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

But a little more than a year ago, Jianjun Yin, now at Florida State University, suggested that it might be something else: a weakening of the “overturning” that drives major ocean currents. In the Atlantic, it works like this: Warm surface water - the Gulf Stream - flows north and east until it reaches the area between the United Kingdom and Greenland, where it cools, thus becoming denser, and sinks. It flows south and west, deep below the surface. Eventually, it rises again, warms, and heads back north.

If any part of this flow is significantly interrupted, the current will slow. Global warming has the potential to do just that, in two ways. First, a warmer North Atlantic won’t let the surface water cool so easily, interfering with its tendency to sink. Second, fresh water from Greenland’s shrinking ice cap dilutes the surrounding waters; since fresh water is less dense than salty water, there’s a further impediment to sinking.

Since the Gulf Stream warms northern Europe, the slowing could cool that part of the world. But the slowing would also force water to pile up behind what amounts to a partial blockage of the overturning current. That could force sea level along the US coast to rise another 8 or so inches over the next century beyond the global average, given a medium-emissions scenario.

When he first heard about this idea, says Stouffer, “it was one of those ‘duh’ moments for me. I said, ‘Yeah, that makes sense.’” He ended up co-authoring a paper that appeared in Nature Geoscience last March, laying out the argument.

Then, however, Stouffer experienced another “duh” moment. “I’m somewhat embarrassed by that paper,” he says, “because here we were focused on this relatively little problem, and there’s this great big gorilla in the room, and I missed it. But I had a lot of company.” (This last point is crucial: Stouffer is among the most experienced and respected modelers in the world, so a “duh” moment for him means the surprise is widespread.)

The gorilla Stouffer refers to - an effect so large that it overwhelms the others - is something called the geoid. It’s an imaginary surface that maps the strength of Earth’s gravitational field, and it’s as bumpy as the surface of the actual planet. Orbiting satellites don’t move around the Earth in perfect circles, or even perfect ellipses; their height changes when they go over the extra gravity exerted by a mountain range, and changes again when they orbit over a valley.

And because water is a liquid, the surface of the sea is also warped to follow the contours of the geoid. The extra gravitational attraction of an undersea mountain range pulls water toward it, creating a literal, permanent bump on the surface of the sea, while the deficit of gravity near an undersea valley creates a depression in the water up above.

The same sort of thing happens when there’s an excess of mass on land that lies near the ocean. A coastal mountain range pulls the water in its direction, raising sea level nearby. So do the massive icecaps that smother Greenland and Antarctica. Indeed, Antarctica’s polar ice sheet is so massive that it is three miles thick in places and covers an area one-and-one-half times the size of the United States, including Alaska.

These polar ice caps are Stouffer’s gorillas. They keep sea level higher than it would otherwise be for thousands of kilometres around both land masses, and correspondingly lower elsewhere.

If the polar ice sheets shrink, though - as they’re currently doing, especially in Greenland and West Antarctica - their gravitational pull weakens and so does their hold on the surrounding water. About a year ago, Jerry Mitrovica, a geophysicist who teaches an entire course on sea level at Harvard, co-authored a paper in Science that laid out what would likely happen if the West Antarctic ice sheet, the smaller of the two sheets that cover the Antarctic continent, were to melt. (Like a complete shutdown of the Gulf Stream, this is not considered likely anytime soon. But recent satellite measurements have shown that glaciers that drain the ice sheet have begun moving faster towards the sea).

If you simply spread the resulting increase in sea level evenly around the world, it would amount to about 5 metres’ worth. But the ice sheet’s gravity is currently keeping sea level artificially low in the Northern Hemisphere, so if it disappeared, the actual increase along the US mid-Atlantic coast would be more like 6.3 metres. In other words, as the West Antarctic Ice Sheet melts and loses mass, its pull on the surrounding ocean will lessen. Seas will drop around Antarctica and parts of the Southern Hemisphere, and that water will be displaced to more northerly areas, such as the east coast of the US.

Now that the gorilla has made its presence known, Stouffer is working with Mitrovica to understand its effects in greater detail. A joint paper, due out in a few months, will look into the gravitationally driven sea-level changes a melting Greenland could trigger. “The signal is so large,” says Stouffer, “that if you own beachfront property in Iceland, and all of the ice on Greenland melts and adds seven metres to average sea level, you end up with more beach. But in Hawaii, you get your seven metres of sea-level rise plus an extra two or three on top of that. It’s phenomenal to me that it matters that much.”