Climate change makes natural disasters that much worse than they otherwise would. A new study that was published in Science Advances used data from the Community Earth System Model Large Ensemble. It found that warming doubled the likelihood of a catastrophic event and only increased the future threat as more water vapor in the atmosphere increases every day as the climate system breaks down.
In the modern era, California is used to water scarcity rather than overabundance, note the researchers. “Observed extreme precipitation and severe subregional flood events during the 20th century—including those in 1969, 1986, and 1997—hint at this latent potential, but despite their substantial societal impacts, none have rivaled (from a geophysical perspective) the benchmark “Great Flood of 1861–1862”. That event was characterized by weeks-long flooding events in winter storms and catastrophic flooding in all low-lying areas of the state. The San Joaquin and Sacramento valleys became temporary inland sea and inundated the counties of present-day Los Angeles and Orange counties built on the coastal plain. “The primary physical mechanism responsible for this projected regional intensification of extreme precipitation is an increase in the strength of cool-season atmospheric river (AR) events (19–21). Previous analyses have suggested that the thermodynamically driven increase in atmospheric water vapor with warming is directly responsible for most of this projected AR intensification [e.g., (16)], with the remainder contributed by shifts in regional atmospheric circulation.”
The Capital Weather Gang, as usual, writes a must-read post in The Washington Post. Up to eight feet of rain in many parts of California could fall.
On the West Coast, there commonly are atmospheric rivers, or streams of moisture-rich air at the mid-levels of the atmosphere with connections to the deep tropics. For a California megaflood to occur, you’d need a nearly stationary zone of low pressure in the northeast Pacific, which would sling a succession of high-end atmospheric rivers into the California coastline.
“These would be atmospheric river families,” Swain said. “You get one of these semi-persistent [dips in the jet stream] over the northeast Pacific that wobbles around for a few weeks and allows winter storm after winter storm across the northeast Pacific into California.”
snip
“We have multiple scenarios. The future one is much larger, consistent with [climate change],” he said. “In the historical scenario, the lesser one, certain parts of the Sierra Nevada see 50 to 60 inches of liquid-equivalent precipitation … but in the future event, some places see 70 to 80 and a few see 100 in a 30-day period. Even places like San Francisco and Sacramento could see 20 to 30 inches of rain, and that’s just in one month.”
snip
A warmer atmosphere has a greater capacity to store moisture. In the absence of storms, that means the air can more quickly dry up the landscape — hence California’s prolonged drought — but should rain occur, the deck is stacked to favor an exceptional event.
“Moisture isn’t the limiting factor in California,” Swain said. “There’s plenty of moisture around even in the drought years. The absence is a lack of mechanism. It’s a lack of storms rather than moisture.”
The NY Times also writes on the rising threat to the Golden State.
This vapor plume will be enormous, hundreds of miles wide and more than 1,200 miles long, and seething with ferocious winds. It will be carrying so much water that if you converted it all to liquid, its flow would be about 26 times what the Mississippi River discharges into the Gulf of Mexico at any given moment.
When this torpedo of moisture reaches California, it will crash into the mountains and be forced upward. This will cool its payload of vapor and kick off weeks and waves of rain and snow.
One storm after another will slam the cities and rural areas alike.
The coming superstorm — really, a rapid procession of what scientists call atmospheric rivers — will be the ultimate test of the dams, levees and bypasses California has built to impound nature’s might.
But in a state where scarcity of water has long been the central fact of existence, global warming is not only worsening droughts and wildfires. Because warmer air can hold more moisture, atmospheric rivers can carry bigger cargoes of precipitation. The infrastructure design standards, hazard maps and disaster response plans that protected California from flooding in the past might soon be out of date.