Hannah Hickey
現在的南極點以及南極東部的其他地區是非常寒冷的地方，而在20000年前的末次冰期甚至還要更加嚴寒―但是溫度並沒有之前認為的那麼低。
Emma Kahle手上的是來自地下1500公尺的冰塊，也是2016年1月南極點鑽探計畫最初的目標深度。新研究利用這根冰芯來計算54000年前至今的溫度變化。圖片來源：Eric Steig/University of Washington
華盛頓大學的冰河學家參與的兩篇論文分析了南極的冰芯，藉此瞭解末次冰期期間該座大陸的氣溫。結果有助於理解在氣候劇烈轉變的時候南極會有怎樣的反應。
其中一篇論文於6月3日發表在《科學》(Science )，作者包括了三名華盛頓大學的研究人員在內的國際團隊。他們分析了涵蓋南極東部與西部的七根冰芯，結果顯示冰河期的時候南極東部的溫度比之前認為的還高。
團隊裡面包括了美國、日本、英國、法國、瑞士、丹麥、義大利、南韓與俄羅斯的科學家。
「想要回答這項問題，不同國家的研究人員互相合作相當重要，因為分析這些來自南極各地的冰芯需要用到許多不同的分析設備及方法，」第二作者，華盛頓大學地球與太空科學系的助理教授T.J. Fudge表示。
南極在今日是地球上最冷的地方，而在末次冰期時甚至還要更加寒冷。數十年來，最先進的科學研究認為南極在冰河期的氣溫平均來說比現代低了9℃；相較之下，當時的全球氣溫平均而言則比現在低了5到6℃。
之前的研究結果顯示南極西部的溫度比現在低了11℃。而這篇發表在《科學》的新論文則認為南極東部某些地方的溫度只有比現在低4到5℃，大概是之前估計值的一半而已。
「這是我們首次具有充足且彼此相容的證據來回答整個南極的狀況，」主要作者，奧勒岡州立大學的助理教授Christo Buizert表示。「這項發現的驚人之處在於降溫幅度會依據你在南極的不同地方而有巨大的差異。這種降溫模式的成因可能是冰河期和今日冰層的高度改變所導致。」
這類發現相當重要，因為全球氣候模型的模擬結果和它們越符合，就代表模型重現全球氣候大型變化的能力越可信。
另一篇由華盛頓大學領導，六月發表在《地球物理研究期刊：大氣科學》(Journal of Geophysical Research: Atmospheres)的論文則著重在2016年從南極點鑽探出來，最近才完成分析的冰芯數據。前述的《科學》論文也有納入這項結果。
「由於南極東部特別高且氣候較乾，因此它的溫度勢必會比南極西部還低，但關鍵在於氣候變暖的時候，兩個區域的溫度會各自如何變化？」主要作者Emma Kahle表示。她最近剛完成華盛頓大學地球與太空科學系的博士學業。
南極點位在南極洲分水嶺的附近。這篇著重在南極點鑽探出來的冰芯的論文發現，此處在冰河期的溫度大約比現在低了6.7℃。而《科學》的論文則發現南極東部的整體平均溫度比現在低了6.1℃，顯示南極點可以代表此區域。
「對於末次冰期南極東部的溫度，這兩篇研究得到的數字都比先前的成果還要高出許多――最新的教科書中寫說當時比現在冷了9℃，」華盛頓大學地球與太空科學系的教授Eric Steig表示，他是這兩篇論文的共同作者。「這項結果相當重要。因為之前氣候模型得出的溫度通常也較高，所以現在數據和模型變得更加一致。」
「這項發現跟氣候模型對該時期的模擬結果十分一致，因此我們對於模型模擬地球氣候的能力也增添了更多信心，」Kahle表示。
之前的研究是分析層層堆積的冰層中的水分子，它們的行為基本上就像溫度計一樣，因此可以用來重建過去的溫度。但是此方法需要其它技術來進行獨立的校正。
新論文運用兩種技術來提供必要的校正。第一種方法稱為井孔測溫法(borehole thermometry)，研究人員在鑽取冰芯之後留下來的井孔中測量不同深度的溫度，就能得到冰層的縱向溫度變化。Fudge 說因為南極的冰層非常厚，因此它能記住過往冰河期的低溫，經過測量之後就能加以重建。
第二種技術則是探討雪堆累積起來之後，緩慢轉變成冰塊的過程中發生的性質變化。南極東部的雪堆厚度可以從50公尺到120公尺，其中含有數千年前降下的雪，它們逐漸壓縮的過程對溫度相當敏感。
「隨著我們鑽取更多南極的冰芯並進行研究，過去氣候變遷的整體圖像也變得越加清晰，使得我們對地球全體的氣候系統有更深的理解，」Fudge 表示。
South Pole and East Antarctica warmer than previously thought during last ice age, two studies show
The South Pole and the rest of East Antarctica is cold now and was even more frigid during the most recent ice age around 20,000 years ago — but not quite as cold as previously believed.
University of Washington glaciologists are co-authors on two papers that analyzed Antarctic ice cores to understand the continent’s air temperatures during the most recent glacial period. The results help understand how the region behaves during a major climate transition.
