原文網址：https://news.climate.columbia.edu/2021/11/08/why-did-glacial-cycles-intensify-a-million-years-ago/
By Marie Denoia
Aronsohn
大約一百萬年前地球發生了某件大事。公轉軌道變化對於地球氣候系統的影響出現了重大改變，這次事件被稱為「中更新世轉型期」(Mid-Pleistocene
Transition, MPT)。在MPT之前，冰期與間冰期大概每41000年循環一次；在MPT之後，冰河期變得更加嚴酷，冷到足以讓北半球形成持續10萬年的冰層。這讓地球在人類時代之前一直處於規律的冰河期循環當中。
一項新研究認為一百萬年前，冰河開始更加牢固地附著在岩床，引發了冰期更長的循環。照片中從冰島Breiðamerkurjökull冰河流出來的冰塊正在往大西洋移動。(Kevin Krajick/Earth Institute)
科學家長久以來對引發此轉變的成因感到疑惑。一個可能的解釋稱為「米蘭科維奇循環」：地球軌道以及面對太陽方向的周期性變化會影響地球吸收的能量多寡。雖然科學家一致同意其為數百萬年來冷暖時期交替出現的主因。然而，研究也顯示米蘭科維奇循環在一百萬年前並未經歷任何一種大型變化，因此當時發生的可能是別種作用。
在MPT發生的同時，有助於傳輸熱量到全球各處的一道大型海洋環流系統也經歷了嚴重的弱化。這道流經大西洋把熱量運送到北方的系統稱為「大西洋經向翻轉環流」(Atlantic
Meridional Overturning Circulation, AMOC)。它的速度減慢跟冰河期的改變有關嗎？如果是的話原因為何，又是透過什麼樣的方式？這些都是還沒解開的問題。不過今日發表在期刊《美國國家科學院院刊》(Proceedings
of the National Academy of Sciences)的新論文提出了一種答案。
研究人員分析了採自南北大西洋的深海沉積物岩芯，過往的深層海水曾經流過這些地方而留下了化學記號。「我們發現在環流停擺不久之前，北大西洋的行為跟這座海盆的其他地方之間有著相當大的差異，」主要作者Maayan
Yehudai表示。她在哥倫比亞大學的拉蒙特多爾蒂地球觀測站就讀博士時進行了這項研究。
在海洋環流停擺之前，北半球的冰層開始更加穩固地附著在下方的岩床之上，使得冰河可以成長到比之前更加深厚。這反過來造成全球冷化比以往更加劇烈，並且阻斷了大西洋的熱量傳輸帶。Yehudai說結果便是更加嚴酷的冰河期以及冰期循環的周期出現變化。
這項研究支持了一項長久以來帶有爭議的假說：堆積在陸地上容易滑動的土壤於過往的冰河期逐漸被侵蝕掉，使得接下來的冰層可以緊緊附著在下方堅硬、古老的結晶岩之上，因此變得更加穩定且長到更厚的高度。這項發現指出在AMOC弱化不久之前，冰河的厚度增加且變得更加穩定，創造出全球氣候呈現的樣貌。
「這是地球進入冰河時代以來最大的一次氣候變遷，我們的研究解決了有關於此的問題中最大的其中一道，」Yehudai表示。「這是過去最重大的氣候轉型事件之一，而我們對它還沒有完全瞭解。我們的發現指出這道改變的起源為北半球，冰河在此處的演變促使氣候模式轉變成我們今日觀察到的樣貌。這是非常重要的一步，讓我們可以瞭解此轉變的發生原因以及源自何方。結果也顯現出北大西洋地區與海洋環流對於現今與未來的氣候變遷來說非常重要。」
Why did glacial cycles intensify a
million years ago?
Something big happened to the planet
about a million years ago. There was a major shift in the response of Earth’s
climate system to variations in our orbit around the Sun. The shift is called
the Mid-Pleistocene Transition. Before the MPT, cycles between glacial (colder)
and interglacial (warmer) periods happened every 41,000 years. After the MPT,
glacial periods became more intense—intense enough to form ice sheets in the
Northern Hemisphere that lasted 100,000 years. This gave Earth the regular
ice-age cycles that have persisted into human time.
Scientists have long puzzled over what triggered
this. A likely reason would be a phenomenon called Milankovitch cycles—cyclic
changes in Earth’s orbit and orientation toward the Sun that affect the amount
of energy that Earth absorbs. This, scientists agree, has been the main natural
driver of alternating warm and cold periods for millions of years. However,
research has shown that the Milankovitch cycles did not undergo any kind of big
change a million years ago, so something else likely was at work.
Coinciding with the MPT, a large system of ocean
currents that helps move heat around the globe experienced a severe weakening.
That system, which sends heat north through the Atlantic Ocean, is the Atlantic
Meridional Overturning Circulation (AMOC). Was this slowdown related to the
shift in glacial periods? If so, how and why? These have been open questions. A
new paper published today in the journal Proceedings
of the National Academy of Sciences proposes an answer.
The researchers analyzed cores of deep-sea sediments
taken in the south and north Atlantic, where ancient deep waters passed by and
left chemical clues. “What we found is the North Atlantic, right before this
crash, was acting very differently than the rest of the basin,” said lead
author Maayan Yehudai, who did the work as a PhD. student at Columbia
University’s Lamont-Doherty Earth Observatory.
Prior to that oceanic circulation crash, ice sheets
in the Northern Hemisphere began to stick to their bedrock more effectively.
This caused glaciers to grow thicker than they had before. This in turn led to
a greater global cooling than before, and disrupted the Atlantic heat conveyor
belt. This led to both stronger ice ages and the ice-age cycle shift, says
Yehudai.
The research supports a long-debated hypothesis that
the gradual removal of accumulated slippery continental soils during previous
ice ages allowed ice sheets to cling more tightly to the older, harder
crystalline bedrock underneath, and grew thicker and more stable. The findings
indicate that this growth and stabilization just before the weakening of the
AMOC shaped the global climate.
“Our research addresses one of the biggest questions
about the largest climate change we had since the onset of the ice ages,” said
Yehudai. “It was one of the most substantial climate transitions and we don’t
fully understand it. Our discovery pins the origin of this change to the
Northern Hemisphere and the ice sheets that evolved there as driving this shift
towards the climate patterns we observe today. This is a very important step
toward understanding what caused it and where it came from. It highlights the
importance of the North Atlantic region and ocean circulation for present and
future climate change.”
原始論文：Maayan
Yehudai, Joohee Kim, Leopoldo D. Pena, Maria Jaume-Seguí, Karla P. Knudson,
Louise Bolge, Alberto Malinverno, Torsten Bickert, Steven L. Goldstein. Evidence
for a Northern Hemispheric trigger of the 100,000-y glacial cyclicity. Proceedings
of the National Academy of Sciences, 2021; 118 (46): e2020260118 DOI: 10.1073/pnas.2020260118
引用自：Earth Institute at Columbia University. "Why
did glacial cycles intensify a million years ago?”
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