San Francisco Chronicle - (Sunday)
Climate change ramping up winter storms
A powerful winter storm buried the Sierra last weekend, with wet weather continuing for days in the Bay Area and Central Coast. Thunderstorms Wednesday drenched Salinas, dropping an entire inch in just 25 minutes.
After historic weather last year, intense California storms have persisted this winter, with strong downpours causing widespread flooding in San Diego and damaging landslides in places like Los Angeles.
Many ingredients contribute to extreme storm activity, but scientists agree that climate change is already amping up winter rains — and may bring even wilder weather in the future.
Warmer and wetter atmosphere
A historic storm stalled over Southern California earlier this winter, dropping more than 7 inches of rain on downtown Los Angeles on Feb. 4 and 5, its thirdhighest two-day total going back to 1877. Some places in the San Gabriel and Santa Monica mountains logged more than a foot of rain over 3 days. Downpours produced widespread flooding and mudslides.
Climate change probably had a hand in the extraordinary storm totals, experts say, because a warmer planet can make for juicier storms.
The atmosphere can hold about 7% more moisture for each degree Celsius — 1.8 degrees Fahrenheit — of warming, said Amir AghaKouchak, professor of civil and environmental engineering at UC Irvine. That means in a warmer climate storms can produce extreme rainfall more often than in the past.
“We have places where average precipitation has remained more or less the same, but we see more and more extreme events,” AghaKouchak said.
In the case of the storm that drenched Los Angeles, climate change likely boosted rainfall, said UCLA climate scientist Daniel Swain during a briefing.
“Probably there was an extra inch or two of rain from this event … that wouldn’t have occurred had the climate not warmed as much as it has in the past century,” Swain said.
Warmer temperatures also mean elevations accustomed to snow during winter storms are instead receiving rain, as snow is restricted to higher, chillier peaks. That happened much of this winter as storms tracked across the Pacific to California.
But where and when it’s cold enough, extra moisture in the atmosphere can enhance snow totals too. A recent storm that dipped down to California from the Gulf of Alaska, brought epic snow in the Sierra Nevada: Sugar Bowl Resort tallied 10.5 feet of snow over four days.
“In general we should probably expect that the amount of precipitation in some of these really big, individual precipitation events — whether they’re rain or snow events — is probably around 10% or so greater, than it would have been prior to the amount of warming that we’ve seen,” Swain said in a briefing. “Ten percent is not a small number.”
Ocean and storm feedback cycles
Earlier this winter, an atmospheric river-fueled storm made landfall in Southern California, causing dangerous flash flooding and millions of dollars in damage. San Diego tallied 2.73 inches of rain on Jan. 22, the city’s fourth-wettest day since 1850. Nearby locations logged 2 to 3 inches of rain in an hour, rates with a 1-in-100 to 1-in-1,000 chance of happening any given year.
Abnormally warm waters off the coast in January allowed the atmospheric river to absorb more heat and water vapor, said Marty Ralph, director of the Center for Western Weather and Water Extremes at UC San Diego’s Scripps Institution of Oceanography, by email.
This “strengthened the precipitation and contributed to the flooding,” he added.
For the world’s oceans, 2023 was the hottest year on record by a long shot. Warmer Pacific water impacts marine life, and may be giving storms like January’s an extra injection of energy and moisture just before they make landfall in California.
Scientists are just beginning to study the connection between warming oceans and intensifying storms. For a series of atmospheric rivers during a sweltering marine heat wave in 2014, researchers found the warmer waters evaporated more readily, donating additional moisture to storm clouds overhead.
“Our model simulations suggest that in some areas along the coast of Southern California, at least for that December’s storms, you can get up to double the amount of rain due to the presence of the marine heat wave,” said Christoph Renkl, an oceanographer at Woods Hole Oceanographic Institution.
“What’s going to happen 10, 50 years from now, when we have an even warmer ocean?” said Arthur Miller, Renkl’s collaborator at Scripps.
Future extremes
While storms are expected to become stronger in a warmer world, that doesn’t mean droughts will become a thing of the past. Instead, wet and dry periods will probably both grow more extreme, resulting in “hydroclimate whiplash” that swings more dramatically than in the past.
More extreme precipitation in the future could impact infrastructure like dams and storm drain systems, which weren’t designed for these increasingly severe — and frequent — weather events.
“We need to … come up with solutions (and) ideas to protect ourselves against future extremes,” AghaKouchak said.