格陵蘭冰層的融化速率達到過去四個世紀以來的巔峰
根據2018.12.5發表在期刊《自然》(Nature)的新研究，格陵蘭厚達數公里的冰層表面融化速率從19世紀中期開始增加，並在接下來的20世紀至21世紀早期一路劇烈飆升，完全沒有減緩的跡象。這項研究提供了新的證據顯示氣候變遷對北極融化與全球海平面上升的衝擊。
格陵蘭每個夏季在表面都會形成大型河流，而把冰層的融水快速帶到海洋。圖片來源：Sarah
Das，伍茲霍爾海洋研究所
「格陵蘭的冰層正在高速融化。造成格陵蘭的融冰對海平面上升的貢獻就算不是過去數千年來，也是三個半世紀以來的新高。」研究主要作者Luke
Trusel表示。這位任職於羅文大學的冰河學家之前在伍茲霍爾海洋研究所進行博士後研究。「冰層開始加速融化的時間大概跟我們開始改變大氣成分的時間一樣，都是在19世紀中葉。」
「從歷史的角度來看今日的融化速率達到新高，而這項研究證明了的確如此。」研究共同作者，伍茲霍爾海洋研究所的冰河學家Sarah
Das表示。「我們發現從工業革命開始到現在，冰層融水的流量總共增加了百分之五十，光從20世紀至今就增加了30%。」
格陵蘭的冰層流失是全球海平面上升的關鍵推手之一。重新回到海洋並造成海平面上升的水有一部分是來自從冰河邊緣崩解到海中的冰山。但是，從冰層進入海洋的水有一半以上是來自冰層最上方的積雪或是冰河融化產生的流水。研究人員認為越來越暖和的夏季造成格陵蘭的冰層以前所未見的速率快速融化。他們表示如果這種情形繼續下去，上升速度已經很快的海平面將會更加快速上漲。
Trusel表示：「不像氣候暖化是穩定升溫，氣溫每升高一度格陵蘭的融化速度就會越來越快。隨著氣候持續暖化，預計未來的冰層融化與海平面上升速度或許會遠遠超出我們現今觀察到的速度。」
為了測出過去數個世紀以來格陵蘭冰層的融化程度，研究人員利用跟紅綠燈柱差不多粗的鑽井，從冰層本身以及附近一座海岸邊的冰帽鑽取出冰芯。他們選擇高於海拔6000英呎(約1800公尺)的地點作業，以確保鑽出的冰芯可以保有過去融化程度的紀錄，使他們的紀錄可以延伸到17世紀。格陵蘭夏季氣溫較高的時候，大部分的冰層表面多少都會融化。低海拔地區的融化程度最為嚴重，融水會流出冰層導致海平面上升，但這些融水並不會留下任何紀錄。然而在高海拔地區，夏季融水在接觸到下方低於冰點的積雪時會快速地重新結凍。這讓融水無法流出冰層；反之，它們會形成特殊的條帶，隨著時間經過而層層堆積成密實的冰塊。
這些冰芯樣品取出後被運送回冰芯實驗室，包括美國科羅拉多州丹佛的美國國家科學基金會冰芯中心、麻薩諸塞州伍茲霍爾的WHOI、麻薩諸塞州諾頓的惠頓學院、內華達州雷諾的沙漠研究院。科學家在這些實驗室中測量冰芯各段的物理化學性質，定出融冰形成的層位厚度和年代。這些劃過冰芯的水平深色條帶就像尺上的刻度一樣，使科學家用看得就能知道年復一年冰層表面的融化程度如何變化。較厚的融冰層代表當年的融化情形較嚴重，而較薄的則代表當年的融化程度較輕微。
科學家將多段冰芯的結果組合起來，配合衛星觀測到的融化現象以及精密的氣候模型，他們證明觀察到的年融冰層厚度不只可以用來推出鑽取地點的融化程度，也能推及至更大範圍的格陵蘭。這項突破讓團隊可以重建冰層邊緣的低海拔地區，也就是導致海平面上升的主要地區的融水流量。
共同作者，MIT-WHOI
合作計畫的研究生Matt
Osman說冰芯記錄提供了相當重要的歷史文獻。因為科學家現今用來了解融冰速率跟氣候變遷的關係時用的衛星測量數據，大概是從1970年代晚期才開始進行。
「我們知道最近數十年有非常大量的冰層融化，但我們並沒有可以用來比較許久之前融化速率的依據。」Osman表示，「透過冰芯樣品，我們可以回推的時間長度是衛星資料的10倍以上。這讓我們得到更加清楚的圖像，可以看出最近數十年的融化速度跟過去相比有多麼不尋常。」
Trusel表示新的研究提供了證據，指出最近數十年來冰層的快速融化從歷史上來看是極為異常的事件。
「若要解出格陵蘭未來的可能遭遇，我們需要先瞭解格陵蘭過往對氣候變遷的反應。」他說，「我們的冰芯顯示即使只跟50年前相比，格陵蘭目前所處的狀態對於溫度的提升是更加敏感。」
Das表示這些發現有一個值得注意的地方是，目前的溫度只要上升一點點就能導致冰層的融化速率再創高峰。
「近幾年的氣溫只要有一絲變化就能導致融化速度以指數上升。」她說，「因此人類導致的暖化對冰層的影響是非線性的。」Trusel總結：「暖化在今日造成的影響比過去更加重大。」
其他共同作者包括惠頓學院的Matthew
J. Evans、華盛頓大學的Ben
E. Smith、列日大學的Xavier
Fettweis、沙漠研究院的Joseph
R. McConnell、烏特勒支大學的Brice
P. Y. Noël 與Michiel
R. van den Broeke。
研究經費來自美國國家科學基金會、資助機構包括羅文大學、伍茲霍爾海洋研究所、美國國防部、荷蘭科學研究組織、荷蘭地球系統科學中心以及比利時國家科學研究基金會。
Greenland ice sheet melt 'off the
