原文網址：www.sciencedaily.com/releases/2016/12/161206111535.htm

在上次暖化期間南極的升溫幅度是整個地球平均的二到三倍

暖化在極區增幅的現象跟現今氣候模型做出的結果一致

根據一組美國地球物理學家進行的新研究，在20,000年前的末次冰河期高峰之後，南極暖化的幅度是世界溫度平均升高量的二到三倍左右。

南極溫度在20,000年前至10,000年前之間提高了大約攝氏11度，將近華氏20度；然而全世界的平均溫度僅升高了攝氏4度，約為華氏7度。這種不一致突顯出無論是暖化或冷化，在兩極地區(包括北邊的北極與南邊的南極)氣候變遷的效應都會增強。

此計算結果跟大多數氣候模型的預測一致，顯示這些模型在估計過往氣候條件時表現良好。因此，它們很有機會也能準確預測未來氣候變遷與全球暖化時代中的氣候條件會是如何。

「雖然結果並不特別令人驚訝，但當你去檢視用來預測未來全球暖化的氣候模型，對地球20,000年前的樣貌的分析結果時，它們平均而言都能良好重現當時南極地區有多冷。」第一作者Kurt Cuffey說。他是加州大學柏克萊分校的冰河學家，同時也是地理學和行星及地球科學的教授。「這項研究相當值得注意，也證實了我們的確瞭解氣候系統是如何運作。」

這些模型目前預測現今氣候變遷造成的結果中，雖然在數百年內還不會到達最高值，南極的升溫幅度會是地球其餘地方的兩倍之多。假設人類仍依照目前速率排放溫室氣體，則到了2100年，最有可能發生的氣候變遷景象是全球平均溫度會升高攝氏3度(華氏5度)，而預測中南極地區則最終會升高攝氏6度(華氏10度)左右。

這項新研究首度精確計算了南極地區於冰河期的氣溫，以及從那之後的溫度增加量。Cuffey表示研究結果可以淘汰一些對回饋效應考慮不足，因而無法準確重現極區升溫較多的氣候模型。

Cuffey和共同研究人員，包括任職於美國科羅拉多州萊克伍德的美國地質調查局的Gary Clow，將他們的研究成果刊登於上週《美國科學院院刊》(Proceedings of the National Academy of Sciences)的線上預告版。

南極的冰河消退

這項分析的原理為末次冰河期於20,000年前達到最為寒冷之後全球開始逐漸暖化，此時南極冰河深處的冰的升溫速率會比地表升溫還要慢，就像把一隻冷凍火雞直接送進熾熱的烤爐，就算火雞表面已經到達爐子的溫度，但裡面還是處於凍結的狀態。透過測量冰河深處的餘溫和表層溫度的差距，科學家可以根據純冰的升溫速率來預估深處冰層形成時的溫度。在南極西部的冰層，深處於20,000年前形成的冰比表層低了攝氏1度。

於2011年結束的南極西部冰層分冰嶺冰芯計畫，在為期八年的計畫期間有鑽探一座深3.4公里(2英里)的冰芯鑽井。Clow分別在2011和2014年各測量了一次內部溫度。鑽井最深處的冰堆積於70,000年前；全長距底部六分之一處為50,000年前形成；距表面三分之一處則形成於20,000年前。

Cuffey根據熱在冰塊中的擴散方式，結合古老冰層的同位素測量結果，發展出一種方法可以把這些溫度測量結果合併成一條平滑的溫度曲線。從中可以估計出從上次冰河期的深度開始，溫度已經升高了攝氏11.3度，誤差為正負1.8度。

有趣的是，末次冰盛期之後，南極溫度的提升速率遠高於北極。在15,000年前，南極氣溫已經達到今日的75%，而北極還要再花上3,000至4,000年才會暖化至此程度。原因主要是北半球擁有大片冰層可以減緩暖化作用，另外海洋洋流以及地球軌道狀態的變化也加速了南半球暖化。

Cuffey表示隨著全球溫度升高，海洋洋流的變化會造成深海富含二氧化碳的海水上湧，導致全球二氧化碳濃度增加。而南極對於這種變化也更加敏感。

他說現今發生的全球暖化主要是由人類燃燒化石燃料所釋放的二氧化碳促成，這種狀況跟自然循環有所不同。海洋吸收二氧化碳的速度無法跟上大氣溫室氣體攀升的速度，意味著除非人類停止排放二氧化碳，否則二氧化碳含量和全球溫度就不會停止升高。

During last warming period, Antarctica heated up two to three times more than planet average

Amplification of warming at poles consistent with today's climate change models

Following Earth's last ice age, which peaked 20,000 years ago, the Antarctic warmed between two and three times the average temperature increase worldwide, according to a new study by a team of American geophysicists.

