原文網址：https://news.harvard.edu/gazette/story/2020/11/when-ice-sheets-melt-its-a-seesaw-effect/
分析發現一座冰層的融化可以影響數千公里外的另一座冰層
By Juan Siliezar
若要了解地球各處之間的關聯究竟有多深，沒有一種方法比探討北半球和南極的廣大冰層要來得更好。
圖片來源：Kenichiro Tani
相距數千公里的南北極幾乎稱不上是隔壁鄰居，但是由一組國際科學團隊進行的新研究得到了令人驚訝的結果。他們指出在冰層擴張與融化這方面，其中一處發生的某些變化竟然會直接對另外一處造成巨大的影響。此團隊由哈佛大學2014年的博士Natalya
Gomez領導，成員包括了哈佛大學的教授Jerry
X. Mitrovica。
這項發表在《自然》(Nature)的分析結果，首度證實了過去四萬年來，由於北半球的冰層融化而造成的海平面變化，後續帶來的影響會讓南極的冰層產生變化。研究人員表示隨著全球暖化促使大型冰層與冰帽加速融化，瞭解其中的運作原理有助於氣候科學家掌握未來的變化。
研究人員模擬了這種蹺蹺板效應的運作方式。他們發現大約20000到26000年前，冰期到達最近一次巔峰的時候，長期處在冰封狀態的北半球冰層造成南極的海平面下降，並讓該處的冰層增長。過了這次巔峰之後氣候開始變暖，北半球的冰層融化，造成南半球的海平面跟著上升。上漲的海水導致南極的冰層開始後退，在數千年之內便縮小到跟現在差不多，以地質時間來看這是相當快的反應速度。
南極冰層在這段暖期為什麼融化得這麼快，一直以來都是個謎團。
「研究的這個部份讓人相當興奮，」地球與行星科學系的教授Mitrovica表示。「這些南極冰雪大量流失的事件究竟是由什麼促成？研究顯示最根本的原因並非來自當地，而是極為遠處的冰層融化，造成南極海平面上升而引發這些事件。南極冰層的穩定性與北半球冰層長期融化有什麼樣的關係之前並未受到重視，而這篇研究結果將兩者成功連結了起來。」
Gomez現為麥基爾大學地球與行星科學的助理教授。他和同僚之前預測了南極洲海平面的變化模式，得到的結果和北半球冰層後退的時期一致。他們的下一步是拓展這項研究的結果，觀察還有哪些地方的冰層後退會導致另一處的冰層跟著變化。這可以讓我們更加瞭解過去其他時間點的冰層穩定性，甚至是未來的。
「觀察過去對我們理解冰層和海平面的運作方式有很大的幫助，」Gomez表示。「這可以讓我們更加了解整個地球系統的運作方式。」
進行這項計畫的科學家團隊除了Gomez和Mitrovica，還有奧勒岡州立大學和德國波恩大學的研究人員。他們著眼的對象是一種稱為漂冰碎屑的石頭。這些石頭原本是在南極的冰層裡面，冰山崩解出來之後會把它們帶到南大洋。研究人員的目標是找出它們是從冰層的哪個地方，在什麼時候釋放出來。他們結合冰層與海平面的模擬結果，以及從南極周遭的海底取出的沉積物岩芯樣品來證實他們的發現。此外，他們也探討了過去海岸線留下的標記，以瞭解冰層前緣的後退模式。
Gomez進入哈佛大學文理學院成為Mitrovica團隊的研究生之後，便一直從事和冰層有關的研究。2010年她主持的研究顯示冰層的重力效應十分巨大，使得冰層融化的時候，進入海洋的全體雪水應該在附近地區造成的海平面上升，反而會被冰層的重力抵銷掉。Gomez指出如果南極西部的冰層全部融化掉，附近的海平面實際上會降低多達300英尺(約90公尺)，但是北半球海平面的上升幅度卻會比預估中的高出許多。
該論文進一步讓研究人員想問在氣候系統當中某部分的冰層融化，會對另外一部份的冰層造成什麼樣的影響。在這個問題上，研究人員探討了北半球曾經覆蓋北美與北歐的冰層。
研究人員把海平面上升與冰層融化模型的數據，結合冰河期從南極崩解出來的冰山所留下的碎屑，藉此模擬過去四萬年來兩個半球的海平面與冰層的動力學是如何變化。
如此一來研究人員就能解釋過去二萬年南極幾度變得不穩定的時期，在這些時期當中南極的冰層歷經了稱為「融雪脈衝」的快速融化階段。事實上，他們的模型顯示現在面積幾乎有1400萬平方公里、重量約有26000兆噸的南極冰層如果沒有歷經這些快速後退的時期，那它會變成比現在更為壯觀的龐然大物。
研究中的大部分地質紀錄是由波恩大學的Michael
Webster蒐集。研究人員利用地質紀錄可以確認模型預測出來的變化歷程，並且看出南極的海平面變化與質量減少，可以對應至北半球冰層融化的時期。
數據讓Gomez大為驚訝，但最重要的是，她對這些冰天雪地的系統又變得更加好奇。
「冰層是地球的氣候當中非常活躍的一部份，讓人十分感興趣且帶來許多驚喜。想像一座覆蓋整片大陸、厚達數公里的冰塊就令人讚嘆不已，而它在任何時間尺度下的演變過程都會影響到整個地球，」Gomez說。「光憑這點，就讓我們有動力去試著更加瞭解這些離我們千里之外的巨大系統。」
Evidence of the interconnectedness of
global climate
Analysis finds melting ice sheet
affects a second thousands of miles away
To see how deeply interconnected the
planet truly is, look no further than the massive ice sheets on the Northern
Hemisphere and South Pole.
