畫作、太陽黑子與冬季博覽會—重新審視小冰期

原文網址：www.sciencedaily.com/releases/2017/04/170404084420.htm

畫作、太陽黑子與冬季博覽會—重新審視小冰期

一群研究太陽與氣候的科學家宣稱「小冰期」(Little Ice Age)傳達的整體概念會令人誤解，原因是跟現今的全球暖化相比，當時的變化規模不但較小、具有季節性且程度比較輕微。

對於16至19世紀地球氣候曾一度冷化的解釋包括了太陽活動降低、火山噴發、人類土地利用方式的改變以及自然氣候變遷。

但刊登於英國皇家天文學會的內部期刊《天文學與地球物理學》(Astronomy & Geophysics)的新論文中，雷丁大學的教授Mike Lockwood和其同事指出，跟最近數十年肇因於溫室氣體排放的溫度變化相比，當時的程度還比較輕。另外，雖然太陽活動低落可能是成因之一，但絕不會是唯一因素。

Lockwood教授表示：「人們在評論氣候變遷時常常會提及小冰期。我們想要進行一份通盤研究來看看那些氣候變冷的證據可信度有多高、它真正造成的影響有多大，以及此事件是由太陽活動造成的證據究竟有多強。」

「即使像英國這類地方發生寒冬的比例變高，小冰期整體而言是以特定區域為主的現象，且氣溫下降不到失控的程度。我們的研究認為並沒有一個單獨原因可以解釋以下現象：夏季多數時候跟現今一樣依然溫暖，也不是每個冬季都很寒冷」

研究人員仔細檢閱歷史紀錄，像是泰晤士河凍結時舉行「冬季博覽會」(frost fair)的報告；同時觀察當時所繪的畫作，比方說老彼得˙布勒哲爾在《雪中獵人》裡描繪的嚴冬景觀。這兩種紀錄時常被引用來支持小冰期的概念。

在1650至1710年左右的蒙德極小期(Maunder Minima)，以及1790至1825年程度較輕的道爾頓極小期(Dalton Minima)，太陽黑子數量經常變得相當少，意味著太陽表面的溫度稍微降低了一些。此外部影響通常被提出來解釋此時的氣候條件為何較冷。

由雷丁大學率領的團隊更加仔細檢查各類證據。他們將直接測量氣溫的紀錄以及其他代用指標，像是冰雪含有的紀錄，對比至泰晤士河結凍的年份(不論冬季博覽會是否舉辦)以及太陽活動的指標。

歷史上的氣候變遷可以經由許多方法得知。英格蘭中部氣溫記錄(Central England Temperature, CET)追蹤了自1659年以來的氣溫，使它成為世上由儀器所記錄的連續氣象資料序列中年代最久也最長者。其他對氣候變遷敏感的生物指標的研究，像是樹輪、珊瑚、昆蟲數量和軟體動物，則可以用來補足這份直接觀測紀錄。

作者把結果跟冰層本身的年代比對。他們可以從南極冰層取出的冰芯得知全球氣溫，方法是測量一種較重的氫原子—氘(²H)，以及一種較重的氧原子—¹⁸O，跟它們質量較輕的「正常」對應原子之間的比例。要將重原子比例較多的水蒸發需要更多能量，而在到達極區堆積於冰雪之前，它們也更容易提早一步從雨水中落下。因此，這些原子之間的比例變化讓研究人員可以得知數百萬年以來氣溫的變動情形。

科學家經由比對結果，宣稱將這段時期比喻為冰河期會令人誤解，因為當時的氣溫下降幅度遠遠小於真正的冰河期。在小冰期(Little Ice Age, LIA)，北半球的平均氣溫大約只有下降0.5℃。相比之下，在最近一次約莫於12,000年前結束的大型冰河期，全球氣溫一般都比今日還要低了8℃。

