Climate change has forced Mother Nature to make it rain … on southern New England. In the past decade, the region has witnessed an increase in extreme rain and snowfall events. All this extra water is having an impact on river flow and making damaging floods more frequent.

David Vallee, a hydrologist based out of the National Weather Service’s Taunton, Mass., office, called this shift in the distribution of precipitation “remarkable.” He didn’t mean that in a wicked awesome kind of way. He pointed to the beating the Rhode Island coast has taken in recent years as a prime example.

“We’re getting the snot kicked out of us on the coast,” he said, noting the damage caused by Superstorm Sandy and Hurricane Irene alone. “We’re on the wet side of the spectrum now and we don’t know where we’re going. That’s the most troubling aspect.”

We do know, however, that the past 10 years have been uncharacteristically wet. Three days in late March 2010 brought 12 inches of rain and flooded the Pawtuxet and Blackstone rivers and numerous other rivers, lakes and streams in the area. The flooding caused by this deluge was the worst in Rhode Island history. At 16.3 inches of rain, March 2010 is the wettest month on record.

But the increase in flooding in southern New England can’t be solely blamed on a changing climate. “The greater frequency of more intense flooding is more complex than climate change. It’s also about urbanization and land-use patterns,” said Vallee during a talk late last year entitled “The Changing Precipitation & Temperature Landscape in New England and Its Impact on River Flooding.”

The Cumberland, R.I., resident also said we should be concerned about what is happening in Greenland, and that is where climate change enters this complex equation. The melting Greenland ice sheet — a vast body of ice covering 660,000 square miles that is the second largest body of ice in the world after the Antarctic ice sheet — is changing ocean temperatures and currents.

“When the ice cover begins to be reduced in the polar region, the behavior of the jet stream changes,” Vallee explained. “There’s a slowing down of atmospheric flow. Moisture plumes are moving more slowly through the region, and the more moisture there is in these plumes the more opportunities there are to get rained on in dramatic fashion.”

These slow-moving weather systems are blocking up the atmosphere, creating intense storms and hurricanes that like to linger. Even modest changes in air and ocean temperatures can allow the atmosphere to hold more moisture, according Vallee.

The aforementioned March 2010 rains were so heavy that four of Rhode Island’s counties were declared emergency disaster zones. The Warwick Mall was flooded with 20 inches of water. A sewage treatment plant in Warwick failed, contaminating rainwater with raw sewage. Hundreds of homes in Warwick, Cranston and Johnston were flooded by a foot of this contaminated mixture.

In the two subsequent years, Irene in 2011 and Sandy in 2012 caused more flooding and more damage. Sandy’s damage was intensified because the superstorm stuck around through several tide cycles.

“When Sandy hit Rhode Island, it was barely a tropical storm, but it hung out for so long that it created a lot of damage,” said Laura Dwyer, the public educator and information coordinator for the state’s Coastal Resources Management Council (CRMC). “Sandy was nothing close to the Hurricane of 1938 and look at the damage it caused.”

In fact, Sandy’s storm surge was 5 feet lower than that of the 1938 hurricane. “We are overdue for a storm of 1938 magnitude,” Dwyer said. “What are the implications?”

All wet
In Rhode Island, annual precipitation has increased by 12 inches since 1905, mostly during non-summer months, according to the University of Rhode Island/Rhode Island Sea Grant Climate Change Collaborative. In fact, the amount of rainfall during extreme precipitation events — defined as the heaviest 1 percent of precipitation events — has increased by 67 percent during the past 50 years.

Previous climatologies have been based on the premise that extreme rainfall events don’t change over time. The last comprehensive climatology of extreme rainfall events at the national level was published in 1961 by the U.S. Weather  Bureau — now the National Weather Service.

However, recent analyses show that old assumptions don’t hold water, particularly in New York and New England where the frequency of 2-inch rainfalls has increased since the 1950s and storms once considered a 1-in-100-year event have become more frequent, according to the Northeast Regional Climate Center at Cornell University.

With increasing rainfall that comes with a changing climate, Block Island’s bluffs, for example, will face more rapid erosion, according to Janet Freedman, coastal geologist with the CRMC. Increased storm-related flooding and the erosion of bluffs, beaches and riverbanks could impact public access. Increased precipitation also can lead to changes in salinity and water levels of the region’s fragile marine and freshwater ecosystems.

Infrastructure concerns
By 2100, precipitation in the Northeast during non-summer months may increase by an additional 7 percent to 14 percent above the current average, according to the Climate Change Collaborative. The region’s roads, infrastructure and drainage systems aren’t designed to address the future conditions being caused by climate change. Transportation, energy and wastewater infrastructure will likely sustain more frequent damage and potential service shutdowns caused by more frequent and intense flooding.

Typically, southern New England storms came and went in one tide cycle. But, as was the case with Sandy, today’s nor’easters, winter storms and hurricanes are often parking themselves offshore and hanging around through more than one high tide — a big problem considering some areas of the region are already experiencing flooding during normal high tide.

The region also is being hit more frequently by consecutive severe weather events, and one-day events are dumping more rain than before. “We’re getting hit multiple times in close proximity of each other,” Vallee said. “Back-to-back rain events are happening more often and massive amounts of rain are falling. We’re seeing multiple weather systems that dump on us in a short period of time. We’re living through change in the Northeast.”

This change in precipitation patterns, for example, is now filling the Scituate Reservoir, the largest inland body of water in Rhode Island, by early March — a full three months before the old norm, according to Vallee.

All of this climate-change induced precipitation and the acres of impervious surface that exacerbate damaging flooding can hinder emergency services, contaminate drinking water and increase mosquito-borne disease. It also can contaminate coastal waters with storm drain and/or wastewater treatment facility overflows, leading to the closure of beaches and shellfish harvesting areas.

Smaller watersheds and those with significant urbanization are most vulnerable to this increased flooding.

To deal with increased flooding caused by more intense and frequent weather events, Cranston, Cumberland, Warwick and Westerly are buying out vulnerable structures in the floodplain, in cooperation with the Federal Emergency Management Agency (FEMA). North Kingstown, Narragansett and Middletown have participated in the National Flood Insurance’s Community Rating System, taking advantage of incentives to reduce flood insurance premiums for each policyholder in the community by reducing risks.

“The Northeast has become a hot spot for record floods and heavy rainfall,” Vallee said. “We have to look at this holistically. Watersheds are living organisms. We have to make smart choices. What we do in one area impacts the entire system.”