碳的「外洩」或許使地球11000年來處於溫暖的狀態，因而助長了人類文明誕生
Liz Fuller-Wright
從數十年至數千年的時間尺度來看，海洋在儲存大氣二氧化碳這方面是最重要的地方。一組國際研究團隊發表的結果指出，海洋封存溫室氣體的作用會因為南大洋的活動而有所減弱，因此南大洋活動增加可以解釋過去11000年來，地球為何能神奇地一直保持在溫暖狀態。
普林斯頓大學Sigman實驗室的研究人員從化石中取出微量的氮，以建立南大洋的活動在全新世期間(大約始於距今11000年前)如何演變的模型。他們研究的化石包括圖片中的三種：左起為泡抱球蟲(Globigerina bulloides)，為一種浮游有孔蟲、一種圓心矽藻、Desmophyllum dianthus，為一種深海珊瑚。圖片來源(從左至右)：馬克斯普朗克化學研究所的Ralf Schiebel、馬克斯普朗克化學研究所的Anja Studer、美國地質調查局的Dann Blackwood。
普林斯頓大學的地質和地球物理學教授Daniel Sigman表示，這段期間氣候維持在溫暖狀態是由地球的二氧化碳濃度逐步上升造成，這讓科學家對於二氧化碳上升的背後原因相當感興趣。
科學家曾提出許多假說來解釋二氧化碳濃度為何上升，但根本成因仍然沒有解答。最近一個由普林斯頓大學和馬克斯普朗克化學研究所的科學家領導的國際合作計畫，將原因指向南大洋的湧升流增加。他們的研究成果發表於本期的《自然―地質科學》(Nature Geoscience)期刊。
「我們認為也許我們找到了解答。」Sigman表示，「南大洋的環流活動增加促使二氧化碳漏到大氣之中，進而造成地球暖化。」
「Danny為此存在已久的問題帶入一個全新的觀點。」未參與此研究的哥倫比亞大學地質學教授Wallace
Broecker表示，「在二氧化碳於大氣和海洋的交換過程中，南大洋的作用就像是閥門一樣。多虧Danny，我們現在知道此閥門在這段時期實際上是有在運作的，而非過往認為的處在停止狀態。」
他們關於海洋變化的這項發現在預測全球暖化對海洋環流造成的影響，以及燃燒化石燃料會讓大氣的二氧化碳濃度上升多少時也有所啟發。
多年來研究人員知道浮游植物成長然後死亡沉入海底之後會將二氧化碳帶到海洋深處，這種作用經常被稱作「生物幫浦」(biological
pump)。「生物幫浦大部分是由低緯度的海洋驅動，但越靠近極區效能就會越低。這是因為此處的深層海水會快速上湧至表層，而把二氧化碳排放回大氣。」Sigman表示。其中最大的阻力便是環繞南極的南大洋。Sigman說：「我們常把南大洋視為生物幫浦上面的破洞。」
Sigman和他的同事發現南大洋的湧升流增加或許可以解釋全新世(Holocene)的穩定氣候。全新世係指工業革命之前長達10000多年的時光(精確來說為距今11700年前)。
大多數科學家同意全新世的溫暖氣候對於人類文明的發展來說相當重要。過去數百萬年中發生的冰河期循環中偶爾會穿插幾段氣候較為溫暖的時期，而全新世即為這類「間冰期」的其中之一。冰河消融為人類展開了更為遼闊的新天地，同時大氣二氧化碳的濃度提高也讓農業的生產量增加，使得人類可以減少狩獵採集活動而建造永久居所。
研究人員表示全新世和其他間冰期在某些關鍵層面上有所差異。比方說它的氣候異常穩定，並沒有展現出其他間冰期以冷化為主的典型變化趨勢。其次，這段期間大氣中的二氧化碳濃度從早全新世的260 ppm(百萬分之一)上升至晚全新世的280 ppm，總共上升了大約20 ppm；然而其他間冰期的二氧化碳濃度通常是維持穩定或者持續下滑。
作為比較，從工業革命開始至今，大氣中的二氧化碳濃度因為燃燒化石燃料的緣故，已經從280 ppm升高至超過400 ppm。
「就此看來，二氧化碳在全新世中觀察到的上升20 ppm似乎不怎麼多。」Sigman表示，「然而，科學家認為這種幅度微小卻相當顯著的上升，起到了關鍵作用防止全新世往逐漸冷化的方向前進，這或許有利於人類發展出更加複雜的文明。」
為了探討全新世二氧化碳濃度上升的可能原因，研究人員研究了從南大洋數個不同區域採集到的三種化石：分別為矽藻和有孔蟲這兩種可以在海洋發現的有殼微生物，以及深海珊瑚。
這些化石由礦物形成的殼體中會包覆含量極少的有機物。科學家從有機物的氮同位素組成可以重建出過去1萬年來，南大洋表層海水的營養鹽濃度如何演變。
研究第一作者Anja Studer說：「我們用來分析這些化石的方法不僅獨一無二，還提供了一種新方式來研究海洋環境在過去是如何變化。」她在Sigman的實驗室擔任研究生時進行了這項研究。
量測這些化石含有的氮同位素指出全新世期間，南大洋有更多富含營養鹽和二氧化碳的海水從深海湧升至表層。雖然湧升流活動提高的成因尚未明瞭，但他們認為最有可能的作用是「咆嘯40度」，也就是環繞南極往東方吹送的盛行風帶出現了變化。
由於南大洋的湧升流活動增加，使得全新世期間的生物幫浦較為乏力，造成更多二氧化碳從深海流失到大氣當中。這或許能解釋大氣二氧化碳的濃度在全新世為何提高了20 ppm。
「此作用使得某些儲藏在深部的碳重新返回大氣當中。」Sigman表示，「而我們基本上正在把生物幫浦捅成千瘡百孔。」
全新世大氣二氧化碳濃度上升造成的影響，抵銷了過往多數間冰期主要呈現的逐漸冷化趨勢。因此，這項新的研究結果提出全新世氣候「特別穩定」的原因或許得歸功於海洋。
今日，相同的作用仍在運行當中：海洋正在吸收二氧化碳以減緩燃燒化石燃料造成的大氣二氧化碳濃度上升，而南大洋的湧升流仍在讓某些被吸收的二氧化碳重新流回大氣。
Sigman說：「如果我們從全新世得到的發現可以用來預測南大洋的湧升流在未來會如何變化，那麼我們就能更加準確地預測大氣二氧化碳濃度的變化，進而增進對全球氣候的預測。」
