Over the last 100 years, the global mean sea level has been estimated to have risen by four to eight inches (ten to 20 centimetres) with sea level rise over the past two decades occurring at double the speed of the preceding 80 years. This increase has been largely put down to both ocean warming – which causes the oceans to expand – and the melting of polar ice. But not everyone is feeling the effects of these twin causes equally.
The east coast of the United States is one such example. Throughout the 20th century, sea levels have risen by a foot and a half in coastal communities near Cape Hatteras in North Carolina and along the Chesapeake Bay in Virginia. Further north, New York City and Miami have experienced only a one-foot rise over the same period, while sea levels further north again in Portland, Maine, rose by approximately six inches (or half a foot).
Researchers from the Woods Hole Oceanographic Institution, based in Massachusetts, have been seeking the answer to this variation. By analysing a range of data, including historic tidal gauge measurements, GPS sensor data and radio carbon dating of salt marsh sediments, they discovered that it is largely due to a geological phenomenon known as post-glacial rebound, or glacial isostatic adjustment – a process that takes place over millennia in response to the waxing and waning of the ice ages.
In short, during the last ice age (which came to an end roughly 11,500 years ago) large sheets of ice covered much of Canada and the northeastern part of the US. Because of the weight of this ice, the Earth’s crust directly beneath was depressed causing the land to sink. At the same time, material lower down still, in the Earth’s mantle, was pushed outwards, resulting in the land around the edges of the ice sheet rising up. Now, with the ice sheets gone, the reverse is taking place. The land once covered by ice is rebounding – a process that can take thousands of years – and the land around the edges that was once pushed up is sinking. In the places where this sinking is taking place, it appears that sea levels are rising faster than in other coastal areas. What’s key for those regions already struggling with floods, such as Cape Hatteras – a thin spit of land that juts into the Atlantic Ocean – is that the combined effect of rising sea levels and sinking land means that the illusion of a faster sea level rise is being felt in real terms.
On the plus side, scientists are able to predict with some confidence how the land will act in the future. Woods Hole assistant scientist Christopher Piecuch explains that this is possible due to the timescales involved. ‘One thing that’s really neat about our study is that this glacial isostatic adjustment process plays out over really long time scales, over centuries if not millennia. If we can have a really good idea in the present tense what glacial isostatic adjustment is doing, we can be pretty sure that in 100 years it will be doing more or less the same thing. This gives us some sense of predictability.’
Unfortunately, it’s still bad news for some. ‘In places such as the Cape Hatteras region, we know that they’ll feel another foot and a half of sea level rise over the next century just due to this process,’ says Piecuch. ‘You can take that number and put it on top of other predictions for sea level rise and get a sense of what parts of those regions will be inundated in the next 50 to 100 years.’
Also on the downside, there are still other factors affecting sea level rise that, for now, make predictability more difficult. While Piecuch is confident that post-glacial rebound is the most important factor when it comes to understanding the variation in sea level rise on the east coast of the US, it is not the whole story. Even when the effects of glacial rebound are removed from the picture, there are still spacial variations evident at different points along the coast. His next step is to explore in more detail why this is the case in the hope of completing the picture. In the meantime however, it seems that the residents of Cape Hatteras can expect no respite from the rising waters of the Atlantic.
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