原文網址：https://www.uvm.edu/news/story/greenland-melted-recently-shows-higher-risk-sea-level-rise
塵封許久的冰芯揭露格陵蘭大部分地方在416,000年前是一片綠地
By Joshua Brown
佛蒙特大學主持的新研究顯示在不遠的過去(從地質上來說)，格陵蘭大部分地方的景色是片沒有冰雪的苔原，或許還有樹木生長並有長毛象漫步其中。這意味格陵蘭的冰原可能比過去認為的還要容易受到人為導致的氣候變遷影響，並且在未來幾個世紀很有可能以非常快的速度融化而無法回復。
綠色大地
冷戰期間，美國在格陵蘭西北的世紀營執行了一項秘密任務：他們從地表開始，一路往下鑽過這座冰封島嶼厚達4560英尺的冰層。接著還繼續往下鑽，取出一管12英尺長，位於冰層底下的土壤與岩石。但是，這管冰冷的沉積物接著就被放在冷凍庫數十年而遭人淡忘，到了2017年才在偶然之間被重新找出來。分析顯示它不是單純的沉積物，還含有樹葉與苔蘚——這是未被冰雪覆蓋的大地，或許還有北方針葉林殘留下來的痕跡。
但是今日的格陵蘭表面是座兩英里厚，面積有三個德州大的冰層，那麼這些植物是在什麼時候生長於此？
一組國際科學團隊訝異地發現不過是在416,000年前(誤差約為38,000年)，格陵蘭還是一片綠色大地。
他們的新研究2023年7月21日發表在《科學》(Science)期刊。
鐵證如山
不久之前，地質學家仍相信格陵蘭就像座冰雪蓋成的堡壘，大部分地區在過去數百萬年來都未曾融化。但兩年前此團隊運用重新找到的世紀營冰芯，顯示格陵蘭在不到一百萬年前曾經融化過。其他在格陵蘭中心地帶進行研究的科學家，蒐集到的數據也顯示該處的冰層在最近110萬年至少融化過一次。但是在本研究之前，沒有人知道冰層消失的確切時間。
藉著螢光科技的進展以及稀有同位素分析，團隊最近建立了更加明確的圖像：大部分格陵蘭冰層融化的時間比一百萬年前還要接近現在許多。新研究直接證實了緊鄰冰層底部的沉積物是在沒有冰雪的環境中由流水沉積，時間是在海洋同位素第11階，從424,000持續到374,000年前的中度暖化時期。這些融冰導致全球海平面至少上升了5英尺。
「這是第一次有如此堅固的證據顯示格陵蘭大部分的冰層曾經在它變暖時完全消失，」佛蒙特大學的科學家Paul
Bierman表示。他和主要作者，在他實驗室擔任博士後研究員的地球科學家Drew
Christ共同主持了這篇新研究。其他成員包括猶他州立大學的教授Tammy
Rittenour、佛蒙特大學地理與地球科學系的教授Nicholas
Perdrial、佛蒙特大學的研究科學家Lee
Corbett，以及另外16位來自各國的科學家。
在預測格陵蘭的巨大冰層隨著未來的氣候暖化會有什麼變化，以及以融化速度有多快的時候，瞭解它的過往是相當重要的。由於格陵蘭的冰層融化等於讓海平面上升23英尺左右，因此全世界的臨海地帶都會處在危險之中。這項新研究提供了確切且強力的證據，顯示格陵蘭對於氣候變遷的敏感程度比過去所理解的還高，而且正面臨完全融化而無法回復的巨大風險。
「在這12英尺長的凍土之中保存了一段過往的格陵蘭，它透露了未來的地球可能是顆溫暖、潮濕、大部分地區都沒有冰雪覆蓋的星球，」Bierman表示。他是佛蒙特大學環境與自然資源學院的地球科學家，也是岡德環境研究所的研究員。「除非我們能把大氣中的二氧化碳濃度給大幅降低。」
天光之下
團隊發表在《科學》的新研究連同先前的成果，使他們對於格陵蘭冰層的歷史有了相當不同的想法並感到擔憂。「我們過去都假設格陵蘭的冰層大約在兩百五十萬年前形成，在那之後就一直存在著，處於非常穩定的狀態，」這篇新研究的共同作者之一，猶他州立大學的科學家Tammy
Rittenour表示。「也許它的邊緣曾經融化，或是有較多積雪而稍微變厚——但它不會消失也不會快速融化。然而這篇文章顯示這些變故確實發生過。」
在Rittenour的實驗室，他們利用「螢光訊號」(luminescence signal)法來檢視世紀營岩芯中的的沉積物。當砂礫與石塊被風或水搬動時會暴露在陽光之下，基本上這可以清空它們先前累積的所有螢光訊號。之後再次被埋藏在岩石或冰層下方的時候，黑暗中沉積物裡的石英和長石礦物會在晶體之中累積自由電子。Rittenour的團隊在特別設計的暗房裡，取出一些冰芯中的沉積物並用藍綠光或紅外光照射，把困在其中的電子釋放出來。利用精密儀器與測量方法來重複進行多次試驗之後，釋放出來的電子數量會形成某種形式的時鐘，精準地指出這些沉積物上一次見到陽光是什麼時候。「要在世紀營看到陽光的唯一方式就是消除上方一英里厚的冰層，」Rittenour說，「此外，植物生長也需要陽光。」