Thousands of miles apart, they are hardly next-door
neighbors, but according to new research from a team of international
scientists — led by Natalya Gomez, Ph.D. ’14, and including Harvard Professor
Jerry X. Mitrovica — what happens in one region has a surprisingly direct and
outsized effect on the other, in terms of ice expanding or melting.
The analysis, published in Nature, shows for the first time that changes in the Antarctic ice
sheet were caused by the melting of ice sheets in the Northern Hemisphere. The
influence was driven by sea-level changes caused by the melting ice in the
north during the past 40,000 years. Understanding how this works can help
climate scientists grasp future changes as global warming increases the melting
of major ice sheets and ice caps, researchers said.
The study models how this seesaw effect works.
Scientists found that when ice on the Northern Hemisphere stayed frozen during
the last peak of the Ice Age, about 20,000 to 26,000 years ago, it led to
reduced sea levels in Antarctica and growth of the ice sheet there. When the
climate warmed after that peak, the ice sheets in the north started melting,
causing sea levels in the southern hemisphere to rise. This rising ocean
triggered the ice in Antarctica to retreat to about the size it is today over
thousands of years, a relatively quick response in geologic time.
The question of what caused the Antarctic ice sheet to
melt so rapidly during this warming period had been a longstanding enigma.
“That’s the really exciting part of this,” said
Mitrovica, the Frank B. Baird Jr. Professor of Science in the Department of
Earth and Planetary Sciences. “What was driving these dramatic events in which
the Antarctic released huge amounts of ice mass? This research shows that the
events weren’t ultimately driven by anything local. They were driven by sea
level rising locally but in response to the melting of ice sheets very far
away. The study establishes an underappreciated connection between the
stability of the Antarctic ice sheet and significant periods of melting in the
Northern Hemisphere.”
The retreat was consistent with the pattern of sea
level change predicted by Gomez, now an assistant professor of earth and
planetary sciences at McGill University, and colleagues in earlier work on the
Antarctic continent. The next step is expanding the study to see where else ice
retreat in one location drives retreat in another. That can provide insight on
ice sheet stability at other times in the history, and perhaps in the future.
“Looking to the past can really help us to understand
how ice sheets and sea levels work,” Gomez said. “It gives us a better
appreciation of how the whole Earth system works.”
Along with Gomez and Mitrovica, the team of
scientists on the project included researchers from Oregon State University and
the University of Bonn in Germany. The rocks they focused on, called ice-rafted
debris, were once embedded inside the Antarctic ice sheet. Fallen icebergs
carried them into the Southern Ocean. Researchers determined when and where
they were released from the ice sheet. They combined ice-sheet and sea-level
modeling with sediment core samples from the ocean bottom near Antarctica to
verify their findings. And researchers also looked at markers of past
shorelines to see how the ice sheet’s edge has retreated.
Gomez has been researching ice sheets since she was a
Graduate School of Arts and Sciences student in the Mitrovica Group. She led a
study in 2010 that showed that gravitational effects of ice sheets are so
strong that when ice sheets melt, the expected sea level rise from all that
meltwater entering the oceans would be counterbalanced in nearby areas. Gomez
showed that if all of the ice in the west Antarctic ice sheet melted, it could
actually lower sea level near the ice by as much as 300 feet, but the sea level
would rise significantly more than expected in the Northern Hemisphere.
This paper furthered that study by asking how melting
ice sheets in one part of the climate system affected another. In this case,
the researchers looked at the ice sheets in the Northern Hemisphere that once
covered North America and Northern Europe.
By putting together modeling data on sea-level rise
and ice-sheet melting with the debris left over from icebergs that broke off
Antarctica during the Ice Age, the researchers simulated how sea levels and ice
dynamics changed in both hemispheres over the past 40,000 years.
The researchers were able to explain several periods
of instability during the past 20,000 years when the Antarctic ice sheet went
through phases of rapid melting known as “meltwater pulses.” In fact, according
to their model, if not for these periods of rapid retreat, the Antarctic ice
sheet, which covers almost 14 million square kilometers and weighs about 26
million gigatons, would be even more of a behemoth than it is now.
With the geological records, which were collected
primarily by Michael Webster from the University of Bonn, the researchers
confirmed the timeline predicted by their model and saw that this sea-level
change in Antarctica and the mass shedding corresponded with episodes of
melting of ice sheets in the Northern Hemisphere.
The data caught Gomez by surprise. More than
anything, though, it deepened her curiosity about these frozen systems.
“These ice sheets are really dynamic, exciting, and
intriguing parts of the Earth’s climate system. It’s staggering to think of ice
that is several kilometers thick, that covers an entire continent, and that is
evolving on all of these different timescales with global consequences,” Gomez
said. “It’s just motivation for trying to better understand these really
massive systems that are so far away from us.”
Natalya Gomez, Michael E. Weber, Peter U. Clark,
Jerry X. Mitrovica, Holly K. Han. Antarctic
ice dynamics amplified by Northern Hemisphere sea-level forcing. Nature, 2020 DOI: 10.1038/s41586-020-2916-2
引用自：Harvard
University. “Analysis finds melting ice sheet affects a second thousands of
miles away.”
沒有留言:
張貼留言