Carbon ‘leak’ may have warmed the planet for 11,000 years, encouraging
human civilization
The oceans are the planet’s most important depository for atmospheric
carbon dioxide on time scales of decades to millenia. But the process of
locking away greenhouse gas is weakened by activity of the Southern Ocean, so
an increase in its activity could explain the mysterious warmth of the past
11,000 years, an international team of researchers reports.
The warmth of
that period was stabilized by a gradual rise in global carbon dioxide levels,
so understanding the reason for that rise is of great interest,
said Daniel Sigman, the Dusenbury Professor of Geological
and Geophysical Sciences at Princeton.
Scientists
have proposed various hypotheses for that carbon dioxide increase, but its
ultimate cause has remained unknown. Now, an international collaboration led by
scientists from Princeton and the Max Planck Institute for Chemistry point to
an increase in Southern Ocean upwelling. Their research appears in
the current issue of the journal Nature
Geoscience.
“We think we
may have found the answer,” said Sigman. “Increased circulation in the Southern
Ocean allowed carbon dioxide to leak into the atmosphere, working to warm the
planet.”
“Danny brings
an entirely new slant to a long-standing question,” said Wallace Broecker,
the Newberry Professor of Geology at Columbia University, who was not
involved in this research. “As the Southern Ocean acts as a throttle in the
transfer of CO2 between ocean and atmosphere, we owe thanks to
Danny, for we now know the throttle was at work during what has been viewed as
a period of quiescence.”
Their findings
about ocean changes could also have implications for predicting how global
warming will affect ocean circulation and how much atmospheric carbon dioxide
will rise due to fossil fuel burning.
For years,
researchers have known that growth and sinking of phytoplankton pumps carbon
dioxide deep into the ocean, a process often referred to as the “biological
pump.” The biological pump is driven mostly by the low latitude ocean but is
undone closer to the poles, where carbon dioxide is vented back to the
atmosphere by the rapid exposure of deep waters to the surface, Sigman said.
The worst offender is the Southern Ocean, which surrounds Antarctica. “We often
refer to the Southern Ocean as a leak in the biological pump,” Sigman said.