這些有力的新數據結合了Bierman在佛蒙特大學的實驗室，經由研究世紀營岩芯裡的石英所得出的見解。當地面暴露在天空之下而被宇宙射線擊中的時候，石英會累積鈹和鋁元素的稀有形式(稱為同位素)。科學家透過探討鈹和其他同位素的比例，可以得知岩石暴露在地表的時間vs.埋藏在冰層下方的時間。這份數據讓科學家得知世紀營的沉積物堆積在冰層下方之前，看見天空的時間不到14000年，使他們可以把格陵蘭這部分沒有冰雪的時間範圍給縮小。
冰層之下
世紀營在1960年代是一座軍事基地，隱藏在格陵蘭冰層下方的地道裡。這座營地的戰略目的之一為最高機密的軍事行動「冰蟲計畫」，旨在蘇聯附近的冰層下方藏匿數百顆核彈。作為掩護，美國軍方宣稱世紀營為一處北極科學研究站。
雖然核彈任務以失敗告終，但是科學團隊卻完成了前所未有的研究，包括鑽出了一根將近一英里長的冰芯。世紀營的科學家將重點放在冰塊本身，其為瞭解地球過去的冰河期與暖期(間冰期)的研究成果之一。至於從冰芯下方取出的12英尺長沉積物，他們並不太感興趣。接著是一段曲折離奇的故事：這段冰芯在1970年代從軍方的冷凍庫移到水牛城大學，然後在1990年代移到另一座位於丹麥的冷凍庫。在此它被遺忘了數十年，直到這座冷凍庫裡面的冰芯要移到新冷凍庫的時候才重新發現它的存在。關於這段岩芯遭到遺忘，從一些餅乾罐裡重新找到它，最後由齊聚在佛蒙特大學岡德環境研究所的國際團隊加以研究的故事細節，可以參照《冰層下的秘密》(Secrets
Under the Ice)。
海平面
世紀營位在距離海岸138英里遠的內陸，距離北極點只有800英里。這篇經費來自美國國家科學基金會，發表在《科學》上的新研究顯示在過去一段長時間、溫度跟現在相似或者高一些的間冰期——海洋同位素第11階期間，該區域的冰層曾經完全融化並且覆有植被。團隊利用這項資訊進行模擬，顯示當時冰層的融化量足以使海平面上升至少五英尺，最高可能到達二十英尺。此研究跟利用另外兩根1990年代從格陵蘭中央取出的冰芯所做出的發現一致。這些冰芯裡的沉積物透露出不遠的地質時間以前，格陵蘭巨大的冰層曾經融化過。結合較早的冰芯研究以及從世紀營得出的新觀點，顯現出整個格陵蘭冰層的本質是脆弱的——不論是在過去(大氣二氧化碳濃度小於等於280
ppm)或是現在(422
ppm，還在上升中)。
「就算格陵蘭的冰層只有融化一部份，海平面也會劇烈上升，」猶他大學的Tammy
Rittenour表示。「模擬未來的融化速度以及高濃度二氧化碳的影響，我們看到海平面將會上升數公尺甚至數十公尺。再看看紐約市、波士頓、邁阿密、阿姆斯特丹的海拔高度；看看印度以及非洲——全世界的人口大部分都集中在海平面附近。」
「四十萬年前的海邊沒有任何一座城市，」佛蒙特大學的Paul
Bierman說。「而現在海邊到處都有城市。」
Greenland melted recently, shows
higher risk of sea level rise
Long-lost ice core reveals that most
of Greenland was green 416,000 years ago
A large portion of Greenland was an
ice-free tundra landscape—perhaps covered by trees and roaming woolly
mammoths—in the recent geologic past, new UVM-led research shows. This
indicates that the ice sheet on Greenland may be more sensitive to human-caused
climate change than previously understood—and will be vulnerable to
irreversible, rapid melting in coming centuries.
