原文網址：https://news.wisc.edu/ancient-farmers-spared-us-from-glaciers-but-profoundly-changed-earths-climate/

古代的農夫使我們免受冰河的侵擾，卻也深深改變了地球氣候

 Kelly April Tyrrell 

數千年前，古代的農夫開始拓墾土地以種植小麥、玉米、馬鈴薯和南瓜。此外，他們還灌溉土地來種植稻米，並開始蓄養牲畜。不知不覺之中，他們可能也澈底改變了地球氣候。

一篇發表在《科學報告》(Scientific Reports)上的研究提出了新證據，顯示古代的農業活動造成大氣中可以保溫的氣體――二氧化碳和甲烷的濃度升高。跟地球地質史上的其他時間完全不同，這些氣體的升高趨勢從那時開始便一直沒有停下來過。

研究也顯示如果沒有人類從工業革命開始持續帶來的影響，地球有可能會邁入另一個冰河期。

「如果沒有人類早期的農業活動，地球現在的氣候會寒冷許多。」主要作者Stephen Vavrus表示。他是美國威斯康辛大學麥迪遜分校環境研究所的氣候研究中心的資深科學家。「農耕的根源十分古老，而其產生的二氧化碳和甲烷多到可以影響環境。」

團隊利用複雜的氣候模型得到這項發現。他們比較我們現今所處的地質時代(全新世)和另一個類似的地質時代――80萬年前的MIS19。結果顯示時間較早的MIS19對應至全新世的1850年左右，全球平均溫度低了1.3℃。此現象在北極更加明顯，模型顯示當時北極的溫度比全新世的北極溫度低了5-6.1℃。

利用冰芯蘊含的資料重建氣候，模型也指出雖然二氧化碳和甲烷濃度在MIS19和全新世開始的時候都差不多，但是這兩種溫室氣體的濃度在MIS19期間整體呈現穩定下降的趨勢，而全新世的趨勢卻在距今5000年前往反方向變化，並在1850年時雙雙達到高峰。研究學者刻意把模擬停止的時間放在工業革命開始的時候，因為工業革命之後有極為大量的溫室氣體排放到大氣當中。

在地球45億年歷史的大部分時間，氣候大體上是取決於稱為米蘭科維奇循環(Milankovitch cycles)的自然現象。它描述了地球繞日軌道形狀會從趨向橢圓至趨向圓形之間周期性的變化；此外，地球自轉軸的擺動和傾斜程度也有各自的週期變化。

天文學家可以精準算出這些週期，而在地質和古生態紀錄中也可以觀察到它們的存在。這些週期會影響陽光分配到地球的方式，使地球處在較為寒冷的冰河期或者較為溫暖的間冰期。上次冰河期約在12000年前結束，自此之後地球便處在全新世間冰期中。而米蘭科維奇循環在MIS19的特徵跟全新世是相當類似的。

包括MIS19在內，科學家研究過的所有間冰期一開始的甲烷和二氧化碳濃度都比較高，然後在接下來的數千年逐漸降低，使得地球氣候越來越冷。最後冷到一個地步時便會進入下一個冰河期。

15年前，共同作者美國維吉尼亞大學的名譽古氣候學家William Ruddiman，研究數萬年前南極冰層中的甲烷和二氧化碳時觀察到某種不尋常的現象。

「我注意到甲烷的濃度大概在10000年前左右開始下降，然後在5000年前往反方向上升，同時二氧化碳也是在10000年前開始下降，然後在7000年前上升。」Ruddiman表示，「這讓我警覺到現今這段間冰期必定有什麼特異之處――我能想到的唯一解釋便是早期農業的發展，它讓溫室氣體進入大氣當中，從而導致了這一連串的現象。」

Ruddiman將其命名為早人類世假說(Early Anthropogenic Hypothesis)，最近的數篇研究也顯示其有可信之處。研究證實歐洲大約從6000年前開始大規模地除林；中國在7000年前出現了大型農業聚落；再加上5000年前稻田這種重要的甲烷來源開始散佈到東南亞各處。

Ruddiman和其他研究人員正持續進行研究來驗證這項理論。他和氣候模擬的專家Vavrus已經合作多年，在最新的研究中，他們利用社區氣候系統模型第4版(Community Climate System Model 4)來模擬如果人類沒有發展農業的話，全新世的氣候會是什麼樣子。此模型的解析度比團隊過往使用的氣候模型還要高，因此可以對冰河期背後的物理作用提供新的見解。

舉例來說，對MIS19的模擬結果顯示出冰河期的發展一開始是北極氣溫劇烈下降，接著海冰開始擴張，陸地也終年被冰雪覆蓋。模型顯示此過程最先發生的區域是包括巴芬島在內的加拿大群島，該地的夏季溫度下降了超過2.8℃。

Vavrus說：「這和地質證據呈現出來的一致。」

今日的北極正不斷暖化。但在我們誇獎古代的農夫延緩全球冷化之前，Vavrus和Ruddiman警告從基礎層面改變全球氣候循環對人類來說是完全未知的領域。

「有些人說我們(的研究)傳遞出錯誤的訊息，但科學最終會帶領我們得到真相。」Vavrus表示，「情況已經大為失控。過去2000年來發生的事物遠超出自然的範疇，我們跟所謂的自然已經大幅脫節。」

真相是我們不知道未來會發生什麼。而冰河自古以來便是地球上主要的淡水來源。

「氣候學界大體上都同意由於人類的關係，在可預見的長遠未來中不會發生下一次冰河期，這是因為就算我們停止把二氧化碳排放到大氣，它們仍會留存許久。」Ruddiman表示，「我們從現象得出來的事實是人類可能阻擋了地球氣候的重要周期變化，使我們深陷於越來越暖和的間冰期當中。」

Ancient farmers spared us from glaciers but profoundly changed Earth’s climate

Millenia ago, ancient farmers cleared land to plant wheat and maize, potatoes and squash. They flooded fields to grow rice. They began to raise livestock. And unknowingly, they may have been fundamentally altering the climate of the Earth.

