Climate change: Worst-case global warming scenario could make California’s storms trillions of gallons wetter, study says

As damaging as it has been that more than 32 trillion gallons of rain and snow has fallen on California since Christmas, a worst-case global warming scenario could amplify similar future downpours by a third by the middle of this century, a new study says.

California’s strongest storms, consisting of atmospheric fluxes, long and wide plumes of moisture that form over an ocean and flow through the sky over land, would likely see a 34% total increase in total precipitation, or an additional 11 trillion gallons more than just fall. That’s because if fossil fuel emissions grow unchecked, rain and snow in places that really get doused are likely to concentrate 22% more and fall over a significantly larger area, according to a new study Thursday Journal of Nature Climate Change.

The entire western United States would likely see a 31% increase in precipitation from these worst of the worst storms in a souped-up, warming world due to more intense and widespread rainfall, the study found.

Scientists say the worst-case scenario of warming of around 4.4 degrees Celsius since pre-industrial times looks a little less likely as efforts are made to curb emissions. If countries do what they promise, temperatures are on track to rise by about 2.7 degrees Celsius (4.9 degrees Fahrenheit), according to Climate Action Tracker.

The National Weather Service calculated that from Dec. 26 to Jan. 17, California averaged 11.47 inches of rainfall statewide — including 18.33 inches in Oakland and 47.74 inches at a spot 235 miles north of San Francisco — due to a range nine devastating atmospheric rivers caused power outages, floods, levee failures, washouts and landslides. At least 20 people died.

“It could be worse,” said study author Ruby Leung, a climate scientist at the US Pacific Northwest National Lab. “We have to start planning how to deal with it.”

Leung used computer simulations at the regional scale to predict what the worst winter storms in the West between 2040 and 2070 will be like in a scenario where carbon emissions run amok. She looked at total precipitation, how concentrated rain and snow would be at peak times, and which area would be affected. All three factors are growing for the West in general. California is forecast to have the highest increase in precipitation peaks, while the Southwest is likely to see more rain due to a large increase in rainfall. The Pacific Northwest would see the least juicing of the three areas.

Total precipitation is reduced somewhat by adding all the factors, as the study says precipitation at the edges of the storms is expected to weaken as peak precipitation increases.

There are two types of storms she worries about, Leung said: flash floods caused by intense rain concentrated in a small area, and slower, larger floods caused by rain and snow piled up over a large area . Both are bad, but the flash floods are causing more damage and injuring people more, she said.

And those flash floods are likely to be exacerbated by what Leung’s article calls a “sharpening effect” occurring in a warming world. This means more precipitation is concentrated in the central areas of the storms and falling at higher hourly rates, while precipitation is slightly weaker on the outer edges.

This happens because of the physics of rainstorms, Leung said.

Not only can the atmosphere hold 4% more moisture per degree Fahrenheit (7% per degree Celsius), but it’s what happens in the storm that changes and causes precipitation to fall even more, Leung said. You have air rising within the storm, with more water vapor condensing to create rain and snow; It then releases heat “which causes the storm to get stronger and stronger,” she said.

As water vapor condenses, it comes down as rain and snow at the edges of the storm, but the warming kind of pushes that falling precipitation toward the center, Leung said.

“The concepts and implications of how precipitation characteristics are likely to change are well quantified and well explained,” said David Gochis, an expert on the effects of water on weather at the National Center for Atmospheric Research in Boulder, Colorado, who was not involved with the study.

Using computer simulations, Leung chose the worst-case scenario for global carbon emissions to increase. It’s a scenario that used to be called business as usual, but the world isn’t on that path anymore. After years of climate negotiations and the growth of renewable fuels, the globe is heading for less-than-worst-case warming, according to climate scientist Zeke Hausfather of technology company Stripe and Berkeley Earth.

“We’re providing more of a worst-case scenario, but understand that we could end up doing better if we take action to reduce emissions in the future,” Leung said. “If we control emissions and reduce global warming in the future, we can limit the impacts of climate change on society, specifically flooding and extreme rainfall, which we talk about in this study.”