As heavy rains and wind pound Massachusetts and forecasters warn of potential flooding, power outages, and downed trees, new research affirms something many have long suspected: Ordinary rain storms are getting more intense in the eastern United States.
The study, published in the journal Geophysical Research Letters this week, links the findings to the climate crisis.
The authors, researchers at Northwestern University, analyzed rainfall data from some 1,700 weather stations across the US. They found that in the swath of the US east of the Rocky Mountains, rainstorms dropped about 5 percent more water on average in the years 1991 to 2020 than they did 1951 to 1980. The increase was even stronger in New England, where storms brought up to 6 percent more rain, the authors say.
Researchers have repeatedly confirmed climate models’ predictions of wetter, more powerful extreme rain events like hurricanes, caused by warmer air, which can hold more moisture. The new study’s authors wanted to see if mundane rain events are getting wetter, too.
“Our study focused on precipitation intensity, so on days that it rained, how much did it rain,” said Ryan Harp, a postdoctoral research fellow at Northwestern University who led the study.
Previously, most researchers aiming to answer that question relied on theoretical numbers supplied by climate models fed with historical data. But the authors wanted to determine if changes are happening in the real world.
The percentage increases may not seem large to anyone but experts in the field, but the study’s authors said even small shifts can have a big effect.
Changes in rainfall can take a toll on crop yields and also increase communities’ vulnerability to landslides and floods, said Harp. And much of the nation’s infrastructure isn’t designed to withstand the kind of rainfall they identified “so these shifts in precipitation intensity could have impacts on our roads and structures,” he said.
The authors hope the study shows the urgent need to prepare for climate-fueled weather shifts.
“Simply being aware of the fact that we’re already seeing the anticipated changes of a warmer world, and factoring that into decisions, can go a long way,” said Harp.
The increase in rainfall was particularly strong in southern and midwestern states and along the east coast.
“The line connecting climate change with more frequent heavy precipitation events in the U.S. was already solid, and this study thickens it even more,” said Jennifer Francis, acting deputy director of the Falmouth-based research organization Woodwell Climate Research Center, who did not work on the report. “And the link makes perfect sense. Additional heat trapped by accumulating greenhouse gases causes evaporation to increase, which sucks moisture from the land and oceans, moistening the air. Any storm that comes along now has more moisture to feed on.”
The findings are also consistent with the laws of atmospheric physics. Back in the 19th century, scientists worked out that for every degree Fahrenheit air temperatures rise, the atmosphere can hold 4 percent more water — an equation known as the Clausius-Clapeyron relationship.
But though there was a clear observable trend across most of the country, the authors didn’t detect an increase in precipitation along the Pacific Coast or in the Rockies.
That might be because shifts in the overall placement of weather systems are “dampening” the effects of warming on rainfall, but “the jury is still out on this question,” said Harp.
Since the study only looked at how much precipitation fell on rainy days, it doesn’t necessarily indicate an overall increase in rain, as there may be places where precipitation is becoming more intense but more sporadic. There is evidence of this trend in Massachusetts.
The authors, in a planned follow-up study, are setting out to determine whether there’s been an increase in total rainfall.