原文網址：https://www.whoi.edu/press-room/news-release/study-finds-6%e2%81%b0c-cooling-on-land-during-the-last-ice-age-with-implications-about-future-global-warming/
最近發表在《自然》的新研究透過分析溶解在地下水的惰性氣體，發現末次冰盛期(Last Glacial Maximum，LGM)時中低緯度的低海拔地區平均而言下降了5.8 ± 0.6℃。
2 004年的野外調查中從華北平原一座水井流出的古老地下水，透過測量其中的惰性氣體可以重建過去的溫度。圖片來源：Werner Aeschbach
距今約20000年前的LGM是最近一次全球氣候長時間處於穩定狀態的時期，當時的溫度也比現在低了許多。之前依據多種代用指標來重建這段時期海洋以及低海拔陸地的溫度，得出了某些著名的成果，但是這篇新研究估計出來的數值卻比它們低了許多。
「從實際層面來看，我們這篇論文的重要之處在於顯示先前的研究嚴重低估了過往冰河期的降溫程度，這會拉計地球氣候對溫室氣體有多敏感的低估值，」 共同作者，加州大學聖地亞哥分校斯克里普斯海洋研究所的地球科學教授Jeffrey Severinghaus表示。「前人研究有所缺陷的主因是他們非常仰賴過去生物的豐富程度。但是就和人類一樣，生物通常也會往氣候適合它們的地方移動。比方說雪鳥在冬天就會從加拿大飛往亞利桑那州。因此生物並不是非常良好的溫度計。」
這篇論文大體而言支持了Tierney等人去年對於海洋代用指標的研究成果，他們發現低緯度地區的降溫程度比之前的結果還要嚴重許多，這也意味著氣候敏感度比之前認為的還要更高。Tierney等人的論文提出大氣二氧化碳濃度每乘以二，反應達到平衡之後會讓全球地表的均溫提高3.4℃，這和最先進的氣候模型估計出來的範圍一致，但是卻比一般最好的估計值3.0℃還要高出一些。
「我們的結果提出了更高的氣候敏感度，就未來的全球暖化來說可不是件好消息，因為這代表相較於先前利用最佳的估計值所進行的預測，未來的暖化程度可能還要更加嚴重。尤其是我們對於全球的評估結果也支持了幾篇單一惰性氣體的個案研究，它們發現熱帶地區的溫度在末次冰盛期時比現在低了許多。這對未來而言相當糟糕，代表世上最熱的地區也不能免於變得更加溫暖。」
這篇論文利用的技術可以測量古老的地下水中溶解的惰性氣體，使他們直接得出過往地表溫度的數值。大氣中的惰性氣體不容易與其他化學物質或生物發生反應，且在研究關注的40000年間並沒有大量產生或是遭到消耗的方法。惰性氣體會溶解到地下水中，而它們達到平衡時的濃度絕大部分取決於當時的溫度。作者蒐集除了南極洲以外各大陸40多年來的地下水惰性氣體數據，以及某些重要的熱帶地區之前未發表的數據，最後彙整出LGM時全球從惰性氣體推算出來的溫度紀錄(NGTs)。
「惰性氣體是非常強力的古溫度計，因為它們根據的是物理法則，不太受到短期的極端事件與生物影響，而生物總是會讓事情變得更複雜，」文章共同作者Martin Stute表示。他是巴納德學院環境科學系的教授與拉蒙特―多爾蒂地球觀測所的兼任高級研究員。「它們提供了數百年至數千年間的平均溫度。在利用惰性氣體重建低緯度地區溫度的結果中，我在1990年代進行的早期研究與最新的研究之間得到了非常一致的結果，這讓我在驚訝之餘也感到我的研究是有意義的。」
作者表示這項研究支持了透過分析惰性氣體來重建古溫度的方法，此外也能讓氣候模型得出更加可信的結果。
「我們研究的另一項重要目標，是評估這些『惰性氣體古溫度計』重建末次冰期的陸地溫度整體來說有多準確。原則上，運用這些工具了解過去時的信心程度，跟它們分析現在的數據表現多好有關。我們取得了年代較近的地下水現今觀測到的溫度，接著利用惰性氣體進行獨立的估計，結果發現惰性氣體溫度計在2到33⁰C左右的大範圍裡都相當準確。這讓我們對於LGM降溫的估計成果添加了更多的信心。」研究主要作者，伍茲霍爾海洋研究所海洋化學與地球化學部門的助理科學家Alan Seltzer表示。
Seltzer 進一步表示新的分析成果的重要之處，在於「氣候模型提供了政策制定者一種強力工具來決定如何因應未來的環境變遷。之前有人擔憂模型的預測可能誇大了全球平均溫度因為二氧化碳而產生的升溫，但這項LGM的代用指標數據減輕了這份疑慮。事實上，根據我們的研究以及最近彙整海洋代用指標的結果，越來越能看出古氣候的代用指標和模型的分析是一致的。」
Study finds 6℃ cooling on land during the Last Ice Age
Low-to-mid latitude land surfaces at low elevation cooled on average by 5.8 ± 0.6℃ during the Last Glacial Maximum (LGM), based on an analysis of noble gases dissolved in groundwater, according to a new study published in Nature.
