遭到遺忘許久的冰芯提供了直接證據，顯示在過去一百萬年之間這座巨大的冰層曾經完全消融，也顯示了它在暖化的氣候之下有多麼岌岌可危
1966年，美國陸軍的科學家鑽入格陵蘭西北部將近一英里厚的冰層，並從底下取出一管十五英吋長的泥土。這些冰封的沉積物之後在冰庫中消失了數十年，直到2017年才在無意之間被人重新發現。
今日的格陵蘭大部分都被冰層給覆蓋住。但是一篇新研究指出在過去一百萬年之間，格陵蘭曾經完全融化而成為一片綠色的苔原，可能就像這幅格陵蘭東部海邊的景色一樣。這項研究提出了強烈證據，顯示格陵蘭比過往認為得還要更容易受到氣候變遷的影響，並且有融化而無法回復的風險。(圖片來源：Joshua Brown)
2019年，佛蒙特大學的科學家Andrew
Christ透過顯微鏡觀察這些沉積物，眼前所見令他難以置信：其中不只有砂土和石塊，還有樹枝和葉片。代表從地質時間來看，不久之前這塊跟阿拉斯加一樣大，厚達一英里的冰層所在之處曾經完全看不到冰雪――取而代之的是一片蒼翠的景象，或許還有針葉林佇立其中。
Christ參與了佛蒙特大學的Paul
Bierman、哥倫比亞大學的Joerg
Schaefer和哥本哈根大學Dorthe
Dahl-Jensen領導的科學團隊。他們去年一整年都在探討這些取自於格陵蘭底下獨一無二的沉積物和植物化石。結果顯示在過去的一百萬年之間，甚至可能不到數十萬年，格陵蘭全境或者大部分地方曾經完全看不到冰雪。
「一般來說冰層會粉碎並摧毀在它路徑上的任何東西，」Christ表示，「但我們發現這些細緻的植物構造卻保存地相當完整。雖然它們是化石，看起來就像昨天才凋謝一樣。這顆時光膠囊保存了過往活在格陵蘭的生物，我們可能再也找不到類似的東西。」
這項發現有助於證實一項嶄新卻也令人擔心的見解：地球歷史最近的暖期當中，格陵蘭的冰層曾經完全融化――而我們造成的人為氣候變遷也正在形成類似於當時的情況。
有鑑於格陵蘭冰層蘊含的水份能讓海平面上升二十幾英尺，它融化的話全世界的臨海城市都會處在危險當中。在預測格陵蘭的冰層未來會如何隨著氣候變遷而變化，以及會以多快的速度融化時，瞭解它的過往相當重要。這篇新研究提出了迄今最為強力的證據，顯示格陵蘭比過往認為的更加脆弱，而且對氣候變遷十分敏感――也代表它有極大的風險會不斷融化而無法回復。
「這並非經過二十個世代才會浮現出來的問題，」佛蒙特大學文理學院、環境與自然資源學院及岡德環境學研究所的地球科學家Paul
Bierman表示，「這是未來50年之內就得面對的迫切問題。」
這項新研究3月15日發表在《美國國家科學院院刊》(Proceedings of the National Academy of
Sciences)。
冰層之下
這項發表在《美國國家科學院院刊》的新研究用的材料取自於「世紀營」(Camp
Century)，一座1960年代冷戰時期建立於北極圈深處，挖進冰層內部的軍營。建立這座營地的真正目的是一項極為機密的計畫――「冰蟲計畫」(Project
Iceworm)，意圖是把600枚核彈藏在靠近蘇聯的冰層下方。作為掩護，美國陸軍讓這座營地表面上像是一座極區科學研究站。
雖然軍事任務以失敗告終，但是科學團隊卻達成了重要的研究成果，包括鑽取出一根4560英尺的冰芯。當時探討地球許久以前的冰河期的研究正在蓬勃發展，世紀營的科學家做為其中的一份子，他們關注的對象為冰塊本身。顯然地，對於從冰芯之下蒐集到的一丁點泥土，他們並沒有多大興趣。接著在一連串極為戲劇化的轉折之下，這根冰芯1970年代從美國陸軍的冰櫃移到水牛城大學，然後在1990年代又被移交至丹麥哥本哈根的一座冰櫃，接著便在那裡沉寂數十年――最後才在被移往另一座新的冰櫃的過程當中重見天日。
關於這根冰芯消失，從幾個裝餅乾的罐子裡再次發現它的存在，然後2019年佛蒙特大學召集的國際團隊開始進行研究，這中間的更多內容可以閱讀以下文章：Secrets
Under the Ice。
