中世紀的僧侶在觀察夜空的行為中，不經意地把史上幾件最劇烈的火山爆發記錄了下來。由日內瓦大學領導的國際研究團隊利用12到13世紀歐洲與中東編年史裡的條目，結合冰芯與樹輪的數據，準確得出世人見過的火山爆發中幾個最大的發生時間。他們發表在期刊《自然》(Nature) 的研究結果對於地球歷史上火山活動最頻繁的時期之一得出了新的資訊。有些人認為這段期間的火山爆發促成「小冰期」發生——一段長期低溫而可以見到歐洲冰河擴展的時期。
這張14世紀晚期或是15世紀早期的畫作描繪了兩個人正在觀察月食。標題為《La lune avant est eclipsee》，中文為《被掩蝕的月亮》。圖片來源：gallica.bnf.fr / BnF
研究人員花了將近五年檢視數百本歐洲與中東各地的年鑑與編年史，尋找提及月全食與月亮顏色的資料。當月球進入地球的影子便會發生月全食，通常這時候的月球仍然可見，就像顆紅色的珠子，原因是月球仍會籠罩在被地球大氣折射的陽光之下。但是在非常大型的火山爆發過後，平流層(大氣層的中段，起點大概是商用飛機的飛行高度)裡可能會有極為大量的塵埃，使得月食期間的月球幾乎消失。
中世紀的編年史記載並描述了各式各樣的歷史事件，像是國王和教宗的事蹟、重要的戰爭、天災、飢荒……等。作為這類災厄的潛在預兆，天文現象也成為了值得記錄的對象。當時的僧侶因為留心《啟示錄》提到末日的景象中月亮會像血一樣紅，所以特別仔細地記錄月亮的顏色。在西元1100到1300年之間歐洲發生了64次月全食，這些編年史如實記載了其中51次。此外，他們也提到在這當中的5次月亮變得格外黯淡。
日本文官的貢獻
這項研究的主要作者是日內瓦大學環境科學研究所的資深研究員Sébastien
Guillet。當問到為什麼會把僧侶記錄下來的月食亮度與顏色，跟火山造成的天色昏暗連結起來，他說：「我在聽平克•佛洛伊德的『月球陰暗面』專輯時，領悟到極為昏暗的月食都是發生在大型火山爆發後大約一年之內。既然我們能確定月食的日期，就有機會利用這些目擊事件來刪減出哪些時段勢必發生了火山爆發。」
研究人員發現日本的文官也同樣地關注月全食。享負盛名的藤原定家寫下在西元1229年12月2日觀察到一次昏暗程度前所未見的月全食：「人們過往從來沒有看過跟這次類似的，月面所在之處變得空無一物，就像是在月食期間消失了一般……這真的讓人感到十分恐懼。」大型火山爆發噴到平流層裡的灰燼不只造成了月亮消失，也會減少到達地表的陽光使得夏季溫度降低，進而對農作物帶來毀滅性的影響。
交叉比對文字與數據
「我們從前人研究知道強烈的熱帶火山爆發可以讓全球溫度下降大約1°C並持續好幾年，」共同設計此研究的文章最末作者Markus
Stoffel表示。他是日內瓦大學環境科學研究所的正教授，專長為將樹輪的測量結果轉化為氣候數據。「此外它們也會造成降雨異常。某個地方乾旱的同時，另一地卻有洪水。」
就算造成了這些影響，當時的人們並無法想像農作物歉收以及不尋常的月食跟火山有任何關係——唯一沒有被記載下來的就是火山爆發本身。「因為火山爆發在南極與格陵蘭的冰層中留下痕跡，我們才知道它們的存在，」共同作者，劍橋大學地理系的教授Clive
Oppenheimer表示。「結合從冰芯得到的訊息以及中世紀文本裡的敘述，我們現在可以更加準確地估計那段時期最大的幾次火山爆發是在何時何地發生。」
對於氣候及社會的影響
為了充分利用這些統整後的數據，Sébastien
Guillet與氣候模擬學者合作來計算火山爆發最有可能發生的時間點。他說：「我們必須知道火山爆發的季節，因為季節會影響火山灰如何散播、溫度下降以及其他與爆發有關的氣候異常。」
除了協助縮減這些事件的發生時間與強度範圍，冰芯的證據顯示西元1100到1300年是歷史上火山活動最活躍的時期之一，也是這項發現的重要之處。新研究評估的15場爆發事件中，有一場可以匹敵西元1815年著名的坦博拉火山爆發事件，其造成了西元1816年變成「無夏之年」。中世紀火山爆發對氣候造成的集體效應也許促成了之後的小冰期，該時期歐洲結冰的河川上會舉辦冬季冰上市集。「若要理解過往的火山活動對於中世紀的氣候與社會是否會造成影響，以及影響的層面為何，深入瞭解這些原本相當神秘的爆發事件是相當重要的。」
The unexpected contribution of
medieval monks to volcanology
By observing the night sky, medieval
monks unwittingly recorded some of history’s largest volcanic eruptions. An
international team of researchers, led by the University of Geneva (UNIGE),
drew on readings of 12th and 13th century European and Middle Eastern
chronicles, along with ice core and tree ring data, to accurately date some of
the biggest volcanic eruptions the world has ever seen. Their results, reported
in the journal Nature, uncover new
information about one of the most volcanically active periods in Earth’s
history, which some think helped to trigger the Little Ice Age, a long interval
of cooling that saw the advance of European glaciers.
