The California drought may have put water in short supply, but debate about it is in surplus. Water use has come under even greater scrutiny as Californians struggle to deal with the current and future reality. Groundwater overuse during the drought has reached epic proportions, with the land surface in some locations sinking almost two inches per month as a result. In addition to arguing over how to use the little water they have, people are also debating the question of whether humans are partly to blame not just for water supply issues, but for the drought itself.

Late last year, a NOAA report concluded that climate change wasn’t required to explain the lack of rainfall, while a separate tree ring study found that the drought looked to be the most severe in 1,200 years. The rains have been fended off by a persistent pattern of high air pressure above the northeastern Pacific that seems to have been a product of ocean surface temperature patterns farther west.

But rainfall isn’t the only factor that contributes to drought. The heat of the day sucks moisture out of the soil, helped along by blowing winds. Boost either of those factors, and you’ll need more rainfall to keep the drought account from going in the red. Since the globe is warmer now than it was a century ago and California is part of that globe, it’s fair to guess that climate change isn’t helping.

To cut to the heart of the question, a group of researchers led by Lamont-Doherty’s Park Williams designed a study to quantify the contribution of global warming to the current drought.

Using a number of weather observation datasets that provide good coverage of the state back to 1901 (and through 2014), they found no long-term trend in precipitation. Climate models don’t predict that precipitation should be decreasing in California because of climate change either, so the researchers focused on temperature.

They calculated potential evaporation and plant transpiration rates as well as a metric for drought conditions. Then using four different estimates of the anthropogenic contribution to increasing temperatures, they estimated the portion of the current drought that can be blamed on anthropogenic warming.

2014 saw the third-lowest precipitation of any year in the record, but 2012 to 2014 was the lowest three-year stretch. Potential evaporation was highest in 2014, with the three-year period coming in at either number one or number two. Overall, 2014 was the worst year for drought conditions. Averaged over the whole state, the three-year stretch was actually second worst (behind 2007 to 2009), but a record was set in the agricultural Central Valley.

Potential evaporation has trended upward since 1901, and that’s mostly because the area has warmed. (Interestingly, evaporative cooling—the way sweat cools your skin—from irrigation in the Central Valley has offset this trend locally. Assuming we’ve hit the peak of inefficient irrigation, that’s likely to change.) Greater evaporation and plant transpiration now consume the equivalent of six to eight centimeters of rainfall more than they did in 1950. That’s about 10 percent of the average annual rainfall.

In the end, the researchers calculated that anthropogenic global warming was responsible for 8 to 27 percent of the extremity of the drought between 2012 and 2014, with the dearth of rainfall chipping in around 70 percent. Because of the way they estimated anthropogenic impacts, the researchers say these numbers are conservative estimates.

If you think in terms of the probability of a drought this extreme taking place, the anthropogenic contribution is enough to make it about twice as likely. The researchers write that “this analysis illustrates the general fact that the anthropogenic drying trend, while still small relative to the range of natural climate variability, has caused previously improbable drought extremes to become substantially more likely.”

Park Williams told Ars that adding 2015 to this analysis probably wouldn’t change much. “I have not yet run the numbers because the summer drought season that we focus on extends through August, but I am reasonably confident that the main conclusions would be about the same. That is, the four-year contribution of warming to the drought anomaly would have still been in the 15-20 percent range. I say this because I think the state-wide mean drought conditions in 2015 have been right around the same as in 2013/2014, and the absolute contribution of global warming is not going to change much from year to year.”

In their paper, Williams and his colleagues conclude, “Anthropogenic warming has intensified the recent drought as part of a chronic trend toward enhanced drought that is becoming increasingly detectable and is projected to continue growing throughout the rest of this century.”

Geophysical Research Letters, 2015. DOI: 10.1002/2015GL064924 (About DOIs).