在涵蓋過去260萬年稱為更新世的寒冷時期當中，格陵蘭有一部份大多數時間都覆蓋在冰層之下，即使是在較為暖和的短暫時期――「間冰期」時也是如此。上述的故事大綱多半是科學家鑽探附近的海洋，蒐集從格陵蘭沖刷到海洋的泥巴與石頭，找出其中的證據拼湊而成，然而這些證據是間接的。最近一次溫暖的間冰期結束於12萬年前，對於在此之前格陵蘭冰層的覆蓋範圍以及島上的生態系樣貌，科學家仍然所知不多且有許多爭議。
世紀營位在距海大約75英里處的內陸，距離北極點只有800英里。這項由美國國家科學基金會提供經費的新研究證實此處深厚的冰層在過去一百萬年當中至少曾經完全融化過一次，取而代之的是苔癬之類的植被，或許還有樹木。這跟另外兩組1990年代從格陵蘭中心地帶取出的冰芯數據相符，它們底下的沉積物也顯示當地的冰層在近代地質史曾經消失一段時間。結合格陵蘭中央的兩根冰芯以及這篇從極為西北的世紀營得出的最新瞭解，研究人員得以對整個格陵蘭冰層的多變命運有了前所未有的見地。
這群科學家利用一系列新穎的分析技術來探討世紀營冰芯底部的沉積物、化石和葉片表面的蠟質，這些都是五十年前的研究人員無法進行的。舉例來說，他們測量石英裡面鋁和鈹原子較少見的形式(同位素)佔的比例有多少。由於這些同位素是地面露出在天空之下，被宇宙射線撞擊時才會產生的，因此它們的佔比可以告訴科學家地表露出在外相對於埋藏在冰層之下的時間有多長。這項分析就像時鐘一樣，可以用來測量格陵蘭過去發生的事件。此外，他們也取出了沉積物裡的冰，測量其中有多少氧是較少見的形式，結果顯示當時的雨水必定是下在比現今的冰層還要低矮的地方，團隊寫道：「這是冰層消失的證據。」結合上述技術以及其他研究，包括透過螢光來估計沉積物距離上次照到光線已經相隔多久；對冰裡面的木屑進行放射性碳定年；分析含有冰塊與碎屑的層位如何排列，這些成果讓團隊清楚證明過去一百萬年之間，至少曾有一次格陵蘭的冰層就算不是全部，大部分也都融化了――造成格陵蘭(Greenland)因為苔癬和地衣，或許還有雲杉和冷杉，而真的變成綠色(green)了。
新的研究也顯示過去的生態系並沒有因為長年刮過上方的冰河與冰層而遭到挖除抹滅。相反的，這些形成年代相對來說較近的冰塊將下方生機盎然的地景凍結在原地，使它們維持原貌而保存下來。
1960年代美國陸軍拍攝了一部關於世界營的電影，其中的旁白提到「格陵蘭超過百分之九十亙古以來都被冰封在極區的冰帽之下。」這項新研究揭露了冰層並不像我們過去認為的如此永恆。「我們的研究顯示格陵蘭跟我們過去認為的不同，它對自然發生的氣候暖化還要敏感許多――而我們知道人類正讓地球的溫度有如失控一般的增加，速率遠遠超出自然的暖化，」佛蒙特大學文理學院與岡德環境學研究所的博士後研究員Christ表示。
「格陵蘭看起來好像離我們很遠，」佛蒙特大學的Paul
Bierman說，「但是它可能會以高速融化，造成大量的水進到海中，使得紐約、邁阿密、達卡，或者是你想得到的隨便一座城市，通通泡在水裡。」
Scientists stunned to discover plants
beneath mile-deep Greenland ice
Long-lost ice core provides direct
evidence that giant ice sheet melted off within the last million years and is
highly vulnerable to a warming climate
In 1966, US Army scientists drilled down
through nearly a mile of ice in northwestern Greenland—and pulled up a
fifteen-foot-long tube of dirt from the bottom. Then this frozen sediment was
lost in a freezer for decades. It was accidentally rediscovered in 2017.
