原文網址：https://www.dri.edu/early-human-activities-impacted-earths-atmosphere-more-than-previously-known/
By Kelsey
Fitzgerald
沙漠研究所的Joe McConnell博士、Nathan Chellman博士以及英國南極調查局的Robert Mulvaney博士，數年前分析南極詹姆斯羅斯島的冰芯樣品時注意到某些不尋常的事物：黑碳的含量大約從西元1300年開始大幅增加，並且持續至今。
一般稱為煤灰的黑碳是種可以吸收光線的顆粒。它們是燃燒作用的產物，像是遭到焚毀的生質(例如森林大火)；而以近代來說，則來自於燃燒化石燃料。在與英國、奧地利、挪威、德國、澳洲、阿根廷和美國的科學家組成的國際團隊合作之下，McConnell、Chellman和Mulvaney試圖揭曉南極冰層捕捉到的黑碳含量突然增加，是由什麼樣的來源造成。
團隊的成果發表於本周的《自然》(Nature)。他們指出的來源令人出乎意料：紐西蘭古代毛利人慣用的火耕法，影響規模涵蓋了南半球大部分地方的大氣。在過去兩千年當中，該地區其他工業革命以前的碳排放源都無法匹敵。
「當時歷史上的人類經由清理土地的活動，使得大氣中的黑碳發生如此劇烈的變化。這種概念確實會讓人感到十分驚訝，」沙漠研究所的水文學教授Joe McConnell表示。他負責設計並領導這項研究。「通常我們認為如果回到數百年前，會看到一個原始、尚未工業化的世界。但是這項研究清楚指出至少從七百年前開始，人類就持續對南大洋與南極半島上方的環境造成了衝擊。」
追蹤黑碳的來源
為了找出黑碳的來源，研究團隊利用沙漠研究所獨有的連續冰芯分析系統，對採自詹姆斯羅斯島以及南極大陸的六根冰芯進行分析。而冰裡面有多少黑碳的分析方法則是McConnell在2007首先發展出來。
詹姆斯羅斯島的冰芯顯示出黑碳含量在西元1300年左右開始顯著增加，經過700年之後變成了原本的三倍，高峰則出現在16、17世紀。然而同一期間，南極大陸冰芯的黑碳含量卻保持得相對平穩。
維也納大學的Andreas
Stohl博士負責建立大氣模型來模擬黑碳在南半球的運輸與沉降過程，結果也支持了這項發現。
「我們的模型以及南極冰芯中的沉降模式都清楚指出，從西元1300年左右開始增加的黑碳，最有可能的排放源為巴塔哥尼亞、塔斯馬尼亞和紐西蘭，」Stohl表示。
一一參照這三個地方的古火紀錄之後，只留下一個符合的選項：紐西蘭。此處的木炭紀錄顯示西元1300年左右開始火災的次數大幅增加，之前估計毛利人也是在這個時間點到達紐西蘭並定居下來，接著焚燒紐西蘭大部分的林地。
這項結論令人驚訝，因為紐西蘭的面積相對來說並不大，而且煙霧必須要傳輸很長一段距離(將近4500英里)才能到達詹姆斯羅斯島的冰芯採樣位置。
「跟其他地方發生的自然火災比較起來，像是亞馬遜、南非或是澳洲，你可能不會預期毛利人在紐西蘭造成的火災具有重大影響，但是南大洋與南極半島的上空確實受到了很大的衝擊，」沙漠研究所的博士後研究員Chellman表示。「我們成功運用冰芯紀錄指出整個南大洋的大氣化學都受到影響，並找出原因是七百多年前毛利人到達並定居在紐西蘭，這些成果真的非常驚人。」
研究帶來的影響
這項研究的發現有幾個重要之處：首先，研究結果可能會深深影響我們對於地球大氣與氣候的理解。當代的氣候模型預測未來的時候需要關於過往氣候的精確資訊，尤其是可以吸收光線的黑碳，濃度與排放量和地球的輻射平衡之間有什麼樣的關聯。雖然過去經常推測工業革命以前人類造成的影響相較於背景值，也就是自然發生的火災，是可以忽略不計的，但這項研究卻提出了新的證據，顯示跟人類有關的火災所排出的黑碳，可能比之前猜想的還要早就對地球大氣與氣候造成了影響，規模也更加龐大。
其次，生質燃燒之後產生的落塵帶有豐富的微量營養素，像是鐵。在南大洋的多數地方，浮游植物的生長條件受限於養分，因此毛利人燃燒森林使得落塵增加，有可能造成這幾個世紀以來南半球許多地方的浮游植物大量生長。
第三，這項結果也更加確定了毛利人到達紐西蘭的時間――此處為地球適合居住的地方，人類最晚殖民的其中之一。之前根據放射性碳定年，毛利人到達的時間範圍為13到14世紀，但是透過可以定年更加精確的冰芯紀錄，早期紐西蘭毛利人開始大規模焚林的時間為1297年，誤差為30年。
「從這項研究以及我們團隊之前的成果，像是北極兩千年前就有來自古羅馬的鉛汙染，可以清楚看出在瞭解人類過去對環境造成的衝擊時，冰芯具有相當寶貴的價值，」McConnell表示。「在工業革命之前，即使是地球最偏僻的地方也未必是塊淨土。」
Early human activities
impacted Earth’s atmosphere more than previously known
Several years ago, while analyzing ice
core samples from Antarctica’s James Ross Island, scientists Joe McConnell,
Ph.D., and Nathan Chellman, Ph.D., from Desert Research Institute, and Robert
Mulvaney, Ph.D., from the British Antarctic Survey noticed something unusual: a
substantial increase in levels of black carbon that began around the year 1300
and continued to the modern day.
