffenbaugh added.

Danielle Touma, a graduate student and co-author of the study, explained, "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. Warm conditions reduce snowfall, increase snowmelt and increase water loss from soils and plants."

The researchers say their findings predict worse weather to come.

"We found that essentially all years are likely to be warm—or extremely warm—in California by the middle of the 21st century," said study co-author 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."