In 2019, University of Vermont scientist Andrew
Christ looked at it through his microscope—and couldn’t believe what he was
seeing: twigs and leaves instead of just sand and rock. That suggested that the
ice was gone in the recent geologic past—and that a vegetated landscape,
perhaps a boreal forest, stood where a mile-deep ice sheet as big as Alaska
stands today.
Over the last year, Christ and an international team
of scientists—led by Paul Bierman at UVM, Joerg Schaefer at Columbia University
and Dorthe Dahl-Jensen at the University of Copenhagen—have studied these
one-of-a-kind fossil plants and sediment from the bottom of Greenland. Their
results show that most, or all, of Greenland must have been ice-free within the
last million years, perhaps even the last few hundred-thousand years.
“Ice sheets typically pulverize and destroy
everything in their path,” says Christ, “but what we discovered was delicate
plant structures—perfectly preserved. They’re fossils, but they look like they
died yesterday. It’s a time capsule of what used to live on Greenland that we
wouldn’t be able to find anywhere else.”
The discovery helps confirm a new and troubling
understanding that the Greenland ice has melted off entirely during recent warm
periods in Earth’s history—periods like the one we are now creating with
human-caused climate change.
Understanding the Greenland Ice Sheet in the past is
critical for predicting how it will respond to climate warming in the future
and how quickly it will melt. Since some twenty feet of sea-level rise is tied
up in Greenland’s ice, every coastal city in the world is at risk. The new
study provides the strongest evidence yet that Greenland is more fragile and
sensitive to climate change than previously understood—and at grave risk of
irreversibly melting off.
“This is not a twenty-generation problem,” says Paul
Bierman, a geoscientist at UVM in the College of Arts & Sciences,
Rubenstein School of Environment & Natural Resources, and fellow in the
Gund Institute for Environment. “This is an urgent problem for the next 50
years.”
The new research was published March 15 in the Proceedings of the National Academy of
Sciences.
BENEATH THE ICE
The material for the new PNAS study came from Camp Century, a Cold War military base dug
inside the ice sheet far above the Arctic Circle in the 1960s. The real purpose
of the camp was a super-secret effort, called Project Iceworm, to hide 600
nuclear missiles under the ice close to the Soviet Union. As cover, the Army
presented the camp as a polar science station.
The military mission failed, but the science team did
complete important research, including drilling a 4560-foot-deep ice core. The
Camp Century scientists were focused on the ice itself—part of the burgeoning
effort at the time to understand the deep history of Earth’s ice ages. They,
apparently, took less interest in a bit of dirt gathered from beneath the ice
core. Then, in a truly cinematic set of strange plot twists, the ice core was
moved from an Army freezer to the University of Buffalo in the 1970s, to
another freezer in Copenhagen, Denmark, in the 1990s, where it languished for
decades—until it surfaced when the cores were being moved to a new freezer.
More about how the core was lost, rediscovered in
some cookie jars, and then studied by an international team gathered at the
University of Vermont in 2019 can be read here: Secrets Under the Ice.
