New Report: Extreme Downpours and Snowstorms Up 25% in Colorado

Environment Colorado Research and Policy Center

Denver, CO— A little over a year after two storms flooded Denver, closing streets and leaving tens of thousands without power, a new Environment Colorado Research and Policy Center report confirms that extreme rainstorms and snowstorms are happening 25% more frequently in Colorado than they were in 1948.  

“As the old saying goes, when it rains, it pours—especially in recent years as bigger storms have hit State more often,” said Bessie Schwarz, Field Organizer for Environment Colorado Research and Policy Center.  “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.”  

Based on an analysis of state data from the National Climatic Data Center, the new report found that heavy downpours and snowstorms that used to happen once every 12 months on average in state now happen every 9.6 months on average.  Moreover, the biggest storms are getting bigger. The largest annual storms in Colorado now produce 8% more precipitation, on average than they did 65 years ago.

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.  

Schwarz pointed to the rainstorms that hit Denver last month as an illustration of what more extreme rainstorms and snowstorms could mean for the state.  That rain and hail storm of torrential rain and golf ball-sized hail, dumped 8 inches of hail in total on the area. Snowplows were called back into commission and in Colorado Springs, 40 people were rescued from vehicles submerged in icy water.

 The new Environment Colorado Research and 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.  

Key findings for Colorado and the mountain region include:
•    Extreme rainstorms and snowstorms are becoming more frequent. Colorado experienced a 25 percent increase in the frequency of extreme rainstorms and snowstorms from 1948 to 2011.  In other words, heavy downpours or snowstorms that happened once every 12 months on average in 1948 now happen every months, on average.  
•    Storms with extreme precipitation increased in frequency by 26% percent in the mountain region during the period studied.
•    The biggest rainstorms and snowstorms are getting bigger. The amount of precipitation released by the largest annual storms in Colorado increased by 8% percent from 1948 to 2011.

Dr. Kevin E. Trenberth, Distinguished Senior Scientist in the Climate Analysis Section at the National Center for Atmospheric Research commented on the report and provides a global perspective: “Changes in precipitation and water availability are likely the biggest pressure point on society as the climate changes. Increases in extremes are already observed and projected to get worse.  As well as more intense rains (and snows), the risk of severe drought also goes up. We have experienced the latter in the United States this year while floods have occurred in Japan, Europe and China.  Management of water is already a major challenge and adding climate change makes it more so.”

Schwarz was careful to note that an increase in the frequency and severity of extreme rainstorms does not mean more water will be available for human use. Hotter temperatures fuel extreme rainstorms by increasing rates of evaporation. At the same time, however, that evaporation increases soil dryness. Moreover, 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.

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 Colorado Research and Policy 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.  

“How serious this problem gets is largely within our control – but only if we act boldly to reduce the pollution that fuels global warming,” said Schwarz.  “We applaud the Obama administration for their proposals to cut carbon pollution from vehicles and new power plants, and urge them to move forward with finalizing these critical initiatives this year.”

View the report.