Weather Report: Why Do the Seas Rise?

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A certain amount of sea level rise is locked in. So what are we going to do about it? 

The phrases “climate change” and “sea level rise” nearly go hand in hand, particularly when discussed in coastal communities like Martha’s Vineyard. Many of the cliché images of global warming often involve the seas, whether it be a polar bear stranded on a melting iceberg or dramatic artistic depictions of seas consuming cities. There is quite a bit of misunderstanding and misinformation on how, how much, and how fast the seas will rise as our climate warms. Many people don’t have a great sense of how much the seas will rise in the future and if you ask around, you often can get wildly different answers or no answers at all. Contributing to this confusion is the fact that the future rate of climate change is somewhat uncertain mostly due to the uncertainty in how much society acts now to reduce future carbon emissions.

Despite that uncertainty, seas are rising and will continue to rise for decades to come. But why the seas are rising is a surprisingly complicated story. Most of us easily understand that as the glaciers and polar ice caps melt, that meltwater eventually flows into the oceans and leads to sea-level rise (SLR). Just like adding water to a bathtub. But that is only about half of the story. There are five other factors at play to contribute to SLR, some of which are rather surprising. The relative importance of each of the six factors depends on where one is on the earth. It also depends on how far into the future one looks with some of the factors being important now, while others have a delayed response and kick more into the future. 

Global Mean Sea Level Rise

Two of the six factors that contribute to SLR pretty much are uniform across the oceans. The other four vary by region and in some places can help reduce the degree of SLR locally. Unfortunately for Martha’s Vineyard (and New England, in general), all six factors contribute to rising seas here, making our region particularly vulnerable. 

The two globally-uniform factors happen to be the biggest of the six, and combined will account for about three-quarters of the SLR we expect in the coming decades. They are:

1. Melting ice 

As mentioned earlier, most people readily understand this factor. As the ice sheets melt, the water runs into the oceans and will account for about half of the expected SLR by 2100. Melting ice from Antarctica and Greenland make up about 80% of the meltwater, with glacial ice from elsewhere (e.g., in mountains) making up most of the remainder. The melting of Arctic Ocean ice near the North Pole does not contribute much to rising seas since much of that ice is already floating in the sea. Hence as it melts, it doesn’t add much new volume to the seas. It’s the melting of ice that’s not currently in the oceans that matters most.

2. Thermal expansion 

Climate change warms both the atmosphere and the oceans. In fact, a vast majority of the heat from climate change is held by the oceans because their capacity to hold heat is so much greater than the atmosphere. Unfortunately, as water warms, it also expands and that causes the sea to rise since there is nowhere for the expanding water to go but up. Expanding seas will contribute about one-quarter of the total SLR seen here in New England this century. However, the exact pace of this expansion depends on how fast the deep oceans warm in the future, and figuring that out is a major focus of ongoing climate research. 

Local Sea Level Rise

The other four factors that contribute to SLR vary depending on where one is on the planet. Combined, these factors will contribute the remaining one-quarter of the SLR we expect to see here by 2100. They are, in rough order of significance:

1. Slowing of the Gulf Stream 

The Gulf Stream carries warm water northward in a narrow “river” off the eastern seaboard of North America. It is a major factor in maintaining the overall climate balance of our planet. An interesting feature of the Gulf Stream is that sea levels are about three feet higher on the east side of the Gulf Stream than on the western side near our shores. As the climate warms, the Gulf Stream will weaken, which will cause the change in sea levels across it to lessen. This results in seas along our coastline to rise a bit, and to fall on the eastern side over the open Atlantic. 

2. Changes to Earth’s Gravity Field 

The amount of ice currently locked up in Greenland and Antarctica is so enormous that it actually has its own small gravitational fields that are strong enough to pull a bit of water from the open oceans toward their shores. But as these massive ice sheets melt, that pull weakens and that water that they have been holding close to their shores begins to slosh back across the oceans, leading to rising seas far elsewhere, including here in New England.

3. Sinking Land 

During the last ice age, the weight of the Laurentian Glacier across much of eastern North America was so large that it forced the continental land to sink into the earth’s crust a bit. Along the edge of that sinking, the land bulged up a bit in response and Martha’s Vineyard sits on that bulge. Even though most of that glacier melted some 10,000 years ago, the continental land is still slowly adjusting back to “normal” and, as a result, the bulge under Martha’s Vineyard is still settling back down. While this isn’t sea-level rise per se, the impacts are effectively the same since what we really care about is where the land meets the sea. Further, this effect isn’t human-induced SLR, but a continued delayed reaction to natural climate change from tens of thousands of years ago. Nonetheless, it compounds the impacts from the current human-induced climate change we are seeing now.

4. Changing Average Weather Patterns 

As the climate changes, so will our weather patterns including the average direction and speed of the wind. Since winds can push or pull water from our shores (as they do in dramatic fashion during storm surges), changes in the average wind patterns will lead to small changes in average sea levels along the shores. This factor is expected to be relatively small and will be a bit different in different parts of the Island. 

What about storms?

