在人類之前型塑氣候的是火山

原文網址：https://www.unibe.ch/news/media_news/media_relations_e/media_releases/2019/medienmitteilungen_2019/volcanoes_shaped_the_climate_before_humankind/index_eng.html

在人類之前形塑氣候的是火山

19世紀早期，地球發生了五次大型火山爆發。根據伯恩大學主持的研究，這些事件造成季風地區變得乾冷，並讓阿爾卑斯山的冰河擴張。研究也顯示工業革命之前的氣候並不穩定，如果我們以這段低溫時期作為現今全球暖化的起點，那麼氣候的暖化程度已經超越我們目前討論結果中的估計值。

1808至1835年，熱帶地區在短時間之內就發生了五次大型火山噴發，對全球氣候造成了深遠影響。圖片來源：維基百科―1883 eruption of Krakatoa

1808至1835年，熱帶地區的火山就跟發瘋一樣――短時間之內，不只印尼的坦博拉火山劇烈噴發，還有四座火山也發生了大型噴發事件。一連串不尋常的火山爆發造成非洲陷入長期乾旱，也讓阿爾卑斯山的冰河在小冰期出現最後一次的擴張。Stefan Brönnimann是瑞士伯恩大學的氣候學教授，也是該校厄施格爾氣候變遷研究中心的成員之一。由他領導的國際研究團隊在發表於期刊《自然―地球科學》(Nature Geosciences)的文章中，發現了這一連串火山爆發對海洋以及後續大氣循環的影響。Brönnimann說：「頻繁的火山爆發確實改變了全球氣候系統的運作模式。」

印非少雨，歐洲多雨

研究人員進行這項研究時，分析了考量大氣環流在內的最新氣候重建結果，並且比較實際觀測到的數據。最後他們輔以模擬結果，確認在19世紀早期的氣候變遷中海洋扮演了什麼腳色，並且證明一連串火山爆發對海洋造成的影響長達數十年。他們的結論是火山爆發造成歐洲和印度的季風系統持續減弱；而大西洋至歐洲上空的大氣環流也產生了變化，造成歐洲中部的低壓系統增強。大氣環流變化加上溫度變低使得雨量變多，形成1820年代至1850年代的畫作以及十分古老的相片中呈現出來的景象：阿爾卑斯山的冰河範圍最後一次往外擴張。但到了19世紀晚期，全球氣溫再次上升，最終全球暖化的第一階段終結了小冰期，並於1940年代達到顛峰。這次全球暖化大部分得歸因於人類。

定義「工業革命前的氣候」時的重要性

這項伯恩大學的新研究不只解釋了19世紀早期的氣候為何會是如此，也跟現今的氣候有所關連。「有鑑於19世紀早期的氣候具有如此劇烈的變化，造成定義『工業革命前的氣候』變得相當困難。」主要作者Stefan Brönnimann加以解釋，「而我們在制定所有氣候政策時，都會提到這個概念。」結果對於政策制定者在決定氣候政策的目標時相當重要。目前是希望把全球的升溫幅度限制在最多1.5到2℃，但如果選用不同的參考時期，氣候暖化的程度可能已經比氣候會議估計的高出許多。原因為今日的氣候通常是跟1850至1900年的氣候相比，來定量現在的暖化程度。以此為前提，全球的平均氣溫已經增加了1℃。Stefan Brönnimann指出：「1850至1900年確實是個好選項。但如果我們跟19世紀上半葉，這段因為頻繁火山爆發使得氣溫大幅降低的時期相比，目前全球溫度已經提高了將近1.2℃。」

Volcanoes shaped the climate before humankind

Five large volcanic eruptions occurred in the early 19th century. They caused cooling and – as a study led by the University of Bern shows – to drying in the monsoon regions and glaciers growing in the Alps. The study shows that the pre-industrial climate was not constant: if one takes this cold period as the starting point for current global warming, the climate has already warmed up more than assumed in the current discussions.

The volcanoes in the tropics went crazy between 1808 and 1835: Not only did Tambora erupt in Indonesia during this short period of time but there were also four other large eruptions. This unusual series of volcanic eruptions caused long-lasting droughts in Africa and contributed to the last advance of Alpine glaciers during the Little Ice Age. "Frequent volcanic eruptions caused an actual gear shift in the global climate system," says Stefan Brönnimann, head of the international research team that discovered the effects of the series of eruptions on the oceans and thus on atmospheric circulation. Brönnimann is Professor of Climatology at the University of Bern and a member of the Oeschger Centre for Climate Research. Their research has been published in the "Nature Geosciences" journal.

Less rain in Africa and India, more rain and snow in Europe

For their investigations, the researchers analyzed new climate reconstructions that include atmospheric circulation and compared the results to observation-based data. Model simulations finally helped to pin down the role of the oceans in climate change in the early 19th century and showed that they could not recover from the effects of the sequence of eruptions for several decades. The consequences: the persistent weakening of the African and Indian monsoon systems and a shift of atmospheric circulation over the Atlantic-European sector. This led to an increase in low-pressure systems crossing Central Europe. The last glacier advance in the Alps from the 1820s to the 1850s, depicted in paintings and even old photographs, is a consequence of increased precipitation due to the altered circulation in combination with low temperatures. However, global temperature increased again from the late 19th century onward. The Little Ice Age was eventually superseded by a first phase of global warming, culminating in the 1940s and with a significant manmade contribution.

Important for the definition of "pre-industrial climate"

The new Bern study not only explains the global early 19th century climate, but it is also relevant for the present. "Given the large climatic changes seen in the early 19th century, it is difficult to define a pre-industrial climate," explains lead author Stefan Brönnimann, "a notion to which all our climate targets refer." And this has consequences for the climate targets set by policymakers, who want to limit global temperature increases to between 1.5 and 2 degrees Celsius at the most. Depending on the reference period, the climate has already warmed up much more significantly than assumed in climate discussions. The reason: Today's climate is usually compared with a 1850-1900 reference period to quantify current warming. Seen in this light, the average global temperature has increased by 1 degree. "1850 to 1900 is certainly a good choice but compared to the first half of the 19th century, when it was significantly cooler due to frequent volcanic eruptions, the temperature increase is already around 1.2 degrees," Stefan Brönnimann points out.

原始論文：Stefan Brönnimann, Jörg Franke, Samuel U. Nussbaumer, Heinz J. Zumbühl, Daniel Steiner, Mathias Trachsel, Gabriele C. Hegerl, Andrew Schurer, Matthias Worni, Abdul Malik, Julian Flückiger, Christoph C. Raible. Last phase of the Little Ice Age forced by volcanic eruptions. Nature Geoscience, 2019; DOI: 10.1038/s41561-019-0402-y

引用自：University of Bern. "Volcanoes shaped the climate before humankind."