Rising Sea Levels -June 2019
Scientists say sea-level rise is already happening. The question is how high will the oceans rise in the next few decades and what can we do about it?
by: John Stanton
photography by: Nicole Harnishfeger
The tide gauge sits unobtrusively just off Steamboat Wharf. In a time of ongoing political arguments about climate change, the gauge is a silent sentry whose work is non-partisan. It simply measures and records water levels. This happens all along the coast of the United States.
The work is overseen by the National Oceanic and Atmospheric Administration and has been going on for more than a century in one form or another. The Nantucket tide gauge was first installed in 1963. The present version was installed in 1990. In any discussion of climate change and sea-level rise, in any discussion of what this town and this island should be preparing for over the next 50 years or so, the data recorded by what NOAA lists as tide gauge 8449130 is good a place as any to begin.
Because numbers, sometimes, have consequences.
On Jan. 4, 2018, a nor’easter called Winter Storm Grayson hammered the island. At the storm’s height, the tide gauge measured 3.8 feet above what is called the mean higher high-water mark (MHHW). According to NOAA’s glossary of terms, MHHW is the average of what is called the higher high water for each tidal day, observed over a 19-year time period.
Those numbers played out this way: Flood waters cut off Brant Point lighthouse so that it appeared to be on an island of its own. The water was nearing three feet deep as it rolled up Easton Street. It flooded the three buildings that make up Coast Guard Station Brant Point and put its floating dock under hip-deep seawater.
When a storm and flooding are predicted, the Coast Guard spends about two days cleaning out the basements of its buildings and moving the equipment to an upper floor. Sometimes, faced with a storm, a decision is made to move the motor-rescue boats over to the Nantucket Boat Basin, but this time they stayed at the dock. Soon, it was obvious that getting personnel down Easton Street in a vehicle was going to be a problem. The answer was to run a boat over to the Town Pier to pick up enough off-duty personnel to bring the Coast Guard to full staff.
“The whole infrastructure will have to be redone here if we see a significant rise in sea level,” Brant Point chief Chris Swiatek said a year later. “This station is going to be a problem. We are typically under three to four feet of water in high-tide cycles, usually ones that correlate to lunar events. But five or six years down the road something is going to have to be done.”
Cars parked in the flood zone were totaled by seawater, homes were shifted on their foundations, families displaced. Nantucket firefighters evacuated 20 people from flooded neighborhoods.
At Jim Weimann’s coffee shop on Broad Street, 18 inches of seawater flooded the space and damaged the refrigeration system. The water rolled up the street as far as the Nantucket Whaling Museum. Washington Street was under water.
That is only a brief accounting. It is not a stretch to say it was the worst flooding in recent memory, perhaps in two decades. Those water levels, of course, were driven by a powerful storm surge. It is exactly that combination of rising sea levels and increasingly-powerful storms, driven by climate change, that has created a need to look 30, 50, 75 years into the future, while beginning to make decisions about how to deal with the impact.
“It’s happening. Sea-level rise is not stopping. Sea levels have picked up over the last several decades, in terms of overall rates, and those rates are expected to keep happening or to increase,” NOAA oceanographer William Sweet, Ph.D., said. “To what extent that happens is largely about the amount global heating increases. That’s what drives sea levels.”
Most scientific models used to predict sea-level rise offer three different scenarios: a best-case scenario, which is a low number; a medium scenario; and a high-end scenario. The differing predictions are fueled by potential climate-change rates. The best-case scenario has a higher probability of occurring. But the worst-case scenario cannot be ruled out, Sweet said, and to not prepare for it would be a mistake.
The big question is how close to that benchmark 3.8 feet above MHHW will the sea level rise, when, and how will it manifest itself as it approaches that height? A second question, nestled right there in all the predictive models and scenarios, is what will it take to spur people to action?
“More and more, the question is how to prepare for something whose impact might be three decades into the future,” Sweet said. “When it floods and flooding happens in lowlying parts of town, people often just shrug. But at what point does something become intolerable?