Each of the above six factors will lead to long-term average SLR. There will also continue to be shorter-term fluctuations in sea levels associated with passing storms. (Lunar tides and their natural variations also contribute to short-term fluctuations in sea levels, and this will continue. However, these natural tidal fluctuations will not be impacted by climate change much.) These storms can lead to short-term sea level rises or falls depending on the orientation of the winds to the shore. It is generally during these storms that we tend to notice sea levels most as this is when flooding and erosion are most prominent. Unfortunately, as the climate warms, storms (particularly hurricanes) are expected to get a bit stronger on average, even if they may not get more frequent. So, while we might not get more storms impacting our Island in the future, those that we do get might be in a stronger category (e.g. Cat 2 rather than Cat 1). As a result, the temporary storm surges we see with these storms will often be a bit larger. When combined with the long-term, general rising of the seas will make future storms we see here that much more impactful.

So how much and how fast?

Now that we know all the reasons the seas will rise, the big question is how much and how fast? To start, it’s good to know how much seas have already risen. Since about 1850, or roughly the start of the industrial era and when sea level rise began in earnest, global mean sea levels have risen just over a foot, or about ¾” per decade. Near Martha’s Vineyard, SLR has averaged closer to 1” per decade because of all the added effects noted here. So seas today are about 1 ½ feet higher today than in 1850.

Unfortunately, SLR is accelerating and over the past two decades, the rate of rise has increased to about 1 ½” each decade. And therein lies the real problem. Sea level rise is a so-called delayed response to global warming. It takes time for ice to melt, oceans to warm and expand, and many of the other factors to kick in. Even if we could suddenly stop adding carbon to our atmosphere today, the delayed response will cause seas to continue to rise at about 1 1/2” per decade well into the next century. But we are very far from shutting off our carbon emissions, so the rate of SLR will continue to accelerate for decades to come. Based on the most recent projections, SLR is expected to increase to between 3” and 5” per decade by 2050 regardless of what actions we take now. And by 2100, the rate of SLR could approach nearly one foot per decade if carbon emissions stay anywhere near current levels. Even with reasonably aggressive carbon emission reductions, the rate of SLR is almost certainly going to accelerate to at least 6” per decade by the end of this century. 

Putting it all together, between now and 2050, it is a near certainty that here on Martha’s Vineyard, seas will rise between 6” and 12”. (Occasionally, higher sea level rise numbers are quoted, but those are either the “worst case scenario” rather than the more likely scenarios, or in some cases, may be relative to an earlier year rather than now. For example, the recent NOAA (2022) Global and Regional Sea Level Rise Scenarios for the United States uses a baseline year of 1970. Hence one must subtract about 6-8” from those projections to account for the rise that has already occurred since 1970 to estimate the expected further rises from where sea levels are today.) In the near-worst cases, by the end of the century, SLR will rise at least between 1.5’ and 5’ over today’s levels, depending on how aggressive our carbon reductions are.  And this is even before we account for the short-term impacts of the expected stronger storms.

What does this mean for our Island?

Even in the most optimistic SLR circumstance, a 1.5’ rise in mean sea level will have dramatic impacts on the Island. Without any adaptation measures, the current nuisance flooding at Five Corners or along Dock St. would become almost normal. Our beaches, particularly on the south side of the Island, will continue to erode and retreat at increasingly faster rates. Atlantic Ave. along South Beach will almost certainly begin to be overtaken by retreating dunes by 2050 and eventually the sea by the end of the century. The Menemsha commercial fishing docks will go from being underwater one or two times a month, to being underwater one or two times a week by 2050 and nearly constantly by 2100.  

The impacts will be most dramatic during our strongest storms. It is impossible to predict exactly when the next one of these major storms will occur, but we know it is inevitable. Martha’s Vineyard has not been struck directly by the eye of a Category 4 hurricane in recorded history, but the odds of that occurring increases slightly in the coming decades. If such a storm occurs with a 10’-15’ storm surge on top of the overall mean SLR we are seeing, the destruction to our Island could be unprecedented and almost unimaginable. Much of infrastructure near the water could be lost and large portions of our communities near the shore could be underwater.

What You Can Do?

Prepare and adapt, it’s as simple as that. While we must continue to curb greenhouse gases as much as possible to limit the degree of future climate change and SLR; unfortunately significant future SLR is inevitable regardless of what we do now. Therefore, it is critical that we plan for and adapt to these changes. Master plans need to accommodate how our communities will look and operate with higher seas. Specific projects for new or modified infrastructure vulnerable to SLR need to start soon. In some cases, those plans may call for a strategic retreat from the shore, while others may only require reinforcements or elevation of the infrastructure.  

The Martha’s Vineyard Commission has started a Climate Adaptation Plan (CAP) to create a framework for both Island-wide climate adaptation and to assist and coordinate the towns in their local adaptation efforts. You can see details here. Such a coordinated framework to SLR-related priorities is key to effective actions, since there is so much vulnerable infrastructure that we collectively depend on, such as ferries, critical roads, and the hospital. While we may not be able to prevent significant SLR in the coming decades, effective planning now such as the CAP can help us make smart choices that will minimize the impact it has on our island and livelihoods. 

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Dr. Peter P. Neilley
Dr. Peter P. Neilley
Dr. Peter Neilley is Head of Weather Forecasting Sciences and Technologies for The Weather Company, an IBM Business. He earned his PhD in Atmospheric Sciences at MIT in part through the Joint Program with Woods Hole Oceanographic Institute, is a Fellow of the American Meteorological Society, an IBM Distinguished Engineer and a year round resident of Chilmark. He’ll be reporting regularly on the interplay between weather and climate.

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