When it comes to California, the concept of an average winter wet season is as useless as a bike on Los Angeles’ 405 freeway. Witness the recent multi-year drought, followed by one of the wettest winters on record in 2016-17. Or just check out this past winter, which started out bone dry before a wild string of storms deluged the state.
Climate change will likely only further degrade the concept of average, according to new findings published in Nature Climate Change on Monday. Researchers are calling for the coming decades to be full of “precipitation whiplash,” with winters swinging wildly between extremely dry or wet conditions. Climate change is also likely increase the odds of a winter like that of 1861-62, when it snowed and rained so hard that Los Angeles received four times its annual average precipitation and Sacramento spent months under muddy floodwaters.
The findings have implications for Californian of all stripes, from people living in wildfire-prone areas or in the shadows of dams, to farmers, to anyone who likes to have water when they turn the tap on.
Climate models have shown that California’s precipitation isn’t expected to change much, averaged over the 21st century. Daniel Swain, a climate research at the University of California, Los Angeles who led the new research, told Earther he had a hunch that this fact was covering up a much more interesting and important reality.
He and his colleagues decided to look at changes in the characteristics of precipitation over November-March, which is traditionally when California gets the lion’s share of its precipitation. Rather than look for a linear trend over time, they looked at modeled changes in the frequency of extremely wet and extremely dry winters , which they define as those having a four percent chance of happening in a given year without climate change. The findings are startling.
They show extremely wet winters could become 100-200 percent more common by the end of the century. Dry winters could become up to 140 percent more common, with the biggest risk of drought years coming in Southern California. The odds of wild swings from year to year are also likely to increase, meaning those recent examples of whiplash could basically become the norm in Southern California by 2100.
The character of individual wet seasons is also likely to change. Shoulder season precipitation likely to dry up, something which happened this past fall and led to the largest wildfire in California’s history.
But perhaps even more important is what the study predicts will happen to California’s atmospheric rivers, storms that tap moisture from the tropical Pacific and dump copious mountain snows and valley rain. The storms fill reservoirs, but can also be incredibly destructive.
According to Swain, the odds a winter like 1861-62 increase substantially due to climate change. That winter was marked by a cold series of storms that dumped prolific snows, followed by warm atmospheric rivers that melted them out. The result was a thunderous flood that killed cattle, drowned the state capital, and altered a way of life.
“We found that at least one such occurrence is more likely than not between now and 2060—which is really not a long time horizon from a planning perspective (essentially 40 years),” Swain said. “That’s a statistic that will probably raise some eyebrows in the water management community, especially considering some of the recent infrastructure failures California has recently experienced during events of a much lesser magnitude.”
Researchers at the U.S. Geological Survey have modeled what an 1862-type flood event would mean for California today. Their research shows it could cause upwards of $725 billion in damage, dwarfing what a big earthquake could do. Despite this, much of the state’s infrastructure isn’t ready for a scenario of this magnitude, according to the USGS report.
“This paper nicely shows this pattern emerging in precipitation for climate change projections over California,” Benjamin Cook, a researcher at NASA and Columbia University, told Earther in an email. “This change is particularly important in this region, because people and ecosystems in the state really are vulnerable to both dry (e.g., recent multi-year droughts) and wet (flooding, landslides) extremes, which can often occur one year after the other.”
The state is already struggling to deal with the climate shocks of today. The drought years of 2012-16 caused billions in agricultural losses and turned reservoirs into mud pits. Then came the 2016-17 winter, which filled those reservoirs and almost caused a the Oroville Dam in Northern California to collapse. The wet weather also caused vegetation to grow, which was great until the dry start to the 2017-18 wet season led to the most destructive wildfires in state history in Northern California, followed by the largest wildfire in state history in Southern California.