A study published in the journal Scientific Reports provides new evidence that ancient farming practices led to a rise in the atmospheric emission of the heat-trapping gases carbon dioxide and methane – a rise that has continued since, unlike the trend at any other time in Earth’s geologic history.

It also shows that without this human influence, by the start of the Industrial Revolution, the planet would have likely been headed for another ice age.

“Had it not been for early agriculture, Earth’s climate would be significantly cooler today,” says lead author, Stephen Vavrus, a senior scientist in the University of Wisconsin–Madison Center for Climatic Research in the Nelson Institute for Environmental Studies. “The ancient roots of farming produced enough carbon dioxide and methane to influence the environment.”

The findings are based on a sophisticated climate model that compared our current geologic time period, called the Holocene, to a similar period 800,000 years ago. They show the earlier period, called MIS19, was already 2.3 degrees Fahrenheit (1.3 C) cooler globally than the equivalent time in the Holocene, around the year 1850. This effect would have been more pronounced in the Arctic, where the model shows temperatures were 9-to-11 degrees Fahrenheit colder.

Using climate reconstructions based on ice core data, the model also showed that while MIS19 and the Holocene began with similar carbon dioxide and methane concentrations, MIS19 saw an overall steady drop in both greenhouse gases while the Holocene reversed direction 5,000 years ago, hitting peak concentrations of both gases by 1850. The researchers deliberately cut the model off at the start of the Industrial Revolution, when sources of greenhouse gas emissions became much more numerous.

For most of Earth’s 4.5-billion-year history, its climate has largely been determined by a natural phenomenon known as Milankovitch cycles, periodic changes in the shape of Earth’s orbit around the sun – which fluctuates from more circular to more elliptical – and the way Earth wobbles and tilts on its axis.

Astronomers can calculate these cycles with precision and they can also be observed in the geological and paleoecological records. The cycles influence where sunlight is distributed on the planet, leading to cold glacial periods or ice ages as well as warmer interglacial periods. The last glacial period ended roughly 12,000 years ago and Earth has since been in the Holocene, an interglacial period. The Holocene and MIS19 share similar Milankovitch cycle characteristics.

All other interglacial periods scientists have studied, including MIS19, begin with higher levels of carbon dioxide and methane, which gradually decline over thousands of years, leading to cooler conditions on Earth. Ultimately, conditions cool to a point where glaciation begins.

Fifteen years ago, study co-author William Ruddiman, emeritus paleoclimatologist at the University of Virginia, was studying methane and carbon dioxide trapped in Antarctic ice going back tens of thousands of years when he observed something unusual.

“I noticed that methane concentrations started decreasing about 10,000 years ago and then reversed direction 5,000 years ago and I also noted that carbon dioxide also started decreasing around 10,000 years ago and then reversed direction about 7,000 years ago,” says Ruddiman. “It alerted me that there was something strange about this interglaciation … the only explanation I could come up with is early agriculture, which put greenhouse gases into the atmosphere and that was the start of it all.”

Ruddiman named this the Early Anthropogenic Hypothesis and a number of studies have recently emerged suggesting its plausibility. They document widespread deforestation in Europe beginning around 6,000 years ago, the emergence of large farming settlements in China 7,000 years ago, plus the spread of rice paddies – robust sources of methane –  throughout northeast Asia by 5,000 years ago.

Ruddiman and others have also been working to test the hypothesis. He has collaborated with Vavrus, an expert in climate modeling, for many years and their newest study used the Community Climate System Model 4 to simulate what would have happened in the Holocene if not for human agriculture. It offers higher resolution than climate models the team has used previously and provides new insights into the physical processes underlying glaciation.

For instance, in a simulation of MIS19, glaciation began with strong cooling in the Arctic and subsequent expansion of sea ice and year-round snow cover. The model showed this beginning in an area known as the Canadian archipelago, which includes Baffin Island, where summer temperatures dropped by more than 5 degrees Fahrenheit.

“This is consistent with geologic evidence,” says Vavrus.

Today, the Arctic is warming. But before we laud ancient farmers for staving off a global chill, Vavrus and Ruddiman caution that this fundamental alteration to our global climate cycle is uncharted territory.

“People say (our work) sends the wrong message, but science takes you where it takes you,” says Vavrus. “Things are so far out of whack now, the last 2,000 years have been so outside the natural bounds, we are so far beyond what is natural.”

The reality is, we don’t know what happens next. And glaciers have long served as Earth’s predominant source of freshwater.

“There is pretty good agreement in the community of climate scientists that we have stopped the next glaciation for the long, foreseeable future, because even if we stopped putting carbon dioxide into the atmosphere, what we have now would linger,” says Ruddiman. “The phenomenal fact is, we have maybe stopped the major cycle of Earth’s climate and we are stuck in a warmer and warmer and warmer interglacial.”

原始論文：Stephen J. Vavrus, Feng He, John E. Kutzbach, William F. Ruddiman, Polychronis C. Tzedakis. Glacial Inception in Marine Isotope Stage 19: An Orbital Analog for a Natural Holocene Climate. Scientific Reports, 2018; 8 (1) DOI: 10.1038/s41598-018-28419-5

引用自：University of Wisconsin-Madison. "Ancient farmers spared us from glaciers but profoundly changed Earth's climate."