潛藏在深海的碳造成過往的氣候模式轉變

原文網址：https://blogs.ei.columbia.edu/2019/04/08/carbon-deep-ocean-ancient-climate-switch/

潛藏在深海的碳造成過往的氣候模式轉變

大西洋環流減慢使地球陷入嚴寒之中

By Kevin Krajick

距今一百萬年前，當時維持已久、冰河期和暖期交替出現的氣候模式發生了重大變化：冰河期變得更長而且更加寒冷。科學家長久以來認為這跟一種重要的大西洋環流系統減緩下來有關，而且此系統今日又再次出現減緩的跡象。新研究探討了大西洋底的沉積物，發現環流的減緩，以及從大氣被帶到深海的碳大量累積於大西洋底，兩者之間有直接關聯。當環流系統全速運作，碳會以較快的速度滲回大氣；但在這段減速時期，碳卻淤積在大西洋深處。結果便是大氣中的碳減少了，地球溫度因而下降――這跟我們現在面臨的，因為人類把碳排放到大氣而造成的溫室效應正好相反。然而，就算環流目前的減緩趨勢持續下去，我們也不該期望這可以幫忙減少我們排放的碳――甚至還有可能造成反效果。這項研究由拉蒙特―多爾蒂地球觀測所的研究人員主持，結果刊登於本周的《自然―地球科學》(Nature Geoscience)期刊。

大西洋經向翻轉環流的簡單示意圖。大西洋經向翻轉環流會把溫暖的表層海水帶往北方，直到格陵蘭和北歐附近的海域(紅色箭頭)；接著，這些海水會下沉並往南流(黃色箭頭)，多數會在南大洋重新回到海水表層。來源：Francesco Muschitiello/拉蒙特―多爾蒂地球觀測所

科學家關注的環流系統稱為大西洋經向翻轉環流(Atlantic meridional overturning circulation)，簡稱AMOC。此環流系統中，表面的海水會往北流，把赤道附近溫暖且高鹽度的海水送到格陵蘭和北歐附近的高緯度地區。這些海水在此跟北極較冷的海水交會，使它們的密度上升並下沉到深海，連帶把海水從大氣中吸收的許多碳也送下去。沉到深處的水接著往南回流，大部分會在南大洋重新回到表層，並把碳釋放回大氣。整個過程需耗時數十年到數百年。

2014年，一項由拉蒙特―多爾蒂地球觀測所的科學家Steven Goldstein和Leopoldo Pena(他們也是這篇新研究的共同作者)進行的研究指出，距今95萬年前AMOC突然減緩下來。而這項新研究則顯示北大西洋深處累積大量的碳，使得空氣中的碳減少，和AMOC的減緩有直接關聯。這起事件似乎造成之後的冰河期以10萬年為間隔重複出現，在此之前的冰河期則是每4萬年左右出現一次；而且之後的冰河期形成的冰層也比之前的更加廣大。科學家稱這個轉捩點為「中更新世轉型期」(Mid-Pleistocene Transition)，此後的新型氣候模式一直持續到於15000年前左右結束的末次冰期。雖然還不清楚這種氣候模式可以一直維持的確切原因，但這篇研究清楚呈現出碳從大氣跑到海洋裡面，會對氣候產生重大影響。

「這是種一對一的關係，就像按下開關，機器就會啟動一樣。」主要作者Jesse Farmer表示。他在拉蒙特―多爾蒂地球觀測所就讀博士時進行了這項研究。「結果告訴我們海洋儲存了多少碳，跟氣候的運作狀況有密切關係。」

研究人員採取的方法是分析從大西洋南部和北部鑽取出來的深海沉積物岩芯。過往的深層海水流經這些地方的時候，會在微小生物的殼體成分中留下線索，顯示海水的組成。結果確認了2014年的研究結論，也就是大約95萬年前，AMOC弱化到前所未見的的程度，而且持續了非常長的時間。因為如此，深海囤積的碳比之前的冰河期還要多出500億噸，大約相當於全世界海洋目前為止吸收的人類排放量的三分之一。(詳細來說，我們排放的碳大約有四分之一被海洋吸收；三分之一由陸地和植物吸收；其他的則留在大氣裡。)

測量冰芯的結果顯示，在此之前的暖期，大氣中的二氧化碳約為280 ppm；而AMOC減緩使得空氣中的二氧化碳降到180 ppm。雖然以往的冰河期期間二氧化碳濃度也有下降，但只是從280 ppm降到約210 ppm。(由於過去兩世紀人類不斷排放二氧化碳到大氣當中，使得以往暖期二氧化碳濃度為280 ppm的常態不復存在。現今大氣裡的二氧化碳濃度已經高達410 ppm。)

某些時候環流曾再次復甦，使得氣候暫時變暖，但在10萬年後又會再次回到極為嚴寒的冰河期。「有很多說法試著解釋這些變化的成因，但要指出最初的觸發條件卻相當困難。」研究共同作者，也是Farmer的指導教授Bärbel Hönisch表示。「只有幾個因素的時候還能想辦法處理，但這個問題涉及的變因實在太多了。」

