太平洋的長遠記憶

原文網址：http://www.whoi.edu/news-release/the-long-memory-of-the-pacific-ocean 

太平洋的長遠記憶

歷史上的寒冷時期仍在太平洋深處上演

海洋有著超群的記憶力：太平洋深處的海水上次見到陽光的時候，神聖羅馬帝國的皇帝為查理曼大帝、中國由宋朝統治，而牛津大學才剛開辦第一門課程。在9至12世紀的那段時間，地球的氣候整體而言較為溫暖；接著在16世紀左右，地球進入了較為寒冷的小冰期。如今海洋表面的溫度又再次升高，但海洋深處知道現在的狀況嗎？

伍茲霍爾海洋研究所和哈佛大學的研究人員，發現太平洋深處的海水在溫度變化方面慢了好幾個世紀，還在適應小冰期的開端。雖然現在大部分的海洋都隨著氣候暖化而改變，太平洋深處卻可能會越來越冷。

WHOI的海洋物理學家Jake Gebbie表示：「這些相當古老的海水長久以來都沒有回到海洋表層附近，使得它們的記憶仍然停留在數百年前，當時歐洲歷經了史上最冷的幾個冬天。」以他為主要作者的這篇研究2019.1.4發表在期刊《科學》(Science)。

研究共同作者，哈佛大學地球與行星科學系的教授Peter Huybers補充：「氣候的變遷可以跨越各種時空尺度。某些區域性的暖化與冷化模式相當有名，像是小冰期和中世紀暖期。我們的目標是發展出一套模型，用來了解海洋內部的性質如何回應地表氣候的變化。」

模擬出來的結果讓他們大為驚訝。

Gebbie表示：「如果過去一千年來，海洋表面大部分的溫度整體來說是降低的，則海裡最不受當今暖化影響的部分可能仍在降溫。」

當然，這個模型不過是真實海洋的簡化版本。為了證實這項預測，Gebbie和Huybers比較了模型呈現的冷化趨勢以及海洋溫度的實際測量結果，包括了1870年代「挑戰者號」船上科學家的測量結果，以及1990年代「世界海洋環流實驗」做出的近期觀測。

挑戰者號是艘英國的木製三桅帆船，原本的設計是當作軍艦，之後卻成為第一艘用來進行現代科學考察的船隻，探索了全世界的海洋和海床。在1872年至1876年的航次期間，船上人員把溫度計下放至海洋深處，並在航海日誌中記下了超過5000筆的溫度測量結果。

Huybers說：「我們把這些歷史數據中的異常值給篩選掉，並仔細考慮許多方法來修正壓力對溫度計的影響，以及下放溫度計時麻繩拉伸造成的深度錯估。」

研究人員接著比較挑戰者號的數據和近代的觀測結果，發現全世界海洋的溫度大部分都升高了。這和預期中的相符，因為20世紀整個地球都在暖化；不過，在太平洋深處大約2公里的地方，溫度卻正在下降。

Gebbie表示：「模型預測結果跟觀測到的趨勢之間有很高的一致性，使我們很有信心地確定此現象是真實存在的。」

這項發現暗示了在現代的全球暖化開始之前的地表氣候變化，仍會影響到目前的氣候暖化程度。之前預估地球在上個世紀吸收多少熱量的時候，會先假設工業革命開始的時候海洋處於熱平衡的狀態。但是Gebbie和Huybers預計太平洋深處的冷化趨勢，使得20世紀海洋吸收的熱量必須要重新下修大約30%。

「要讓海洋跟擁有更多溫室氣體的大氣達到平衡所需的熱量，有一部份似乎早就存在於太平洋深處。」Huybers表示，「這項發現使我們有更大的動機瞭解中世紀暖期和小冰期的成因，進而對現在的暖化趨勢有更多瞭解。」

The long memory of the Pacific Ocean

Historical cooling periods are still playing out in the deep Pacific

The ocean has a long memory. When the water in today’s deep Pacific Ocean last saw sunlight, Charlemagne was the Holy Roman Emperor, the Song Dynasty ruled China and Oxford University had just held its very first class. During that time, between the 9th and 12th centuries, the earth’s climate was generally warmer before the cold of the Little Ice Age settled in around the 16th century. Now ocean surface temperatures are back on the rise but the question is, do the deepest parts of the ocean know that?

Researchers from the Woods Hole Oceanographic Institution (WHOI) and Harvard University have found that the deep Pacific Ocean lags a few centuries behind in terms of temperature and is still adjusting to the entry into the Little Ice Age. Whereas most of the ocean is responding to modern warming, the deep Pacific may be cooling.

“These waters are so old and haven’t been near the surface in so long, they still ‘remember’ what was going on hundreds of years ago when Europe experienced some of its coldest winters in history,” said Jake Gebbie, a physical oceanographer at WHOI and lead author of the study published Jan. 4, 2019, in the journal Science.

"Climate varies across all timescales,” adds Peter Huybers, Professor of Earth and Planetary Sciences at Harvard University and co-author of the paper. “Some regional warming and cooling patterns, like the Little Ice Age and the Medieval Warm Period, are well known. Our goal was to develop a model of how the interior properties of the ocean respond to changes in surface climate.”

What that model showed was surprising.

“If the surface ocean was generally cooling for the better part of the last millennium, those parts of the ocean most isolated from modern warming may still be cooling,” said Gebbie.

The model is, of course, a simplification of the actual ocean. To test the prediction, Gebbie and Huybers compared the cooling trend found in the model to ocean temperature measurements taken by scientists aboard the HMS Challenger in the 1870s and modern observations from the World Ocean Circulation Experiment of the 1990s.

The HMS Challenger, a three-masted wooden sailing ship originally designed as a British warship, was used for the first modern scientific expedition to explore the world’s ocean and seafloor. During the expedition from 1872 to 1876, thermometers were lowered into the ocean depths and more than 5,000 temperature measurements were logged.

“We screened this historical data for outliers and considered a variety of corrections associated with pressure effects on the thermometer and stretching of the hemp rope used for lowering thermometers,” said Huybers.

The researchers then compared the HMS Challenger data to the modern observations and found warming in most parts of the global ocean, as would be expected due to the warming planet over the 20th Century, but cooling in the deep Pacific at a depth of around two kilometers.

“The close correspondence between the predictions and observed trends gave us confidence that this is a real phenomenon,” said Gebbie.

These findings imply that variations in surface climate that predate the onset of modern warming still influence how much the climate is heating up today. Previous estimates of how much heat the Earth had absorbed during the last century assumed an ocean that started out in equilibrium at the beginning of the Industrial Revolution. But Gebbie and Huybers estimate that the deep Pacific cooling trend leads to a downward revision of heat absorbed over the 20th century by about 30 percent.

"Part of the heat needed to bring the ocean into equilibrium with an atmosphere having more greenhouse gases was apparently already present in the deep Pacific,” said Huybers. "These findings increase the impetus for understanding the causes of the Medieval Warm Period and Little Ice Age as a way for better understanding modern warming trends."

1.    原始論文：G. Gebbie, P. Huybers. The Little Ice Age and 20th-century deep Pacific cooling. Science, 2019; 363 (6422): 70 DOI: 10.1126/science.aar8413

引用自：Woods Hole Oceanographic Institution. "The long memory of the Pacific Ocean."