After years of punishing drought, historic amounts of rain and snow fell on California in the winter of 2016–17. The season's deluge was followed by one of the hottest and driest summers on record. Firestorms raged across the state in the fall, and when the first intense rains hit Southern California in December, mudflows swept down the mountainside in Montecito, damaging hundreds of structures and homes and killing 21 people. It's been a wild few years of weather in California, and a new study published this week in Nature Climate Change finds that these kind of drought-to-deluge swings will only become more frequent in the future.
California's climate naturally fluctuates between wet winters and dry summers, and, historically, precipitation extremes like severe droughts and floods were relatively rare—though not unheard of. In 1862, for example, a series of storms dropped so much rain over just 45 days that the Central Valley and Los Angeles Basin were practically turned into an inland sea up to 300 miles long and 60 miles wide. Climate change is expected to make such extreme events both more common and more severe in California and around the world.
In the new study, a team of researchers from the University of California–Los Angeles looked at extreme historical events, like the flood of 1862 and the drought of 1976–77, and models of future climate scenarios to find out not only how common precipitation extremes might become by the end of the century, but also how often drastic transitions between drought and flood conditions, like the one that occurred in the winter of 2016–17, will occur. The authors call these rapid transitions "whiplash events."
Under a business-as-usual emission scenario (which is, unfortunately, still the scenario that appears most likely given current emissions trends), the model suggests that California's seasons will become "sharper," with wetter winters and drier summers. Extreme events at both ends of the spectrum will become more frequent—a finding in line with previous studies on California's future climate. The risk of events on par with the 1862 flood is expected to rise by 300 to 400 percent. In other words, storms of that magnitude might hit the state every 50 years or so, instead of every 200 years.
Indeed, one projection the authors found particularly alarming is that it's "more likely than not" that such an event will strike California's cities sometime before 2060. This would almost certainly be an economic disaster: The flood of 1862 bankrupted the state, and a repeat today could cost the state upwards of $1 trillion.
Meanwhile, whiplash events are expected to increase by 100 percent in Southern California by the end of the century, and up to 25 percent in Northern California. Perhaps most disconcerting, the authors write that their analysis found a "substantial increase in the projected risk of extreme precipitation events exceeding any that have occurred over the past century—meaning that such events would be unprecedented in California’s modern era of extensive water infrastructure."
This finding is particularly disconcerting given incidents like the Oroville Dam disaster, in which the dam's spillway broke apart in February of 2017 following a particularly intense winter storm, forcing nearly 200,000 people to evacuate.
The good news is the worst impacts could still be averted—provided big emitters around the world, including the United States, get it together to drastically reduce carbon emissions, which maybe makes it not such good news after all.