For much of the Pleistocene—the icy period covering
the last 2.6 million years—portions of the ice on Greenland persisted even
during warmer spells called “interglacials.” But most of this general story has
been pieced together from indirect evidence in mud and rock that washed off the
island and was gathered by offshore ocean drilling. The extent of Greenland’s
ice sheet and what kinds of ecosystems existed there before the last
interglacial warm period—that ended about 120,000 years ago—have been hotly
debated and poorly understood.
The new study makes clear that the deep ice at Camp
Century—some 75 miles inland from the coast and only 800 miles from the North
Pole—entirely melted at least once within the last million years and was
covered with vegetation, including moss and perhaps trees. The new research,
supported by the National Science Foundation, lines up with data from two other
ice cores from the center of Greenland, collected in 1990s. Sediment from the
bottom of these cores also indicate that the ice sheet was gone for some time
in the recent geologic past. The combination of these cores from the center of
Greenland with the new insight from Camp Century in the far northwest give
researchers an unprecedented view of the shifting fate of the entire Greenland
ice sheet.
The team of scientists used a series of advanced
analytical techniques—none of which were available to researchers fifty years
ago—to probe the sediment, fossils, and the waxy coating of leaves found at the
bottom of the Camp Century ice core. For example, they measured ratios of rare
forms—isotopes—of both aluminum and the element beryllium that form in quartz
only when the ground is exposed to the sky and can be hit by cosmic rays. These
ratios gave the scientists a window onto how long rocks at the surface were
exposed vs. buried under layers of ice. This analysis gives the scientists a
kind of clock for measuring what was happening on Greenland in the past.
Another test used rare forms of oxygen, found in the ice within the sediment,
to reveal that precipitation must have fallen at much lower elevations than the
height of the current ice sheet, “demonstrating ice sheet absence,” the team writes.
Combining these techniques with studies of luminescence that estimate the
amount of time since sediment was exposed to light, radiocarbon-dating of bits
of wood in the ice, and analysis of how layers of ice and debris were
arranged—allowed the team to be clear that most, if not all, of Greenland
melted at least once during the past million years—making Greenland green with
moss and lichen, and perhaps with spruce and fir trees.
And the new study shows that ecosystems of the past
were not scoured into oblivion by ages of glaciers and ice sheets bulldozing
overtop. Instead, the story of these living landscapes remains captured under
the relatively young ice that formed on top of the ground, frozen in place, and
holds them still.
In a 1960’s movie about Camp Century created by the
Army, the narrator notes that “more than ninety percent of Greenland is
permanently frozen under a polar ice cap.” This new study makes clear that it’s
not as permanent as we once thought. “Our study shows that Greenland is much
more sensitive to natural climate warming than we used to think—and we already
know that humanity’s out-of-control warming of the planet hugely exceeds the
natural rate,” says Christ, a postdoctoral researcher in the College of Arts
& Sciences and Gund Institute.
“Greenland may seem far away,” says UVM’s Paul
Bierman, “but it can quickly melt, pouring enough into the oceans that New
York, Miami, Dhaka—pick your city—will go underwater.”
原始論文：Andrew J.
Christ, Paul R. Bierman, Joerg M. Schaefer, Dorthe Dahl-Jensen, Jørgen P.
Steffensen, Lee B. Corbett, Dorothy M. Peteet, Elizabeth K. Thomas, Eric J.
Steig, Tammy M. Rittenour, Jean-Louis Tison, Pierre-Henri Blard, Nicolas
Perdrial, David P. Dethier, Andrea Lini, Alan J. Hidy, Marc W. Caffee, John
Southon. A multimillion-year-old record of Greenland vegetation and
glacial history preserved in sediment beneath 1.4 km of ice at Camp Century. Proceedings
of the National Academy of Sciences, 2021; 118 (13): e2021442118 DOI: 10.1073/pnas.2021442118
引用自：University of Vermont. "Scientists stunned
to discover plants beneath mile-deep Greenland ice.”
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