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

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

Dallas joined four other US communities that have recently experienced hot — and suddenly very humid — summers.

Extreme rains hit the Dallas area on Monday, killing at least one person, requiring hundreds of rescues and adding the city to a list of communities across the country that have seen rainfall so intense it was only expected once in a millennium.

Some parts of Dallas have recorded more than 13 inches of precipitation in 12 hours, according to the city’s water utility — enough to exceed the marker set by the National Oceanic and Atmospheric Administration for a 1-in-1,000-year precipitation 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 to 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 that this will happen, based on historical data.

But climate researchers say a warmer atmosphere has increased the potential for extreme rainfall and damaging flooding. While 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 quickly changed the likelihood of extremes so much that some say these numbers are losing their relevance. . benchmarks because they are changing so fast.

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

Image: (LM Otero/AP)

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 highest rainfall recorded during a 24-hour period in any season or year. The average rainfall for the airport in August is around 2.18 inches.

Rainfall totals across the metro area were uneven, according to Jennifer Dunn, a meteorologist for the Weather Service’s Fort-Worth.

A rain gauge near a creek in east Dallas measured more than 15 inches of rain during the storm, she said. In the same period and 40 kilometers to the north, “they didn’t get an inch of rain”.

The affected areas experienced flash flooding, particularly on roads and concrete infrastructure not designed for such intense rainfall, Dunn said.

Prior to Monday, northern Texas remained mired in an exceptional drought, according to the US Drought Monitor.

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

Each US region has its own recipe for heavy rain, Prein said. Along the west coast, atmospheric river storms, which draw moisture from the tropical Pacific, cause deluges. In the Midwest, mesoscale convective systems (a set of thunderstorms that function as a system) are responsible for the most intense rainfall.

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

Image: Massive flooding devastates eastern Kentucky after heavy rains (Michael Swensen/Getty Images)

Image: Massive flooding devastates eastern Kentucky after heavy rains (Michael Swensen/Getty Images)

“No matter where you are, you see the same changes. We have higher rainfall intensities now than we did 50 years ago,” Prein said, adding that it is a trend seen in both meteorological observations and climate modelling.

There are several reasons for the trend.

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

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

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 amplify as the world warms as a result of humanity’s dependence on oil and gas.

Some research suggests that climate change is also causing the growth of storms.

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

In the future, researchers expect more intense droughts, punctuated by heavy bursts of precipitation.

“Rainfall variability is increasing,” Prein said. “We expect there will be longer dry periods and short very wet periods.”

Of the five regions that recorded rainfall every 1,000 years this summer, four suffered droughts of varying intensities.

These trends have confounded the probabilities that meteorologists, engineers, and ordinary people use to contextualize the probability of floods or exceptional rainfall. As the climate warms, the risks change.

“What does a 1 in 1,000 year event mean?” said Robert Jnglin Wills, research scientist in the Department of Atmospheric Sciences at the University of Washington. “This can be calculated in the climate of the past or in the distant past, before greenhouse gases affected the climate. This is continually 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 considering the increase in warming from the early 20th century to today, “the probability of such an event has increased by a factor of between 1.5 and 5,” Wills said, citing data from several studies, the results of which vary by with the 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 baseline changes have important implications for infrastructure and how the public perceives risk.

“If you want to build a dam, a highway or a hospital, it’s difficult to build your infrastructure in a way that makes sure it doesn’t get flooded. The risk is changing so quickly,” Prein said. “We are seeing that our infrastructure is not sustainable. Making our cities more flood resistant is really important.”

The world is on track for average global temperatures to rise 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 would help mitigate extreme consequences,” Prein said.

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