Climate scientists say little chance Colorado floods caused by human CO2, cause was unusual confluence of atmospheric events-CS Monitor

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9/25/13, "Were Colorado floods result of global warming? Probably not." CSMonitor.com, Amanda Paulson

"The statistics for the storm that produced the Colorado floods are stunning. But there was a very similar storm in 1938, suggesting nature can surprise even without human help."

"The record-breaking storm that caused the Colorado floods has been called a “thousand-year event,” leading to speculation about whether the storm and flooding owed anything to climate change. The answer, of course, is impossible to know for sure. But according to a panel of climate scientists from Colorado, the storm likely had little to do with climate change and more to do with an unusual confluence of atmospheric events.

It’s possible that some shifts due to climate change, such as increased water vapor in the air, may have exacerbated the effects of the storm slightly. But a storm in September 1938 was very similar in its footprint, its timing, and the type of rainfall (which was not the brief, intense thunderstorms typical here).
  

“Having seen the September 1938 analog should somewhat humble us, and remind us that nature has a way of delivering without human intervention in the climate system,” says Martin Hoerling, a research meteorologist at NOAA’s Earth System Research Laboratory in Boulder, Colo. “Climate change is operating, there’s no question about that, and water vapor has gone up,” but in this instance, climate change “really wasn’t a factor.”Most striking about the recent storm was the sheer amounts of rain that fell in some areas. It set a new one-day record for rainfall in Boulder, 9.08 inches, that is almost double the previous record of 4.8 inches, set in 1919. The town, which averages just over 20 inches of rainfall in a year, has received 17.59 inches so far in September.

 

The storm was caused by a confluence of several unusual factors. A low-pressure system along the Utah-Nevada border helped pull a heavy plume of tropical moisture up from Mexico, a high-pressure system to the east pushed up even more moisture from the Gulf of Mexico, and a stalled front generated lift. All those factors set up a “blocking pattern” that helped keep the storm hovering over the same period for a long time.

Those events combining are highly unusual, said Klaus Wolter, a NOAA climate researcher who spoke at a panel discussion hosted by the Cooperative Institute for Research in Environmental Sciences (CIRES) at the University of Colorado and NOAA.",,,via Junk Science


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