Black carbon, commonly referred to as soot, is a
light-absorbing particle that comes from combustion sources such as biomass
burning (e.g. forest fires) and, more recently, fossil fuel combustion. Working
in collaboration with an international team of scientists from the United
Kingdom, Austria, Norway, Germany, Australia, Argentina, and the U.S.,
McConnell, Chellman, and Mulvaney set out to uncover the origins of the
unexpected increase in black carbon captured in the Antarctic ice.
The team’s findings, which published this week in Nature, point to an unlikely source:
ancient Māori land-burning practices in New Zealand, conducted at a scale that
impacted the atmosphere across much of the Southern Hemisphere and dwarfed
other preindustrial emissions in the region during the past 2,000 years.
“The idea that humans at this time in history caused
such a significant change in atmospheric black carbon through their land
clearing activities is quite surprising,” said McConnell, research professor of
hydrology at DRI who designed and led the study. “We used to think that if you
went back a few hundred years you’d be looking at a pristine, pre-industrial
world, but it’s clear from this study that humans have been impacting the
environment over the Southern Ocean and the Antarctica Peninsula for at least
the last 700 years.”
Tracing the black
carbon to its source
To identify the source of the black carbon, the study
team analyzed an array of six ice cores collected from James Ross Island and
continental Antarctica using DRI’s unique continuous ice-core analytical
system. The method used to analyze black carbon in ice was first developed in
McConnell’s lab in 2007.
While the ice core from James Ross Island showed a
notable increase in black carbon beginning around the year 1300, with levels
tripling over the 700 years that followed and peaking during the 16th and 17th
centuries, black carbon levels at sites in continental Antarctica during the
same period of time stayed relatively stable.
Andreas Stohl, Ph.D., of the University of Vienna led
atmospheric model simulations of the transport and deposition of black carbon
around the Southern Hemisphere that supported the findings.
“From our models and the deposition pattern over
Antarctica seen in the ice, it is clear that Patagonia, Tasmania, and New
Zealand were the most likely points of origin of the increased black carbon
emissions starting about 1300,” said Stohl.
After consulting paleofire records from each of the
three regions, only one viable possibility remained: New Zealand, where charcoal
records showed a major increase in fire activity beginning about the year 1300.
This date also coincided with the estimated arrival, colonization, and
subsequent burning of much of New Zealand’s forested areas by the Māori people.
This was a surprising conclusion, given New Zealand’s
relatively small land area and the distance (nearly 4,500 miles), that smoke
would have travelled to reach the ice core site on James Ross Island.
“Compared to natural burning in places like the
Amazon, or Southern Africa, or Australia, you wouldn’t expect Māori burning in
New Zealand to have a big impact, but it does over the Southern Ocean and the
Antarctic Peninsula,” said Chellman, postdoctoral fellow at DRI. “Being able to
use ice core records to show impacts on atmospheric chemistry that reached
across the entire Southern Ocean, and being able to attribute that to the Māori
arrival and settlement of New Zealand 700 years ago was really amazing.”
Research impacts
The study findings are important for a number of
reasons. First, the results have important implications for our understanding
of Earth’s atmosphere and climate. Modern climate models rely on accurate
information about past climate to make projections for the future, especially
on emissions and concentrations of light-absorbing black carbon linked to
Earth’s radiative balance. Although it is often assumed that human impacts
during preindustrial times were negligible compared to background or natural
burning, this study provides new evidence that emissions from human-related
burning have impacted Earth’s atmosphere and possibly its climate far earlier,
and at scales far larger, than previously imagined.
Second, fallout from biomass burning is rich in
micronutrients such as iron. Phytoplankton growth in much of the Southern Ocean
is nutrient-limited so the increased fallout from Māori burning probably
resulted in centuries of enhanced phytoplankton growth in large areas of the
Southern Hemisphere.
Third, the results refine what is known about the
timing of the arrival of the Māori in New Zealand, one of the last habitable
places on earth to be colonized by humans. Māori arrival dates based on
radiocarbon dates vary from the 13th to 14th century, but the more precise
dating made possible by the ice core records pinpoints the start of large scale
burning by early Māori in New Zealand to 1297, with an uncertainty of 30 years.
“From this study and other previous work our team has
done such as on 2,000-year old lead pollution in the Arctic from ancient Rome,
it is clear that ice core records are very valuable for learning about past
human impacts on the environment,” McConnell said. “Even the most remote parts
of Earth were not necessarily pristine in preindustrial times.”
原始論文：Joseph
R. McConnell, Nathan J. Chellman, Robert Mulvaney, Sabine Eckhardt, Andreas
Stohl, Gill Plunkett, Sepp Kipfstuhl, Johannes Freitag, Elisabeth Isaksson,
Kelly E. Gleason, Sandra O. Brugger, David B. McWethy, Nerilie J. Abram,
Pengfei Liu and Alberto J. Aristarain. Hemispheric
black carbon increase after the 13th-century Māori arrival in New Zealand. Nature, 2021; DOI: 10.1038/s41586-021-03858-9
引用自：Desert
Research Institute. “Early Human Activities Impacted Earth’s Atmosphere More
Than Previously Known.”
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