重建過去氣候從而得知未來氣候變化的線索
Kayla Zacharias
在過去6600萬年最溫暖的時期中，溫室氣體都是推動氣候變化的主要因素。這項發現對於長期氣候變化背後的驅動因子提供了新的見解。
南極洲跟澳洲大約在始新世(5600萬至2290萬年前)末期分離，造成兩者之間形成了一條深沉的水道而改變了海洋環流模式。有些研究人員相信始新世的「暖房」環境結束之際的溫度下降就是由此變化驅使，但有些研究人員則認為應該要歸因於二氧化碳濃度的下降。
如果冷化是因為海洋環流的變化導致，由於地球的熱量會重新分配，所以赤道地區的溫度會上升而極區則會下降。但如果是溫室氣體的濃度發生變化，則會影響到留在地球大氣的總熱量，造成包含熱帶的每個地區都會冷化――而這便是發表於期刊《自然》(Nature)的研究中所發現到的現象。
「我們重建出來熱帶和極區的溫度同步演變現象只能由溫室氣體造成的作用來解釋。」論文第一作者，荷蘭烏特勒支大學的博士候選人Margot Cramwinckel表示，「跟我們的發現相符的假說只有一個：始新世的長期冷化是由溫室氣體變化導致。這讓我們對於長期氣候變遷背後的驅動因素有了更加長足的瞭解，在預測未來氣候變遷會如何發展時，這點相當重要。」
氣候變遷在極區造成的影響通常比地球上其他地方都還要強烈，這種現象稱作「極區放大效應」(polar amplification)。
雖然始新世的氣候大多時候都極為溫暖，造成極區附近幾乎或根本就沒有冰層，但研究發現始新世時極區附近的溫度變化仍比熱帶劇烈許多。
Cramwinckel表示：「就算是在一個幾乎無冰的世界中，溫度下降時極區的變冷程度仍然比熱帶還要高。這代表溫室氣體光憑自身作用就能造成極區放大效應。」
研究人員還有一個關於極區放大效應的問題：它會達到某個極限嗎？
論文共同作者，普渡大學地球、大氣與行星科學的教授Matthew Huber表示：「我們的結果支持了這項說法――在溫暖的氣候下，極區放大效應會在某個程度到達飽和，而不會隨著進一步的暖化繼續增加下去。」
研究人員從象牙海岸附近鑽出的深海沉積物岩芯中，探討一種生活在海洋表層的單細胞生物「奇古菌」(Thaumarchaeota)細胞膜上的脂質。由於奇古菌會隨著溫度變化改變它們的細胞膜成分，使得它們可以做為溫度指標。
他們把觀察結果結合Huber在普渡大學的團隊開發出來的氣候模型，來交互映證出始新世期間的溫度變化歷程。
「為了歸納出在不同二氧化碳濃度下氣候達到平衡時的狀態，我們用了將近四年的時間持續運算這項模型。」Huber表示，「這是氣候模型首次能獲得熱帶海洋表層溫度的主要趨勢，以及涵蓋各種氣候條件下，將近2000萬年中的氣溫變化梯度。唯一的問題是模型需要的二氧化碳變化量比觀測出來的還要高，顯示此模型對於二氧化碳還不夠靈敏。」
研究人員在重現始新世期間熱帶和寒帶之間的溫度梯度時一直以來都遇到了相當困難。而新的氣候模型能夠克服過去模型所遇到的多數問題。
Reconstruction of past climate provides clues
about future climate change
Greenhouse gases were
the main driver of climate throughout the warmest period of the past 66 million
years, providing insight into the drivers behind long-term climate change.
Antarctica and Australia separated around the end of the Eocene
(56 to 22.9 million years ago), creating a deep water passage between them and
changing ocean circulation patterns. Some researchers believe these changes
were the drivers of cooling temperatures near the end of the Eocene
"hothouse" period, but some think declining levels of carbon dioxide
were to blame.
If the cooling had been caused by changes in ocean circulation,
regions around the equator would have warmed as the polar regions cooled,
shifting the distribution of heat on Earth. But changing the concentration of
greenhouse gases would affect the total heat trapped in Earth’s atmosphere,
causing cooling everywhere (including in the tropics), which is what the
researchers found. The findings were published
in the journal Nature.
“The synchronized evolution of tropical and polar temperature we
reconstructed can only be explained by greenhouse gas forcing,” said Margot
Cramwinckel, a Ph.D. candidate at Utrecht University in the Netherlands and
first author of the paper. “Our findings are uniquely compatible with the
hypothesis that the long-term Eocene cooling was driven by greenhouse gases.
This greatly improves our understanding of the drivers behind long-term climate
change, which is important in order to predict the development of future
climate change.”
Climate change often has more intense effects near the poles
than elsewhere on the planet, a phenomenon known as polar amplification.
The study found that temperature change was more dramatic near
the poles than in the tropics during the Eocene, even though most of the period
was extremely warm, leaving little to no ice near the poles.
“Even in a largely ice-free world, the poles cooled more than
the tropics as temperature dropped,” Cramwinckel said. “This indicates that
greenhouse gas forcing by itself can cause polar amplification.”
The researchers had one more question about polar amplification:
does it reach some sort of limit?
“Our results support the idea that polar amplification saturates
out at some point in warm climates and does not continue to increase with
further warming,” said Matthew Huber, a professor of earth, atmospheric and
planetary sciences at Purdue University and co-author of the paper.
As a proxy for temperature, the research team looked at membrane
lipids of simple, sea-surface dwelling organisms called Thaumarchaeota that
change their membrane composition as temperatures change in deep sea sediment
cores drilled near the Ivory Coast.
They combined these observations with climate models, produced
by Huber’s team at Purdue, to mesh together a timeline of temperature
throughout the Eocene.
“The simulations took about four years of continuous computing
to achieve equilibrated climate states at various carbon dioxide levels,” Huber
said. “For the first time, the climate model is capable of capturing the main
trends in tropical sea surface temperatures and temperature gradients across a
range of climate encompassing nearly 20 million years. The only problem is that
the simulations required more carbon dioxide changes than observed, which
demonstrates that this model is not sensitive enough to carbon dioxide.”
Historically, researchers have had trouble reproducing
temperature gradients between the tropics and the poles throughout the Eocene.
These new climate models are capable of overcoming most of the issues faced by
past models.
原始論文：Margot J.
Cramwinckel, Matthew Huber, Ilja J. Kocken, Claudia Agnini, Peter K. Bijl,
Steven M. Bohaty, Joost Frieling, Aaron Goldner, Frederik J. Hilgen, Elizabeth
L. Kip, Francien Peterse, Robin van der Ploeg, Ursula Röhl, Stefan Schouten,
Appy Sluijs. Synchronous tropical and polar temperature evolution in
the Eocene. Nature, 2018; DOI: 10.1038/s41586-018-0272-2
引用自：Purdue University. "
Reconstruction of past climate
provides clues about future climate change."