Sigman and his
colleagues have found that an increase in the Southern Ocean’s upwelling could
be responsible for stabilizing the climate of the Holocene, the period reaching
more than 10,000 years before the Industrial Revolution.
Most
scientists agree that the Holocene’s warmth was critical to the development of
human civilization. The Holocene was an “interglacial period,” one of the rare
intervals of warm climate that have occurred over the ice age cycles of the
last million years. The retreat of the glaciers opened a more expansive
landscape for humans, and the higher concentrations of carbon dioxide in the
atmosphere made for more productive agriculture, which allowed people to reduce
their hunter-gathering activities and build permanent settlements.
The Holocene
differed from other interglacial periods in several key ways, say the
researchers. For one, its climate was unusually stable, without the major
cooling trend that is typical of the other interglacials. Secondly, the
concentration of carbon dioxide in the atmosphere rose about 20 parts per
million (ppm), from 260 ppm in the early Holocene to 280 ppm in the late
Holocene, whereas carbon dioxide was typically stable or declined over other
interglacial periods.
For
comparison, since the beginning of industrialization until now, the carbon
dioxide concentration in the atmosphere has increased from 280 to more than 400
ppm as a consequence of burning fossil fuels.
“In this
context, the 20 ppm increase observed during the Holocene may seem small,” said
Sigman. “However, scientists think that this small but significant rise played
a key role in preventing progressive cooling over the Holocene, which may have
facilitated the development of complex human civilizations.”
In order to
study the potential causes of the Holocene carbon dioxide rise, the researchers
investigated three types of fossils from several different areas of the
Southern Ocean: diatoms and foraminifers, both shelled microorganisms found in
the oceans, and deep-sea corals.
From the
nitrogen isotope ratios of the trace organic matter trapped in the mineral
walls of these fossils, the scientists were able to reconstruct the evolution
of nutrient concentrations in Southern Ocean surface waters over the past
10,000 years.
“The method we
used to analyze the fossils is unique and provides a new way to study past
changes in ocean conditions,” says Anja Studer, first author of the study, who
performed the research while a graduate student working with Sigman’s lab.
The
fossil-bound nitrogen isotope measurements indicate that during the Holocene,
increasing amounts of water, rich in nutrients and carbon dioxide, welled up
from the deep ocean to the surface of the Southern Ocean. While the cause for
the increased upwelling is not yet clear, the most likely process appears to be
a change in the “Roaring 40s,” a belt of eastward-blowing winds that encircle
Antarctica.
Because of the
enhanced Southern Ocean upwelling, the biological pump weakened over the
Holocene, allowing more carbon dioxide to leak from the deep ocean into the
atmosphere and thus possibly explaining the 20 ppm rise in atmospheric carbon
dioxide.
“This process
is allowing some of that deeply stored carbon dioxide to invade back to the
atmosphere,” said Sigman. “We’re essentially punching holes in the membrane of
the biological pump.”
The increase
in atmospheric carbon dioxide levels over the Holocene worked to counter the
tendency for gradual cooling that dominated most previous interglacials. Thus,
the new results suggest that the ocean may have been responsible for the
“special stability” of the Holocene climate.
The same processes
are at work today: The absorption of carbon by the ocean is slowing the rise in
atmospheric carbon dioxide produced by fossil fuel burning, and the upwelling
of the Southern Ocean is still allowing some of that carbon dioxide to vent
back into the atmosphere.
“If the
findings from the Holocene can be used to predict how Southern Ocean upwelling
will change in the future, it will improve our ability to forecast changes in
atmospheric carbon dioxide and thus in global climate,” said Sigman.
原始論文：Anja S.
Studer, Daniel M. Sigman, Alfredo Martínez-García, Lena M. Thöle, Elisabeth
Michel, Samuel L. Jaccard, Jörg A. Lippold, Alain Mazaud, Xingchen T. Wang,
Laura F. Robinson, Jess F. Adkins, Gerald H. Haug. Increased nutrient
supply to the Southern Ocean during the Holocene and its implications for the
pre-industrial atmospheric CO2 rise. Nature Geoscience, 2018;
DOI: 10.1038/s41561-018-0191-8
引用自：Princeton University. "Carbon 'leak' may
have warmed the planet for 11,000 years, encouraging human civilization”
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