Green Land
During the Cold War, a secret U.S. Army mission, at
Camp Century in northwestern Greenland, drilled down through 4560 feet of ice
on the frozen island—and then kept drilling to pull out a twelve-foot-long tube
of soil and rock from below the ice. Then this icy sediment was lost in a
freezer for decades. It was accidentally rediscovered in 2017 and shown to hold
not just sediment but also leaves and moss, remnants of an ice-free landscape,
perhaps a boreal forest.
But how long ago were those plants growing—where
today stands an ice sheet two miles thick and three times the size of Texas?
An international team of scientists was amazed to
discover that Greenland was a green land only 416,000 years ago (with an error
margin of about 38,000 years).
Their new study was published in the journal Science on July 21, 2023.
Bulletproof
Evidence
Until recently, geologists believed that Greenland
was a fortress of ice, mostly unmelted for millions of years. But, two years
ago, using the rediscovered Camp Century ice core, this team of scientists
showed that it likely melted less than one million years ago. Other scientists,
working in central Greenland, gathered data showing the ice there melted at
least once in the last 1.1 million years—but until this study, no one knew
exactly when the ice was gone.
Now, using advanced luminescence technology and rare
isotope analysis, the team has created a starker picture: large portions of
Greenland’s ice sheet melted much more recently than a million years ago. The
new study presents direct evidence that sediment just beneath the ice sheet was
deposited by flowing water in an ice-free environment during a moderate warming
period called Marine Isotope Stage 11, from 424,000 to 374,000 years ago. This
melting caused at least five feet of sea level rise around the globe.
“It's really the first bulletproof evidence that much
of the Greenland ice sheet vanished when it got warm,” says University of
Vermont scientist Paul Bierman. He co-led the new study with lead author Drew
Christ, a post-doctoral geoscientist who worked in Bierman’s lab; Professor
Tammy Rittenour from Utah State University; UVM professor Nicholas Perdrial in
the Department of Geography and Geosciences; UVM research scientist Lee
Corbett; and sixteen other scientists from around the world.
Understanding Greenland’s past is critical for
predicting how its giant ice sheet will respond to climate warming in the
future and how quickly it will melt. Since about twenty-three feet of sea-level
rise is tied up in Greenland’s ice, every coastal region in the world is at
risk. The new study provides strong and precise evidence that Greenland is more
sensitive to climate change than previously understood—and at grave risk of
irreversibly melting off.
“Greenland’s
past, preserved in 12 feet of frozen soil, suggests a warm, wet, and largely
ice-free future for planet Earth,” says Bierman, a geoscientist in UVM’s
Rubenstein School of the Environment and Natural Resources and a fellow in the
Gund Institute for Environment, “unless we can dramatically lower the
concentration of carbon dioxide in the atmosphere.”
Into the Light
The team’s new study in Science, combined with their earlier work, is causing a major and
worrisome rethinking of the history of Greenland’s ice sheet. “We had always
assumed that the Greenland ice sheet formed about two and a half million years
ago—and has just been there this whole time and that it’s very stable,” says
Tammy Rittenour, a scientist at Utah State University and co-author on the new
study. “Maybe the edges melted, or with more snowfall it got a bit fatter—but
it doesn't go away and it doesn't dramatically melt back. But this paper shows
that it did.”
