The devastating flooding in Maryland over Memorial Day Weekend not only sent a wall of water through downtown Ellicott City, Maryland, but reignited a debate over global warming and floods.
Is global warming making floods, like in Ellicott City and surrounding towns worse, or do other factors play a dominant role? Meteorologists seemed to come down on both sides of the matter.
NBC Today Show weatherman Al Roker said that while Ellicott City’s unique geography made it particularly flood-prone, warming was leading to more heavy rainfall events. A warmer atmosphere holds more moisture, leading to bigger storms, he said.
“And that just continues to grow as we start to continue to see climate change and more warm air making its way with moisture and causing bigger storms,” Roker said on air Tuesday. “That’s what’s going on around the country.”
On the other hand, AccuWeather Meteorologist Brian Lada said it was “such a localized, small-scale event that the big climate debate wouldn’t really have an impact.”
“This was about training thunderstorms,” Lada told USA Today.
Ellicott City has had at least 15 major floods since the late 1700s, the most devastating being one in 1868 where 43 people were killed after the Patapsco River rose 21.5 feet. Rainfall west of Ellicott city brought a wall of water through Main Street, which sits in a valley where to streams meet the Patapsco River.
This is the second major flood event in Ellicott City in the last two years. The city saw devastating flooding in 2016 that some also blamed on global warming.
Liberal news outlets began linking the most recent Ellicott City flood to man-made warming, citing the latest National Climate Assessment special report that found statistically significant increases in heavy rainfall events across the U.S.
However, the 2017 report also noted “there is low confidence in attributing the extreme precipitation changes purely to anthropogenic forcing.” Still, it’s often claimed man-made warming is probably playing some role.
“Statistically, over the long term, these types of extreme floods are probably becoming more common, in areas that are normally rainy as a result of global warming,” Capital Weather Gang’s Jeff Halverson wrote in a piece detailing the meteorology behind the weekend flood.
This begs the question, how has global warming affected flooding across the world, including the U.S.? Well, it turns out there’s a lack of evidence that warming is having any impact on flooding.
The Intergovernmental Panel on Climate Change’s most recent report, released in 2013, found “there continues to be a lack of evidence and thus low confidence regarding the sign of trend in the magnitude and/or frequency of floods on a global scale.”
For the U.S., the NCA special report found the “[d]etectable changes in some classes of flood frequency have occurred in parts of the United States and are a mix of increases and decreases.”
“Extreme precipitation, one of the controlling factors in flood statistics, is observed to have generally increased and is projected to continue to do so across the United States in a warming atmosphere,” reads the special report. “However, formal attribution approaches have not established a significant connection of increased riverine flooding to human-induced climate change, and the timing of any emergence of a future detectable anthropogenic change in flooding is unclear.”
U.S. government stream gauge data also shows a mix of increases and decreases in flooding. The Environmental Protection Agency’s chart on river flooding showed a decrease in 60 percent flood magnitude and frequency since 1965.
“As noted above, precipitation increases have been found to strongly influence changes in flood statistics,” reads the NCA special report. “However, in U.S. regions, no formal attribution of precipitation changes to anthropogenic forcing has been made so far, so indirect attribution of flooding changes is not possible.”
“Hence, no formal attribution of observed flooding changes to anthropogenic forcing has been claimed,” reads the special report.
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