The secret of sea level rise: it will vary greatly by region

As the world warms, sea levels could easily rise three to six feet this century. But increases will vary widely by region, with prevailing winds, powerful ocean currents, and even the gravitational pull of the polar ice sheets determining whether some coastal areas will be inundated while others stay dry.

For at least two decades now, climate scientists have been telling us that CO2 and other human-generated greenhouse gases are warming the planet, and that if we keep burning fossil fuels the trend will continue. Recent projections suggest a global average warming of perhaps 3 to 4 degrees C, or 5.4 to 7 degrees F, by the end of this century.

But those same scientists have also been reminding us consistently that this is just an average. Thanks to all sorts of regional factors - changes in vegetation, for example, or ice cover, or prevailing winds - some areas are likely to warm more than that, while others should warm less.

What’s true for temperature, it turns out, is also true for another frequently invoked consequence of global warming. Sea level, according to the best current projections, could rise by about a metre by 2100, in large part due to melting of the Greenland and West Antarctic ice sheets. But that figure, too, is just a global average. In some places - Scotland, Iceland, and Alaska for example - it could be significantly less in the centuries to come. In others, like much of the eastern United States, it could be significantly more.

And among the most powerful influences on regional sea level is a surprising force: the massive polar ice sheets and their gravitational pull, which will lessen as the ice caps melt and shrink, with profoundly different effects on sea level in various parts of the globe.

If the idea of local differences in sea level comes as a surprise, it’s probably because the experts themselves are only now beginning to fully realise what might cause such differences, and how significant they might be. One factor, which they’ve have been aware of for decades, is that the land is actually rising in some places, including northern Canada and Scandinavia, which are still recovering from the crushing weight of the Ice Age glaciers that melted 10,000 years ago. That makes sea-level increases less than the global average would suggest, since these land areas are rising a few millimetres a year.

Around the periphery of where the glaciers sat, by contrast - places like Chesapeake Bay and the south of England - the land was actually squeezed upward during the Ice Age by the downward pressure nearby. The resulting “glacial forebulge” has been sinking back ever since, also at an average rate of a few millimeters a year, so sea level rise is greater than average in these regions.

And in some coastal areas - most notably along the Gulf of Mexico in Louisiana - the land is falling as well: Thanks to massive oil and gas extraction, the continental shelf is collapsing like a deflated balloon. “The rate of subsidence measured at Grand Isle, Louisiana,” says Rui Ponte, of the private consulting firm Atmospheric and Environmental Research, Inc, “is almost 10 millimetres per year, compared with two or three in other areas”. That’s especially problematic for a city like New Orleans, which already lies partly below sea level.

Ponte said that these local instances of rebound or subsidence will subtract or add a couple of inches to the global increase in sea level over the next century, depending on the region.

A bigger effect will come from changes in prevailing winds, which can push water consistently toward the land or keep it at bay. The trade winds that blow west across the tropical Pacific, for example, move water in the same direction, boosting average sea levels by as much as 24 inches on the western side of the ocean - in places such as the Philippines - compared with those in northern South America. If those winds shift with climate change, so would local sea levels.

Ocean currents can also create significant local effects. During preparations for the Intergovernmental Panel on Climate Change’s Second Assessment Report back in the mid-1990s, Ronald Stouffer - a climate modeler at the US Geophysical Fluid Dynamics Laboratory in Princeton, NJ - and several colleagues were comparing projections of regional sea-level rise from different models.

“They were splattered all over the place,” he recalls, “and the differences had no rhyme or reason. We speculated that they had to do with differences in how the models treated changes in the prevailing winds.”