“Please don’t come to Ellicott City,” Baltimore Sun reporter Kevin Rector tweeted on Sunday, minutes after a powerful flash flood rocked the Baltimore suburb. In the course of about three hours, eight inches of rain gutted roads, smashed windows, felled buildings, and killed at least one person. Emergency crews searched submerged cars for trapped passengers; a bride and groom waded through floodwaters after being evacuated from their venue. “It is a disaster,” Rector wrote.

The meteorological probability of rain events like this are about 1 in 1000. (This doesn’t mean they always happen once every 1000 years; just that they have about a 0.1 percent chance of happening.) But Ellicott City’s last 1000-year flood was in 2016, when six inches of rain fell in two hours, killing two people. That such a major storm would hit the exact same location in two years “boils down to rotten luck,” wrote The Washington Post’s meteorologist Jeff Halverson. “Personally, I would not have expected to ‘relive’ (vicariously) such a devastating flood, in the same spot, over the course of my lifetime,” he said. “Yet, it happened. Sometimes lightning does strike twice.”

The lightning metaphor implies that the second flooding of Ellicott City in two years was only bad luck. But humans have increased the odds of such devastation, especially in the Northeastern United States.



National Climate Assessment

One contributing factor is the global rise in greenhouse gas emissions. Since the Industrial Revolution, humans have emitted about 535 billion metric tons of carbon dioxide. Those gases have increased the water vapor content of the atmosphere, thus making rainy areas like the Northeast more susceptible to severe rain. (The Northeast gets more precipitation than any other region in America.)



As Halverson pointed out, “scientific studies have shown a statistically meaningful uptick in the frequency of extreme rain events over the eastern United States.” Such events are not only becoming more frequent, but more intense: The 2014 National Climate Assessment shows a 71 percent increase in the amount of precipitation in the heaviest rainstorms and snowstorms in the Northeast between 1958 and 2012. Climate scientists and meteorologists made similar observations on Twitter following the Ellicott City flooding.