Warming Could Push the Atlantic Past a ‘Tipping Point’ This Century

本世纪全球变暖将触发大西洋气候危险“临界点”

The last time there was a major slowdown in the mighty network of ocean currents that shapes the climate around the North Atlantic, it seems to have plunged Europe into a deep cold for more than a millennium.

 上一次影响北大西洋周边气候的强大洋流网络出现大减速时，似乎令欧洲陷入了长达一千多年的严寒。

That was roughly 12,800 years ago, when not many people were around to experience it. But in recent decades, human-driven warming could be causing the currents to slow once more, and scientists have been working to determine whether and when they might undergo another great weakening, which would have ripple effects for weather patterns across a swath of the globe.

 那大约是在1.28万年前，当时的人类亲历者了了。但近几十年来，人类活动导致的气候变暖有可能令洋流再次减速，科学家们一直在努力搞清楚，洋流是否以及何时会再次经历大幅减弱，这将对全球大片地区的天气模式产生连锁反应。

A pair of researchers in Denmark this week put forth a bold answer: A sharp weakening of the currents, or even a shutdown, could be upon us by century’s end.

 丹麦的两名研究人员本周给出了一个大胆的回答：到本世纪末，洋流可能会急剧减弱，甚至可能会完全停止流动。

It was a surprise even to the researchers that their analysis showed a potential collapse coming so soon, one of them, Susanne Ditlevsen, a professor of statistics at the University of Copenhagen, said in an interview. Climate scientists generally agree that the Atlantic circulation will decline this century, but there’s no consensus on whether it will stall out before 2100.

 其中一名研究者、哥本哈根大学的统计学教授苏珊娜·迪特莱夫森在接受采访时表示，他们的分析表明，洋流的崩溃可能很快就会到来，这令研究人员也大吃一惊。气候科学家普遍认为大西洋环流本世纪会慢下来，但对于是否会在2100年之前停止流动尚未达成共识。

Which is why it was also a surprise, Ditlevsen said, that she and her co-author were able to pin down the timing of a collapse at all. Scientists are bound to continue studying and debating the issue, but Ditlevsen said the new findings were reason enough not to regard a shutdown as an abstract, far-off concern. “It’s now,” she said.

 迪特莱夫森说，这就是为什么她和合著者能够确定洋流停止流动的时间也是一个令人意外的地方。科学家们肯定会继续研究、讨论这个问题，但迪特莱夫森说，新的发现足以证明，不能把洋流停止流动视为一个抽象的、遥远的问题。“就在眼前了，”她说。

The new research, published Tuesday in the journal Nature Communications, adds to a growing body of scientific work that describes how humankind’s continued emissions of heat-trapping gases could set off climate “tipping points,” or rapid and hard-to-reverse changes in the environment.

越来越多的科学工作指出，人类持续排放的吸热型气体会如何触发气候“临界点”，也就是环境发生快速且难以逆转的变化。周二发表在《自然-通讯》杂志上的这项新研究为这些科学工作增加了新的内容。

Abrupt thawing of the Arctic permafrost. Loss of the Amazon rainforest. Collapse of the Greenland and West Antarctic ice sheets. Once the world warms past a certain point, these and other events could be set into swift motion, scientists warn, though the exact thresholds at which this would occur are still highly uncertain.

 北极永冻层突然解冻。亚马孙雨林消失。格陵兰岛和南极洲西部的冰盖崩塌。科学家警告，一旦全球变暖越过某个点，这些以及其他事件可能会迅速发生，尽管发生这种情况的确切阈值仍非常不确定。

In the Atlantic, researchers have been searching for harbingers of tipping-point-like change in a tangle of ocean currents that goes by an unlovely name: the Atlantic Meridional Overturning Circulation, or AMOC.

 在大西洋，研究人员一直在一个乱如麻的洋流中寻找出现临界性变化的先兆，这个洋流有一个惹人厌的名字：大西洋经向翻转环流(AMOC)。

These currents carry warm waters from the tropics through the Gulf Stream, past the southeastern United States, before bending toward northern Europe. When this water releases its heat into the air farther north, it becomes colder and denser, causing it to sink to the deep ocean and move back toward the equator. This sinking effect, or “overturning,” allows the currents to transfer enormous amounts of heat around the planet, making them hugely influential for the climate around the Atlantic and beyond.

 这些洋流从热带携带温暖的海水穿过墨西哥湾流，经过美国东南部，然后转向北欧。在更为靠北的位置，当这些海水将携带的热量释放到空气中后，它会变得更冷、密度更大，导致它沉入深海，并重新向赤道方向移动。这种下沉效应，或者说“翻转”，使得洋流可以在地球上转移大量的热量，对大西洋周围及其他地区的气候产生巨大影响。

As humans warm the atmosphere, however, the melting of the Greenland ice sheet is adding large amounts of freshwater to the North Atlantic, which could be disrupting the balance of heat and salinity that keeps the overturning moving. A patch of the Atlantic south of Greenland has cooled conspicuously in recent years, creating a “cold blob” that some scientists see as a sign that the system is slowing.

