目前氣候暖化的速度是過去兩千年來的新高

原文網址：https://www.unibe.ch/news/media_news/media_relations_e/media_releases/2019/medienmitteilungen_2019/the_climate_is_warming_faster_than_it_has_in_the_last_2000_years/index_eng.html

目前氣候暖化的速度是過去兩千年來的新高

根據瑞士伯恩大學進行的兩篇研究結果，目前由人為造成的氣候變遷與工業革命之前的氣候波動不同，是全世界同時發生的現象。此外，至少就過去兩千年來說，全球暖化的速度也達到了歷史新高。

「小冰期」的嚴冬因為許多荷蘭畫家以其為主題的畫作，加上其他事物而深植於歐洲人的集體意識之中。這幅《The Pleasures of Winter》的作者為17世紀的荷蘭畫家艾維坎普(Hendrick Avercamp，1585-1634)。圖片來源：The Bridgeman Art Library, Object 39769, Wikimedia

許多人對於「小冰期」(Little Ice Age，大約從西元1300~1850年)有相當明確的意像。當時的畫作描繪出人們在荷蘭的運河上溜冰，以及冰河大幅擴張至高山谷地，都顯示了小冰期的存在。不只是過去的畫作，利用樹輪之類的材料大量重建出的溫度紀錄，也證實了歐洲經歷了數個世紀的嚴寒時期。由於北美重建出來的溫度紀錄也有類似結果，使得人們推測「小冰期」以及同樣著名的「中世紀暖期」(Medieval Warm Period，大約是西元700~1400年)是發生於全世界的現象。但是瑞士伯恩大學厄施格爾氣候變遷研究中心的Raphael Neukom領導的國際研究團隊，最近把這類號稱影響全球的氣候波動描繪成十分不同的樣貌。在甫發表於知名科學期刊《自然》(Nature)的論文，以及發表於《自然―地球科學》(Nature Geoscience)的補充論文中，研究團隊表示過去2000年來，沒有證據可以支持全世界曾經發生同時變暖或變冷的時期。

過去不同地區發生的氣候波動具有差異

「全世界在小冰期時的溫度普遍較低，這點是沒有問題的。」Raphael Neukom如此解釋。「但每個地方的發生時間並非相同。工業革命以前的最暖期與最冷期在不同地區其實發生在不同時候。」最近被推翻的假說認為過去全球各地會在相同時間進入同樣的氣候階段。這位伯恩大學的氣候科學家指出，該假說的形成原因為歐洲和北美的氣候史給人的印象。由於缺乏地球其他區域的氣候資料，這種概念便被推及至全世界，使人們推測過去兩千年來相對較暖或較冷的時期，都是在世界各地同步發生。但他們現在證明實情並非如此。

這篇發表在《自然》的研究中，作者提出的解釋認為工業革命之前各個區域的氣候會有差異，主要是受到氣候系統本身具有的隨機波動影響。火山爆發或太陽活動這類外部因素的強度，不足以讓全球氣溫明顯降低或升高數十年，甚至數個世紀。

研究人員為了探討工業革命之前五個不同的氣候時期，運用了國際研究組織PAGES(Past Global Changes, www.pastglobalchanges.org)的資料庫，其中概括了過去2000年來各方面的氣候資料。除了樹輪之外，該資料庫也含有冰芯、湖泊沉積物和珊瑚的資料。Raphael Neukom領導的研究團隊為了驗證他們的結果，運用六種不同的統計模型來分析資料，比過往的研究都還要多。這讓他們不只可以算出確切的溫度，也能算出長達數十年或數個世紀的極端高低溫出現的機率。結果顯示研究期間之內並沒有出現全球一致的圖像。Raphael Neukom表示：「不同區域的最高溫和最低溫都不一樣。」因此地區性的溫度變化現象，像是歐洲和北美經常提到的「中世紀暖期」發生的時候，並不能推論當時全球各地也達到極端高溫。

目前是首度在世界各地發生的暖期

研究結果中的近代歷史看起來則相當不同。兩篇研究皆顯示20世紀可能是過去2000年來最溫暖的時期，而且地球表面98%的地方都是如此。這再次證明當今的氣候變遷無法用氣候的隨機波動來解釋，得歸咎於人為排放的二氧化碳和其他溫室氣體。此研究讓我們知道至少以過去2000年來說，20世紀的全球平均氣溫是這段期間的新高，也是首次同時影響整個地球的暖期。另外，目前全球暖化的速度也是前所未見。

The climate is warming faster than it has in the last 2,000 years

In contrast to pre-industrial climate fluctuations, current, anthropogenic climate change is occurring across the whole world at the same time. In addition, the speed of global warming is higher than it has been in at least 2,000 years. That’s according to two studies from the University of Bern.

