With flashier floods and stronger hurricanes, ‘I don’t think we know what normal is anymore’

Climate scientists say the extreme rainfall and flash floods in Leominster are now expected on a warming planet — and there’s much more to come

Paul Sullivan stood over the waterfall that developed in his front yard from a flooded stream near the back of his property on Apple Tree Lane in Leominster.John Tlumacki/Globe Staff

INTO THE RED: Climate and the fight of our lives

The trouble, it seems, started with the soil.

A summer of relentless rain had inundated the ground in Leominster, drenching the earth so thoroughly that when the real rain came — nearly a foot, in just a matter of hours — there was nowhere for it to go.

In less time than a movie double-feature, enough water fell to wash out roads and train tracks, deluge homes and businesses in several feet of water, and force evacuations— the consequences of flash flooding that experts say is a hallmark of climate change.

Home in Leominster is blocked off after flooding undermined the foundation and tore up the street.John Tlumacki/Globe Staff

“Is this just a new normal? I don’t think we know what normal is anymore,” said Michael Rawlins, associate director of the Climate System Research Center at the University of Massachusetts Amherst. “What happened [Monday] night is an obvious manifestation of a warming climate.”

It’s the latest in a season of extremes, in which Massachusetts saw its second-rainiest June through August, the planet had its hottest summer on record by a large margin, and ocean surface temperatures soared.

Earlier this summer, as much as 9 inches of rain fell across parts of Vermont over 48 hours, resulting in flash floods that washed out roads, necessitated more than 200 urban and swift water boat rescues, and flooded downtown Montpelier. The same storm brought devastating floods to Western Massachusetts, where some farmers lost their livelihoods under inches of water, and to New York’s Hudson Valley.

And as the Northeast on Wednesday experienced another round of rainfall, other parts of the globe are also underwater.

Thousands are feared dead in Libya, where two dams burst following torrential rains. In Greece, entire towns in the Thessalian plain are underwater from the same storm that also devastated parts of Turkey and Bulgaria. In Asia, Hong Kong and southern China saw extreme rainfall and flooding in recent days. Dozens of people died in flooding in southern Brazil, and catastrophic flooding hit Spain, too, in recent weeks.

“The fact that you’re seeing these extreme events — record-breaking events, record-breaking temperatures — really suggests that these events are being exacerbated by climate change and by warmer temperatures,” said Erin Dougherty, a hydrometeorologist studying the current and future climate at the National Center for Atmospheric Research in Colorado.

In a 2020 study, Dougherty and a coauthor found that by the end of the century, the Eastern United States could experience a nearly 18 percent increase in both rainfall and runoff under a worst-case scenario of global warming. Another study, published last year, found that floods in the United States could become 7.9 percent “flashier” — meaning more sudden — by the end of the century in a scenario in which emissions continue unchecked.

Already, these flash floods seem to be happening more frequently. And if worldwide fossil fuel use continues to rise through the end of the century, the Northeast can expect an average of 52 percent more extreme precipitation, compared with the period from 1976 through 2005, according to a study released earlier this summer. The study found that most of that additional precipitation will come from an increase in the number of days with extreme precipitation, though it also projects a small increase in the amount of rain or snow on each extreme precipitation day.

These extreme events are expected to increase because of a simple fact of physics: the warmer the ocean, the more water that evaporates into the air; and the warmer the air, the more moisture it holds. As a result, when rainstorms happen on a warmer planet, they drop more rain. And when significant rainfall happens in a short period, flash flooding becomes more likely.

“If you have really intense rainfall, it can exceed the capacity of the soil to hold any of that moisture and you get things like really devastating flooding,” Dougherty said.

A wrinkle in all this, as evidenced by last summer’s drought in Massachusetts, is that the warming climate is also making dry periods drier, bringing confounding swings between extremes.

As scientists are increasingly able to attribute specific events to a warming climate, they have been able to point to specific deluges that are fueled by warming. Among the most extreme examples was 2017′s Hurricane Harvey, which dropped as much as 60 inches of rain in the Houston area over four days. Kerry Emanuel, a professor of meteorology at MIT, found that climate change had increased the odds that such a storm would occur, and that future warming would make them more likely in the future.

While no such analysis has been done for the recent storms in Massachusetts, Emanuel said the extreme rainfall in Leominster is in line with what is expected on a warming planet: rain may not happen as frequently, but when it does, it will be heavier.

Another factor that contributes to such extreme rainfalls, he said, is that as the climate warms, wind speeds during storms are dropping.

“That means that storms are moving more slowly, so if you happen to be unlucky enough to be under one, it’s going to rain longer,” Emanuel said. “And that’s also going to contribute to flooding problems.”

Now, New England is waiting to learn whether Hurricane Lee, which is churning east of the Bahamas in the Atlantic Ocean, will bring even greater rainfall. Already, the storm stands out because it is one of just three hurricanes on the historical record to intensify from a Category One storm to a Category Five in just 24 hours. (It later weakened.)

Such rapid strengthening came as no surprise to climate scientists, because hurricanes are fueled by warmth at the top of the ocean and the North Atlantic has had anomalously high surface temperatures all summer. On Monday, sea surface temperatures in the North Atlantic were a degree higher than the last record high for that day, which was in 2022, and were more than 2 degrees higher than the historic average, according to data from the Climate Change Institute at the University of Maine.

With these intensifying storms, it’s only more likely that extreme rainfall will happen again. But it’s impossible to know when or where. Experts say that policy makers and planners should be taking this into account, building in extra green spaces and natural features like rain gardens and natural channels for runoff that can allow for slower drainage so infrastructure doesn’t get overwhelmed.

But when it comes to the very extreme rainfall events, even those measures might not be enough.

“We designed these systems at their best for certain storm sizes,” said Martin Pillsbury, environmental planning director at the Metropolitan Area Planning Council. “When you get 10 or 11 inches, all bets are off. No system is prepared for that.”