原始網址：https://www.sciencemag.org/news/2016/08/atomic-bombs-and-oil-addiction-herald-earth-s-new-epoch-anthropocene
Atomic bombs and oil addiction herald Earth’s new epoch: The Anthropocene
原子彈及油癮宣告地球進入了嶄新的紀元：人類世
By Paul Voosen, Aug. 24, 2016 , 9:00 AM
二次世界大戰結束之後不久，當原子彈從天而降及我們對煤炭和石油的需求轉變成永不饜足的癮頭之時，也意味著地球進入了「人類世」(Anthropocene)。這個嶄新的地質時代標示出人類對環境的影響開始在全世界的沉積物中留下了痕跡。這是由許多研究學者組成的人類世工作小組(Anthropocene Working Group)發表的主要結論。他們在過去7年來潛心研究「人類世」這條已經融入大眾文化的名詞，是否應該申請成為正式的地質年代。
在結算完甫於本月舉行的投票結果之後，工作小組決議要以1940年代晚期至1950年代初期的戰後全盛時期(postwar boom)作為人類世的起始年代。他們將會請求統籌地質年代表的官方組織，即國際地層學委員會(International Commission on Stratigraphy，ICS)正式認可「人類統」為一個新的地層單位(「統」在地層學上相當於地質時間的「世」)，其位階等同於年代更早以前的全新統和更新統。任職於英國 Keyworth的英國地質調查局的地質學家 Colin Waters，同時也是此工作小組的幹事將會在8月29日於南非開普敦舉行的國際地質大會上發表他們的提案。
但工作小組還不會提交一套正式方案。要提出正式方案他們必須要從地球各地採集許多沉積物岩芯，並證明這些岩芯中的地球化學示蹤物(tracer)在同一時期有出現十分急遽的轉變，並且有機會成為岩石紀錄的一部分而永久保存下來。它們之中能夠最清楚呈現轉折之處的岩芯爾後會被公認為「金色之釘」(golden spike)[1]，即為人類世起點的標記。這些岩芯的來源可能是湖床、海床、冰層，甚至是珊瑚或樹輪。小組召集人，英國萊斯特大學的地質學家Jan Zalasiewicz表示這些岩芯還必須捕捉到「劇烈加速度」(Great Acceleration)[2]的痕跡，也就是戰後人類開始大量燃燒化石燃料的時期。「我們已經準備好動身並不畏艱難來尋找可以讓我們提出正式方案的岩芯剖面了。」
由於人類世提案深受來自地層學家的質疑，因此這些剖面必須含有許多種類的訊號互相佐證。「國際地層學委員會中具有投票權的成員會嚴格審核這些證據。」 ICS的主委Stan Finney說。他也是美國加州州立大學長灘分校的地質學教授。
他和其他地層學家懷疑他們的準則是否能適用於形成年代不過數十年的泥巴和粉砂，這些物質可不像以往記錄了較古老地層邊界的堅硬岩層。對地層學家而言他們的研究是要從沉積岩中以一致的標準劃分出地質年代表，因此他們質問人類世在地層學有何價值可言。他們之中有些人也對推動這份提案的人士當中有來自其他領域的專家，像是氣候學家而感到冒犯，並視這份提案為帶有政治意味的議題。
若ICS否決人類世存在的提案，有些地層學家擔憂他們恐怕會因此而飽受惡評攻訐。「我感覺自己就像是巨大海嘯將至時首當其衝的一座燈塔。」 Finney說道。英國劍橋大學的地層學家 Phil Gibbard是在這項提案中投下反對票的工作小組成員，他也對後續反彈感到擔憂。「我們都為此坐立難安。」他如此表示。
這個由35名地質學家、氣候學家、考古學家及其他領域研究人員共同組成的工作小組考慮過數個不同的時間點。他們曾經表決將人類世起點定在較早的7000年前，此時人類開始砍伐大片森林以供畜牧及農耕，大氣二氧化碳濃度紀錄中可能因此而出現了一道峰值。另一個年代則是3000年前，這時人們發展出的冶鉛技術汙染了土壤。他們最近還考慮了當新世界的植物花粉開始出現在歐洲的西元1610年，以及工業革命展開序幕的1800年代早期。但最多票數還是流向了「劇烈加速度」。
美國夏洛茨維爾的維吉尼亞大學環境科學名譽教授 Bill Ruddiman坦言，他對工作小組決議將人類世的起始設定在近代的一個特定時間點感到失望。「將人類世依附在一個特定時間點以使其正名的做法完全是項錯誤，」他說。「尤其人類大幅改造地球表面的歷史中會有很長一部分被這項提議給徹底摒除在外。」許多考古學家也偏好選擇早期人類開始改變地球表面時的7000年前。然而，工作小組要尋找的是人類引發的全球性變化殘留在岩石紀錄中的標記，而非第一道顯示人類改變了局部地貌的痕跡。
在今年稍早刊登於期刊《科學》(Science)的研究中，工作小組點出了他們認為最符合資格的指標。優先度第一的對象是從1950年代開始大量使用的物質，像是塑膠和鋁元素。在西元1951年的土壤中首次可見，由大氣核武試驗產生的鈽則會在衰變至鈾然後是鉛的過程中，接下來的10萬年間都會殘留於全世界的沉積物。但從近期對世界各地71座湖床進行的研究中指出，運用潛力最高的指標或許是1950年代高溫燃燒煤炭和石油產生的粉煤灰殘渣(fly ash residue)高峰值。「這是一種可以永遠存在的信號，」 Waters說。「這些粒子不會遭到分解。」他補充說煤灰跟人類驅使的二氧化碳增加有著直接關係，而此現象正好是人類世這個概念最初會大放光明的原因[3]。
直到今年之前，工作小組並沒有尋找金色之釘的意圖；取而代之的是，他們傾向僅以單一起始時間點來定義人類世。地層學家在界定前寒武紀(Precambrian，超過5.4億年前)的時間單位時才會使用這種方法，因為已經無法在形成於當時的岩石中找到任何可以用來劃分年代的清晰訊號。然而，包括 Finney在內的數名ICS成員清楚表明若要讓此提案有一絲機會能被贊同，工作小組勢必要拿出金色之釘。
