New Report: Extreme Downpours Up 35% in California

Scientists Link Trend to Global Warming
For Release

The snowstorm that closed down the Grapevine in 2011: the Ridgecrest flood of 2008: the La Conchita mudslide of 2005: extreme rain and snow storms are on the rise in many parts of California according to a new Environment California Research & Policy Center report that confirms extreme rainstorms are happening 35 percent more frequently since 1948. 

“As the old saying goes, when it rains, it pours—especially in recent years as bigger storms have hit California more often,” said Bernadette Del Chiaro, director of global warming programs with Environment California Research & Policy Center.  “This isn’t just in our imaginations. This research shows that the weather is becoming more extreme.”

Based on an analysis of state data from the National Climatic Data Center, the new report found that heavy downpours that used to happen once every 12 months on average now happen every 10.7 months on average statewide.  Southern California and parts of the Central Valley are experiencing up to 72% increase in the frequency of extreme storms with a 7 percent increase in the intensity of the storms as well. In other words, in the south, storms are happening more frequently and are bigger in size.   

Scientists have concluded that the rise in the frequency and severity of heavy rainstorms and snowstorms is linked to global warming.  Warming increases evaporation and enables the atmosphere to hold more water, providing more fuel for extreme rainstorms and heavy snowstorms. 

“We need to heed scientists’ warnings that this dangerous trend is linked to global warming, and do everything we can to cut carbon pollution today,” said Del Chiaro. “How serious this problem gets is largely within our control – but only if we act now to reduce the pollution that fuels global warming through solutions like increasing clean energy and implementing key policies like AB 32.”

The new Environment California Research & Policy Center report, When It Rains, It Pours: Global Warming and the Increase in Extreme Precipitation from 1948 to 2011, examines trends in the frequency of and the total amount of precipitation produced by extreme rain and snow storms across the contiguous United States from 1948 to 2011.  Using data from 3,700 weather stations and a methodology originally developed by scientists at the National Climatic Data Center and the Illinois State Water Survey, the report identifies storms with the greatest 24-hour precipitation totals at each weather station, and analyzes when those storms occurred. The report also examines trends in the amount of precipitation produced by the largest annual storm at each weather station.

Nationally, the report found that storms with extreme precipitation increased in frequency by 30 percent across the contiguous United States from 1948 to 2011. Moreover, the largest annual storms produced 10 percent more precipitation, on average.  At the state level, 43 states show a significant trend toward more frequent storms with extreme precipitation, while only one state (Oregon) shows a significant decline. 

An increase in the frequency and severity of extreme rainstorms does not mean more water will necessarily be available for human use. Furthermore, the hotter temperatures that fuel extreme rainstorms by increasing rates of evaporation also lead to increased soil dryness. Scientists expect that, as global warming intensifies, longer periods with relatively little precipitation will tend to mark the periods between heavy rainstorms. As a result, droughts are likely to become more frequent and severe in some regions of the United States. Currently, more than half of the lower United States is suffering through prolonged drought, aggravated by the fact that the last six months have been the hottest January-June period on record.

 Key findings for California include:

  • Extreme rainstorms and snowstorms are becoming more frequent. California experienced a 35percent increase in the frequency of extreme rainstorms from 1948 to 2011 on average.  In other words, heavy downpours that happened once every 12 months on average in 1948 now happen every 10.7 months, on average. Areas like San Diego are experiencing and Santa Barbara are experiencing a 64 and 72 percent respective increase in frequency of extreme storms. Bakersfield is up 63%, Riverside is up 53%, Fresno is up 39% and Madera 45% and the Hanford region 61%.
  • Storms with extreme precipitation increased in frequency by up to 72 percent in Southern California and parts of the Central Valley and high desert regions during the period studied.
  • Northern California is seeing a decrease in extreme precipitation events showing almost a mirror image to the south with areas like Redding, Santa Rosa, and Yuba City seeing a 29, 34, 42 percent respective decrease in the frequency of extreme rainstorms since 1948.  

 According to the most recent science, the United States must reduce its total global warming emissions by at least 35 percent below 2005 levels by 2020 and by at least 85 percent by 2050 in order to prevent the most devastating consequences of global warming.  Environment California Research & Policy Center highlighted two proposals from the Obama administration—carbon pollution and fuel efficiency standards for cars and light trucks through model year 2025, and the first ever carbon pollution standards for new power plants—as critical steps toward meeting these pollution reduction targets. Meanwhile, the group continues to urge California decision makers to continue to keep the state on track to achieving, and then going beyond, the goals of the cap on carbon pollution mandated by AB 32.