Goldstein的團隊和其他某些人擁護的說法如下：在北方，冰河重複的擴張會刮除陸地上的所有事物，直到岩床暴露出來。之後形成的冰河便能緊緊固著在岩床上，使它們累積的厚度更勝以往。這些體積更大的冰河進入海洋成為冰山的時候，會引入更多淡水跟AMOC混和，造成AMOC的密度變低而無法下沉。另一方面，南極的冰層增長讓更多冰山進入海中，造成海水溫度降低、鹽度減小，而有利於讓更多海冰形成。理論上，海冰增加會把海水表層蓋住，使得深層海水無法到達表層並釋放其中含有的碳。然而，就算整個機制真得是如此運作，他們仍無法說明其中任何一個步驟要如何開始或結束――這個問題就像是「先有雞還是先有蛋」。

一般認為AMOC的強度本來就會有所變動，但20世紀中葉以來強度減少15%就顯得相當不尋常。沒有人可以肯定背後的原因為何，以及繼續減弱下去會造成什麼樣的影響。拉蒙特―多爾蒂的科學家上個月發表的另一個研究顯示，大約13000年前末次冰期即將結束的時候，AMOC減弱使得400年之後的氣候變得非常寒冷，而且持續了數個世紀。

Farmer現為普林斯頓大學的博士後研究員。「在把古今進行類比的時候必須十分謹慎。」他說。「現在我們也看到AMOC出現類似的減弱現象。或許會有人因此說：『太棒啦！海洋環流可以幫我們解決氣候暖化！』但這是錯的，因為還得考慮氣候系統不同部分之間的互動方式。」Farmer說如果AMOC持續減弱，含有碳的海水在北方下沉的量會變少；在此同時，已經到達深海的碳則可以毫無阻礙的繼續從南大洋冒出來。結果便是碳會持續累積在大氣當中，而非儲存在深海。

研究人員強調AMOC只不過是更為廣大的全球環流系統中的一部分。今年辭世的拉蒙特―多爾蒂科學家Wallace Broecker創造了專有名詞「大洋輸送帶」(Great Ocean Conveyor)來稱呼全球環流系統，而目前許多研究的基礎工作也是由他奠下的。科學家對於印度洋和太平洋的碳是如何運送有更多不瞭解的地方。由於這兩者的總和遠遠超出大西洋，因此整幅圖像當中仍有許多地方還沒拚上。拉蒙特―多爾蒂正在進行的研究目標，便是要在未來數年從其他水體中取得更多的碳定年數據。

研究共同作者包括拉蒙特-多爾蒂的Laura Haynes、Heather Ford、Maureen Raymo、Maria Jaume-Seguí、Steven Goldstein、Maayan Yehudai和Joohee Kim；英國愛丁堡大學的Dirk Kroon、Simon Jung和Dave Bell；西班牙塞隆納大學的Leopoldo Pena。

Carbon lurking in deep ocean threw ancient climate switch, say researchers

Slowdown of Atlantic circulation sent planet into deep freeze

A million years ago, a longtime pattern of alternating glaciations and warm periods dramatically changed, when ice ages suddenly became longer and more intense. Scientists have long suspected that this was connected to the slowdown of a key Atlantic Ocean current system that today once again is slowing. A new study of sediments from the Atlantic bottom directly links this slowdown with a massive buildup of carbon dragged from the air into the abyss. With the system running at full speed, this carbon would have percolated back into the air fairly quickly, but during this period it just stagnated in the depths. This suggests that the carbon drawdown cooled the planet—the opposite of the greenhouse effect we are seeing now, as humans pump carbon into the atmosphere. But if the current keeps slowing now, we should not expect it to help us out by storing our emissions; possibly to the contrary. The study, led by researchers at Columbia University’s Lamont-Doherty Earth Observatory, appears this week in the journal Nature Geoscience.

The scientists targeted a system of currents called the Atlantic meridional overturning circulation, or AMOC. Flowing northward near the surface, it transports warm, salty water from near the equator into the latitudes near Greenland and northern Europe. Here, it hits colder water from the Arctic, becomes denser and sinks into the abyss, taking with it large amounts of carbon absorbed from the atmosphere. The deep water then circles back south, where much of it re-merges in the Southern Ocean, to release carbon back to the air. The journey takes place over decades to centuries.

A 2014 study by Lamont-Doherty scientists Steven Goldstein and Leopoldo Pena–both of whom also are coauthors of the new study–showed that this current system abruptly slowed around 950,000 years ago. The new study shows that this slowdown correlated directly with a huge buildup of carbon in the deep Atlantic, and corresponding decline of carbon in the air. This event was the apparent trigger for a series of ice ages that came every 100,000 years, versus previous ones that occurred about every 40,000 years, and which built up less ice than those that came later. Scientists call this turning point the Mid-Pleistocene Transition, and the new pattern has persisted right through the last ice age, which ended about 15,000 years ago. Exactly why the pattern has continued no one knows, but the study clearly demonstrates that the carbon missing from the air ended up in the ocean, and had a powerful effect on climate.

