Climate change could mean a much wetter California

Californians have been bracing themselves for a drier future accompanying a warming climate. But research by scientists at the University of California, Riverside, suggests that the state may actually get wetter in the event of severe climate change.

The study, published July 6 in Nature Communications, reports that more years in the state could look like El Niño ones, when California typically has wetter winters. The authors found that average annual precipitation in California could increase by about 12 percent if nothing is done to curb carbon emissions. Average precipitation in the winter could increase by as much as 30 percent. A wetter climate, however, might not offset the overwhelmingly negative effects of a warming climate.

That wetter scenario would take place in a future where atmospheric carbon dioxide reaches almost 1,000 parts per million, more than doubling what it is now and corresponding to the planet heating by between 5 and 9 degrees Fahrenheit. So far, the planet has warmed about 2 degrees Fahrenheit since the late 1800s. The climate change treaty signed by 195 countries in Paris aims to keep warming to 3.6 degrees Fahrenheit this century.

The area north of Santa Rosa could receive 14 percent more precipitation, the study found. Central California, stretching to just south of San Luis Obispo, could see increases of about 15 percent, while precipitation in Southern California would decrease by about 3 percent.

Most previous studies predicted that California would be drier with climate change, said Robert Allen, one of the authors of the paper and an assistant professor of climatology at UC Riverside.

California stretches between two different climate zones, making it especially difficult to predict changes to future precipitation. The Pacific Northwest to the north is expected to get wetter with a warming planet, while drier weather is expected in Southern California and Mexico.

Allen and co-author Rainer Luptowitz analyzed dozens of climate models, focusing on those that best captured the climate processes behind California’s precipitation. To figure out which models were best, they used them to recreate data from California’s past.

Climate models work by breaking up the surface of the earth and the atmosphere above it into millions of boxes. Those boxes let scientists watch how water, air and heat flow among them over time, and how these flows change with increasing greenhouse gas emissions. Essentially, the models keep track of future weather, for instance at 15-minute increments, to monitor how the climate will change over a hundred years.

The authors linked increases in California precipitation to changes in temperature in the tropical Pacific Ocean that are similar to those of El Niño years. The warming of the atmosphere above the tropical Pacific pushes the jet stream southeast to steer storms toward California.

“Just because we have these projections of precipitation increase, no one should leap to the conclusion that we aren’t going to be severely impacted in our water resources,” says David Neelin, a professor of atmospheric and oceanic sciences at UCLA who was not affiliated with the work. “It’s a reduction in the bad news for California.”

In a much warmer climate, other changes to the water cycle could work against precipitation increases, Neelin said. Those include faster melting of snowpack and more evaporation and transpiration, which is water released through the leaves of plants. The increases in precipitation may come as rain, which doesn’t help build snowpack in the mountains. Also, projected increase are an average over many years. The precipitation may be split between wet and dry years.

Allen said the projections shouldn’t change the way policymakers plan for the future just yet. More researchers need to reproduce the study’s results first.

This story was originally published July 19, 2017 at 8:00 AM with the headline "Climate change could mean a much wetter California."