原文網址：https://www.sciencedaily.com/releases/2021/01/210113120656.htm
科學家聲稱他們找到了冰河期引發過程中的「失落環節」。
卡地夫大學的團隊表示南極冰山融化是一系列連鎖反應中的關鍵環節，造成地球陷入一段處在低溫的漫長時期。
由世界各地許多大學聯合組成的科學團隊今日在《自然》(Nature)
發表了他們的發現。
科學家早已知道地球繞日軌道的周期性變化會改變到達地球表面的太陽輻射多寡，因而決定了冰河期循環的步調。
然而，為什麼太陽能的微小變動就可以讓地球氣候產生如此劇烈的變化，目前仍然是個謎團。
團隊在這項研究中提出當地球繞日軌道的形狀恰到好處，南極的冰山便會開始加速融化並且從南極漂到更遠的地方，而把大量的淡水從南大洋帶到大西洋。
隨著南大洋越來越鹹而大西洋越來越淡，大尺度的海洋環流模式便會開始發生劇烈的改變，使得海洋吸收大氣中的二氧化碳並降低溫室效應。
結果便是把地球推入冰河期的狀態。
科學家在這項研究中的一部份，同時利用多種技術來重建過往的氣候條件，其中一種是辨識從融化的冰山裡面掉落到開放海洋，源自南極的微小石頭碎屑。
這些石頭碎屑採自於國際海洋發現計畫361航次取回的沉積物，其代表了160多萬年的地球歷史，也是記載南極冰山的詳細紀錄中時間最長的之一。
利用一種微小的深海化石――有孔蟲的化學性質可以重建海洋深層環流的變化。研究發現這些稱作漂冰碎屑的沉積物似乎總是比深海的變化還早發生。
團隊也利用新的氣候模型進行模擬以驗證他們的假說，結果顯示冰山確實可以搬運大量的淡水。
研究主要作者，卡地夫大學地球與環境科學院的Aidan
Starr表示：「我們發現過去160萬年當中每次冰河期開始的時候都有這種先後關係，這令我們感到十分訝異。」
同為地球與環境科學院的Ian
Hall教授是研究共同作者，也是該IODP航次的共同主持科學家。他說：「氣候系統會因為地球繞日軌道而自然產生節律變化，這會讓南極與南大洋產生什麼樣的反應？我們的結果提出了其中的失落環節。」
過去300萬年地球會規律地陷入冰河期的狀態，不過我們現在所處的則是溫度較高的間冰期。
然而，由於人類排放二氧化碳造成全球溫度不斷升高，研究人員認為南大洋會變得太過溫暖使得南極的冰山難以漂到遠方，海洋環流因此不易產生變化，無法形成發展冰河期的所需條件。結果便是冰河期循環的自然節律可能會受到擾亂。
Hall教授認為這項結果可以用來瞭解未來的氣候會因為人為的氣候變遷而出現什麼樣的反應。
「同樣地，我們現在也觀察到南極大陸正在流失許多冰塊，而南大洋的冰山也跟著變多，原因便是現代人類排放溫室氣體造成的暖化。我們的研究強調出在發展相當穩健的模型，來預測冰山未來對於海洋環流和氣候的影響時，瞭解冰山的漂流途徑與融化模式是十分重要的。」
Melting icebergs key to sequence of an
ice age, scientists find
Scientists claim to have found the
‘missing link’ in the process that leads to an ice age on Earth.
Melting icebergs in the Antarctic are the key, say
the team from Cardiff University, triggering a series of chain reactions that
plunges Earth into a prolonged period of cold temperatures.
The findings have been published today in Nature from an international consortium
of scientists from universities around the world.
It has long been known that ice age cycles are paced
by periodic changes to Earth’s orbit of the sun, which subsequently changes the
amount of solar radiation that reaches the Earth’s surface.
However, up until now it has been a mystery as to how
small variations in solar energy can trigger such dramatic shifts in the
climate on Earth.
In their study, the team propose that when the orbit
of Earth around the sun is just right, Antarctic icebergs begin to melt further
and further away from Antarctica, shifting huge volumes of freshwater away from
the Southern Ocean and into the Atlantic Ocean.
As the Southern Ocean gets saltier and the North
Atlantic gets fresher, large-scale ocean circulation patterns begin to
dramatically change, pulling CO2 out of the atmosphere and reducing
the so-called greenhouse effect.
This in turn pushes the Earth into ice age
conditions.
As part of their study the scientists used multiple
techniques to reconstruct past climate conditions, which included identifying
tiny fragments of Antarctic rock dropped in the open ocean by melting icebergs.
The rock fragments were obtained from sediments
recovered by the International Ocean Discovery Program (IODP) Expedition 361,
representing over 1.6 million years of history and one of the longest detailed
archives of Antarctic icebergs.
The study found that these deposits, known as
Ice-Rafted Debris, appeared to consistently lead to changes in deep ocean
circulation, reconstructed from the chemistry of tiny deep-sea fossils called
foraminifera.
The team also used new climate model simulations to
test their hypothesis, finding that huge volumes of freshwater could be moved
by the icebergs.
Lead author of the study Aidan Starr, from Cardiff
University’s School of Earth and Environmental Sciences, said: “We were
astonished to find that this lead-lag relationship was present during the onset
of every ice age for the last 1.6 million years."
Professor Ian Hall, co-author of the study and
co-chief scientist of the IODP Expedition, also from the School of Earth and
Environmental Sciences, said: “Our results provide the missing link into how
Antarctica and the Southern Ocean responded to the natural rhythms of the
climate system associated with our orbit around the sun.”
Over the past 3 million years the Earth has regularly
plunged into ice age conditions, but at present is currently situated within an
interglacial period where temperatures are warmer.
However, due to the increased global temperatures
resulting from anthropogenic CO2 emissions, the researchers suggest the natural
rhythm of ice age cycles may be disrupted as the Southern Ocean will likely
become too warm for Antarctic icebergs to travel far enough to trigger the
changes in ocean circulation required for an ice age to develop.
Professor Hall believes that the results can be used
to understand how our climate may respond to anthropogenic climate change in
the future.
“Likewise as we observe an increase in the mass loss
from the Antarctic continent and iceberg activity in the Southern Ocean,
resulting from warming associated with current human greenhouse-gas emissions,
our study emphasises the importance of understanding iceberg trajectories and
melt patterns in developing the most robust predictions of their future impact
on ocean circulation and climate,” he said.
原始論文：Aidan Starr,
Ian R. Hall, Stephen Barker, Thomas Rackow, Xu Zhang, Sidney R. Hemming, H. J.
L. van der Lubbe, Gregor Knorr, Melissa A. Berke, Grant R. Bigg, Alejandra
Cartagena-Sierra, Francisco J. Jiménez-Espejo, Xun Gong, Jens Gruetzner,
Nambiyathodi Lathika, Leah J. LeVay, Rebecca S. Robinson, Martin Ziegler. Antarctic
icebergs reorganize ocean circulation during Pleistocene glacials. Nature,
2021; 589 (7841): 236 DOI: 10.1038/s41586-020-03094-7
引用自：Cardiff
University. "Melting icebergs key to sequence of an ice age."
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