“There is no answer to it. But collectively, people need to face the fact that flood frequencies now are different then they were in the past, and will they change in the future? Is there a sweet spot where (you get flooding) a number of times per year and people begin to say that’s enough?”
We often use the idea of a hundred-year storm as a storm with a likelihood of happening every 100 years. But scientists use that phrase to describe a storm with a one-percent chance of happening every year.
Rising sea levels might eventually mean that the storm surge that drove the flood height of that January storm would not be a necessary element of flooding, Sweet said.
“The way it works, is as sea levels starts rising, you get these events (like the January 2018 storm) five or 10 times a year and it becomes less to do with a storm surge and more to do with tidal flooding,” he said, trying to put his finger on what to a layman might be called the new normal.
The question can be put on a spreadsheet. How much did the storm of last Jan. 4 cost the town in man hours for emergency workers like police and firefighters and public-works employees at the height of the storm, in cleanup, in lost commerce, in repairs after the water receded? And what happens when sea levels begin to creep up to a point where what is called a nuisance or sunny-day flooding becomes the norm, when the odds of a big storm are no longer one percent each year?
According to Sweet, by 2050 local sea levels are likely to rise between one and 1.7 feet. If that happens, the kind of flooding seen last year is likely to go from a one-percent chance of happening annually to between 10-50 percent. By 2100, he said, local sea levels are likely to rise between two and 4.3 feet. If that happens, this type of flooding is likely to occur “somewhere between annually and daily.”
“This is not an end-of-the-century discussion,” Sweet said. “Maybe the sea-level rise now is not as disconcerting, but at what point do they become damaging and alarming?”
Kirk Bosma is a coastal engineer at The Woods Hole Group. He has done work for the Boston Green Ribbon Commission, a comprehensive study of the potential effects of sea-level rise on the city of Boston which brings together scientists, city officials, civic groups and businesses.
“If your political affiliation is geared to denying climate change, it can be a little difficult to sell. I would maintain that they don’t really deny there is climate change, they just have political motivation for denying the causes,” Bosma said, asked about the politics of climate change.
“Frankly, if we think about what I’m concerned with, that the sea level is going up, then who cares what’s causing it, the point is to prepare for that. Then you can eliminate some of the political elements of the debate.”
That preparation, he said, begins with understanding the risk to communities faced with the effects of sealevel rise. Clearly defining that risk is a combination of two elements: What is the probability of that risk, or in other words, determining how often and when will storms, sea-level rise and the combination of the two occur? And what is the consequence of that event?
“If this particular road is inundated, is it just wet for a while or damaged? If either thing happens, what are the consequences of that? The same thing with pumps or shelters or electric stations, or wastewater-treatment plants, the same for natural resources, dunes, marshes, beaches, everything you can think of under the sun. What are the consequences?” he asked.
“You can then begin to determine what your overall risk is. The reason you want to do that is to begin to prioritize how to build resiliencies for your community or town, across a whole wideranging group of assets.”
The way to do that, he added, is to bring together “very high-level scientific assessment of the changing climate and how that is not only going to impact the community today, but how
it will evolve and change over time.”
“At minimum, you want to be prepared. We know there are some problems you’re going to get today. Even if you don’t believe what the future holds at least you should know what people are saying and be prepared to think about that. So that as the climate changes, or doesn’t, you can have a toolkit to build resilience.”
Almost a year to the day after Winter Storm Grayson hit the island, the ballroom of the Nantucket Hotel was filled with town officials, Select Board members and other interested parties, for what was called a community-resilience-building workshop. Chuck Larson, the town’s former special-projects manager, and two representatives from the civil-engineering and landscape-architecture firm Milone & MacBroom, who were hired by the town to lead it through the first steps of the process, stood on the stage and welcomed the assembly. ///
John Stanton is a documentary filmmaker living on Nantucket. He is a regular contributor to Nantucket Today and The Inquirer and Mirror, Nantucket’s newspaper since 1821.