California is warming to make torrential rains even wetter, study says

As damaging as more than 32 trillion gallons of rain and snow have fallen on California since Christmas, a worst-case global warming scenario could increase future similar downpours by a third by the middle of this century, according to a new study. .

California’s strongest storms from atmospheric rivers, long, wide clouds of moisture forming over an ocean and flowing from the sky over land, would likely take up an overall 34% increase in total precipitation, or 11 trillion gallons more than fallen. That’s because rain and snow are 22% more concentrated at their peak in places that are truly drenched, according to a new study published Thursday in the journal Nature Climate Change, and will fall much more widely if fossil fuel emissions increase uncontrollably. .

All regions of the western US will likely see a 31% increase in precipitation from the worst of these worst storms in a world that is warming due to more intense and widely spreading precipitation, the study found.

The worst-case scenario, which has been about 4.4 degrees Celsius (7.9 degrees Fahrenheit) of warming since pre-industrial times, seems somewhat more likely as efforts have been made to rein in emissions, the scientists say. Temperatures are on track to reach around 2.7 degrees Celsius (4.9 degrees Fahrenheit) if countries deliver on their promises, according to Climate Action Tracker.

The National Weather Service calculated that California averaged 11.47 inches of precipitation From December 26 to January 17, statewide — including 18.33 inches in Oakland and 47.74 inches at a point 235 miles north of San Francisco — caused power outages, flooding, levees, flooding, and landslides. due to a series of nine destructive atmospheric rivers. At least 20 people died.

“It could be worse,” said study author Ruby Leung, a climate scientist at the US Pacific Northwest National Laboratory. “We need to start planning how we can deal with this.”

Leung used regional-scale computer simulations to predict what the worst winter storms in the west would be like between 2040 and 2070 in a scenario where carbon emissions go wild. It looked at total precipitation, how intense the heavy rain and snowfall would be, and the affected area. All three factors are growing for the West in general. California is predicted to receive the highest increase in precipitation, while the Southwest is likely to see more rain due to the large jump in precipitation. The Pacific Northwest will see the least juice among the three areas.

Adding all factors together, total precipitation decreased slightly, according to the study, as precipitation at the edges of storms is predicted to weaken as peak precipitation increases.

There are two types of storms that Leung says he’s worried about: flash floods caused by heavy rain concentrated in a small area, and slower, larger floods caused by rain and snow accumulating over a large area. Both are bad, but flash floods cause more damage and hurt people more, he said.

And these flash floods will likely be exacerbated by what Leung’s article calls the “sharpening” effect, which is occurring in an increasingly hot world. This means more precipitation concentrated in the central areas of the storms and falling at higher rates per hour, while precipitation is slightly weaker at the outer edges.

Leung said this is because of the physics of rainstorms.

Leung said the atmosphere is not only holding 4% more moisture per degree Fahrenheit (7% per degree Celsius), but what changes in the storm, causing precipitation to drop even more. There is air rising inside the storm as more water vapor condenses to form rain and snow; then it releases heat “causes this type of storm to be more severe and stronger,” he said.

When water vapor condenses, it travels down as rain and snow along the edges of the storm, but warming compresses the falling precipitation toward the middle, Leung said.

“The concepts and effects of how precipitation characteristics can change are well quantified and well explained,” said David Gochis, who is not part of and an expert on how water affects weather at the National Center for Atmospheric Research in Boulder, Colorado. of your study.

Using computer simulations, Leung chose the most serious worst-case scenario of how the world’s carbon emissions would increase. A scenario that used to be called business as usual, but the world is no longer that way. After years of climate negotiations and the rise of renewable fuels, the world is heading towards less warming than the worst-case scenario, according to climate scientist Zeke Hausfather of tech company Stripe and Berkeley Earth.

“We present more of the worst-case scenario, but we understand that we can do 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 effects of climate change on society, especially the flooding and heavy rains we talk about in this study.”

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