Heavy rain to become more frequent with climate change, experts say: ‘We’re heading into a new normal’

These once-in-100-years severe climate events are now likely to occur once every 37 years, data show.

Tennis courts were flooded at the Oxbow Marina on the Connecticut River in Northhampton Tuesday.Barry Chin/Globe Staff

Four days last week were recorded as the hottest days on Earth in modern history, with the global average temperature reaching 17.23 degrees Celsius, or 63.01 degrees Fahrenheit.

Then came the rain. Storms pummeled New England and millions were placed under flood watches through Massachusetts, New York, and Vermont. The torrential rain has been compared to the likes of Hurricane Irene, with residents being evacuated from homes and vehicles. One fatality has been reported in New York.

Experts predict that this extreme weather will become the new norm as the climate warms. The fluctuating heat and rain are closely tied to climate change, and expected to become frequent in the Northeast in the coming years.

“The Northeast has one of the signals for the highest increase in precipitation,” said Zachary Zobel, risk associate director at the Woodwell Climate Research Center.

The 2022 Massachusetts Climate Change Assessment predicted that precipitation in the Boston area would increase by at least 6 percent in the summers and 12 percent in the winters by 2070. What once may have been a once-in-100-years event is predicted to become a once-in-37-years event, according to a report from the First Street Foundation, or FSF, a New York-based nonprofit that collects climate risk data.

As temperatures rise with climate change, the chance for rain increases because the atmosphere is able to retain more moisture. For every degree increase in Celsius, the atmosphere can hold 7 percent more water vapor in the same air volume, according to the FSF report. The atmosphere has already warmed more than 1 degree Celsius since the Industrial Revolution, according to NASA. And this impact is compounded with every additional degree, said Peter Huybers, professor of earth and planetary sciences at Harvard University.

“Sea surface temperatures offshore, changes in circulation patterns, and the availability of moisture are all threads that tie into this increased risk,” Huybers said.

Then when it rains, there is a greater volume of rainfall. With climate change, wet areas will get wetter and dry areas will get drier, according to Michael Rawlins, associate director of the Climate System Research Center at University of Massachusetts Amherst. For the Northeast region, a typically wet area, rainfall may become more frequent or more severe. Summers can bring periods of more rainfall because of the heightened temperatures, but this doesn’t mean that each summer will be the hottest or wettest on record; rather, it’s the general trend of greater precipitation throughout the century.

“The number of days each year with a rainfall of greater than an inch has increased,” Rawlins said, “and we can attribute this to climate change.”

These extended periods of rain are connected to inconsistencies in the jet stream, which is the river of wind in the atmosphere that shapes our weather, Zobel said. As the Arctic warms, the jet stream experiences dips that keep the weather stagnant over an area for longer periods of time than normal. This irregularity in the jet stream was one of the causes behind the south’s heat dome last month, and the early wildfire season in Canada.

New England’s infrastructure wasn’t built with such climate events in mind, Huybers said. It was built to withstand such events only once every 100 years, but these events are growing in frequency. Developing climate resiliency must be a priority, and projects like road upgrades and culvert expansions can help us handle the extreme weather conditions in the coming years, Rawlins said.

“We’re heading into a new normal,” Zobel said. “The climate you’ve experienced previously is not the climate you’re going to be living in for the next 30 years.”