How climate change is increasing the number of '1,000-year' storms like the 1 that just hit New York and Vermont

Extremely heavy rain in parts of Vermont, New York and Connecticut caused dramatic flash flooding Sunday night and Monday that washed out roads and suspended rail service. Hundreds of motorists had to be rescued from cars, and at least one person was killed in the floodwaters.

“They’re calling this a once-in-a-thousand-year event,” New York Gov. Kathy Hochul noted in a Monday press conference. The torrential downpour dumped up to 10 inches of rain in West Point, N.Y., an amount that has only a one-in-1,000 chance of occurring in any year, according to data from the National Oceanic and Atmospheric Administration.

As global temperatures continue to rise, however, such extreme rainfall events are no longer rarities. In one week last summer, the United States saw three 1-in-1,000-year rains: 8 to 12 inches of rain in Southern Illinois in 12 hours, 6 to 10 inches of rain in the St. Louis area and 14 inches of rainfall in eastern Kentucky.

What causes such extreme rainfall events?

According to NOAA, deluges like those seen in New York and Vermont this week are typically caused by high moisture and an “atmospheric disturbance” such as a storm, hurricane or cold front colliding with a warm front.

“The longer these conditions persist in the same place, the more extreme the rainfall you’re likely to have,” said Dr. David Novak of NOAA’s Weather Prediction Center in a 2018 interview.

And that’s what has happened in the Northeast this week.“The atmospheric pattern is stuck,” the meteorological website Weather Underground reported Tuesday morning.

A high-pressure system near Greenland is “keeping a large, spinning gyre of low pressure in place over central Canada into early next week,” Weather Underground explained.

Is climate change also to blame?

Climate change is increasing the frequency of severe storms in two ways, experts say. First, warmer air causes more water to evaporate, making precipitation events more extreme. Second, rising global temperatures are weakening the jet stream, the fast-flowing eastward band of air that is powered by the difference in temperature between the equator and the poles. As the poles are warming faster than the equator, that temperature difference is being reduced and the jet stream is weakening.

“Human-caused warming from fossil fuel burning is impacting these events in several ways,” Michael Mann, director of the Center for Science and Sustainability and the Media at the University of Pennsylvania, told Yahoo News. “A warmer atmosphere holds more moisture, so when it rains there’s potential for much more of it. And the pattern of warming is impacting the jet stream in such a way that we get more of these very stagnant or ‘stuck,’ wavy patterns which are associated with persistent weather extremes (both heat/drought/wildfire and flooding, depending on your location).”

Does a 1-in-1,000-year storm mean it won’t happen more than once in a millennium?

No. Unfortunately, a one-in-1,000-year rainfall event has a one-in-1,000 chance of happening every single year. While the amount of rain that fell on New York and Vermont this week is only seen once every 1,000 years on average, a similar event could happen at any time. Climate scientists say that climate change will cause more extreme rainfall events and, ultimately, make us reevaluate the probability of such storms.

“Those numbers indeed ignore the fact that climate change has made these events far more frequent,” Mann said. “While the numbers will have to be crunched to look at the specifics of this event, we know there are many other one-in-a-thousand or even one-in-tens-of-thousand year events (like the 2021 Pacific Northwest heat dome) where something that simply wouldn’t have occurred in the absence of human-caused warming is now becoming a regular occurrence. So, in many cases, what might have been a ‘thousand-year event’ before fossil fuel burning and human-caused warming, is now, say, a ‘10 year event’ (something we expect once in a decade), and with yet more warming it could be an annual event (we see something like that pretty much every summer). That’s where we’re headed in the absence of substantial climate action.”