科學家在暖期氣候的重建上終於有了等待許久的突破性進展

原始網址：www.sciencedaily.com/releases/2016/07/160718133009.htm

Long-awaited breakthrough in the reconstruction of warm climate phases

科學家在暖期氣候的重建上終於有了等待許久的突破性進展

Scientists from the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research (AWI) have overcome a seeming weakness of global climate models. They had previously not been able to simulate the extreme warm period of the Eocene. One aspect of this era that particularly draws interests to climatologists: It was the only phase in recent history when greenhouse gas concentration was as high as researchers predict it to be for the future. The AWI scientists have now found that the apparent model weakness is due to a misinterpretation of the temperature indicator TEX86. These molecules, which are produced by archaea do not record the surface temperature of the ancient ocean as expected, but rather the temperature of water depths up to 500 metres. In the current issue of the journal Nature Geoscience, the scientists report on this new finding which has now made it possible to correctly simulate the temperature distribution of the Eocene in climate models.

阿爾弗雷德•韋格納研究所暨亥姆霍茲極地與海洋研究中心(AWI)的科學家克服了全球氣候模型中一個相當明顯的缺陷：先前科學家無法重新模擬出始新世(Eocene)時的極端暖期。這段時期特別吸引氣候學家的其中一個方面是，這是近代史上唯一溫室氣體濃度相當高，跟氣候模型預測的未來溫室氣體濃度相仿的時期。AWI的科學家發覺模型中的明顯缺陷是來自於錯誤解讀溫度指標TEX86的含意。這些由古菌(Archaea)製造的分子並不如預期記錄了古代海洋的表層溫度，而是深達500公尺的水溫。在當期《自然—地質科學》(Nature Geoscience)期刊中，科學家發表的這項新發現讓氣候模型有機會能正確模擬出始新世時的全球氣溫分布模式。

Climate scientists often hear the same complaint: How can climate models accurately predict the future of our planet if it is not even possible to correctly reproduce the climate of the past? One of the unsolved problems was that all previous attempts to simulate the extreme temperatures of the Eocene with climate models failed.

氣候學家經常聽到人們一再質疑說：如果氣候模型連正確地重現過去的氣候都無法做到，那怎麼可能用它來準確預測地球未來的命運？在種種尚未解決的難題中，其一便是過往嘗試以氣候模型模擬始新世的極端高溫時都以失敗收場。

At that time, 49 to 55 million years ago, the carbon dioxide content of the air was likely more than 1000 ppm (parts per million) -- i.e. at least two times the current greenhouse gas concentration. The earth warmed up so strongly that the icesheets on Greenland and Antarctica disappeared. Instead of ice crystals, palm trees grew there. "Until recently, we believed that the sea surface temperature near the North Pole at the time was 23 degrees Celsius; in Antarctica, it was believed to have been more than 30 degrees Celsius," says Dr Thomas Laepple, climate researcher at the AWI Potsdam.

在4900萬年至5500萬年前，當時大氣二氧化碳濃度可能超過1000 ppm(百萬分之一)，也就是現今溫室氣體濃度的至少兩倍以上。地球加溫的情形之嚴重，連覆蓋在格陵蘭和南極大陸的冰棚都融化殆盡。當時在那裏見到的可不是冰晶，而是繁茂生長的棕櫚樹。「直到最近我們仍然確信當時北極附近的海水表面溫度為23℃；在南極則可能超過30℃。」波茨坦大學的AWI氣候研究人員， Thomas Laepple表示。

These temperature estimates were based on data from the climate indicator TEX86. This abbreviation stands for a ratio of specific organic compounds produced by archaea, depending on the water temperature in which they lived. "Archaea are unicellular organisms that can in part withstand surprisingly high ambient temperatures. The molecules of the organisms that were living at that time are still preserved in the sedimentary layers of the seafloor. They are one of our most important archives for warm climate conditions, but as we have seen, we decoded them wrongly in the past," says Thomas Laepple.

