Why ‘1,000 year’ floods suddenly seem so common

Dallas has joined four other U.S. communities that recently experienced hot — and then suddenly very wet — summers.

Extreme rainfall lashed the Dallas area on Monday, killing at least one person, requiring hundreds of rescues and adding the city to a list of communities in the country that saw rainfall so heavy it was expected only once in a millennium.

Some areas of Dallas saw more than 13 inches of rain in 12 hours, according to the city’s water utility — enough to exceed the benchmark set by the National Oceanic and Atmospheric Administration for a 1-in-1,000-year rainfall event.

Elsewhere, heavy rains left 37 people dead in eastern Kentucky, closed all roads in Death Valley National Park near the California-Nevada border, forced rescues in suburban St. Louis and sent vehicles into ditches in southern Illinois. Each of these storms has been described as a 1 in 1,000 year event, meaning that each year, there is a 0.1% chance of occurring, based on historical data.

But climate researchers say a warmer atmosphere has neutralized the potential for extreme rainfall and catastrophic flooding. Although it is difficult for scientists to immediately interpret the link between a single weather event – or a series of events – and climate change, human-caused warming has rapidly shifted the likelihood of extreme events so much that some of them say that these numbers lose their meaning. benchmarks because they change so quickly.

“It’s very worrying, this whole concept of the return period – the 1 in 1,000 year storm – doesn’t hold up anymore. The climate has changed,” said Andreas Prein, a researcher at the National Center for Atmospheric Research in Boulder, Colorado.

Image: (LM Otero/AP)

Rainfall in Dallas nearly set the city’s all-time record for 24-hour rainfall, with 9.19 inches of rain recorded at Dallas-Fort Worth International Airport, according to the National Weather Service. It was the second most rainfall recorded during a 24-hour period in any season or year. The average rainfall for the airport in August is about 2.18 inches.

Rainfall totals across the metro area were uneven, according to Jennifer Dunn, a Fort Worth-based meteorologist with the weather service.

A rain gauge near a creek in east Dallas measured more than 15 inches of rain during the storm, he said. During the same time period and 25 miles to the north, “it didn’t rain an inch.”

Hard-hit areas experienced flash flooding, particularly on roads and concrete infrastructure not designed for such heavy rainfall, Dunn said.

Before Monday, North Texas remained mired in extreme drought, according to the U.S. Drought Monitor.

“We’ve seen some very high rainfall totals that are certainly helping to alleviate some of the ongoing drought conditions,” Dunn said. “In some areas, it won’t be enough to erase the deficits.”

Each region of the U.S. has its own recipe for heavy rain, Prein said. Along the West Coast, atmospheric river storms, drawing moisture from the tropical Pacific, cause flooding. In the Midwest, mesoscale convective systems (a collection of storms that work as a system) are responsible for the heaviest rainfall.

For Dallas on Monday, two sources of moisture from the Gulf of Mexico and the Pacific Ocean combined with a slow-moving front produced the torrential rains.

Image: Major flooding ravages eastern Kentucky after heavy rains (Michael Swensen/Getty Images)

Image: Major flooding ravages eastern Kentucky after heavy rains (Michael Swensen/Getty Images)

“Wherever you are, you see the same changes. We have higher rainfall intensities now than we did 50 years ago,” Prein said, adding that it’s a trend seen in both weather observations and climate modeling.

There are several reasons for the trend.

Chief among them: A warmer atmosphere can simply hold more moisture. “We’ve known this for almost 200 years from physics,” Prein said.

For every degree Fahrenheit of warming, the atmosphere can absorb about 4.5 percent more water, Prein said. For each degree Celsius, this percentage is about 7%. Human activities are responsible for about 1.1 degrees Celsius of global warming so far, according to a recent United Nations climate change report.

Under the right conditions, “you can extract more moisture from the atmosphere” today than in the past, he said. This is a trend that will continue to strengthen as the world warms, a result of humanity’s dependence on oil and natural gas.

Some research shows that climate change is also causing storms to develop.

“The storms themselves are also changing. We have some studies that show that storms that increase heavy rainfall are increasing in size,” Prein said.

In the future, researchers expect more severe droughts, characterized by intense bursts of rainfall.

“Precipitation variability is increasing,” Prein said. “We expect there to be longer periods that are dry and short periods that are very wet.”

Of the five regions that saw 1-in-1,000-year rainfall this summer, four had experienced droughts of varying intensity.

These trends have created a mess in the probabilities that meteorologists, engineers and average people use to determine the likelihood of flooding or extreme rainfall. As the climate warms, the risks are shifting.

“What does a 1 in 1,000 year event mean?” said Robert Jnglin Wills, a researcher in the department of atmospheric sciences at the University of Washington. “This can be calculated in the past climate or the distant past before greenhouse gases affected the climate at all. This is constantly changing with climate change.”

As the world warms, the probability of what was once a 1 in 1,000 year event has changed dramatically.

Just taking into account the increase in warming from the beginning of the 20th century to the present day, “the probability of such an event has increased by a factor of between 1.5 and 5,” Wills said, citing data from several studies whose results vary by region. season and other factors.

Image: (Laurie Skrivan/St. Louis Post-Dispatch via AP)

Image: (Laurie Skrivan/St. Louis Post-Dispatch via AP)

Added Prein: “What was once a 1-in-1,000-year storm 30 years ago is now perhaps a once-in-500-year storm.”

These shifting benchmarks have significant implications for infrastructure and how the public perceives risk.

“If you want to build a dam or a highway or a hospital, it’s hard to build your infrastructure in a way that ensures it doesn’t flood. The risk is changing so quickly,” Prein said. “We see that our infrastructure is not holding up. It is very important to make our cities more flood resistant.”

The world is accelerating the rise in average global temperatures by about 3.2 degrees Celsius (5.8 degrees Fahrenheit) by the end of the century, compared to pre-industrial times.

“If we can limit warming to 2 degrees Celsius in the Paris Agreement, that will help mitigate extremes,” Prein said.

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