雖然南極的冰層正持續消融，但冰層下方的基岩抬升或許能減緩融化的速度
這篇發表在期刊《科學》(Science)的論文標題為「在阿蒙森海灣下方觀測到的基岩迅速抬升現象，可以提高冰層的穩定性」。這篇文章對於此區域下方的地質條件以及地質跟冰層之間的交互作用提出了新的見解。
研究人員表示這項發現對於瞭解並預測冰層的穩定性以及地球的海平面變化來說，具有相當重要的意義。
科羅拉多州立大學的教授Rick
Aster表示：「我們研究了一種驚人且重要的機制――冰河均衡調整(glacial
isostatic adjustment)。這種機制可以抬起西南極冰層之下的基岩和沉積物，或許能夠延緩這座巨大冰層的崩解速度。」Rick
Aster為此研究的共同作者，也是該大學地質科學系的主任。
冰層下方的碗型地勢使其特別容易崩解
科學家一直擔憂這座冰層在面對氣候和海流暖化時會變得特別不穩定，這是因為其底部所在的地面位在海面下方數百至數千公尺以下――這點和東南極或格陵蘭的冰層不同――而且還往內陸傾斜。這種碗型的地勢使它特別容易在數百年或者數千年之內就因為冰層流失而變得不穩定甚至完全崩解。
如果整個南極西部擁有的冰融化的話，足以讓全球海平面平均上升3公尺以上。此外，由於這座冰層的質量相當大，它產生的重力使得此區的海水較為高漲。如果這些海水被釋放出去，會使得北半球某些地區的平均海平面再上升一公尺或者更多。
現在全球每年融化的陸冰中，西南極冰層大約佔了25%。近期利用衛星資料進行的研究中顯示最近數十年這個比例有增加的現象。西南極冰層每年流失的體積約為154立方公里，也就是邊長為5公里多的立方體。
由任職於俄亥俄州立大學的共同作者主導的工作為在此偏遠地區周圍露出的岩石露頭裝設高精度GPS站，目的是要測量冰層厚度變化導致的抬升作用。測量結果顯示岩床的抬升速率高達每年4公分，是冰河地區記錄到的最快速率之一。
Aster表示：「這種非常快的抬升速率或許可以延緩冰層因為流失造成的破壞，以及最終冰層崩毀的時間。」他解釋抬升能穩住關鍵的基線(grounding
line)位置，也就是冰蓋脫離下方岩床或沉積物，變成在水中漂浮的界線。基線的穩定可以抵銷使冰層崩解的作用。
顛覆過往認知的發現
研究人員在這篇研究中也發現抬升有加速的現象，並預測此趨勢將會持續至下個世紀。
Aster表示：「我們的研究指出在過去的研究中，對於此區近期不斷流失的冰層體積大約低估了10%。這是因為利用衛星的測量方法中，並沒有充分考慮到岩床抬升這項因素。」
主要作者Valentina
Barletta是丹麥科技大學DTU
SPACE的博士後研究員。她說跟地球上其他冰河也在融化的地區相比，西南極冰蓋的抬升進行得非常快速。
Barletta表示：「我們看到這類型的抬升現象一般來說是在冰河期過後的數千年之間，以十分緩慢的速度發生。」她繼續說道：「我們的發現告訴我們當冰塊的重量被移除之後，地表下方的黏稠地函會變得更容易流動，而以較快的速度運動。」
俄亥俄州立大學的地球科學名譽教授Terry
Wilson表示，這項發現指出冰層之下的地質條件，可能跟過往科學家認為的大不相同。
「我們發現的抬升速率快得不尋常而讓人感到十分訝異。」她表示，「這顛覆了我們的認知。」
地震波掃描得到的線索跟下方地函有關
Aster、Barletta和研究共同作者利用高靈敏度的地震儀組成覆蓋整個南極洲的大型觀測網，使得他們可以得到南極下方地球深處的地震層析影像(seismic
tomographic image)，就像是對地球進行巨幅的電腦斷層掃描一樣。
這項成果描繪出南極西部地下70公里至700公里深處，相對地殼來說高溫且具有流動性的地函狀況，這些資訊可以幫助科學家解釋並模擬GPS得出的抬升數據。
由此研究團隊之前進行的相關研究也指出南極下方有些溫度較高的構造，可以一直延伸到地球內部超過950公里深的地方。
Aster表示：「之前在南極西部難以解釋的火山活動，像是羅斯島上的埃里伯斯火山，就是由這些溫度較高的地函構造促成。」他接著表示：「這些從冰層底下散發出來的地熱，可以讓某些區域的冰下湖泊一直存在。湖水會潤滑冰層底部，使得冰層一路滑入海洋。」
Aster表示雖然這項研究確實呈現出某種程度的正面訊息，但如果未來的全球暖化越來越劇烈，整座西南極冰層仍會完全融化。
「若要讓全球海平面在這個世紀以內甚至是之後僅上升幾公尺，我們還是必須透過國際合作與科技發展，來限制大氣中的溫室氣體濃度持續上升。」
Antarctic ice sheet is melting, but
rising bedrock below could slow it down
An
international team led by DTU Space at the Technical University of Denmark with
Colorado State University has found that the bedrock below the remote West
Antarctic Ice Sheet is rising much more rapidly than previously thought, in
response to ongoing ice melt.
