The large waves, storm surge, and flooding that Hurricane Earl will spawn as it strikes Massachusetts tonight comes with an added dollop of trouble: sea level rise.
Very gradual, and in some cases accelerating, rises in sea level off our coast over the last century will boost the height of Earl’s storm surge — expected to be 1 to 4 feet — meaning the wall of water will be able to travel that much farther inland and over higher elevations to flood basements, streets, and other low-lying areas.
Although scientists agree that sea level rise contributes to flooding during severe storms, exactly how much can be hard to determine because coastal flooding depends on tides, winds, and topography. Sea levels can also vary by several feet from one region to another, and even from season to season, because of ocean circulation and other factors.
Sea level is rising, scientists say, largely because of a global warming double punch: higher ocean temperatures that expand the volume of water and melting glaciers that add water to the sea. So future hurricanes are likely to cause more widespread flooding.
“Sea level rise is fairly insidious and one of those things that we think is in the background and not in our lifetime or our children’s lifetime, but it keeps adding up,’’ said Greg Berman, coastal processes specialist with Woods Hole Sea Grant and Cape Cod Cooperative Extension. He said sea level rise in the last century has coincided with the mass migration of people to live near the coast, and although “we drew a line in the sand where the sea should stop, it’s not listening to us.’’
In Falmouth, sea level has risen about a foot over the last century, according to Berman. So if Hurricane Earl’s storm surge is 4 feet, that surge would come atop an extra foot of sea level compared with 100 years ago. But forecasters say the Cape and Islands and Southeastern Massachusetts may get a break because the hurricane might be at its fiercest at low tide, giving water time to dissipate as it runs up sloping beaches.
The future threat hasn’t been lost on the state or individual businesses, which are planning for enormous changes in sea level over time and more flooding. New England Aquarium on Boston Harbor moved all its electrical switching equipment upstairs several years ago to prepare for a major flooding event. Boston’s sea level has risen about 11 inches in the last century, said Bud Ris, president of the New England Aquarium. And Deer Island Sewage Treatment Plant, completed in 2001, was built 2 feet higher than originally planned to prepare for sea level rise.
The state is working with communities so the ocean doesn’t go where people don’t want it to be. The state’s StormSmart Coasts program is working with Boston to identify particularly vulnerable low-lying areas that can be protected. This past spring, Hull approved a program to encourage builders to elevate new and renovated structures above predicted flood waters.
Builders there can get a $500 credit to be used toward permit fees if the structure is at least 2 feet above the highest federal or state requirement for flood zones, according to Julia Knisel, who works for the state to make coasts more resilient to climate change. State officials are also promoting new design standards and working to find better ways to manage flood plains.
“We need to continue to build strong partnerships to advance mapping, planning, and policy tools . . . for greater coastal resilience,’’ Knisel said.
Although sea level has been slowing increasing over the long term, scientists say levels can vary from one region to another and fluctuate in the short term.
For example, last summer, National Oceanic and Atmospheric Administration officials observed water levels 6 inches to 2 feet higher than predicted from Maine to Florida over a period of a few weeks. The gain averaged about 6 inches in the Boston area during its peak in early July and as a much as a foot higher off Cape Cod. At the onset, high tides were further amplified by a perigean spring tide, when the moon is closest to the earth and its gravitational pull causes larger ranges of tide.
An unrelated high-sea-level event, this time brought on by changes in weather patterns in the Pacific, was noticed this past winter. Although residents might not notice these types of changes directly, they could exacerbate flooding if they coincide with a large storm.
“Global sea level is a general average, but that [may not be] what is going on locally,’’ said Richard Edwing, director of NOAA’s Center for Operational Oceanographic Products and Services. His office is studying the sea level anomalies to help communities better prepare for storm surges.
The aquarium’s Ris said there is debate about whether hurricanes will become more intense with rising ocean temperatures brought on by manmade global warming.
“But even if you have the same storm regime [we have now] in the future, with sea level rise you have a real problem,’’ he said.
Beth Daley can be reached at email@example.com.