“It’s a one-to-one relationship. It was like flipping a switch,” said lead author Jesse Farmer, who did the work while a PhD. student at Lamont-Doherty. “It shows us that there’s an intimate relationship between the amount of carbon stored in the ocean, and what the climate is doing.”

The researchers reached their findings by analyzing cores of deep-sea sediments taken in the south and north Atlantic, where ancient deep waters passed by and left chemical clues about their contents in the shells of microscopic creatures. Their analysis confirmed the 2014 study showing that the AMOC weakened to an extent not seen before, around 950,000 years ago, and for an unusually long time. Because of this, the deep water collected about 50 billion tons more carbon than it had during previous glaciations—equivalent to about one third of the human emissions that all the world’s oceans have so far absorbed today. (For context, the oceans today absorb roughly a quarter of what we emit; land and vegetation take up a third. The rest stays in the air.)

In the warm period leading up to this event, the atmosphere had held about 280 parts per million carbon; with the slowdown, airborne carbon dioxide went down to 180 ppm, as measured by ice cores. Atmospheric carbon had sunk during previous glaciations as well, but from 280 ppm down only to about 210 ppm. (Because of human emissions during the past two centuries, this normal repeating 280 ppm warm-era figure has become obsolete; atmospheric carbon is now up to about 410 ppm.)

At some point, the current woke up again, and things warmed for a while before dropping back into another similarly extreme ice age, after 100,000 years. “There are lots of ideas about what caused these changes to happen, but it’s hard to say what the trigger was,” said Bärbel Hönisch, Farmer’s advisor and coauthor of the study. “There are several different screws you could imagine turning, and lots of loose screws.”

One idea, espoused by Goldstein’s group among others: In the north, repeated build-ups of glaciers ultimately scrape everything on land down to bedrock. Subsequent glaciers are then able to stick fast to the bedrock and bulk up even more, before discharging icebergs into the ocean. This introduces more freshwater to mix with the AMOC, making it less dense and eventually unable to sink. On the other end, ice would also grow in Antarctica and discharge more icebergs, which would make the ocean waters colder and less salty, thus encouraging the growth of more sea ice. This, theoretically, would cap the surface and keep deep water from rising and releasing its carbon. But if this is indeed the way it works, it is not clear what starts or ends any of the processes; it is a chicken-and-egg kind of question.

The strength of the AMOC is believed to fluctuate naturally, but it appears to have weakened by an unusual 15 percent since the mid-20th century. No one is sure what is behind that, or what effects it might produce if the slowdown continues. Another Lamont-Doherty study last month showed that a slowdown around 13,000 years ago, at the tail end of the last ice age, was followed 400 years later by an intense cold snap that lasted centuries.

“We have to be careful about drawing parallels with that,” said Farmer, now a postdoctoral researcher at Princeton University. “We see a similar weakening today, and one might say, ‘Great! Ocean circulation is going to save us from warming climate!’ But that’s not correct, because of the way different parts of the climate system talk to each other.” Farmer said that if the AMOC continues weakening now, it is probable that less carbon-laden water will sink in the north, at the same time, in the Southern Ocean, any carbon already arriving in the deep water will likely keep bubbling up without any problem. The result: carbon will continue to build in the air, not the ocean.

The researchers point out that the AMOC is only part of a much larger system of global circulation that connects all the oceans—the so-called Great Ocean Conveyor, a term coined by the late Lamont-Doherty scientist Wallace Broecker, who laid the groundwork for much of the current research. Much less is known about the carbon dynamics of the Indian and Pacific, which together dwarf the Atlantic, so there are many missing pieces to the puzzle. Ongoing research at Lamont-Doherty is aimed at building carbon chronologies of those other waters in the next few years.

The study was also coauthored by Laura Haynes, Heather Ford, Maureen Raymo, Maria Jaume-Seguí, Steven Goldstein, Maayan Yehudai and Joohee Kim, all of Lamont-Doherty; Dirk Kroon, Simon Jung and Dave Bell of the University of Edinburgh; and Leopoldo Pena of the University of Barcelona.

原始論文：J. R. Farmer, B. Hönisch, L. L. Haynes, D. Kroon, S. Jung, H. L. Ford, M. E. Raymo, M. Jaume-Seguí, D. B. Bell, S. L. Goldstein, L. D. Pena, M. Yehudai, J. Kim. Deep Atlantic Ocean carbon storage and the rise of 100,000-year glacial cycles. Nature Geoscience, 2019; DOI: 10.1038/s41561-019-0334-6

引用自：Earth Institute at Columbia University. "Carbon lurking in deep ocean threw ancient climate switch, say researchers."