The Role Of Climate Change In Utah And Arizona's Deadly Floods

It's not causing them, but it might be making them worse.

Climate change isn't what's causing the deadly flash floods in Utah and Arizona this week, but it's part of what's making them so catastrophic, one expert warned.

As of Wednesday evening, at least 18 people have been killed by intense flooding near the Arizona-Utah border that began Monday, while others remain missing, including 6-year-old Tyson Lucas Black.

Flooding that powerful is an example of how the warmer atmosphere turns ordinary weather events into more extreme ones, Kevin Trenberth, a senior scientist with the National Center for Atmospheric Research, told The Huffington Post.

"The climate change aspect of this is that the atmosphere is warmer, and for every 1 degree Fahrenheit increase in temperature, the atmosphere can hold about 4 percent more moisture," he explained.

"When the right weather system comes along," such as the storm above Utah and Arizona, Trenberth said, "that weather system can be thought of as a device for reaching out -- quite a ways at times -- and grabbing the available moisture and bringing it in and dumping it down."

"Climate change is not the cause, but it is -- I’m tempted to say -- a minor contributor," he added. "Even that minor contribution can be that straw that broke the camel’s back."

Prolonged drought in Utah and Arizona, which are facing moderate to severe dryness in about 90 percent of each state, is also partly responsible for the water buildup, Trenberth explained.

“In the case of a drought, the ground is often not receptive to moisture," he said, explaining that lighter, more frequent rains would be absorbed more easily. "Not much of it soaks in, and it all tends to accumulate, and the next thing you know, you've got a flood on your hands."

Such extreme floods are occurring more frequently than in previous decades, Trenberth said, pointing to flooding in Japan this week that killed at least seven people and flooding in Texas and Oklahoma in May that killed at least 21 people.

Michael Mann, a climate scientist and director of Penn State's Earth System Science Center, echoed Trenberth's view that climate change exacerbates these weather events when he spoke to HuffPost in May about the Texas floods.

"There are many factors that came together here -- an incipient El Niño event, and the vagaries of weather," he said. "But human-caused climate change is, in many cases, the straw that broke the camel’s back, that extra fuel that takes what would have been a really bad flood and turns [it] into a catastrophic flood."

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