charts' compared with past four centuries
Surface melting across Greenland’s
mile-thick ice sheet began increasing in the mid-19th century and then ramped
up dramatically during the 20th and early 21st centuries, showing no signs of
abating, according to new research published Dec. 5, 2018, in the journal Nature. The study provides new evidence
of the impacts of climate change on Arctic melting and global sea level rise.
“Melting of the Greenland Ice Sheet has gone into
overdrive. As a result, Greenland melt is adding to sea level more than any
time during the last three and a half centuries, if not thousands of years,”
said Luke Trusel, a glaciologist at Rowan University’s School of Earth &
Environment and former post-doctoral scholar at Woods Hole Oceanographic
Institution, and lead author of the study. “And increasing melt began around
the same time as we started altering the atmosphere in the mid-1800s.”
“From a historical perspective, today’s melt rates
are off the charts, and this study provides the evidence to prove this” said
Sarah Das, a glaciologist at Woods Hole Oceanographic Institution (WHOI) and
co-author of the study. “We found a fifty percent increase in total ice sheet
meltwater runoff versus the start of the industrial era, and a thirty percent
increase since the 20th century alone.”
Ice loss from Greenland is one of the key drivers of
global sea level rise. Icebergs calving into the ocean from the edge of
glaciers represent one component of water re-entering the ocean and raising sea
levels. But more than half of the ice-sheet water entering the ocean comes from
runoff from melted snow and glacial ice atop the ice sheet. The study suggests
that if Greenland ice sheet melting continues at “unprecedented rates”—which
the researchers attribute to warmer summers—it could accelerate the already fast
pace of sea level rise.
“Rather than increasing steadily as climate warms,
Greenland will melt increasingly more and more for every degree of warming. The
melting and sea level rise we’ve observed already will be dwarfed by what may
be expected in the future as climate continues to warm,” said Trusel.
