或許冰帽融化並非海流中斷的原因

 原文網址：https://news.wisc.edu/melting-ice-caps-may-not-shut-down-ocean-current/

威斯康辛大學麥迪遜分校主持的新研究表示，北極冰層融化造成海洋環流突然變化的關係上，我們用來模擬未來氣候的模型也許設定得太過敏感。

氣候學家認為全球氣候災難的發生過程中，大西洋經向翻轉環流(Atlantic Meridional Overturning Circulation, AMOC)是最重要的臨界點之一。大西洋環流就像是輸送帶一樣，將熱帶表層溫暖的海水帶往北方，並且把密度較高、溫度較低的深層海水帶往南方。

「我們的教科書一直以來都說把它想像成一條輸送帶――就算是現在的國高中也是這麼教導的――而冰雪融化的淡水流進來之後會讓它停擺，」威斯康辛大學麥迪遜分校氣候研究中心的研究員Feng He表示。

然而基於過往的研究，He說研究人員正在重新審視他們理解中AMOC和極區冰雪融化後的淡水之間的關係。

過去AMOC停滯曾伴隨著突然發生的氣候事件，比方說14500年前全球氣溫飆升的波令―阿勒羅德(Bølling-Allerød)暖化事件。He在2009年還是威斯康辛大學麥迪遜分校的研究生的時候，主導了一個氣侯模型的建立過程，他利用該模型成功地重現了這場事件。

「在古氣候紀錄中可以看到14700年前突然發生了一場暖化事件，在重現它的時候我們確實取得了成功，」現在的He表示，「但是在這場突然變化的時期之後，我們便無法繼續準確地重現下去。」

地球在這場突然暖化的事件之後氣溫再次降低，接著在最近這10000年升到新高並維持穩定，但是2009年的模型沒辦法跟上這些變化。模擬中地球北方整體的暖化狀況，與地質氣候紀錄(像是冰芯)中看到的溫度上升情形並非一致。

在本周發表於期刊《自然―氣候變遷》(Nature Climate Change)的研究，He和奧勒岡州立大學的古氣候學家Peter Clark敘述了新的模擬結果，其符合最近10000年的暖化趨勢。而他們所用的方式並沒有用到大部分科學家認知中，引發AMOC停滯或停擺的原因。

地表氣溫升高會讓北極海的海冰以及格陵蘭的冰層融化，而把淡水釋放到海洋當中。許多科學家相信淡水流入之後會擾亂北大西洋海水的密度差，這股密度差是AMOC往北流的海水下沉並往南回流的原因。

「問題在於從地質紀錄中得到的氣候數據，」He表示。

雖然氣候紀錄顯示覆蓋北美洲與歐洲的冰層最後一次融化的時候釋放出了大量淡水，但是AMOC卻幾乎沒有改變。因此，He把淡水大量流入的假設從模型中移除。

「當模型裡沒有淡水流入造成AMOC減緩的時候，我們得到了更好的模擬結果，它和氣候紀錄中的溫度數據保持一致的時間持續得更久，」He表示。「這項結果的重要之處在於不管是從資料或是模型來看，都顯示AMOC對於淡水的反應不如長久以來認為的那麼敏感。」

在氣候模型評估未來冰雪融化造成更多淡水流入，AMOC 會出現什麼樣的反應時，這項結果顯得格外重要。

「許多模型都有納入這道關係，」He表示，「一般認為大氣二氧化碳增加造成未來氣候進一步暖化，使得海冰融化以及冰層融出更多淡水會讓AMOC的強度減弱。」

AMOC劇烈減弱會造成廣泛的影響，像是北美東岸的海平面迅速上升、整個歐洲的氣溫降低而影響農業、亞馬遜雨林乾枯、亞洲季風受到擾動…….等。這項新的模擬研究預估AMOC的強度不會減弱那麼多，但並未排除會發生突然的變化。

「我們建議除非解決這道難題，否則任何模擬AMOC會受到淡水作用而變化的模擬結果都要審慎看待，」He表示。「雖然我們無法確定AMOC在過去停擺的原因，但我們很肯定這項變化確實發生過，而且未來有可能再次發生。」

 

Melting ice caps may not shut down ocean current

Most simulations of our climate’s future may be overly sensitive to Arctic ice melt as a cause of abrupt changes in ocean circulation, according to new research led by scientists at the University of Wisconsin–Madison.

