研究表示季風的變化造成中國古文明的消長

原文網址：www.sciencedaily.com/releases/2017/02/170207092837.htm

研究表示季風的變化造成中國古文明的消長

強降雨在全新世早期和中期往北移動了400公里

一則新研究的作者表示，每年在約有10億人居住的東亞地區降下雨水的夏季季風於遠古時期曾發生劇烈改變。當時夏季季風往北移動多達400公里，造成中國北域的雨量多出一倍。過去10,000年來，季風的改變很可能對中國的早期人類文明如何演變有相當影響。

拉蒙特―多爾蒂地球觀測所和西安中國科學院的學者，研究了位於中國西北內蒙古的一座封閉湖――洱海的古水位變化。他們發現全新世(始於11,700年前)早期和中期，此湖泊是現今的六倍大且深度多了60公尺。許多人類文明也在這段時期有所演變。

「我認為這項研究的重要之處，在於強調出季風在空間上的波動可以讓中國北方發生重大變化。」論文的主要作者，拉蒙特―多爾蒂地球觀測所的研究生Yonaton Goldsmith表示。「當季風強度變強，它會往北移動並造成中國北部變得一片翠綠。當季風強度減弱，它則會停留在中國南部而北方則陷入乾旱。這麼大的波動必定會劇烈改變中國北部的生態系。」

這篇刊登於本周《美國國家科學院院報》(Proceedings of the National Academy of Sciences)的研究，也把季風的移動跟地球軌道的改變，以及氣候系統中的其他週期性變化連結在一起。研究有助於科學家瞭解季風如何受這些自然循環影響，以及今日發生的氣候變遷未來可能對季風造成的衝擊。

Goldsmith說目前還不清楚季風會如何對全球暖化做出回應。有種看法認為季風應該會變得更強，但區域研究卻顯示最近數十年來中國北部越趨乾燥。他說：「因此在我們將答案確定下來以前，此區域仍有許多研究尚待完成。」

洱海的位置接近於東亞季風所及區域的西北邊界，因此可以反映出季風往北移動時帶來的改變。研究人員探討洱海比現今大上許多時遺留下來的沉積物露頭，配合其他標記他們可以重建出湖面的變化歷程，以及數千年來雨量如何波動。

他們發現湖水的深度於123,000前年曾達到高值，另一次則在58,000年前左右，而11,000年至5000年前之間也發生過一次。這些事件跟季風界線往北移動最多達到400公里，而造成的降雨週期性增加一致。跟之前從中國北方與南方的洞穴沉積物中得到的測量結果相比，洱海的紀錄跟它們之間具有「高度相關性」。

研究人員發現在5500至5000年前，季風減弱使得中國北方整體雨量減少了約50%。他們猜測乾旱造成了此區的文明發生重大轉變。他們的敘述中寫道，兩個新石器時代早期的文明――中國北方的紅山文化以及中國中部的仰韶文化――都在5000年前滅絕。在中國中部，隨之出現的是政治以及社會階級更加分明且複雜的社會，像是龍山文化。而西藏高原東緣之前未受開發的區域也開始有人遷入。於此同時，以小河沿文化為代表，中國東北則經歷了人口大幅衰減。

作者寫道：「這些發現顯示氣候變遷會對人類社會造成重大影響，也強烈指出瞭解全球暖化對中國和全世界的降雨模式會造成何種影響確實有其必要。」

在其他文明的殞落中，雨量的強烈變化可能也扮演了重要腳色。由拉蒙特的科學家Brendan Buckley領導在數年前刊登的論文中，提出伴隨季風移動而發生的長期乾旱可能造成了柬埔寨吳哥窟的古暹粒文明於大約600年前滅絕。在古典馬雅文明的衰敗中，也認為乾旱具有推波助瀾的作用。雖然在此例中，另一則拉蒙特的研究認為是馬雅人興建城市和耕種而進行的除林行為讓自己陷入乾旱的處境。

這篇關於洱海的論文其他作者包括了拉蒙特―多爾蒂地球觀測所的Wallace S. Broecker、 Pratigya J. Polissar和Peter B. deMenocal；中國科學院地球環境研究所，黃土與第四紀國家重點實驗室的徐海、藍江湖、程鵬、周衛建和安芷生；以及以色列地質調查所的Naomi Porat。

Shifting monsoon altered early cultures in China, study says

Heavier rainfall moved 400 km north during early and middle Holocene

The annual summer monsoon that drops rain onto East Asia, an area with about a billion people, has shifted dramatically in the distant past, at times moving northward by as much as 400 kilometers and doubling rainfall in that northern reach. The monsoon's changes over the past 10,000 years likely altered the course of early human cultures in China, say the authors of a new study.

