Warmer weather will make California rainstorms even wetter, study says

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

The strongest of the California storms due to atmospheric rivers, long and wide plumes of moisture that form over the ocean and flow through the sky above the earth, will likely lead to a 34% increase in total precipitation, or another 11 trillion gallons more than just what fell out. That’s because rain and snow are likely to be 22% more concentrated at the peak in places that are actually flooded and fall over a significantly larger area if fossil fuel emissions rise uncontrollably, according to a new study released Thursday. in the journal Nature Climate Change. .

The study says the entire western United States is likely to see a 31% increase in precipitation due to these strongest of the world’s strongest storms, with warming due to more intense and widespread precipitation.

Scientists say the worst-case scenario, which is about 4.4 degrees Celsius (7.9 degrees Fahrenheit) warming since pre-industrial times, looks a little more unlikely as efforts are made to curb emissions. If countries do what they promise, temperatures could rise by about 2.7 degrees Celsius (4.9 degrees Fahrenheit), according to the Climate Action Tracker.

The National Weather Service estimated that California received an average of 11.47 inches of rainfall statewide from December 26 to January 17, including 18.33 inches in Oakland and 47.74 inches at one location 235 miles north of San Francisco. , due to a series of nine destructive atmospheric rivers that caused power outages, floods, dam failures, washouts and landslides. At least 20 people died.

“It could have been worse,” said study author Ruby Leung, a climatologist at the U.S. Pacific Northwest National Laboratory. “We need to start planning how we can handle this.”

Leung used computer simulations on a regional scale to predict what the strongest of the westerly winter storms would be between 2040 and 2070 in a scenario where carbon emissions are out of control. She looked at total rainfall, how concentrated peak rain and snowfall would be, and the areas they hit. All three factors are growing for the West as a whole. California is predicted to have the largest increase in peak rainfall, while the southwest is likely to have more rainfall due to the large jump in rainfall area. The Pacific Northwest will see the least juice out of the three areas.

The total amount of precipitation decreased slightly due to the addition of all factors, because, according to the study, as precipitation increases at the peak, precipitation at the edges of storms is predicted to weaken.

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

And these flash floods are likely to get worse due to what Leung’s article calls the “spiky” effect that is happening in an increasingly warm world. This means that more precipitation is concentrated in the central regions of the storms, falling at a higher rate per hour, while precipitation is slightly lighter at the outer edges.

This is due to the physics of rain showers, Leung said.

Not only can the atmosphere hold 4% more moisture per degree Fahrenheit (7% per degree Celsius), but what happens during a storm changes and causes even more rain to fall, Leung said. You have air rising inside the storm with a lot of water vapor condensing to produce rain and snow; it then releases heat, “which makes the storm more energetic and violent,” she said.

When the water vapor condenses, it falls as rain and snow at the edges of the storm, but warming tends to compress the precipitation towards the middle, Leung said.

“The concepts and implications of how precipitation characteristics can change are well defined 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 part of the study.

When she used computer simulations, Leung chose the most severe worst-case scenario for how the world’s carbon emissions would rise. It’s a scenario that used to be called business as usual, but the world doesn’t follow that path anymore. After years of climate talks and the rise of renewable fuels, the globe is heading towards less than worst-case warming, according to climate scientist Zeke Hausfather of tech company Stripe and Berkeley Earth.

“We are presenting more of a worst-case scenario, but we understand that if we do take action to reduce emissions in the future, we may be better off,” Leung said. “If we control emissions and reduce global warming in the future, we can limit the impact of climate change on society, especially the floods and extreme rainfall that we are talking about in this study.”