兩百萬年前的冰塊提供給我們地球溫室氣體如何變化的片段紀錄
By Sean Nealon
一組研究團隊最近從南極洲發現兩百萬年前形成的冰塊，它們讓我們清楚看到過去溫室氣體和氣候之間有何關聯，並讓科學家能藉此瞭解未來的氣候變遷。
奧勒岡州立大學的Ed
Brook捧著200萬年前形成的冰塊。圖片來源：奧勒岡州立大學
在最近發表於期刊《自然》(Nature)的論文中，一群科學家利用年代達到兩百萬年的冰塊中的氣泡，測量出過去二氧化碳和甲烷這些溫室氣體在空氣中的含量。領導這組團隊的科學家為普林斯頓大學的John
Higgins和Yuzhen
Yan以及緬因大學的Andrei
Kurbatov，成員則包含了奧勒岡州立大學的Ed
Brook和加州大學聖地牙哥分校的Jeff
Severinghaus。
這是科學家首次研究如此古老的冰芯。之前年代最久的完整冰芯紀錄可以提供的數據只能到八十萬年前。過往的研究利用這段冰芯以及其他冰芯，證實過去八十萬年來大氣中的二氧化碳濃度，和南極以及全球氣溫有直接關聯。但更早以前的氣候和二氧化碳濃度之間有什麼關係，科學家就沒有辦法了解得如此透徹。
而最近這篇發表在期刊《自然》的論文改變了這種情形。
在過去一百萬年之間，冰河期接續暖期的循環每十萬年就會發生一次。但是在280萬年前到120萬年前，循環週期卻縮短成四萬年左右，而且冰河期的強度也沒有那麼劇烈。
包括Brook在內的研究團隊想要找出二氧化碳濃度在這段較為古老的時光中是如何變化。在此之前，科學家只能從海底和陸上沉積物的化學性質來間接瞭解它們的變化。
他們發現當時二氧化碳濃度的最高值和最近這些暖期的二氧化碳濃度一致。不過和過去八十萬年來冰河期時相當低的二氧化碳濃度相比，當時的最低值就沒有這麼低了。
Brook說：「這項研究最重要的發現之一，是證實了更早以前的二氧化碳含量和溫度也有關聯。」
科學家透過研究冰層的化學性質，可以得出二氧化碳濃度變化的同時，南極的氣溫是如何隨之改變，因此可以得出上述結論。
Brook說：「在氣候科學的研究以及校正預測未來氣候變遷的模型時，研究成果提供了相當重要的基準。」
這段兩百萬年前的冰芯取自艾倫丘陵，距美國在南極的研究基地麥克默多研究站約有130英里遠(大約為210公里)。之前曾在艾倫丘陵的表面發現相當古老的隕石，使得科學家相信此區的冰層具有年齡相當久遠的冰塊。
在2015至16年的考察季節，團隊鑽到冰層下方200公尺深而取出這段年代有二百萬年的冰芯。要鑽井並取出這樣的冰芯需要用上一至兩個禮拜，而他們也從該地區取回另外幾段冰芯。
團隊未來幾天之內將返回艾倫丘陵進行為期兩個月的後續工作。他們的目標是大量蒐集年代達到二百萬年的冰芯，同時尋找更為古老的樣品。
「我們不知道這個區域的冰塊能有多老，」Brook表示。「某些地方的年代可能會比我們目前找到的樣品更加古老，所以我們才回到這裡繼續研究。如果能夠把時間回推至兩百萬年以前就再好不過了。」
Two
million-year-old ice provides snapshot of Earth’s greenhouse gas history
Two million-year old ice from Antarctica
recently uncovered by a team of researchers provides a clearer picture into the
connections between greenhouse gases and climate in ancient times and will help
scientists understand future climate change.
In a paper published today in Nature, a group of scientists used air trapped in the bubbles in
ice as old as 2 million years to measure levels of the greenhouse gases carbon
dioxide and methane. The group was led by John Higgins and Yuzhen Yan of
Princeton University and Andrei Kurbatov of the University of Maine, and
included Ed Brook at Oregon State University and Jeff Severinghaus at the
University of California, San Diego.
This is the first time scientists were able to study
an ice core that old. Previously, the oldest complete ice core provided data
back to 800,000 years. Past studies using that core and others have shown that
atmospheric carbon dioxide levels are directly linked to Antarctic and global
temperature during the past 800,000 years. Prior to that, the connection
between climate and carbon dioxide levels has not been as well understood.
The paper published today in Nature begins to change that.
During the past one million years the cycle of ice
ages followed by warm periods occurred every 100,000 years. But between 2.8
million years ago and 1.2 million years ago, those cycles were shorter, about
40,000 years, and ice ages were less extreme.
The team that included Brook wanted to find out how
carbon dioxide levels varied during that older time period, which until now was
known only indirectly from the chemistry of sediments in the ocean and on land.
They found that the highest levels of carbon dioxide
matched the levels in warm periods of more recent times. The lowest levels,
however, did not reach the very low concentrations found in the ice ages of the
last 800,000 years.
“One of the important results of this study is to
show that carbon dioxide is linked to temperature in this earlier time period,”
Brook said.
This conclusion is based on studies of the chemistry
of the ice, which provide an indication of temperature change in Antarctica at
the same time as the carbon dioxide variations.
“That’s an important baseline for understanding
climate science and calibrating models that predict future change,” Brook said.
The ice core with the 2 million-year-old ice comes
from an area known as Allan Hills, which is about 130 miles from the U.S.
Antarctic research station known as McMurdo Station. Ancient meteorites had
been found on the surface in this area, leading scientists to believe there
could be ancient ice in the ice sheet.
The core with the 2 million-year-old ice was drilled
to a depth of 200 meters during the 2015-16 field season. It takes one to two
weeks to drill and recover a core like that, and several cores were collected
in the region.
The research team is on its way back to Allan Hills
in the coming days for two months of additional work. They will be collecting
larger quantities of the 2 million year old ice and searching for even older
samples.
“We don’t know the age limit in this area,” Brook
said “It could be much older in some places. That’s why we’re going back.
Pushing beyond two million years would be pretty amazing.”
原始論文：Yuzhen Yan, Michael L. Bender, Edward J. Brook,
Heather M. Clifford, Preston C. Kemeny, Andrei V. Kurbatov, Sean Mackay, Paul
A. Mayewski, Jessica Ng, Jeffrey P. Severinghaus, John A. Higgins. Two-million-year-old snapshots of
atmospheric gases from Antarctic ice. Nature, 2019; 574
(7780): 663 DOI: 10.1038/s41586-019-1692-3
引用自：Oregon State University. "Two
million-year-old ice provides snapshot of Earth's greenhouse gas
history."
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