It took the researchers almost five years to examine
hundreds of annals and chronicles from across Europe and the Middle East, in
search of references to total lunar eclipses and their colouration. Total lunar
eclipses occur when the moon passes into the Earth’s shadow. Typically, the
moon remains visible as a reddish orb because it is still bathed in sunlight
bent round the Earth by its atmosphere. But after a very large volcanic
eruption, there can be so much dust in the stratosphere – the middle part of
the atmosphere starting roughly where commercial aircraft fly – that the
eclipsed moon almost disappears.
Medieval chroniclers recorded and described all kinds
of historical events, including the deeds of kings and popes, important
battles, and natural disasters and famines. Just as noteworthy were the
celestial phenomena that might foretell such calamities. Mindful of the Book of
Revelation, a vision of the end times that speaks of a blood-red moon, the
monks were especially careful to take note of the moon’s coloration. Of the 64
total lunar eclipses that occurred in Europe between 1100 and 1300, the
chroniclers had faithfully documented 51. In five of these cases, they also
reported that the moon was exceptionally dark.
The contribution
of Japanese scribes
Asked what made him connect the monks’ records of the
brightness and colour of the eclipsed moon with volcanic gloom, the lead author
of the work, Sébastien Guillet, senior research associate at the Institute for
environmental sciences at the UNIGE, said: “I was listening to Pink Floyd’s
Dark Side of the Moon album when I realised that the darkest lunar eclipses all
occurred within a year or so of major volcanic eruptions. Since we know the
exact days of the eclipses, it opened the possibility of using the sightings to
narrow down when the eruptions must have happened.”
The researchers found that scribes in Japan took
equal note of lunar eclipses. One of the best known, Fujiwara no Teika, wrote
of an unprecedented dark eclipse observed on 2 December 1229: ‘the old folk had
never seen it like this time, with the location of the disk of the Moon not
visible, just as if it had disappeared during the eclipse... It was truly
something to fear.’ The stratospheric dust from large volcanic eruptions was
not only responsible for the vanishing moon. It also cooled summer temperatures
by limiting the sunlight reaching the Earth’s surface. This in turn could bring
ruin to agricultural crops.
Cross-checking
text and data
“We know from previous work that strong tropical
eruptions can induce global cooling on the order of roughly 1°C over a few
years,” said Markus Stoffel, full professor at the Institute for environmental
sciences at the UNIGE and last author of the study, a specialist in converting
measurements of tree rings into climate data, who co-designed the study. “They
can also lead to rainfall anomalies with droughts in one place and floods in
another.”
Despite these effects, people at the time could not
have imagined that the poor harvests or the unusual lunar eclipses had anything
to do with volcanoes – the eruptions themselves were all but one undocumented.
“We only knew about these eruptions because they left traces in the ice of
Antarctica and Greenland,” said co-author Clive Oppenheimer, professor at the
Department of Geography at the University of Cambridge. “By putting together
the information from ice cores and the descriptions from medieval texts we can
now make better estimates of when and where some of the biggest eruptions of
this period occurred.”
Climate and
society affected
To make the most of this integration, Sébastien
Guillet worked with climate modellers to compute the most likely timing of the
eruptions. “Knowing the season when the volcanoes erupted is essential, as it
influences the spread of the volcanic dust and the cooling and other climate
anomalies associated with these eruptions,” he said.
As well as helping to narrow down the timing and
intensity of these events, what makes the findings significant is that the
interval from 1100 to 1300 is known from ice core evidence to be one of the
most volcanically active periods in history. Of the 15 eruptions considered in
the new study, one in the mid-13th century rivals the famous 1815 eruption of
Tambora that brought on ‘the year without a summer’ of 1816. The collective
effect of the medieval eruptions on Earth’s climate may have led to the Little
Ice Age, when winter ice fairs were held on the frozen rivers of Europe.
“Improving our knowledge of these otherwise mysterious eruptions, is crucial to
understanding whether and how past volcanism affected not only climate but also
society during the Middle Ages,” concludes the researcher.
原始論文：Sébastien
Guillet, Christophe Corona, Clive Oppenheimer, Franck Lavigne, Myriam Khodri,
Francis Ludlow, Michael Sigl, Matthew Toohey, Paul S. Atkins, Zhen Yang, Tomoko
Muranaka, Nobuko Horikawa, Markus Stoffel. Lunar eclipses illuminate
timing and climate impact of medieval volcanism. Nature, 2023;
616 (7955): 90 DOI: 10.1038/s41586-023-05751-z
引用自：Université de Genève. "The unexpected contribution of medieval
monks to volcanology."
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