FLOODSRisks of Megafloods in California Increase

Although California is currently contending with historic drought, a new study indicates that climate change is sharply increasing the risk of a catastrophic megaflood that could submerge large swaths of the state and displace millions of residents.

The study by scientists at the National Center for Atmospheric Research(NCAR) finds that the likelihood of such a calamitous flood has already doubled because of climate change. If society were to follow a worst-case climate scenario of emitting greenhouse gasses at a high rate, the state by later this century could expect to experience a catastrophic megaflood about every 30-35 years, instead of just once in two centuries.

The last time such an event hit California, in 1861-62, the deluge of water transformed much of the state into an inland sea, killing 1% of the population and resulting in devastating economic losses.

“The megaflood risk with climate change is much greater than we expected,” said NCAR scientist Xingying Huang, the co-lead author of the study. “These types of extreme events are rare, but they will happen, as predicted, with more frequency as warming continues.”

The extreme rainfall required to create a megaflood would likely be associated with a series of  atmospheric rivers — long and narrow regions in the atmosphere that are a critical source of cool-season moisture on the West Coast. Previous research has shown that climate change is altering the intensity and path of atmospheric rivers. The authors used a combination of advanced computer models to detail how these changes will affect extreme precipitation and runoff across California.

They found that the state not only faces the threat of more intense winter precipitation but also disproportionately greater volumes of runoff. That is partly because more precipitation will fall as rain instead of snow, thereby immediately rushing down slopes instead of slowly melting into the soil.

“Although California has recently experienced historically severe drought and the broader Southwest is facing an accelerating water scarcity crisis, it’s important to remember that this is still a region susceptible to rare but potentially severe floods,” said co-lead author Daniel Swain, a University of California, Los Angeles scientist who is also affiliated with NCAR and The Nature Conservancy. “It may seem paradoxical that climate change is increasing the risks associated with both droughts and floods in a place like California, but that’s exactly what the scientific evidence suggests.”