To determine how intensely Greenland ice has melted
in past centuries, the research team used a drill the size of a traffic light
pole to extract ice cores from the ice sheet itself and an adjacent coastal ice
cap, at sites more than 6,000 feet above sea level. The scientists drilled at
these elevations to ensure the cores would contain records of past melt
intensity, allowing them to extend their records back into the 17th century. During
warm summer days in Greenland, melting occurs across much of the ice sheet
surface. At lower elevations, where melting is the most intense, meltwater runs
off the ice sheet and contributes to sea level rise, but no record of the melt
remains. At higher elevations, however, the summer meltwater quickly refreezes
from contact with the below-freezing snowpack sitting underneath. This prevents
it from escaping the ice sheet in the form of runoff. Instead, it forms
distinct icy bands that stack up in layers of densely packed ice over time.
The core samples were brought back to ice core labs
at the U.S. National Science Foundation Ice Core Facility in Denver, Colo., WHOI
in Woods Hole, Mass., Wheaton College in Norton, Mass., and the Desert Research
Institute in Reno, Nev. where the scientists measured physical and chemical
properties along the cores to determine the thickness and age of the melt
layers. Dark bands running horizontally across the cores, like ticks on a
ruler, enabled the scientists to visually chronicle the strength of melting at
the surface from year to year. Thicker melt layers represented years of higher
melting, while thinner sections indicated years with less melting.
Combining results from multiple ice cores with
observations of melting from satellites and sophisticated climate models, the
scientists were able to show that the thickness of the annual melt layers they
observed clearly tracked not only how much melting was occurring at the coring
sites, but also much more broadly across Greenland. This breakthrough allowed
the team to reconstruct meltwater runoff at the lower-elevation edges of the
ice sheet—the areas that contribute to sea level rise.
Ice core records provide critical historical context
because satellite measurements—which scientists rely on today to understand melting
rates in response to changing climate—have only been around since the late
1970s, said Matt Osman, a graduate student in the MIT-WHOI Joint Program and
co-author of the study.
“We have had a sense that there’s been a great deal
of melting in recent decades, but we previously had no basis for comparison
with melt rates going further back in time,” he said. “By sampling ice, we were
able to extend the satellite data by a factor of 10 and get a clearer picture
of just how extremely unusual melting has been in recent decades compared to
the past.”
Trusel said the new research provides evidence that
the rapid melting observed in recent decades is highly unusual when put into a
historical context.
“To be able to answer what might happen to Greenland
next, we need to understand how Greenland has already responded to climate
change,” he said. “What our ice cores show is that Greenland is now at a state
where it’s much more sensitive to further increases in temperature than it was
even 50 years ago.”
One noteworthy aspect of the findings, Das said, was
how little additional warming it now takes to cause huge spikes in ice sheet
melting.
“Even a very small change in temperature caused an
exponential increase in melting in recent years,” she said. “So the ice sheet’s
response to human-caused warming has been non-linear.” Trusel concluded,
“Warming means more today than it did in the past.”
Additional co-authors are: Matthew J. Evans, Wheaton
College; Ben E. Smith, University of Washington; Xavier Fettweis, University of
Leige; Joseph R. McConnell, Desert Research Institute; and Brice P. Y. Noël and
Michiel R. van den Broeke Utrecht University.
This research was funded by the US National Science
Foundation, institutional support from Rowan University and Woods Hole
Oceanographic Institution, the US Department of Defense, the Netherlands
Organization for Scientific Research, the Netherlands Earth System Science
Center, and the Belgian National Fund for Scientific Research.
原始論文：Luke D.
Trusel, Sarah B. Das, Matthew B. Osman, Matthew J. Evans, Ben E. Smith, Xavier
Fettweis, Joseph R. McConnell, Brice P. Y. Noël, Michiel R. van den
Broeke. Nonlinear rise in Greenland runoff in response to
post-industrial Arctic warming. Nature, 2018; 564 (7734): 104
DOI: 10.1038/s41586-018-0752-4
引用自：Woods Hole Oceanographic Institution. "Greenland
ice sheet melt 'off the charts' compared with past four centuries."
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