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Climate Change Unlikely To Have Caused 2013 Colorado Floods

Dan Greenwood
A car on a flooded road in Boulder during the 2013 September floods.

After the extreme rains of 2013 hit Colorado, scientists wanted to know if the intense weather event was linked to climate change. After a year's worth of research, they have concluded that the unusual rain event was not made more likely or worsened by human-caused climate change.

"We shouldn't necessarily expect these events to happen more often because we didn't find that climate change made this type of event more frequent," said Martin Hoerling, a research meteorologist at the National Oceanic and Atmospheric Administration’s Earth System Research Laboratory in Boulder, Colorado, and lead author of the new study.

The report, published on Sept. 29 in the Bulletin of the American Meteorological Society, includes 22 individual case studies examining 16 severe weather events from 2013, of which the Colorado floods were one. For the past three years, the Society has published a bulletin examining extreme weather events and their potential links to climate change.

In order to understand whether the floods would have been made more likely on account of climate change, Hoerling and his fellow researchers ran a climate model set up to imitate both past conditions, before climate change, and current conditions, which include human-caused warming. In both models, heavy September rainfall similar to the 2013 floods were equally likely, indicating that climate change did not change the probability of such flooding occurring.

"What we found when we did that analysis is that there was no increase in the frequency of such heavy rains in September over  our area in the recent decades compared to the 19th century. In fact there was a slight decrease," said Hoerling.

Overall, climate change is leading to more water vapor in the atmosphere, which, all things being equal, should lead to heavier and more intense rainfall. But weather is incredibly variable, said Hoerling.

"I think it reminds us of the complexity by which weather behaves and it requires this careful consideration of place-based analysis," he said.

The same study included three separate examinations of whether the California drought was linked to climate change. Those findings were less conclusive, with one study out of Stanford linking a high pressure ridge and low rainfall to human caused climate change, but other studies finding the long term global warming trend not linked to the drought. One of the reports even found that warming could lead to some forces that would keep storms from reaching California, but also others, like increased humidity, that would cause more rainfall.

The report also included analysis of the climate connection between 2013's unusual South Dakota blizzard, record-high spring and summer temperatures in Australia, and severe rainfall in parts of Europe, among other events.

Stephanie Paige Ogburn has been reporting from Colorado for more than five years, primarily from the Western Slope.
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