Temperature estimates in the study are substantially lower than indicated by some notable marine and low-elevation terrestrial studies that have relied on various proxies to reconstruct past temperatures during the LGM, a period about 20,000 years ago that represents the most recent extended period of globally stable climate that was substantially cooler than present.
“The real significance of our paper is that prior work has badly underestimated the cooling in the last glacial period, which has low-balled estimates of the Earth’s climate sensitivity to greenhouse gases,” said paper co-author Jeffrey Severinghaus, a professor of geosciences at Scripps Institution of Oceanography, University of California San Diego. “The main reason that prior work was flawed was that it relied heavily on species abundances in the past. But just like humans, species tend to migrate to where the climate suits them. Think, for instance, of snowbirds moving from Canada to Arizona in winter. So, species aren’t very good thermometers.”
The paper does broadly support a recent marine proxy study by Tierney et al. published last year that found substantially greater low-latitude cooling than previous efforts and, in turn, suggested greater climate sensitivity than prior studies. That earlier paper suggested the equilibrium response of Earth’s global-mean surface temperature is 3.4⁰C per doubling of atmospheric carbon dioxide, in line with the consensus range of estimates from state-of-the-art climate models, but somewhat higher than the usual best estimate of 3.0 °C.
“The rather high climate sensitivity that our results suggest is not good news regarding future global warming, which may be stronger than expected using previous best estimates. In particular, our global review reinforces the finding of several single noble gas case studies that the tropics were substantially cooler during the last glacial maximum than at present. The unpleasant implication for the future is that the warmest regions of the world are not immune to further heating,” commented co-author Werner Aeschbach, a professor at the Institute of Environmental Physics, Heidelberg University, Heidelberg, Germany.
The paper made use of a technique in which measurements of noble gases dissolved in ancient groundwater enable direct and quantitative determination of past surface temperature. Noble gases in the atmosphere are chemically and biologically inactive and have no appreciable sinks or sources over the 40,000-year timescales relevant to this study. They dissolve into groundwater, and their equilibrium concentrations depend strongly on temperature. The authors compiled four decades worth of groundwater noble gas data from every continent except Antarctica, along with previously unpublished measurements from some key tropical locations to produce a global record of noble gas-derived temperatures (NGTs) of the LGM.
“Noble gas paleo temperature records are so powerful because they are based on a physical principle and are not much influenced by life—which always complicates everything— and short term extreme events.” said journal article co-author Martin Stute, a professor in the Environmental Science Department at Barnard College and an adjunct senior research scientist at the Lamont-Doherty Earth Observatory. “They provide a temperature average over hundreds to thousands of years. It is remarkable, and rewarding for me, how consistent noble gas paleo temperature reconstructions are in low latitudes from the early studies that I led in the 1990s to the most recent ones.”
The study bolsters the method of analyzing noble gases to reconstruct paleo temperatures and provides more confidence in climate models, according to the authors.
“Another key goal of our study was to evaluate the overall accuracy of the so-called ‘noble gas paleo-thermometer’ for reconstructing temperatures on land during the last glacial period. Naturally, our ability to confidently use this tool to understand the past is related to how well it works in the present. By comparing modern temperature observations to independent estimates using noble gases in relatively young groundwater, we found that the noble gas thermometer is remarkably accurate over a wide temperature range from around 2 to 33 ⁰C (36 to 91 ⁰F). This adds a good deal of confidence to our estimates of cooling during the LGM,” said the paper’s lead author, Alan Seltzer, an assistant scientist in the Marine Chemistry and Geochemistry Department at the Woods Hole Oceanographic Institution.
Seltzer added that the new analysis is important because climate models “provide an important tool that policy makers can use to decide on how to prepare for future environmental changes. This study alleviates the concern that, based on LGM proxy data, models might over-predict the global mean temperature response to carbon dioxide. In fact, based on both our study and the recent marine-proxy compilation, it is becoming clear that paleoclimate proxies and models are in agreement.”
原始論文：Alan M. Seltzer, Jessica Ng, Werner Aeschbach, Rolf Kipfer, Justin T. Kulongoski, Jeffrey P. Severinghaus, Martin Stute. Widespread six degrees Celsius cooling on land during the Last Glacial Maximum. Nature, 2021; 593 (7858): 228 DOI: 10.1038/s41586-021-03467-6
引用自：Woods Hole Oceanographic Institution. "Study finds six degrees celsius cooling on land during the last Ice Age.”
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