Earth is warming up. So why is California having a record-breaking winter? (2024)

In the Bay Area, the past three months have included historic rains, record-breaking low temperatures, and even snow in places like the Berkeley hills and North Bay highlands. In the Sierra Nevada, storms and frigid temperatures have produced so much snow that this year’s snowpack could become the largest ever recorded for the state, following upcoming storms.

A Pineapple Express — an atmospheric river ferrying moisture from waters off the coast of Hawaii — brought even more wet weather to California on Thursday, prompting Gov. Gavin Newsom to proclaim a state of emergency for an area that includes San Francisco, San Mateo, Santa Clara and Napa counties. Areas at high risk for excessive rains and flash floods are under yet another round of evacuation orders.

“My anxiety is on high alert every time I’ve had to leave my house over the last couple of months,” Ben Lomond resident Cheryl Noon told The Chronicle on Thursday. Noon and her two children have had to evacuate from the Santa Cruz Mountains three times since January.

“I don’t know how long I’m going to be gone,” Noon said.

All this winter weather may seem to be at odds with the hotter, drier California that scientists expect with climate change, as greenhouse gas emissions raise global temperatures. But that trend is taking place over longer timescales, across the entire planet. What happens in California from year to year — or even winter to winter — can vary dramatically and still fit into the bigger story, scientists say.

“Weather does not equal climate,” Erica Siirila-Woodburn, a hydrology research scientist at Lawrence Berkeley National Laboratory, said by email.

Chilly temperatures

Throughout the West, this winter has averaged about 2-4 degrees cooler than normal, said Julie Kalansky, a climate scientist at the Center for Western Weather and Water Extremes at Scripps Institution of Oceanography, UC San Diego.

Bay Area residents especially felt the chill during February. In downtown San Francisco, temperatures averaged well below normal, particularly during the last week of the month.

Back-to-back storms after President’s Day funneled Arctic air toward California, resulting in abnormally chilly temperatures in the Bay Area. One of the coldest days this year was Feb. 24, with a low temperature of 39. That was a record minimum temperature for the date. The previous record of 40 degrees was established in 1891.

The past three months came in as California’s 31st-coldest winter on record, but is only a blip in a longer warming trend dating to the 1890s.

“Within that there is variability,” Kalansky said.

The coldest California winter was in 1949, where the state averaged 36.8 degrees -- 6.8 degrees below normal for the century.

Random chaos in the atmosphere can also produce extreme outcomes.

“Weather systems can do their own thing,” said Isla Simpson, a climate scientist at the National Center for Atmospheric Research.

Some scientists also think that atmospheric warming can change how air masses move around the planet by altering jet streams, strong winds that travel about 5-9 miles above the Earth’s surface.

“As the Earth warms, the general circulation that keeps a nice jet stream and contains that polar air up in the polar regions starts to break down,” said Mike Anderson, state climatologist with the California Department of Water Resources, during a March 3 media briefing.

As a result, cold air masses can move farther south, toward California.

But the influence of global warming on the polar jet stream is still an active area of research, and other scientists disagree on whether the jet stream is becoming more wavy or not.

“At this point, it’s pretty hard to really pull out what is a clear climate change signal — in terms of waviness of the jet stream — in the observational record,” Simpson said.

Historic snow and rain

Cold winter storms in February brought unusual low-elevation snow to the North Bay highlands, East Bay hills and Santa Cruz Mountains. There were even reports of snow on Santa Cruz beaches, though experts confirmed it was a close relative known as graupel.

In the Sierra, these storms piled even more snow atop the deep snowpack deposited during a series of storms in December and January fueled by atmospheric rivers.

All this snow seems to contradict what scientists expect with climate change: a warmer atmosphere that results in precipitation falling as rain rather than snow, and a snowpack more susceptible to melt.

Little to no snow could become persistent in 35-60 years if greenhouse gas emissions remain unchecked, according to model projections in a 2021 review, led by researchers at Lawrence Berkeley National Laboratory. The authors defined a “low-to-no” snow year as one where the amount of water held in snow was in the bottom 30% of historical measurements. They also highlighted the importance for further analyses, using additional models.

As with the average temperatures, California’s winter precipitation can vary dramatically.

“The snowpack conditions of this year — while impressive — are not necessarily outside the realm of what we might expect,” said review co-author Siirila-Woodburn. “The western U.S. climate is historically highly variable from year to year, with a majority of precipitation coming from a handful of storms per year.”

In fact, scientists expect that storms will become more intense in the future due to climate change, as a warmer atmosphere can hold more moisture. That means more fuel for storms.

While storms are expected to produce more precipitation in the future, droughts are also expected to become more severe. Scientists expect this to drive bigger swings between wet and dry conditions — sometimes referred to as “weather whiplash.”

Even this winter, while snow has been bountiful in the Sierra Nevada, it’s low elsewhere in the world.

“Other mountain regions in the world, such as the Swiss Alps, have recorded an all-time low in snowpack conditions since records have been kept,” Alan Rhoades, a hydroclimate research scientist at Lawrence Berkeley National Laboratory and a co-author of the 2021 review, said by email.

That’s why it’s important to keep in mind that this winter in California may be an anomaly in a long trend of warming and snow loss around the planet. As the state swings more dramatically between wet and dry years, it will be even more important that measures are in place to maintain the state’s water supply, Siirila-Woodburn said.

“Last year California had a prolonged midwinter dry period, in stark contrast to conditions this winter,” Siirila-Woodburn said. “When precipitation comes, it’s important — now more than ever — to implement strategies that can preserve that water for times of water scarcity.”

Reach Jack Lee: jack.lee@sfchronicle.com