Sea levels may swell much higher than previously predicted, thanks to feedback mechanisms that are speeding up ice melt in Greenland and Antarctica.
Climate simulations need to take such feedbacks into account, William Hay, a geologist at the University of Colorado Boulder, told the Geological Society of America meeting in Charlotte, N.C., on November 4. So far the models haven’t incorporated such information because “it just makes them much more complicated, ” he says.
Many scientists share Hay’s concerns, says geologist Harold Wanless of the University of Miami. “The rate at which ice melt and sea level rise is happening is far faster than anything predicted, ” he says.
Global sea levels rose an average of about 15 centimeters over the past century. Current data suggest they will rise another 1 meter by the year 2100, and some scientists predict far more. But the 2007 report of the Intergovernmental Panel on Climate Change projected a rise of just 0.2–0.6 meters over the same time period. “The data weren’t available in 2007 to say Greenland and Antarctica were melting, ” says earth scientist Benjamin Horton of the University of Pennsylvania in Philadelphia. “Sea levels are going to be greater than the upper estimate of the 2007 IPCC, but the big question is, when?”
To help answer that question, Hay is looking at underappreciated feedbacks. For one thing, big infusions of freshwater in the Arctic — from melting sea ice and from northern rivers — are driving cold ocean currents away from the North Pole and bringing up warm ones. This process is helping melt Greenland’s ice sheet, which could cause sea level to change very quickly, Hay says. Also, Arctic melt is exposing large areas of dark water, which absorbs the sun’s heat instead of reflecting it like ice does. As a result, warmer temperatures are disrupting local weather. After a record melt in August, for example, the high-pressure weather system that keeps the Arctic in a deep freeze was seasonally replaced by a low-pressure system that sucked in warm air, causing even more melting. To top it all off, the Arctic Ocean and thawing permafrost are releasing methane, a powerful greenhouse gas that traps atmospheric heat and raises temperatures still more.
The last time Earth’s climate was as balmy as today was about 120, 000 years ago, when the planet was 2–3° Celsius warmer and sea levels were 4–6 meters higher. Much of the Greenland ice sheet was melted then. “Those sea level change rates are very, very large, and that was under natural conditions, not the human perturbation that’s going on now, ” says Hay.
And Greenland isn’t the only concern — Antarctica contains a vast amount of ice that, if emptied entirely into the ocean, would cause 80 meters of sea level rise. For years scientists suspected the Antarctic ice was frozen to the ground, but evidence now suggests there is liquid water under many regions, lubricating the ice base like a skating rink. The only things stopping that ice from sliding into the sea are ice shelves, which act like corks in a bottle, Hay says. As these ice shelves break up — as some are already doing — “it’s like taking the cork out of the bottle.”
Even modest sea level rise can have far-reaching impacts. Higher sea levels make it easier for storm surges — like those produced by Hurricane Sandy — to reach further inland and inflict damage, Wanless says. “The future of our coastal cities is at stake.”
海面升高幅度高于预期
海面上升的幅度可能要远远高于先前的预测,原因是反馈机制加速了格陵兰和南极的冰雪融化.
11月4日,科罗拉多博尔德大学的地质学家William Hay在北贝卡莱罗纳州夏洛蒂市举行的美国地质学会会议上说,气候模拟过程中需要将各种此类的反馈机制考虑进去.目前为止的模型并没有包含此类信息,因为”如此的话会使模型变化的非常复杂”.
迈阿密大学的地质学家Harold Wanless称,很多科学家都很关注Hay的担心.他说”在那些冰融化及海面上升发生地地方,其发生的速率比任何预测都要高.
在过去的100年全球海面平均约上升了15cm.现在的数据则显示在2100年海面将升高1m,一些科学预测甚至会上升更多.但IPCC(政府间气候变化专门委员会)2007年报告中预测本世纪末海面上升0.2-0.6m.宾夕法尼亚大学的地学家Benjamin Horton说”在2007年并没有数据显示格陵兰和南极的冰盖在消融”,”海面上升幅度将超过IPCC报告预测的上限，但现在最大的问题是，具体什么时候海面会上升这么多？”
为回答这个问题,Hay正关注以前未被充分重视的反馈机制.他称,一方面,在北极地区,来自海冰融化和北部河流带来的大量淡水输入会迫使冷洋流离开北极点,并使暖洋流上涌.这个过程会促进格陵兰冰盖消融,进一步将是海面产生快速的变化.而且,北极冰雪消融会使大面积出露海面吸收太阳辐射,而非像原来冰雪存在时反射太阳辐射.结果将导致升高的温度绕乱当地的气候.例如,在八月份的一次冰雪消融记录中,使北极处在的冷高压季节性的被吸引暖空气的低压系统替代,结果导致更多的冰雪消融.最甚的是,北冰洋和消融的冻土会释放一种厉害的温室气体--甲烷,甲烷会吸收大气中的热量致使温度上升.
地球最近一次向今天这么温暖的时候大约是在12000年前,当时温度比现在高2-3摄氏度,海面比现在要高4-6m.当时格陵兰冰盖大部分消融掉了.Hay说当时海面变化幅度非常大,但那时仅是在自然原因的作用下,而没有像今天这样的人为作用导致海面波动.
格陵兰并非是唯一关注的地方,在南极也有巨量的冰,如若南极冰盖完全融化并注入大洋中,将是海面上升80m. Hay说,数年来来科学家们一直怀疑南极的冰是否冻到了地面,但最新的证据表明在南极冰盖下很多区域存在液态水,如滑冰场似的起到对冰盖的润滑作用.唯一能阻止冰滑向海洋的的东西就是冰架,其扮演的作用一如瓶子中的软木塞.如果这些冰架损坏的话(尽管已经有一部分正在损坏),就如将瓶塞从瓶子中去掉一样.
即使是最小幅度的海面上升也会导致严重的后果.Wanless说,高海面会使风暴潮容易达到内陆,并造成破化,像飓风桑迪一样. ”我们沿海城市的未来正处在险境中”