冬季博覽會看起來也不是一項可以指出整體氣候狀況的指標，因為就算泰晤士河凍結了，也常常沒有舉辦冬季博覽會。這部分要歸咎於很多原因，包括清教教會的禁止或者是冰融時可能會造成傷亡的安全考量。冬季博覽會走入歷史跟氣候變遷或是太陽活動沒有任何關係，而是因為原先的倫敦橋於1825年毀壞，與第一座維多利亞堤岸於1870年竣工後造成的流量增加有關。這兩項因素造成自此之後即使仍有多次寒冬發生，河流也不致完全凍結。

選擇性地引用歷史證據似乎增強了當時陷入漫長冬季魔咒的幻象。描繪一月景象的《雪中獵人》不過是布勒哲爾《月令圖系列》的其中一幅。這一系列畫作的其中七幅可能已經佚失，但是《陰天》(二月)、《割曬牧草》(七月)和《牧歸》(十一月)三幅畫中皆沒有線索能指出當時氣候特別地冷。與此呼應的是，Lockwood和他的團隊指明即便在小冰期的高峰，歐洲發生嚴寒冬季的同時仍有多次溫暖的夏季。

舉例來說，1701年是小冰期的氣溫最低點，但有報告指出同年倫敦和巴黎的夏季熱得令人難以承受；CET也顯示這年七月為七月氣溫紀錄中排行第十高，其月均溫到達18.3℃。1676年的六月則是以18℃在該月高溫紀錄中排行第二，但它同樣發生在一連串出現寒冷冬季的年份之間。像這般的夏季高溫跟「小冰期」的名號一點都不相稱。

更加劇烈的變化可能是來自於大型火山爆發。1257年印尼的薩馬拉斯火山噴發將大量的塵埃灌入大氣層，造成暫時性的冷化效應。對應到小冰期最冷時段的1570至1730年，也觀察到此時曾發生連續數起程度較低的火山活動，它們可能有助於降低氣溫。毫無疑問的是火山爆發可以同時造成冷夏和嚴冬。最明顯的一個例子便是發生在1815年7月的坦博拉火山爆發(印尼)，此事件造成隔年被稱作「無夏之年」。

Lockwood教授表示：「在我們面臨全球暖化挑戰的當下，這篇研究沒有為我們舒緩多少壓力。雖然太陽活動看似正在減弱，但任何冷卻效應都會被二氧化碳排放量增加的效應所抵銷，也讓我們沒有藉口對現況繼續置之不理。」

Paintings, sunspots and frost fairs: Rethinking the Little Ice Age

The whole concept of the 'Little Ice Age' is 'misleading', as the changes were small-scale, seasonal and insignificant compared with present-day global warming, a group of solar and climate scientists argue.

Explanations for the cooling to Earth's climate, thought to have occurred between the 16th and 19th centuries, include low solar activity, volcanic eruptions, human changes to land use and natural climatological change.

But in a new paper in Astronomy & Geophysics, the house journal of the Royal Astronomical Society, Professor Mike Lockwood, of the University of Reading, and his collaborators, note that the temperature shift was smaller than that seen in recent decades resulting from the emission of greenhouse gases, and that although low solar activity may have been one driving factor, it certainly was not the only one.

Professor Lockwood said: "Commentators frequently refer to the Little Ice Age in discussions on climate change. We wanted to carry out a comprehensive study to see just how reliable the evidence is for a cooler climate, how big an impact it really had and how strong the evidence for a solar cause really was.

"On the whole the Little Ice Age was a manageable downturn in climate concentrated in particular regions, even though places like the UK had a larger fraction of cold winters. Our research suggests that there is no single explanation for this, that warm summers continued much as they do today and that not all winters were cold."

Researchers scrutinised historical records, such as the accounts of 'frost fairs' when the River Thames froze solid, and looked at the paintings from the era, such as the landscapes of Pieter Bruegel the Elder, with 'Hunters in the Snow' depicting a cold winter scene. Both of these are cited in support of the Little Ice Age concept.

From around 1650-1710, and to a lesser extent from 1790-1825, periods respectively known as the Maunder and Dalton Minima, sunspot numbers were unusually low, an indication that the surface of the Sun was slightly cooler. This external influence is often suggested as an explanation for the colder conditions.

