Storms to grow bigger, warmer
California’s wild weather swings, from pounding rain to drought and from fires to floods, are widely expected to worsen as the climate warms. A new study shows just how severe things might get, and it’s not pretty.
The biggest of Pacific storms will dump 40% more rain and snow on parts of the Sierra, boost the hourly rate of precipitation in hills and valleys nearly a third, on average, and be about 4 degrees Fahrenheit warmer upon landfall, the research shows.
The findings, distilled from models of the weather phenomena known as atmospheric rivers, mean that a worstcase storm scenario, on par with the Great Flood of 1862 that turned the Central Valley into a raging sea and took countless lives, is a real possibility.
“We really wanted to get at the specifics,” said Daniel Swain, a climate scientist who has long studied California’s
big storms and is one of three UCLA researchers who authored the new paper. “It’s one thing to say that an atmospheric river will have more moisture. It’s another thing to connect them to the clouds and say this is how much moisture will drop.”
Using localized climate and weather models, Swain and his colleagues deliberately teased out the largest storms that California is likely to face if greenhouse gas emissions — and global warming — continue at their current trajectories. The researchers wanted to reveal the possible perils that lie ahead.
Their work, published Wednesday in the journal Science Advances, is premised on the fact that atmospheric rivers, which are essentially long channels of airborne moisture wicked from the ocean, are becoming more waterlogged as temperatures climb. With every 1.8 degrees of temperature rise, the atmosphere can hold 7% more water vapor.
That additional moisture, combined with a slight increase in highaltitude wind speeds, translates to stronger storms that wring out in differing ways depending on where they land, the researchers found.
Category 5 atmospheric rivers, the largest of these systems, which can drop a dozen or more inches of rain and snow, will each bring about 4 to 6 more inches of precipitation, on average, to the western slopes of the Southern and Central Sierra in coming decades, according to the study. The entire range will see an average of 18% more precipitation from the storms, though some spots will see as much as 40% more.
While the water may be good for filling reservoirs, especially during one of California’s infamous dry spells, the benefits of a Category 5 system are generally outweighed by the damage that can be done. Shorter, less intense bouts of precipitation on a more regular basis are preferable. Category 5 atmospheric rivers have historically occurred once every few years.
Lowerlying areas like the Central Valley and the Bay Area’s TriValley don’t get nearly as much rain as the Sierra. But as a percentage of what they get, they’re expected to see bigger increases with the large atmospheric rivers. Each storm, on average, will deliver 26% more precipitation to the San Joaquin Valley, the study finds.
As far as intensity goes, hourly precipitation during the big storms will increase an average of 32% in lowlying areas around the Sierra and 27% at higher elevations, according to the study. This means as much as an inch more rain per hour will fall in mountainous areas.
Temperatures also will be higher in the future, with the Sierra seeing as much as a 6degree increase when the systems strike, the study finds. As a result, more precipitation will fall as rain instead of snow, leaving less water to melt off into California’s reservoirs after the wet winter season.
While the study did not identify the problems that California might face with changing storms, its authors repeated warnings that other climate scientists have made.
They noted that the state’s snowpack will not provide as much water for the growing population, and that cities and towns might not be prepared to handle harder, longer rainfall and flooding.
“A 20% to 30% increase in precipitation might not sound like a big increase in a lot of ways, but it is for every flood wall, every levee, every dam,” Swain said. “Even very high thresholds (for this infrastructure) are not infinite and eventually you will exceed them.”
Marty Ralph, a climate researcher in Southern California who is not affiliated with the new study, says drilling down into the details of what future atmospheric rivers might look like is vital for planning. The head of the Center for Western Weather and Water Extremes at UC San Diego’s Scripps Institution of Oceanography has also published research showing that the biggest weather systems are getting bigger.
Ralph is currently working to improve shortterm forecasting of the storms.
“The frequency and intensity of atmospheric rivers determine whether California is in drought or flooded,” he said. “They really are the storms we need to watch out for when it comes to climate change.”
While the larger storms ahead may bring more rain, that doesn’t necessarily equate to more cumulative rainfall for the state. Climate models have generally been mixed about whether California’s total precipitation will rise or fall.
“There may be fewer days when it rains significantly and a lengthier dry season,” Swain said. “But we’re still going to have wet winters.”