Researchers from the Lamont-Doherty Earth Observatory and the Chinese Academy of Sciences in Xi'an studied ancient water levels for Lake Dali, a closed-basin lake in Inner Mongolia in the northeast of China. They found that the lake was six times larger and water levels were 60 meters higher than present during the early and middle Holocene -- the period beginning about 11,700 years ago, and encompassing the development of human civilization.

"I think it is important to emphasize that these spatial fluctuations in the monsoon drive large changes in northern China," said Yonaton Goldsmith, a graduate student at Lamont-Doherty Earth Observatory and lead author of the paper. "When the monsoon is strong, it shifts northward and northern China becomes green. When the monsoon is weak, the monsoon stays in the south and northern China dries out. Such large fluctuations must have altered the ecosystems in northern China dramatically."

The study, appearing this week in the Proceedings of the National Academy of Sciences, also ties the shifting monsoon to changes in Earth's orbit and other periodic changes in the climate system. The study should help scientists understand how the monsoon is affected by those natural cycles, and how a changing climate today might influence the monsoon in the future.

Goldsmith said it's still unclear how the monsoon will react to global warming. One view is that the monsoon should grow stronger, but the area studied has been drying out over recent decades, he said, "so there is still a lot that needs to be done in that region before we can get definitive answers."

Dali Lake is located near the northwestern limit of the East Asian monsoon, and so would reflect the changes brought about when the monsoon shifted north. The researchers studied outcrops of sediments left behind when the lake was far larger, and used those and other markers to construct a timeline of lake levels, and the fluctuation of rainfall over millennia.

They found that the lake reached peak levels around 123,000 years ago, again around 58,000 years ago, and once more between 11,000 and 5,500 years ago. They tie the periodic increases in rainfall to the range of the monsoon shifting north by as much as 400 kilometers. The lake record is "highly correlated" with measurements taken earlier from cave deposits in both northern and southern China.

Between 5,500 and 5,000 years ago, the monsoon weakened and rainfall over northern China decreased by 50 percent, the researchers found. They speculate that this drying triggered a major cultural transition in the region. As they describe it, two early Neolithic societies, the Hongshan culture in North China and the Yangshao culture in central China, collapsed around 5,000 years ago. In central China, the following period saw the rise of more stratified and socially and politically complex societies, including the Longshan culture. Previously unoccupied areas on the eastern margin of the Tibetan plateau were populated. Meanwhile, northeast China experienced a sharp population decline, represented by the Xiaoheyan culture.

"These findings show that climate change can have dramatic effects on human societies and highlight the necessity to understand the effect of global warming on rainfall patterns in China and all over the world," the authors write.

Intense variations in rainfall may have played a role in the collapse of other civilizations. A study led by Lamont scientist Brendan Buckley, published several years ago, suggested that extended drought coupled with changes in the monsoon could have doomed Cambodia's ancient Khmer civilization at Angkor nearly 600 years ago. Drought is thought to have played a role in the decline of the Classic Maya civilization, too, though in that case, another Lamont study suggests that the Maya themselves contributed to the drought by clearing forests for cities and crops.

The Lake Dali paper's other authors are Wallace S. Broecker, Pratigya J. Polissar and Peter B. deMenocal of Lamont-Doherty; Hai Xu, Jianghu Lan, Peng Cheng, Weijian Zhou and Zhisheng An of the State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences; and Naomi Porat of the Geological Survey of Israel.

This work was supported by a Gary Comer Science and Education Foundation grant to Yonaton Goldsmith and Pratigya J. Polissar; Columbia's Center for Climate and Life; the National Basic Research Program of China Grant 2013CB955900; the External Cooperation Program of Bureau of International Cooperation, Chinese Academy of Sciences Grant 132B61KYSB20130003; and Lamont-Doherty Earth Observatory Contribution no. 8084.

原始論文：Yonaton Goldsmith, Wallace S. Broecker, Hai Xu, Pratigya J. Polissar, Peter B. deMenocal, Naomi Porat, Jianghu Lan, Peng Cheng, Weijian Zhou, Zhisheng An. Northward extent of East Asian monsoon covaries with intensity on orbital and millennial timescales. Proceedings of the National Academy of Sciences, 2017; 201616708 DOI: 10.1073/pnas.1616708114

引用自：Lamont-Doherty Earth Observatory, Columbia University. "Shifting monsoon altered early cultures in China, study says: Heavier rainfall moved 400 km north during early and middle Holocene." ScienceDaily. ScienceDaily, 7 February 2017.