![\"\"](\"http:\/\/www.cawater-info.net\/all_about_water\/en\/wp-content\/uploads\/2012\/12\/maldives-sea-level-rise.jpg\")
<\/a><\/p>\nBut those same scientists have also been reminding us consistently that this is just an average. Thanks to all sorts of regional factors \u2014 changes in vegetation, for example, or ice cover, or prevailing winds \u2014 some areas are likely to warm more than that, while others should warm less.<\/p>\n
What\u2019s 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 meter 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 \u2014 Scotland, Iceland, and Alaska for example \u2014 it could be significantly less in the centuries to come. In others, like much of the eastern United States, it could be significantly more.<\/p>\n
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.<\/p>\n
If the idea of local differences in sea level comes as a surprise, it\u2019s probably because the experts themselves are only now beginning to fully realize what might cause such differences, and how significant they might be. One factor, which they\u2019ve 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 millimeters a year.<\/p>\n
Around the periphery of where the glaciers sat, by contrast \u2014 places like Chesapeake Bay and the south of England \u2014 the land was actually squeezed upward during the Ice Age by the downward pressure nearby. The resulting \u201cglacial forebulge\u201d 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.<\/p>\n
And in some coastal areas \u2014 most notably along the Gulf of Mexico in Louisiana \u2014 the land is falling as well: Thanks to massive oil and gas extraction, the continental shelf is collapsing like a deflated balloon. \u201cThe rate of subsidence measured at Grand Isle, Louisiana,\u201d says Rui Ponte, of the private consulting firm Atmospheric and Environmental Research, Inc, \u201cis almost 10 millimeters per year, compared with two or three in other areas.\u201d That\u2019s especially problematic for a city like New Orleans, which already lies partly below sea level.<\/p>\n
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.<\/p>\n
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 \u2014 in places such as the Philippines \u2014 compared with those in northern South America. If those winds shift with climate change, so would local sea levels.<\/p>\n
Ocean currents can also create significant local effects. During preparations for the Intergovernmental Panel on Climate Change\u2019s Second Assessment Report back in the mid-1990s, Ronald Stouffer \u2014 a climate modeler at the U.S. Geophysical Fluid Dynamics Laboratory in Princeton, NJ \u2014 and several colleagues were comparing projections of regional sea-level rise from different models.<\/p>\n
\u201cThey were splattered all over the place,\u201d he recalls, \u201cand 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.\u201d<\/p>\n
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 \u201coverturning\u201d that drives major ocean currents. In the Atlantic, it works like this: Warm surface water \u2014 the Gulf Stream \u2014 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.<\/p>\n
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\u2019t let the surface water cool so easily, interfering with its tendency to sink. Second, fresh water from Greenland\u2019s shrinking ice cap dilutes the surrounding waters; since fresh water is less dense than salty water, there\u2019s a further impediment to sinking.<\/p>\n
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 U.S. coast to rise another 8 or so inches over the next century beyond the global average, given a medium-emissions scenario.<\/p>\n
When he first heard about this idea, says Stouffer, \u201cit was one of those \u2018duh\u2019 moments for me. I said, \u2018Yeah, that makes sense.\u2019\u201d He ended up co-authoring a paper that appeared in Nature Geoscience last March, laying out the argument.<\/p>\n
Then, however, Stouffer experienced another \u201cduh\u201d moment. \u201cI\u2019m somewhat embarrassed by that paper,\u201d he says, \u201cbecause here we were focused on this relatively little problem, and there\u2019s this great big gorilla in the room, and I missed it. But I had a lot of company.\u201d (This last point is crucial: Stouffer is among the most experienced and respected modelers in the world, so a \u201cduh\u201d moment for him means the surprise is widespread.)<\/p>\n
The gorilla Stouffer refers to \u2014 an effect so large that it overwhelms the others \u2014 is something called the geoid. It\u2019s an imaginary surface that maps the strength of Earth\u2019s gravitational field, and it\u2019s as bumpy as the surface of the actual planet. Orbiting satellites don\u2019t 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.<\/p>\n
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.<\/p>\n
The same sort of thing happens when there\u2019s 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\u2019s 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.<\/p>\n
These polar ice caps are Stouffer\u2019s gorillas. They keep sea level higher than it would otherwise be for thousands of kilometers around both land masses, and correspondingly lower elsewhere.<\/p>\n
If the polar ice sheets shrink, though \u2014 as they\u2019re currently doing, especially in Grenland and West Antarctica \u2014 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 toward the sea).<\/p>\n
If you simply spread the resulting increase in sea level evenly around the world, it would amount to about 5 meters\u2019 worth. But the ice sheet\u2019s gravity is currently keeping sea level artificially low in the Northern Hemisphere, so if it disappeared, the actual increase along the U.S. mid-Atlantic coast would be more like 6.3 meters. 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 U.S.<\/p>\n
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. \u201cThe signal is so large,\u201d says Stouffer, \u201cthat if you own beachfront property in Iceland, and all of the ice on Greenland melts and adds seven meters to average sea level, you end up with more beach. But in Hawaii, you get your seven meters of sea-level rise plus an extra two or three on top of that. It\u2019s phenomenal to me that it matters that much.\u201d<\/p>\n
Mitrovica agrees.<\/strong><\/p>\n \u201cWhen I give talks about this, people don\u2019t believe me,\u201d says Mitrovica. He doesn\u2019t blame them, either. \u201cIt\u2019s just wacky when you think about it, completely counterintuitive,\u201d he says. \u201cBut it\u2019s true.\u201d<\/p>\n As governments, businesses, and homeowners plan for the future, they should assume that the world\u2019s oceans will rise by at least two meters \u2014 roughly seven feet \u2014 this century.<\/p>\n It\u2019s even measurable, despite the fact that the melting of the ice sheets has barely begun. Even when you correct for other effects, says Mitrovica, you can still see that Europe\u2019s sea level rise is less than you\u2019d expect. \u201cIt\u2019s profoundly puzzling,\u201d he says, \u201cuntil you realize you\u2019re seeing the gravitational signal of Greenland melting.\u201d<\/p>\n When he started looking at regional effects, Mitrovica recalls, some climate-change deniers were noting that sea-level rise was happening at different rates in different regions, arguing that this proved there was no global trend, and thus no global warming. That was already a bogus argument, but now that he and others have begun investigating the gorilla in the living room, it\u2019s even more absurd. The science is so straightforward, he says, that \u201cif you saw that sea level was rising uniformly around the world, it would be proof that the big ice sheets are not melting.\u201d<\/p>\n by Michael D. Lemonick Dear User\/Visitor! Please, answer on our questions: tick off one of the positions – your answer will make us able to improve our site and make it more interesting and useful!<\/strong><\/em><\/p>\n
\nSource: http:\/\/www.climatecentral.org\/<\/a><\/p>\n