Has An Ocean Circulation Collapse Been Triggered?

Has An Ocean Circulation Collapse Been Triggered?

ScienceDaily (Feb. 25, 2008) — Predictions that the 21st century is safe from major circulation changes in the North Atlantic Ocean may not be as comforting as they seem, according to a Penn State researcher.

"The Intergovernmental Panel on Climate Change concluded that it is very unlikely that the North Atlantic meridional overturning circulation (MOC) will collapse in the 21st century. They predict a probability of less then 10 percent," says Klaus Keller, assistant professor of geosciences. "However, this should not be interpreted as an all clear signal. There can be a considerable delay between the triggering of an MOC collapse and the actual collapse. In a similar way, a person that has just jumped from a cliff may take comfort that pain in the next few seconds is very unlikely, but the outlook over the long term is less rosy."

Keller and his colleagues analyzed a possible threshold response for the MOC. A threshold response occurs when a system reacts in a highly nonlinear and potentially abrupt way. For example, a paddler can tip a canoe quite a bit without getting wet. However, pushing that canoe just a bit further can result in a wet paddler. The impacts of pushing the canoe to the side are negligible until the very last small push triggers the overturning of the canoe in a threshold response.

The MOC may also respond to human-made greenhouse gas emissions in a threshold response. The research projects sizeable impacts on patterns of surface air temperatures and precipitation, fisheries and terrestrial ecosystems if a slowdown or complete collapse of the MOC occurs.

"Currently, MOC projections are deeply uncertain. This uncertainty puts a large value on observation systems that could deliver an actionable early warning of an MOC collapse," Keller said February 17 at the annual meeting of the American Association for the Advancement of Science in Boston. "The problem is that information that arrives after the threshold response has been triggered is only of very limited use. For example, warning a person in a canoe about an approaching waterfall can be useful before the waterfall, but is not really useful after the canoe went over the waterfall.

"The problem with the potential MOC collapse is that the signs of an approaching threshold response are very subtle to detect. The noise is large and picking out the signal from the noise is non trivial," he adds.

"There is tantalizing evidence for a recent MOC slow down. However, this is not an open-and-shut case," Keller continues.

The researchers analyzed how they could improve MOC observation systems to result in more skillful MOC projections. For example, optimizing the locations of the observation system can considerably improve the projections.

Improved MOC projections can enable improved climate policies and can have economic value. Keller and colleagues show that investments into an MOC observation system that would provide an early warning of an approaching MOC collapse would likely pass a cost benefit test.

The National Science Foundation and the U.S. EPA supported this work.

Adapted from materials provided by Penn State.

洋流是否已经开始紊乱?

科学日报2008年2月25日文

宾夕法尼亚大学的研究者称:之前对北大西洋洋流在21世纪内不会发生大的变动的预测,可能不想人们想象的那么简单.

"政府间气候变化小组表示,北大西洋经向翻转环流,简称MOC(译者注:Meridional Overturning Circulation,简称MOC.热海水流到北极圈附近后变冷,比重增加,因此便沉到海底,再从海底流回赤道附近,完成一个循环,术语叫做经向翻转环流)不太可能在21世纪发生紊乱.他们预测发生紊乱只有10%的可能性"地球科学助理教授Klaus Keller表示,"但是,这不是一个非常确定的信号,紊乱的形成和刚刚发端之间存在一定延迟,就好像一个刚刚从悬崖上跳下来的人可能会安慰自己说，在接下来的短短数秒中自己是不会有什么疼痛的，但是从长远看来，这张看法显然就不那么乐观了"

Keller和他的同事计算了可能引发MOC发生的情况.MOC发生在当整个系统处于在严重非线性和剧烈跳变的时候.就好像,划独木舟的人,晃动小舟,水不会灌进来.但是如果晃动的再剧烈一些,水就会把小舟上的人打湿.把独木舟向一侧倾斜一点点是可以忽略的,直到倾斜的太过厉害,之后再倾斜一点,船就翻了.

同样温室气体排放到达一定程度会触发MOC的发生.这个研究计划评估地球表面空气温度和降雨的形式以及渔业和陆地生态系统在渐变的或者完全的MOC发生紊乱时候造成的影响.

"现在,MOC的预测非常不确定.这种不确定使得很多数值建立在观测系统之上,这个观测系统可以传达有效的早期预警当MOC发生紊乱的时候."Keller在2月17日波士顿的美国科技进步协会的年会上说."问题在于信息到达响应的临界值触发非常有限,比如说,警告一个在独木舟上的一个人,关于接近瀑布的时候是有用的,但是对于已经进到瀑布底下了,就不那么有用了."

"对于潜在的MOC的紊乱的问题是一个信号对于接近临界响应对观察是非常细微的,噪声干扰非常严重,但是从噪声干扰中分离出信号是非常有意义的"他补充道.

"一些突然出现的迹象能说明最近MOC的减慢.但是,这还不是个一个一目了然的状况"keller继续说.

研究者分析他们如何提高对MOC的观察系统可以导致MOC预测更有准确性.比如说,优化观测系统的观测地址,可以有效的提高预测的结果.
改进的MOC观测可以使得提高的天气政策更具有经济价值.Keller和他的同时指出,对MOC观测系统的投资可能通过投资收益的评估,该系统可以提供一个对于迫近的MOC紊乱早期的预警,

这工作由国家科学基金会和美国环保局提供资助.