The size of rainstorms hitting Los Angeles has been getting bigger over the past 60 years, according to a new report released today by the Environment California Research and Policy Center.
The environmental advocacy group measured rainfall in the Los Angeles metro area since 1948 and found that a storm large enough to occur only once a year decades ago is now happening every 8.8 months.
Similar trends were seen throughout much of California and nationwide. Overall, California experienced a 13 percent increase in extreme rainstorms and snowstorms between 1948 and 2011, one of 43 states to see statistically significant increases.
The report “When It Rains, It Pours” attributed the nationwide rise in extreme storms to global warming, although some experts are still hesitant to link climate change to relatively short-term weather patterns. It”s also unclear what an increase in extreme storms means for Los Angeles” water supply.
Travis Madsen, one of the report”s lead authors and a policy analyst at the Frontier Group, an environmental think tank, called extreme rainfall frequency “one of the clearest ways in which we can see the impact of the change in climate.”
He said that while attributing extreme rainfall to global warming “sounds counterintuitive,” higher temperatures cause more evaporation and allow the atmosphere to store more water vapor, leading to bigger storms.
Over the last few years, scientists have increasingly attributed extreme weather events to global warming.
The Intergovernmental Panel on Climate Change, a group of leading climate scientists from around the world, released a report in November saying that global warming will cause stronger storms, harsher droughts and heavier rainfall.
“It”s a significant trend, and it”s the kind of thing that we can expect more of in the future if we continue to emit lots of global warming pollution,” Madsen said.
Bill Patzert, a climatologist at the National Aeronautics and Space Administration”s Jet Propulsion Laboratory in La Cañada Flintridge, called the report a comprehensive study that correctly identifies an increase in extreme storms in California.
But Patzert questioned whether that trend has been caused by global warming.
Data for Los Angeles and Sacramento going back to 1878, Patzert said, shows that the overall amount of rainfall in California has stayed roughly flat. To look for global warming trends, he said, “you need a much longer record.”
“If you only use 50 years of data, which they did, they”re right. The rainfall is increasing, both in LA and Sacramento,” he said. “But that”s because from 1948 to 1975, it was generally dry in California, and the `80s and `90s were generally wet. And that had nothing to do with global warming.”
Blaming extreme weather events on global warming, Patzert added, is a “huge stretch that”s definitely leapfrogging good science.”
“Does my intuition tell me that”s going to be a preview of coming attractions? Yes it does,” Patzert said. “But is the science airtight at this point? I don”t think so.”
Regardless of what”s driving the increase in extreme storms, the trend could have serious consequences for Los Angeles” water supply and economy.
Madsen noted that strong storms can damage infrastructure and cause landslides and flooding, adding that rainwater from extreme storms is harder to capture.
But David Pettijohn, the Los Angeles Department of Water and Power”s manager of water resources, noted the increased rainfall could also benefit Los Angeles. The city has invested in more efficient stormwater capture, and more rain will help recharge groundwater supplies in the San Fernando Valley, he said.
This strategy and others “will reduce L.A.”s reliance on more expensive imported water and protect our customers from higher costs in the future,” Pettijohn said in a statement.
The report did not identify a specific number of inches of precipitation defining an extreme rain event, but characterized it as among the largest storms since 1948 for any given weather station.
The report found that overall rainfall in California regions south of the Bay Area increased by 11 percent since 1948.
Additionally, the report found that the most extreme storms each year have been getting bigger, with the largest annual storm, in California regions south of the Bay Area, increasing by 7 percent since 1948. Madsen said there was not enough data to determine whether that trend has been seen in Los Angeles specifically.