Climate scientists count the Atlantic Meridional Overturning Circulation (or AMOC) among the biggest tipping points on the way to a planetary climate disaster. The Atlantic Ocean current acts like a conveyor belt carrying warm tropical surface water north and cooler, heavier deeper water south.

“We’ve been taught to picture it like a conveyor belt — even in middle school and high school now, it’s taught this way — that shuts down when freshwater comes in from ice melt,” says Feng He, an associate scientist at UW–Madison’s Center for Climatic Research.

However, building upon previous work, He says researchers are revising their understanding of the relationship between AMOC and freshwater from melting polar ice.

In the past, a stalled AMOC has accompanied abrupt climate events like the Bølling-Allerød warming, a 14,500-year-old, sharp global temperature hike. He successfully reproduced that event using a climate model he conducted in 2009 while a UW–Madison graduate student.

“That was a success, reproducing the abrupt warming about 14,700 years ago that is seen in the paleoclimate record,” says He, now. “But our accuracy didn’t continue past that abrupt change period.”

Instead, while Earth’s temperatures cooled after this abrupt warming before rising again to plateau at new highs for the last 10,000 years, the 2009 model couldn’t keep pace. The simulated warming over the northern regions of the planet didn’t match the increase in temperatures seen in geological archives of climate, like ice cores.

In a study published this week in the journal Nature Climate Change, He and Oregon State University paleoclimatologist Peter Clark describe a new model simulation that matches the warmth of the last 10,000 years. And they did it by doing away with the trigger most scientists believe stalls or shuts down the AMOC.

Warming temperatures on Earth’s surface cause sea ice in the Arctic Ocean and the Greenland Ice Sheet to melt, releasing fresh water into the ocean. Scientists widely believed that the freshwater influx disrupts the density differences in the North Atlantic that make the AMOC’s north-bound water sink and turn back south.

“The problem,” says He, “is with the geological climate data.”

Though the climate record shows an abundance of freshwater that came from the final melting of the ice sheets over North America and Europe, the AMOC barely changed. So, He removed the assumption of a freshwater deluge from his model.

“Without the freshwater coming in making the AMOC slow down in the model, we get a simulation with much better, lasting agreement with the temperature data from the climate record,” He says. “The important result is that the AMOC appears to be less sensitive to freshwater forcing than has long been thought, according to both the data and model.”

This is particularly important to climate models that evaluate how the AMOC will respond to future increases of freshwater from ice melt.

“It’s built into many models,” He says. “Future global warming from increasing carbon dioxide in the atmosphere melts sea ice, and the freshwater from the melting ice is believed to cause the AMOC to weaken.”

The widespread consequences of a drastic weakening of the AMOC include rapid sea-level rise on the eastern coast of North America, cooling over Europe that could disrupt agriculture, a parched Amazon rainforest and disruption of Asian monsoons. The new modeling study anticipates a much smaller reduction in AMOC strength, but that doesn’t rule out abrupt change.

“We suggest until this challenge is solved, any simulated AMOC changes from freshwater forcing should be viewed with caution,” He says. “We can’t be certain why the AMOC shut down in the past. but we are certain it did change. And it can change again.”

原始論文：Feng He, Peter U. Clark. Freshwater forcing of the Atlantic Meridional Overturning Circulation revisited. Nature Climate Change, 2022; DOI: 10.1038/s41558-022-01328-2

引用自：University of Wisconsin-Madison. "Melting ice caps may not shut down ocean current."