Precipitation over much of California and the West has been mostly concentrated in strong storms, with long dry spells in between.
The net effect is to stop it, researchers find in a new study. It’s not just the western United States; the same is true in many parts of the world.
The study is the first to reveal how these extreme rainfall events, such as small, extreme events, shape the landscape.
“The more rain you get, the drier you get,” said Justin Mankin, an associate professor of geography at Dartmouth College who collaborated on the study.
Heavy rainfall is sometimes above the ground, and the ground can absorb a lot at once. Mankin said it’s like “asking the country to drink from a fire hose.”
“As you concentrate the rain into heavy rains, a lot of water stays on the ground so it can evaporate easily,” he said.
This trend is less pronounced in Southern California and more pronounced in the North. The American West is one of the areas where the rain has accumulated the most.
The analysis, published Wednesday in the journal Nature, provides new insight into how precipitation changes as the climate warms.
Scientists have analyzed rainfall worldwide from 1980 to 2022. To determine which areas grew drier or wetter, they used data from satellites that track water changes across the region.
Researchers found that precipitation in the Rocky Mountains has increased by 20%, affecting the Colorado River, California’s largest water source. The river has shrunk significantly since 2000 during a drought that scientists say is probably the worst in 1,200 years.
Experts have long expected global warming to produce less frequent but more intense rainfall. Research shows that precipitation consolidation is already occurring across much of the western US
“It’s consistent with what we expected from climate change, because hot air can hold more water vapor,” said Corey Lesk, who led the study as a researcher at Dartmouth and is now a professor of Earth and atmospheric science at the University of Quebec in Montreal.
As more planet-warming gases come from fossil fuels, rising temperatures also cause more moisture to evaporate from the land and cause plants to absorb more moisture.
California naturally has dramatic and sometimes volatile changes between droughts and floods. Weather models have shown increased rainfall in the state, especially in atmospheric river storms.
As temperatures rise in the future, climate models show that Southern California will become slightly drier and Northern California likely to become slightly wetter, said Alexander Gershunov, a research climatologist at the Scripps Institution of Oceanography at UC San Diego who was not involved in the study.
Warmer temperatures are also reducing the snowpack in the Sierra Nevada, he said, and that means more of the state’s water will come from big trees during high-altitude rivers.
Research shows that rainfall has become more concentrated regardless of whether the region has a wet or dry climate.
The trend of fewer but stronger storms “really reveals the mechanics of how climate change will affect water resources for everyone,” Lesk said.
Other studies have shown that large regions of the world are dry growthincluding the “drier” region that stretches from the western US through Mexico to Central America.
Recent research shows that the amount of water available in a given area depends as much on the concentration of rainfall as it does on the total amount of rainfall, Mankin said.
In California and other western states, he said, the findings suggest current measures to deal with drought and flooding are inadequate.
“This is just another indicator … we’re not keeping up with the weather we’re having, let alone what’s happening,” he said.