科學家利用溫度指標 TEX86的數據來估算當時氣溫。這個縮寫代表由古菌產生的特定數種有機化合物之間的比例，其數值會跟它們生長時的水溫有關。「古菌是一種單細胞生物，其中某些種類可以忍受十分驚人的高溫。生存於當時的這些生物製造的分子至今仍保存在海床沉積物當中。它們是我們用來指出溫暖氣候的最重要紀錄之一，但現在我們了解到，之前我們都誤解了它們的含意。」 Thomas Laepple說。

He and his AWI colleague at the time, Sze Ling Ho, first had doubts about the interpretation of the TEX86 temperature indicator during a comparison of climate data from the most recent ice age. The scientists noticed that the TEX86 temperatures were far too cold compared to other geological evidence. "The discrepancy was so obvious that we started to review the TEX86 values of around 3,000 sediment samples from different ocean basins and from different epochs of the Earth. It soon became apparent that the average temperature change inferred from TEX86 was exaggerated, always and on all time scales, by one and a half to two times. The temperature it showed for cold periods was much too cold and the one for warm periods was much too warm," explains geochemist Sze Ling Ho.

他和當時的同事 Sze Ling Ho在比對最近一次冰河期的氣候資料時，首度對TEX86溫度指標的涵義開始感到懷疑。這兩位科學家注意到TEX86指示的溫度跟其他地質證據相比低了許多。「這個差異是如此顯而易見，使我們開始重新審視將近3000筆不同地球歷史時代以及不同海盆沉積物樣品中的TEX86值。很快地，我們就發現所有時期從TEX86推論而出的溫度變化平均值，都明顯比其他指標放大了1.5到2倍。它所顯示的溫度在寒冷時期過於嚴酷；而在溫暖時期則太過炎熱。」地球化學家Sze Ling Ho如此解釋。

The cause of this pattern had to be of a fundamental nature, a suspicion that was confirmed upon closer analysis. "TEX86 had previously been interpreted as an indicator of sea surface temperature, in spite the fact that the archaea that produce TEX86 rarely directly live at the sea surface. Through the comparison with other climate archives, we have been able to constrain the depth in which the TEX86 signal is produced. We now assume that TEX86 represents the water temperature at a depth of up to 500 metres," Sze Ling Ho explains.

必然有相當基礎的原理可以解釋這種情形，而此猜想也經由更加詳細的分析得到證實。「TEX86以往被當成是海洋表面溫度的指標，儘管生成TEX86的古菌很少直接生存在海洋表面。經由比對其他氣候研究方面的文獻，我們可以界定出製造TEX86訊號的深度。現在，我們推測TEX86代表的水溫深度最深可達500公尺。」 Sze Ling Ho解釋。

At this water depth, the temperature difference between the tropical oceans and the polar seas is smaller than at the surface. This has direct consequences for climate reconstruction, since the information generated from the indicator is differently translated into temperature values. "In practice, the TEX86 extreme values need to be roughly halved in the climate reconstructions. Comparing the corrected temperatures with the models shows that they now reflect the climate of the Eocene in a realistic and physically consistent way," explains Thomas Laepple.

熱帶跟極區海洋之間於此深度的溫度差異較海洋表面小了許多。這在重建氣候時會帶來直接影響，因為從TEX86指標得到的資訊會被轉化成不同數值的溫度。「實際上，重建氣候時必須要將TEX86呈現出的極端數值折半才行。溫度修正後的氣候模型跟之前相比，現在反映出的始新世氣候更加真實且較符合物理法則。」Thomas Laepple這麼解釋。

However, we also have to correct our temperature-conception of the Eocene. Thomas Laepple: "The era remains the warmest period of the past 65 million years. The water temperatures that we assumed for the Arctic and Antarctica, though, were overstated by at least ten degrees Celsius. Now, we know that the water in the Southern Ocean had a temperature of about 20 to 25 degrees Celsius at that time. The region was therefore still warm enough for there to be palm trees sprouting on the beach."

然而，我們對始新世時的氣溫概念也要有所修正。 Thomas Laepple說：「雖然我們之前對南北極的海水溫度至少高估了10℃以上，這仍然是過去6500萬年以來最溫暖的時期。現在我們得知當時南大洋的海水溫度約莫是20至25℃。因此這個區域仍然溫暖到足以使棕櫚樹在沙灘上繁茂生長。」

引用自：Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research. "Long-awaited breakthrough in the reconstruction of warm climate phases." ScienceDaily. ScienceDaily, 18 July 2016.