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Spokane, Washington  Est. May 19, 1883

Extreme rainfall risks could increase under climate change, scientists warn

Evening commuters make their way through roadway flooding caused by heavy rains in Spokane in this 2012 file photo. (Colin Mulvany / The Spokesman-Review)
By Chelsea Harvey Washington Post

U.S. residents may want to start preparing for a stormier future, scientists say. According to a new study, future climate change could cause an increase in extreme precipitation events throughout much of the country – in fact, these events could become up to five times more frequent in some areas.

It’s an outcome many climate scientists have already predicted for the planet as a whole, according to Andreas Prein, a scientist at the National Center for Atmospheric Research and the new study’s lead author.

“We expect that intense rainfall extremes will get more frequent and more intense in the future climate because if we warm up the atmosphere, air can hold more moisture,” he said. In fact, he added, records from the past few decades indicate that we’re already seeing this effect in the warming United States.

“What we were interested in is how these kinds of storms might change in the future,” Prein said.

The new study, published Monday in the journal Nature Climate Change, investigated the relationship between rising temperatures and precipitation throughout the Lower 48 states. Using special climate models, the researchers simulated precipitation across the continent under both current climate conditions and a hypothetical high-warming, or “business-as-usual,” scenario.

They found that the way warming affects precipitation depends a lot on humidity – the amount of moisture that’s available in the air to begin with. In moist locations, rising temperatures tend to cause an increase in extreme precipitation, while the opposite can be true in drier places. In general, there tends to be a “sweet spot” in terms of the temperature that produces the most extreme precipitation events on average in the United States, Prein said – currently, it’s about 80 degrees Fahrenheit.

But, he noted, the study suggests that “in the future climate, storms keep on intensifying up to something like 87 or 88 degrees Fahrenheit.”

This is because under future climate change, many parts of the nation will become both warmer and wetter. As a result, much of the country will see an increase in both the frequency and intensity of extreme precipitation events. This is especially true for parts of the Gulf and Atlantic coasts, as well as some mountain regions like the Rockies, where humidity is high to begin with.

On average across the U.S., extreme precipitation events may become nearly three times more likely, according to Prein. And at highest, their frequency may increase fivefold in some locations. One way to think about this is to imagine the most intense storm of the summer, Prein said. In the future, a storm of that magnitude might occur five times in a season instead of just once.

The researchers expect the intensity of these events, or the amount of precipitation they produce, to spike as well – by as much as 70 percent in some cases. This means a storm that produced 3 inches of rain in the past, for example, might produce more than 5 inches in the future.

This could be a problem for communities across the country in the future, which may need to start thinking about updating their infrastructure to deal with a future uptick in extreme storms. But there’s still hope: The new study relies on a business-as-usual scenario, meaning a situation in which current warming rates continue unabated into the future. With a serious effort at cutting greenhouse gas emissions and slowing the progression of climate change, these situations could still be avoided.

So the new study serves as a warning of what could happen in the future, but not necessarily a definite forecast.

“If we can manage to level out temperature increases at a lower rate than what we see here … then the consequences will be less,” Prein said.