Many people have a clear picture of the "Little Ice Age" (from approx. 1300 to 1850). It’s characterized by paintings showing people skating on Dutch canals and glaciers advancing far into the alpine valleys. That it was extraordinarily cool in Europe for several centuries is proven by a large number of temperature reconstructions using tree rings, for example, not just by historical paintings. As there are also similar reconstructions for North America, it was assumed that the "Little Ice Age" and the similarly famous "Medieval Warm Period" (approx. 700 – 1400) were global phenomena. But now an international group led by Raphael Neukom of the Oeschger Center for Climate Change Research at the University of Bern is painting a very different picture of these alleged global climate fluctuations. In a study which has just appeared in the well-known scientific journal "Nature", and in a supplementary publication in "Nature Geoscience", the team shows that there is no evidence that there were uniform warm and cold periods across the globe over the last 2,000 years.

Climate fluctuations in the past varied from region to region

"It’s true that during the Little Ice Age it was generally colder across the whole world," explains Raphael Neukom, "but not everywhere at the same time. The peak periods of pre-industrial warm and cold periods occurred at different times in different places." According to the climate scientist from Bern, the now-debunked hypothesis of climate phases occurring at the same time across the globe came about because of an impression that is defined by the climate history of Europe and North America. In the absence of data from other parts of the earth, this notion was applied to the whole planet, raising expectations that relatively cold or warm periods throughout the last 2,000 years were globally synchronous phenomena. But it has now been shown that this was not the case.

The authors of the study in "Nature" see the explanation for that as being that regional climates in pre-industrial times were primarily influenced by random fluctuations within the climate systems themselves. External factors such as volcanic eruptions or solar activity were not intense enough to cause markedly warm or cold temperatures across the whole world for decades, or even centuries.

The researchers relied on a database from the international research consortium PAGES (Past Global Changes, www.pastglobalchanges.org), which provides a comprehensive overview of climate data from the last 2,000 years, for their investigation of five pre-industrial climate epochs. In addition to tree rings, it also includes data from ice cores, lake sediments and corals. To really put the results to the test, the team led by Raphael Neukom analyzed these data sets using six different statistical models – more than ever before. This allowed for the calculation of the probability of extremely warm or cold decades and centuries, and not just the calculation of absolute temperatures. The result was that no globally coherent picture emerged during the periods being investigated. "The minimum and maximum temperatures were different in different areas," says Raphael Neukom. So thermal extremes across the world cannot be inferred from regional temperature phenomena like the oft-mentioned "Medieval Warm Period" in Europe and North America.

The current warm period is happening across the world for the first time

The results look very different for recent history. Both studies show that the warmest period of the last 2,000 years was most likely in the 20th century. They also show that this was the case for more than 98 percent of the surface of the earth. This shows – once again – that modern climate change cannot be explained by random fluctuations, but by anthropogenic emissions of CO₂ and other greenhouse gases. What we didn’t know until now is that not only average global temperatures in the 20th century are higher than ever before in at least 2,000 years, but also that a warming period is now affecting the whole planet at the same time for the first time. And the speed of global warming has never been as high as it is today.

原始論文：Neukom, R., Steiger, Nathan, Gómez-Navarro, J. J., Wang, J., & Werner, J. P. No evidence for globally coherent warm and cold periods over the pre-industrial Common Era. Nature, 2019 DOI: 10.1038/s41586-019-1401-2

PAGES 2k Consortium. Consistent multidecadal variability in global temperature reconstructions and simulations over the Common Era. Nature Geoscience, 2019 DOI: 10.1038/s41561-019-0400-0

引用自：University of Bern. " The climate is warming faster than it has in the last 2,000 years."