 然而，随着人类导致大气变暖，格陵兰岛冰盖的融化正在为北大西洋增加大量淡水，这可能会破坏热量和盐度的平衡，阻碍洋流的翻转继续。近年来，格陵兰岛以南的一片大西洋明显变冷，形成了一个“冷水区块”，一些科学家认为就是该系统正在减速的迹象。

Were the circulation to tip into a much weaker state, the effects on the climate would be far-reaching, although scientists are still examining their potential magnitude. Much of the Northern Hemisphere could cool. The coastlines of North America and Europe could see faster sea-level rise. Northern Europe could experience stormier winters, while the Sahel in Africa and the monsoon regions of Asia would most likely get less rain.

 虽然科学家们仍在研究洋流速度减弱的可能幅度，但如果环流进入一个更弱的状态，对气候的影响将是深远的。北半球的大部分地区可能都会变冷。北美和欧洲的海岸线的海平面上升速度可能加快。北欧的冬季可能会有更多的风暴，而非洲的萨赫勒地区和亚洲的季风区的降雨很可能减少。

Evidence from ice and sediment cores indicates that the Atlantic circulation underwent abrupt stops and starts in the deep past. But scientists’ most advanced computer models of the global climate have produced a wide range of predictions for how the currents might behave in the coming decades, in part because the mix of factors that shape them is so complex.

 来自冰和沉积物岩心的证据表明，大西洋环流在很久以前曾经经历过突然的停止和启动。但科学家最先进的全球气候计算机模型已经对未来几十年洋流的行为做出了各种预测，部分原因是塑造它们的因素组合非常复杂。

Ditlevsen’s new analysis focused on a simple metric, based on sea-surface temperatures, that is similar to ones other scientists have used as proxies for the strength of the Atlantic circulation. She conducted the analysis with Peter Ditlevsen, her brother, who is a climate scientist at the University of Copenhagen’s Niels Bohr Institute. They used data on their proxy measure from 1870 to 2020 to calculate statistical indicators that presage changes in the overturning.

 迪特莱夫森的新分析侧重于一个基于海面温度的简单指标，该指标类似于其他科学家用来衡量大西洋环流强度的代理指标。这项分析由她与她的兄弟、哥本哈根大学尼尔斯·玻尔研究所的气候科学家彼得·迪特莱夫森共同进行。他们使用1870年至2020年的代理指标的数据来计算统计指标以预测洋流翻转的变化。

They then used the mathematical properties of a tipping point-like system to extrapolate from these trends. That led them to predict that the Atlantic circulation could collapse around midcentury, though it could potentially occur as soon as 2025 and as late as 2095.

 然后，他们利用类似临界点的系统的数学特性从这些趋势外推。他们由此预测大西洋环流可能会在本世纪中叶左右崩溃，最早可能会在2025年、最迟在2095年发生。

Their analysis included no specific assumptions about how much greenhouse gas emissions will rise in this century. It assumed only that the forces bringing about an AMOC collapse would continue at an unchanging pace — essentially, that atmospheric carbon dioxide concentrations would keep rising as they have since the Industrial Revolution.

 他们的分析没有包含关于本世纪温室气体排放量将增加多少的具体假设。分析仅假设导致AMOC崩溃的力量将以不变的速度持续下去——本质上是在说，大气中的二氧化碳浓度将保持工业革命以来的上升趋势。

In interviews, several researchers who study the overturning applauded the new analysis for using a novel approach to predict when we might cross a tipping point, particularly given how hard it has been to do so using computer models of the global climate. But they voiced reservations about some of its methods and said more work was still needed to nail down the timing with greater certainty.

 由于这项新分析使用了一种新颖的方法来预测我们何时可能跨越临界点，几位研究洋流翻转的研究人员在受访时对此表示赞赏，特别是考虑到使用全球气候的计算机模型作出这样的预测是多么困难。但他们对研究中的某些方法保留意见，并表示仍需要做更多工作才能更确切地确定时间。

Susan Lozier, a physical oceanographer at Georgia Tech, said sea surface temperatures in the North Atlantic near Greenland weren’t necessarily influenced by changes in the overturning alone, making them a questionable proxy for inferring those changes. She pointed to a study published last year showing that much of the cold blob’s development could be explained by shifts in wind and atmospheric patterns.

 佐治亚理工学院的物理海洋学家苏珊·洛齐尔表示，格陵兰岛附近北大西洋的海面温度不一定只受到洋流翻转变化的影响，这就让人怀疑，它们是否可以成为推断这些变化的指标。她指出，去年发表的一项研究表明，冷斑的形成很大程度上可以用风和大气模式的变化来解释。

Scientists are now using sensors slung across the Atlantic to directly measure the overturning. Lozier is involved in one of these measurement efforts. The aim is to better understand what’s driving the changes beneath the waves and to improve projections of future changes.

 科学家们现在正在使用大西洋上空的传感器来直接测量翻转情况。洛齐尔参与了其中一项测量工作。这些工作的目的是更好地了解在海浪之下推动变化的因素，并改进对未来变化的预测。

But the projects began collecting data in 2004 at the earliest, which isn’t enough time to draw firm long-term conclusions. “It is extremely difficult to look at a short record for the ocean overturning and say what it is going to do over 30, 40 or 50 years,” Lozier said.

 但这些项目收集数据的最早日期是2004年，目前还没有足够的时间得出明确的长期结论。洛齐尔说：“从洋流翻转的短暂记录来预测30、40或50年后它会发生什么，是极其困难的。”