雖然小組的提案或許無法滿足 Finney的標準，但 Waters還是期望ICS能好好考量他們選定的金色之釘擁有的優點。「正因為這根金色之釘很細且代表的時間很短，它很明確這一點本身就具有相當重大的意義。」他說。
Zalasiewicz補充就算人類能以某種方法逆轉全球暖化並將半個地球都設成保護區，劇烈加速度造成的痕跡仍然會留存下來。而如果人類維持現今的步調，未來地層學家可能要將人類世提升到地質年代表中更高的位階，他說。「(人類世)作為一個地質世說不定還會被認為太過短暫。」
Just
after World War II, when the atomic bombs fell and our thirst for
coal and oil became a full-blown addiction, Earth entered
the Anthropocene, a new geologic time when humanity’s
environmental reach left a mark in sediments worldwide. That’s the
majority conclusion of the Anthropocene Working Group, a
collection of researchers that has spent the past 7 years quietly
studying whether the term, already popular, should be submitted as a
formal span of geologic time.
After
tallying votes this month, the group has decided to propose the
postwar boom of the late 1940s and early 1950s as the Anthropocene’s
start date. The group will ask the International Commission on
Stratigraphy (ICS), the bureaucracy that governs geologic time, to
recognize the Anthropocene as a series, the stratigraphic equivalent
of an epoch, on par with the Holocene and Pleistocene that preceded
it. Colin Waters, the group’s secretary and a geologist
at the British Geological Survey in Keyworth, will reveal the group’s
recommendations on 29 August at the International Geological Congress
in Cape Town, South Africa.
The
group won’t submit a formal proposal yet. To do so, it must gather
multiple cores of sediment from around the planet and show that they
contain a sharp transition in geochemical tracers that is likely to
persist as a permanent part of the rock record; the core with the
best example of the transition would then serve as a “golden
spike,” marking the Anthropocene’s start. These cores could
come from lakebeds, ocean floors, ice sheets—or even corals or tree
rings. But they must capture the “Great Acceleration”:
the postwar
period when fossil fuel combustion took off, says Jan
Zalasiewicz, a geologist at the University of Leicester in the
United Kingdom who convened the group. “We’ll go and get our
hands dirty, beginning to look for sections that we can formally
propose.”
Those
sections will have to be rich with multiple signatures, as the
Anthropocene proposal faces deep skepticism from stratigraphers. “The
voting members of the International Commission on Stratigraphy look
at these things critically,” says Stan Finney, chair of ICS and a
geologist at California State University, Long Beach.