At Rittenour’s lab, sediment from the Camp Century
core was examined for what is called a “luminescence signal.” As bits of rock
and sand are transported by wind or water, they can be exposed to
sunlight—which, basically, zeros out any previous luminescence signal—and then
re-buried under rock or ice. In the darkness, over time, minerals of quartz and
feldspar in the sediment accumulate freed electrons in their crystals. In a
specialized dark room, Rittenour’s team took pieces of the ice core sediment
and exposed them to blue-green or infrared light, releasing the trapped
electrons. With some advanced tools and measures, and many repeated tests, the
number of released electrons forms a kind of clock, revealing with precision
the last time these sediments were exposed to the sun. “And the only way to do
that at Camp Century is to remove a mile of ice,” says Rittenour, “Plus, to
have plants, you have to have light.”
These powerful new data were combined with insight
from Bierman’s UVM lab. There, scientists study quartz from the Camp Century
core. Inside this quartz, rare forms—called isotopes—of the elements beryllium
and aluminum build up when the ground is exposed to the sky and can be hit by
cosmic rays. Looking at ratios of beryllium and other isotopes gave the
scientists a window onto how long rocks at the surface were exposed vs. buried
under layers of ice. This data helped the scientists show that the Camp Century
sediment was exposed to the sky less than 14,000 years before it was deposited
under the ice, narrowing down the time window when that portion of Greenland
must have been ice-free.
Under Ice
Camp Century was a military base hidden in tunnels
under the Greenland ice sheet in the 1960s. One strategic purpose of the camp
was a top-secret operation, called Project Iceworm, to hide hundreds of nuclear
missiles under the ice near the Soviet Union. As cover, the Army claimed the
camp was an Arctic science station.
The missile mission was a bust, but the science team
did complete first-of-its-kind research, including drilling a nearly mile-deep
ice core. The Camp Century scientists were focused on the ice itself—part of an
effort to understand Earth’s past ice ages and warm periods, the interglacials.
They took little interest in the twelve feet of sediment gathered from beneath
their ice core. Then, in a bizarre story, the ice core was moved in the 1970s
from a military freezer to the University at Buffalo—and then to another
freezer in Denmark in the 1990s. There it was lost for decades—until it was
found again 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’s Gund Institute for
Environment can be read here: Secrets
Under the Ice.
Sea Level
Camp Century is 138 miles inland from the coast and
only 800 miles from the North Pole; the new Science
study shows that the region entirely melted and was covered with vegetation
during Marine Isotope Stage 11, a long interglacial with temperatures similar
to or slightly warmer than today. With this information, the team’s models show
that, during that period, the ice sheet melted enough to cause at least five
feet, and perhaps as much as twenty feet, of sea-level rise. The research,
supported by the U.S. National Science Foundation, lines up with findings from
two other ice cores collected in 1990s from the center of Greenland. Sediment
from these cores also suggest that the giant ice sheet melted in the recent
geologic past. The combination of these earlier cores with the new insight from
Camp Century reveal the fragile nature of the entire Greenland ice sheet—in the
past (at 280 parts per million of atmospheric CO2 or less) and today
(422ppm and rising).
“If we melt just portions of the Greenland ice sheet,
the sea level rises dramatically,” says Utah’s Tammy Rittenour. “Forward
modeling the rates of melt, and the response to high carbon dioxide, we are
looking at meters of sea level rise, probably tens of meters. And then look at
the elevation of New York City, Boston, Miami, Amsterdam. Look at India and
Africa—most global population centers are near sea level.”
“Four-hundred-thousand years ago there were no cities
on the coast,” says UVM’s Paul Bierman, “and now there are cities on the
coast.”
原始論文：Andrew J.
Christ, Tammy M. Rittenour, Paul R. Bierman, Benjamin A. Keisling, Paul C.
Knutz, Tonny B. Thomsen, Nynke Keulen, Julie C. Fosdick, Sidney R. Hemming,
Jean-Louis Tison, Pierre-Henri Blard, Jørgen P. Steffensen, Marc W. Caffee, Lee
B. Corbett, Dorthe Dahl-Jensen, David P. Dethier, Alan J. Hidy, Nicolas
Perdrial, Dorothy M. Peteet, Eric J. Steig, Elizabeth K. Thomas. Deglaciation
of northwestern Greenland during Marine Isotope Stage 11. Science,
2023; 381 (6655): 330 DOI: 10.1126/science.ade4248
引用自：University of Vermont. "Greenland melted
recently: High risk of sea level rise today: Long-lost ice core reveals that
most of Greenland was green 416,000 years ago."
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