In one paper, an international team of researchers, including three at the UW, analyzed seven ice cores from across West and East Antarctica. The results published June 3 in Science show warmer ice age temperatures in the eastern part of the continent.
The team included authors from the U.S., Japan, the U.K., France, Switzerland, Denmark, Italy, South Korea and Russia.
“The international collaboration was critical to answering this question because it involved so many different measurements and methods from ice cores all across Antarctica,” said second author T.J. Fudge, a UW assistant research professor of Earth and space sciences.
Antarctica, the coldest place on Earth today, was even colder during the last ice age. For decades, the leading science suggested ice age temperatures in Antarctica were on average as much as 9 degrees Celsius cooler than the modern era. By comparison, temperatures globally at that time averaged 5 to 6 degrees cooler than today.
Previous work showed that West Antarctica was as cold as 11 degrees C below current temperatures. The new paper in Science shows that temperatures at some locations in East Antarctica were only 4 to 5 degrees cooler, about half previous estimates.
“This is the first conclusive and consistent answer we have for all of Antarctica,” said lead author Christo Buizert, an assistant professor at Oregon State University. “The surprising finding is that the amount of cooling is very different depending on where you are in Antarctica. This pattern of cooling is likely due to changes in the ice sheet elevation that happened between the ice age and today.”
The findings are important because they better match results of global climate models, supporting the models’ ability to reproduce major shifts in the Earth’s climate.
Another paper, accepted in June in the Journal of Geophysical Research: Atmospheres and led by the UW, focuses on data from the recently completed South Pole ice core, which finished drilling in 2016. The Science paper also incorporates these results.
“With its distinct high and dry climate, East Antarctica was certainty colder than West Antarctica, but the key question was: How much did the temperature change in each region as the climate warmed?” said lead author Emma Kahle, who recently completed a UW doctorate in Earth and space sciences.
That paper, focusing on the South Pole ice core, found that ice age temperatures at the southern pole, near the Antarctic continental divide, were about 6.7 degree Celsius colder than today. The Science paper finds that across East Antarctica, ice age temperatures were on average 6.1 degrees Celsius colder than today, showing that the South Pole is representative of the region.
“Both studies show much warmer temperatures for East Antarctica during the last ice age than previous work — the most recent ‘textbook’ number was 9 degrees Celsius colder than present,” said Eric Steig, a UW professor of Earth and space sciences who is a co-author on both papers. “This is important because climate models tend to get warmer temperatures, so the data and models are now in better agreement.”
“The findings agree well with climate model results for that time period, and thus strengthen our confidence in the ability of models to simulate Earth’s climate,” Kahle said.
Previous studies used water molecules contained in the layers of ice, which essentially act like a thermometer, to reconstruct past temperatures. But this method needs independent calibration against other techniques.
The new papers employ two techniques that provide the necessary calibration. The first method, borehole thermometry, takes temperatures at various depths inside the hole left by the ice drill, measuring changes through the thickness of the ice sheet. The Antarctic ice sheet is so thick that it keeps a memory of earlier, colder ice age temperatures that can be measured and reconstructed, Fudge said.
The second method examines the properties of the snowpack as it builds up and slowly transforms into ice. In East Antarctica, the snowpack can range from 50 to 120 meters (165 to 400 feet) thick, including snow from thousands of years which gradually compacts in a process that is very sensitive to the temperature.
“As we drill more Antarctic ice cores and do more research, the picture of past environmental change comes into sharper focus, which helps us better understand the whole of Earth’s climate system,” Fudge said.
原始論文：Emma C. Kahle et al. Reconstruction of temperature, accumulation rate, and layer thinning from an ice core at South Pole using a statistical inverse method. Journal of Geophysical Research: Atmospheres, 2021 DOI: 10.1002/essoar.10503447.2
Christo Buizert, T. J. Fudge, William H. G. Roberts, Eric J. Steig, Sam Sherriff-Tadano, Catherine Ritz, Eric Lefebvre, Jon Edwards, Kenji Kawamura, Ikumi Oyabu, Hideaki Motoyama, Emma C. Kahle, Tyler R. Jones, Ayako Abe-Ouchi, Takashi Obase, Carlos Martin, Hugh Corr, Jeffrey P. Severinghaus, Ross Beaudette, Jenna A. Epifanio, Edward J. Brook, Kaden Martin, Jérôme Chappellaz, Shuji Aoki, Takakiyo Nakazawa, Todd A. Sowers, Richard B. Alley, Jinho Ahn, Michael Sigl, Mirko Severi, Nelia W. Dunbar, Anders Svensson, John M. Fegyveresi, Chengfei He, Zhengyu Liu, Jiang Zhu, Bette L. Otto-Bliesner, Vladimir Y. Lipenkov, Masa Kageyama, Jakob Schwander. Antarctic surface temperature and elevation during the Last Glacial Maximum. Science, 2021; 372 (6546): 1097 DOI: 10.1126/science.abd2897
引用自：University of Washington. "South Pole and East Antarctica warmer than previously thought during last ice age, two studies show."
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