The study, “Observed rapid bedrock uplift in the
Amundsen Sea Embayment promotes ice-sheet stability,” reveals new insights on
the geology of the region and its interaction with the ice sheet and is published
in the journal Science.
Researchers said the findings have important
implications in understanding and predicting the stability of the ice sheet and
Earth’s rising sea levels.
“We studied a surprising and important mechanism,
glacial isostatic adjustment, that may slow the demise of the massive West
Antarctic Ice Sheet by lifting up the bedrock and sediments beneath the ice
sheet,” explained CSU Professor Rick Aster, a co-author of the study and head
of the Geosciences department at the university.
Ice sheet’s
bowl-like position makes it susceptible to collapse
Scientists have been concerned that this ice sheet is
particularly precarious in the face of a warming climate and ocean currents,
because it is grounded hundreds to thousands of feet below sea level — unlike
the ice sheets of East Antarctica or Greenland — and its base slopes inland.
This bowl-like topography makes it susceptible to runaway destabilization and
even complete collapse over centuries or even thousands of years.
The entirety of West Antarctica contains enough ice
that, if it were to melt, would contribute more than 10 feet of average global
sea level rise. In addition, the ice sheet is so massive that it attracts an
ocean bulge, due to gravity. If this mass of water is released, it would lead
to an additional increase of three feet or more to average sea level in parts
of the Northern Hemisphere.
The West Antarctic Ice Sheet is currently
contributing approximately 25 percent of global melting land-based ice each
year, and recent satellite-based studies have shown that this amount has
increased in recent decades. This figure is equivalent to about 37 cubic miles
or a cube of over three miles on each side.
Co-authors of the study based at The Ohio State
University led the installation of sensitive GPS stations on rock outcrops
around the remote region to measure the rise in response to thinning ice and to
directly measure the uplift due to changes in the ice sheet. These measurements
showed that the bedrock uplift rates were as high as 1.6 inches per year, one
of the fastest rates ever recorded in glacial areas.
“This very rapid uplift may slow the runaway wasting
and eventual collapse of the ice sheet,” said Aster. The uplift tends to
stabilize the critical grounding line where the ice sheet loses contact with
underlying bedrock or sediment and goes afloat, he explained. This grounding
line then counteracts the process of the ice sheet collapsing.
‘A game changer’
Researchers also found in this study that the uplift
is accelerating, and predicted that it will continue to do so into the next
century.
“Our research indicates that recent and ongoing ice
loss in the region has been underestimated by approximately 10 percent in past
studies, because this bedrock uplift was inadequately accounted for in satellite
measurements,” said Aster.
Lead author Valentina Barletta, a postdoctoral
researcher at DTU SPACE, Technical University of Denmark, said that this uplift
is occurring quite rapidly, when compared with other regions of the earth where
glaciers are melting.
“Normally we would see this type of uplift happening
slowly, over thousands of years after an ice age,” said Barletta. “What we
found tells us that earth’s underlying viscous mantle is relatively fluid and
moves quickly when the weight of the ice is taken off,” she added.
Terry Wilson, professor emeritus of Earth Sciences at
The Ohio State University, said the findings suggest that the geological
conditions beneath the ice are very different from what scientists had
previously believed.
“The rate of uplift we found is unusual and very surprising,”
she said. “It’s a game changer.”
Seismic scans
uncover clues about the earth’s mantle
Aster, Barletta and the study co-authors have
deployed a large network of sensitive seismographs across Antarctica to produce
seismic tomographic images — analogous to a gigantic CAT scan — of the deep
earth below Antarctica.
The work assisted in the interpretation and modeling
of the GPS uplift data by delineating a vast region of the earth’s mantle, 40
to 400 miles below West Antarctica, that is relatively hot and fluid.
Previous and related studies conducted by the
research team have also revealed that some hotter features below Antarctica
extend still deeper, over 600 miles, into the earth.
“These warmer mantle features drive the previously
enigmatic volcanic activity in West Antarctica, including Mount Erebus on Ross
Island,” said Aster. “This geothermal heat at the base of the ice sheet helps
to sustain subglacial lakes in some regions and lubricates the ice sheet as it
slips towards the ocean,”he added.
Aster said while the research does provide room for a
positive outcome, if future global warming is extreme, the entire West Antarctic
Ice Sheet will still melt.
“To keep global sea levels from rising more than a few
feet during this century and beyond, we must still limit greenhouse gas
concentrations in the atmosphere through international cooperation and
innovation,” he said.
原始論文：Valentina
R. Barletta, Michael Bevis, Benjamin E. Smith, Terry Wilson, Abel Brown, Andrea
Bordoni, Michael Willis, Shfaqat Abbas Khan, Marc Rovira-Navarro, Ian Dalziel,
Robert Smalley, Eric Kendrick, Stephanie Konfal, Dana J. Caccamise, Richard C.
Aster, Andy Nyblade, Douglas A. Wiens. Observed
rapid bedrock uplift in Amundsen Sea Embayment promotes ice-sheet stability.
Science, 2018; 360 (6395): 1335 DOI:
10.1126/science.aao1447
引用自：Colorado
State University. "Antarctic ice sheet is melting, but rising bedrock
below could slow it down."
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