问答题
Think or Swim: Can We Hold Back the Oceans[A] As the world gets warmer, sea levels are rising. It has been happening at a snail’s pace so far, but as it speeds up more and more low-lying coastal land will be lost. At risk are many of the world’s cities and huge areas of fertile farmland. The sea is set to rise a metre or more by the end of this century. And that’s just the start. "Unless there is a rapid and dramatic about-face in emissions—which no one expects—the next century will be far worse than this century," says glaciologist (冰川学家) Bob Bindshadler of NASA’s Goddard Space Flight Center, Maryland.[B] Throwing trillions of dollars at the problem could probably save big cities such as New York and London, but the task of defending all low-lying coastal areas and islands seems hopeless. Or is it Could we find a way to slow the accelerating glaciers, drain seas into deserts or add more ice to the great ice caps of Greenland and Antarctica[C] These ideas might sound crazy but we have got ourselves into such a bad situation that maybe we should start to consider them. If we carry on as we are, sea levels will rise for millennia, probably by well over 10 metres. Slashing greenhouse gas emissions would slow the rise, but the longer we hesitate, the bigger the rise we will be committed to. Even if "conventional" geo-engineering schemes for cooling the planet were put in place and worked as planned, they would have little effect on sea level over the next century unless combined with drastic emissions cuts.[D] In short, if coastal dwellers don’t want their children and grandchildren to have to abandon land to the sea, now is the time to start coming up with Plan C. So New Scientist set out in search of the handful of researchers who have begun to think about specific ways to hold back the waters.[E] One of the reasons why the great ice sheets of Greenland and Antarctica are already shrinking is that the ice is draining off the land faster. Ice floating on the surrounding seas usually acts as a brake, holding back glaciers on land, so as this ice is lost the glaciers flow faster. The acceleration of the Jakobshavn glacier in Greenland is thought to be the result of warm currents melting the floating tongue of the glacier. Other outlet glaciers are being attacked in a similar way.[F] Mike MacCracken of the Climate Institute in Washington DC is one of those starting to think that we shouldn’t just sit back and let warm currents melt ice shelves. "Is there some way of doing something to stop that flow, or cool the water" he asks.[G] Last year, physicist Russel Seitz at Harvard University suggested that the planet could be cooled by using fleets of customised boats to generate large numbers of tiny bubbles. This would whiten the surface of the oceans and so reflect more sunlight. MacCracken says the bubbles might be better arranged in a more focused way, to cool the currents that are undermining the Jakobshavn glacier and others like it. A couple of degrees of chill would take this water down to freezing point, rendering it harmless. "At least that would slow the pace of change," MacCracken says.[H] What about a more direct approach: building a physical barrier to halt a glacier’s flow into the sea by brute force Bindshadler thinks that is a non-starter. "The ice discharge has many sources, mostly remote and in environments where barriers are not likely to work," he says. "Taking just the one example I know best, the Pine Island glacier in Antarctica drains into an ice shelf that at its front is 25 kilometres across and 500 metres thick, and moves at over 10 metres per day. The seabed there is 1000 metres down and is made of sediment (沉淀物) hundreds of metres thick and the consistency of toothpaste." Not your ideal building site.[I] A slightly more subtle scheme to rein in the glaciers was proposed more than 20 years ago by Douglas MacAyeal of the University of Chicago. His idea is to fight ice with ice. The big outlet glaciers feed into giant floating shelves of ice, which break off into icebergs at their outer edges. MacAyeal suggested pumping water up from beneath the ice and depositing it on the upper surface, where it would freeze to form a thick ridge, weighing down the floating ice shelf. Add enough ice in this way, and the bottom of the ice shelf would eventually be forced down onto the seabed. Friction with the seabed would slow down the shelf’s movement, which in turn would hold back the glaciers feeding into it. It would be like tightening an immense valve.[J] "I think it’s quite an inspired idea," says Bindshadler. But nobody has followed it up to work out how practical the scheme would be. "On the back of an envelope it has promise—but these ice shelves are big. You would need a lot of drilling equipment all over the ice shelf, and my intuition is that if you look at the energetics of it, it won’t work," Bindshadler says.[K] Even if we could apply brakes to glaciers, this would only slow down sea level rise. Could we do better than that and reverse it—actually make the sea retreat If you think of the sea as a giant bathtub, then the most obvious way to lower its level is to take out the plug.[L] "One of the oldest notions is filling depressions on the land," says MacCracken. Among the largest of these is the Qattara depression in northern Egypt, which at its lowest point is more than 130 metres below sea level. Various schemes have been proposed to channel water from the Mediterranean into the depression to generate hydroelectric (水力的) power, and as a by-product a few thousand cubic kilometres of the sea would be drained away. Unfortunately, that’s only enough to shave about 3 millimetres off sea level: a drop in the ocean. And there would be grave consequences for the local environment. "The leakage of salt water through fracture systems would add salt to aquifers (含水层) for good," says Farouk El-Baz, a geologist at Boston University who has studied the region.[M] Refilling the Dead Sea is no better. Because of surrounding hills, this depression could be filled to 60 metres above sea level, but even that would only offset the rise by 5 millimetres—and drown several towns into the bargain.[N] The notion of engineering lower sea levels remains a highly abstract topic. "If the world doesn’t control emissions, I’m pretty sure that no geo-engineering solution will work—and it would potentially create other side effects and false promises," says MacCracken. "But if we do get on a path to curbing emissions dramatically—down 50 per cent by 2050, say—then the question becomes, can geo-engineering help with the hump we’re going to go through over the next few centuries"Sea levels are rising very slowly at present.