The Reading-led team looked at the various pieces of evidence in more detail. They compared direct temperature records and proxy data such as ice records, with the years when the Thames was frozen over (whether or not a frost fair took place), and with the indications of solar activity.

Historical climate change is assessed through a variety of means. The Central England Temperature (CET) dataset tracks temperature from 1659, making it the oldest and longest running meteorological instrumental data sequence in the world. This direct record is supplemented by studies of biological proxies such as tree rings, corals, insect numbers and molluscs, all sensitive to climate change.

The authors draw comparisons with the ice ages proper. Cores taken from Antarctic ice allow global temperatures to be inferred, by measuring the proportions of deuterium (2H), a heavier atom of hydrogen, and of the heavier oxygen atom 18O, compared with their lighter 'normal' counterparts. It takes more energy to evaporate water with a higher proportion of these atoms, and they are more easily lost from rainfall, before they are deposited in ice found nearer the poles. The changing proportion of these atoms then allows researchers to assess how the temperature has changed over millions of years.

From these comparisons, the scientists argue that the description of the period as an Ice Age is misleading, as temperatures in that period fell far less than in a glaciation. During the Little Ice Age (LIA), the average temperature in the northern hemisphere fell by around 0.5 degrees. In contrast, in the most recent major glaciation that came to an end around 12,000 years ago, global temperatures were typically 8 degrees Celsius colder than today.

Frost fairs also seem to be a poor indication of overall climate, as they often did not take place despite the Thames freezing, partly for many reasons including puritanical authorities or safety as lives were lost when the ice melted. The ending of the frost fairs had nothing to do with climate change or solar activity, instead being due to the increased river flow when the original London Bridge was demolished in 1825, and the first Victoria embankment opened in 1870. Both of these prevented the river from freezing completely, despite many subsequent cold winters.

Selective use of art historical evidence appears to reinforce the illusion of a prolonged cold spell. Yet 'Hunters in the Snow', depicting a January scene, is part of a series by Bruegel known as 'The Twelve Months'. Seven of these paintings may have been lost, but 'The Gloomy Day' (February), 'Haymaking' (July), and 'The Return of the Herd' (November) all give no indication of unusually cold conditions. Consistent with this, Lockwood and his team note that even at the height of the LIA period, colder European winters were still accompanied by many warm summers.

For example, 1701 is close to the lowest point of the Little Ice Age, yet in both Paris and London the summer was reported as being unbearably hot and the CET for July that year is the 10th hottest on record, with average temperatures for the month reaching 18.3°C. The year 1676 is the second hottest June on record at 18.0°C, yet it too was in the middle of a run of cold winters. Such high summer temperatures do not fit at all with the name "Little Ice Age."

Much more dramatic variations can result from large volcanic eruptions. Samalas, a volcano which erupted in 1257 in what is now Indonesia, ejected large amounts of dust into the atmosphere, causing a temporary cooling effect. The years between 1570 and 1730, corresponding to the coldest part of the LIA, also saw continuous lower level volcanic activity that may have suppressed temperatures. Volcanic eruptions undoubtedly cause both cold winters and cold summers. One of the clearest examples was the Tambora eruption of July 1815, which caused the next year to be called "the year without a summer."

Professor Lockwood said: "This study provides little solace for the future, as we face the challenge of global warming. Solar activity appears to be declining at present, but any cooling effect that results will be more than offset by the effect of rising carbon dioxide emissions, and provides us with no excuse for inaction."

原始論文：Mike Lockwood, Mat Owens, Ed Hawkins, Gareth S Jones, Ilya Usoskin. Frost fairs, sunspots and the Little Ice Age. Astronomy & Geophysics, 2017; 58 (2): 2.17 DOI: 10.1093/astrogeo/atx057

引用自：Royal Astronomical Society. "Paintings, sunspots and frost fairs: Rethinking the Little Ice Age." ScienceDaily. ScienceDaily, 4 April 2017.