Why ‘Hundred-Year' Weather Events Are Happening More Than Once Every 100 Years
Climate change is leading not only to droughts, wildfires, and extreme weather. It's also leading to oxymorons—at least when it comes to what are known as hundred-year storms, floods, and other events.
Long-term weather forecasting—the kind that predicts conditions months or even years or decades in advance—is all about probabilities, factoring together not only current conditions and trends, but the historical record. An area that has seen floods in the past when the spring was unusually rainy or tropical storms were unusually fierce, is likely to see them again if the same conditions recur. Ditto the likelihood of severe storms when the atmosphere is holding a lot of moisture and the oceans are atypically warm.
Environmental scientists have gotten so good at reading weather history that they can characterize some severe storms or floods as likely to occur in a given area only once in 100 years—or even 500 years or a thousand years. That's where the oxymoron comes in. As climate change leads to greater meteorological volatility, the one in 100—or 500 or 1,000—year events are occurring twice or three times or more in those windows. Since 1999, there have been nine storms along the North Carolina coast that qualify as hundred or thousand year events. From 2015 to 2019, one suburb of St. Louis experienced three major floods, two of which met the criteria for hundred-year events. One study by the Montreal-based carbon removal project Deep Sky calculates that the frequency of deadly hurricanes has jumped 300%, with 100-year storms now forecast to occur once every 25 years.
Climate change is also redefining what qualifies as one of these rare and intense events. 'In April, an extreme rainfall event hit the Mississippi Valley, including Arkansas, Mississippi, Kentucky, and Tennessee,' says climate scientist Andrew Pershing, chief program officer at Climate Central, an advocacy and communications group. 'Some of our colleagues at the World Weather Attribution group did a study and calculated that it was a 100-year event based on today's climate, but without climate change it would have been more like a 500-year event.'
Making those kinds of calculations can take some doing—and a fair bit of data modeling—because climate unfolds over the course of millennia and modern weather and climate records barely go back a century. 'Scientists first look at 30 years of data, 50 years of data and figure out how frequently these events occur,' says Pershing. 'The challenge is that when you do that you're using data from the past when it was around two degrees cooler than it is now. When you start to do the calculations for today's climate, you find that events that you might expect to happen once every hundred years might happen once every 20 years.'
The math here gets a little simpler. By definition, a hundred-year storm has a 1% likelihood of occurring in any one year; for a 500-year storm it's 0.2%; for a thousand years it's 0.1%. But every year the probability clock starts anew; if the 1% longshot comes in and a hundred-year storm occurs on the Carolina coast in 2025, that same area would typically have the same 1% chance in 2026—but climate change is making the likelihood even higher. 'It's not like you can calendar one of these events and say you're cool for another 100 years,' says Pershing.
Driving the more frequent events is what Pershing describes as a 'thirstier' atmosphere, one that is hotter and thus capable of holding more moisture. 'We have a supercharged water cycle and that means that when you get a rain event it has a better chance of being a bigger event than it used to be,' says Pershing.
Some of those bigger events could be coming soon—in the form of hurricanes. On May 22, the National Oceanic and Atmospheric Administration (NOAA) released its projections for storm severity in the 2025 Atlantic hurricane season, which runs from June 1 to November 30. NOAA did not attempt to predict 100- or 500- or thousand-year events, but it does see trouble looming. The agency projects a 60% chance of an above-average hurricane season, a 30% chance of an average season, and just a 10% chance of below average. Across the six hurricane months, NOAA predicts 13 to 19 named storms—with winds of 39 mph or higher—up to 10 of which will likely develop into hurricanes with winds of 74 mph or more. Up to five of those could be major hurricanes—category 3, 4, or 5, with winds of 111 mph or more. And the impact could extend far beyond the coastal regions that are usually hardest hit.
'As we witnessed last year with significant inland flooding from hurricanes Helene [in September] and Debby [in August], the impacts of hurricanes can reach far beyond coastal communities,' said acting NOAA administrator Laura Grimm in a statement. Things could get dicey not only in the Atlantic, but in the Pacific as well. Already, tropical storm Alvin is forming off the southwest coast of Mexico, two weeks ahead of the start of the eastern Pacific hurricane season.
In addition to hurricanes, floods, and storms, heat waves, droughts, and wildfires can be projected out over centuries. 'A hotter atmosphere can hold more water, but if you squeeze that moisture out over a mountain range like what happens in the west, then you end up with a much drier air mass,' says Pershing. 'The atmosphere then wants to suck the moisture out of the ground and so droughts get more severe.'
There's no easy fix for a feverish atmosphere. In the short run, adaptation—dikes and levees to protect flood-prone cities, relocating residences away from eroding coasts—can help. In the longer run, shutting off the greenhouse emissions that created the problem in the first place is the best and most sustainable bet for limiting hundred-year storms to their hundred-year timelines. 'We have to quit fossil fuels as fast as we can,' says Pershing. 'This will give the climate a chance to stabilize and us a chance to adjust.'
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