How they "think"-----California drought: Climate change plays a role, study says. But how big?

California drought: Climate change plays a role, study says. But how big? - LA Times
A growing number of scientists have made the claim that climate change is at least partly responsible for California's crippling drought.
Now researchers have estimated the extent to which humans are to blame: between 8% and 27%.
In other words, without heat-trapping greenhouse gas emissions causing temperatures to rise, the drought would have been 8% to 27% less severe than it is, said John Abatzoglou, a University of Idaho climate researcher and coauthor of a study published Thursday in the journal Geophysical Research Letters.
"By knowing how much global warming has contributed to the trend in California drought conditions over the past century, we can reliably predict how the future will play out," said A. Park Williams, a bioclimatologist at Columbia University's Lamont-Doherty Earth Observatory who led the study..."