He
and other stratigraphers doubt
that their standards can be properly applied to decades-old
mud and silt rather than the solid rock that records older
stratigraphic boundaries. They question
the value of the Anthropocene for their science, which seeks
to draw coherent chronologies out of sedimentary rocks.
Some also resent the role that scientists from other disciplines such
as climate science have played in driving the proposal and see it as
a political statement.
Should
ICS decide against the Anthropocene, some stratigraphers fear, they
could be swamped with bad press. “I feel like a lighthouse with a
huge tsunami wave coming at it,” Finney says. Phil Gibbard, a
stratigrapher at the University of Cambridge in the United Kingdom
and a working group member who voted against the proposal, also
worries about a backlash. “We’re nervous,” he says.
The
working group, a mix of 35 geologists, climate scientists,
archaeologists, and others, considered multiple dates. There were
votes for an early start to the Anthropocene, 7000 years ago, when
humanity began converting forests en masse to pastures and cropland,
perhaps causing carbon dioxide (CO2) to spike, and also for 3000
years ago, when lead smelting tainted the ground. More recently, they
considered 1610, when pollen from the New World appeared in
Europe, and the early 1800s, the start of the Industrial Revolution.
But the most votes went to the Great Acceleration.
The
group’s decision to go for a single, recent start date for the
Anthropocene disappoints Bill Ruddiman, an emeritus professor of
environmental science at the University of Virginia in
Charlottesville. “It is a mistake to formalize the term by rigidly
affixing it to a single time,” he says, “especially one that
misses most of the history of the major transformation of Earth’s
surface.” Many archaeologists also favor the 7000-year-old
date, when
early humans began to alter the planet’s surface. But the
working group was looking for a signature of global, human-driven
change that would wind up in the rock record, not the first traces of
human influence on the local landscape.
In
a study published in Science earlier
this year, the
working group highlighted their most likely proxies.
Materials that rose to mass use in the 1950s, such as plastics and
elemental aluminum, are prime targets. Plutonium from atmospheric
nuclear testing, first visible in the soil in 1951, will linger in
sediments globally for the next 100,000 years as it decays into
uranium and then lead. But perhaps the most promising proxy comes
from recent work that has shown, across
71 lakebeds worldwide, a 1950s spike in fly ash residue from the
high-temperature combustion of coal and oil. “This is a permanent
signal,” Waters says. “These particles will not be degrading.”
He adds that the ash is directly tied to the human-driven increase in
CO2 that
sparked the notion of the Anthropocene in the first place.
Until
this year, the group had not sought a golden spike; instead, they
favored defining the Anthropocene simply by a starting date, a method
stratigraphers have only used for units of time within the
Precambrian, more than 540 million years ago, when clear dividing
signals have been impossible to find in the rock. But several ICS
members, including Finney, made
it clear a golden spike would be necessary for any chance of
approval.
The
group’s proposal may not satisfy him. But Waters hopes the ICS will
consider the chosen golden spike on its merits. “Just because it’s
thin and short duration, the fact that it’s very sizable is the
most important thing,” he says.
Zalasiewicz
adds that the marks of the Great Acceleration will endure, even if
somehow humanity reverses global warming and gives half the planet
over to conservation. And if humanity doesn’t change course, then
future stratigraphers might need to elevate the Anthropocene’s rank
in the geological hierarchy, he says. “An epoch would be
thinking too small.”
譯註：
[1]金色之釘(golden
spike)：正式名稱為「全球界線層型剖面和點位」(Global
Boundary Stratotype Section,
GSSP)。為一地層剖面中國際公認的參考點，其標示出一地層「階」(stage，為岩石地層單位的基本。對應於時間地層單位的「期」)的最底部。
[2]劇烈加速度(Great
Acceleration)：指20世紀後半人類的經濟和社會活動開始進展且變化得十分迅速的現象。連帶使人類和地球之間的關係有了深遠且長足變化。
[3]：2000年Crutzen和Stoermer提出我們現在所處的地質時期應稱作人類世，目的為強調從生態學和地質學角度來看，人類都已經在這個星球上占據了主導地位。他們的理由為從冰芯中看到溫室氣體濃度在過去兩世紀以來的上升趨勢，顯示人類自工業革命以來對大氣成分的影響已經到了不容忽視的地步。
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