The disparity -- Antarctica warmed about 11 degrees Celsius, nearly 20 degrees Fahrenheit, between about 20,000 and 10,000 years ago, while the average temperature worldwide rose only about 4 degrees Celsius, or 7 degrees Fahrenheit -- highlights the fact that the poles, both the Arctic in the north and the Antarctic in the south, amplify the effects of a changing climate, whether it gets warmer or cooler.

The calculations are in line with estimates from most climate models, proving that these models do a good job of estimating past climatic conditions and, very likely, future conditions in an era of climate change and global warming.

"The result is not a surprise, but if you look at the global climate models that have been used to analyze what the planet looked like 20,000 years ago -- the same models used to predict global warming in the future -- they are doing, on average, a very good job reproducing how cold it was in Antarctica," said first author Kurt Cuffey, a glaciologist at the University of California, Berkeley, and professor of geography and of earth and planetary sciences. "That is noteworthy and a confirmation that we know how the system works."

These models currently predict that as a result of today's global climate change, Antarctica will warm twice as much as the rest of the planet, though it won't reach its peak for a couple of hundred years. While the most likely climate change scenario, given business-as-usual greenhouse gas emissions, is a global average increase of 3 degrees Celsius (5 degrees Fahrenheit) by 2100, the Antarctic is predicted to warm eventually by around 6 degrees Celsius (10 degrees Fahrenheit).

The new results, which are the first good calculation of Antarctica's ice age temperature and the amount of warming since, do rule out a couple of climate models that do not include enough feedback to accurately reproduce the amplified temperature in the polar regions, Cuffey said.

Cuffey and his colleagues, including Gary Clow of the U.S. Geological Survey in Lakewood, Colorado, published their results online last week in the early edition of the Proceedings of the National Academy of Sciences.

Deglaciation in Antarctica

The analysis is based on the fact that as the world warmed following the coldest part of the last ice age 20,000 years ago, the ice deep inside the Antarctic glaciers warmed more slowly than Earth's surface, just as a frozen turkey put into a hot oven will still be cold inside even after the surface has reached oven temperature. By measuring the remaining difference -- the 20,000-year old ice deep in the West Antarctic ice sheet is about 1 degree Celsius cooler than the surface -- the scientists were able to estimate the original temperature based on how fast pure ice warms up.

Clow measured twice, once in 2011 and again in 2014, the temperature in a 3.4-kilometer-deep (2-mile-deep) borehole from which the West Antarctic Sheet Divide ice core had been drilled during an eight-year project that ended in 2011. Ice at the bottom of the borehole was deposited about 70,000 years ago; ice about one-sixth of the way up about 50,000 years ago; and ice about one-third of the way to the surface 20,000 years ago.

Cuffey developed a technique to combine these temperature measurements, which are smoothed as a result of heat diffusion in the ice, with isotopic measurements of old ice to come up with an estimated temperature of 11.3 degrees, plus or minus 1.8 degrees Celsius, warming since the depths of the ice age.

Interestingly, the Antarctic temperature increased much more rapidly than did Arctic temperatures after the glacial maximum. By 15,000 years ago, Antarctica had warmed to about 75 percent of its temperature today. The Arctic took another 3,000-4,000 years to warm this much, primarily because of the fact that the Northern Hemisphere had huge ice sheets to buffer warming, and the fact that changes in ocean currents and Earth's orbital configuration accelerated warming in the south.

Antarctica was also more sensitive to global carbon dioxide levels, Cuffey said, which increased as the global temperature increased because of changing ocean currents that caused upwelling of carbon-dioxide-rich waters from the depths of the ocean.

The situation today, with global warming driven primarily by human emissions of carbon dioxide from burning fossil fuels, is different from natural cycles, he said. The ability of the oceans to take up carbon dioxide cannot keep up with the rising levels of greenhouse gases in the atmosphere, which means carbon dioxide and global temperatures will continue to increase unless humans cut their carbon dioxide emissions.

原始論文：Kurt M. Cuffey, Gary D. Clow, Eric J. Steig, Christo Buizert, T. J. Fudge, Michelle Koutnik, Edwin D. Waddington, Richard B. Alley, Jeffrey P. Severinghaus. Deglacial temperature history of West Antarctica. Proceedings of the National Academy of Sciences, 2016; 201609132 DOI:10.1073/pnas.1609132113

引用自：University of California - Berkeley. "During last warming period, Antarctica heated up two to three times more than planet average: Amplification of warming at poles consistent with today's climate change models." ScienceDaily. ScienceDaily, 6 December 2016.