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海洋吸熱延緩全球變暖

海洋吸熱延緩全球變暖

Tim Radford 2013-11-28
部分海域的水溫正在上升,最新研究顯示,海洋吸收、儲存了溫室氣體產生的大部分熱量。不過,研究人員警告,海洋只能延緩全球變暖,并不能完全阻止這個趨勢。
????從太空俯瞰太平洋。圖片來源:美國國家航空航天局

????有更多證據表明,大氣升溫的速度明顯放緩可能是因為深海吸收了熱量。

????在東南亞海域的底部,海水溫度正在上升。太平洋一片水域海水溫度上升的速度比過去1萬年快了至少14倍。這個發現目前僅限于太平洋與印度洋交匯的深海區域,如果這個現象在全球都普遍存在,那么氣候變化問題將變得更加緊迫。

????美國新不倫瑞克市羅格斯大學(Rutgers University)的伊爾?羅森塔爾同來自紐約哥倫比亞大學(Columbia University)拉蒙特?多赫帝地球觀測實驗室(the Lamont-Doherty Earth Observatory)以及麻省伍茲霍爾海洋研究所(the Woods Hole Oceanographic Institution in Massachusetts)的同行們在《科學》(Science)雜志上發表文章稱,氣象學家們原以為會被大氣吸收的熱量,很多現在被海洋所吸收,導致深海水溫升高。

????過去幾年,雖然溫室氣體排放量有增無減,但全球變暖速度卻在放緩。現在有證據顯示,溫室氣體所產生的熱量大部分都被海洋吸收、儲存起來。

????透明蟲外殼中的記錄

????不過,現存有關海洋溫度的記錄少得可憐,人們最多只能找到半個世紀前的記錄。羅森塔爾等人于是決定尋找別的可信的方法,他們將目光瞄準了一種古老海洋生物的沉積物。

????一種名為透明蟲(Hyalinea balthica)的單細胞生物只生活在500到1000米深的海域。這種生物身上有一層極其微小的外殼,透明蟲死后,這層外殼便會沉入海底。構成外殼的原料是溶解于海水中的成分,而各種成分的結構隨著海水溫度的變化而變化。水溫越高,海水中鎂元素相對于鈣元素的比重就越大,而這種不同被殘存的透明蟲外殼記錄了下來。

????因此,印尼附近的海洋沉積物保存著隨時間變化的熱量記錄。科學家通過研究海洋中心,解讀自冰河時代結束以來近1萬年的氣候變化模式。從海洋沉積物獲取的數據印證了一系列已知的氣候變化,比如在冰河時代末期曾出現一段很溫暖的時期,又比如“中世紀暖期”曾使得葡萄園在英國興起,再比如在“小冰河期”,倫敦的泰晤士河等河流常常會結冰。

????More evidence has emerged that the apparent slowdown in the rate of atmospheric warming may be explained by heat absorption in the deep ocean.

????Far below the surface, the waters of south-east Asia are heating up. A region of the Pacific is now warming at least 15 times faster than at any time in the last 10,000 years. If this finding – so far limited to the depths where the Pacific and Indian Oceans wash into each other – is true for the blue planet as a whole, then the questions of climate change take on a new urgency.

????Yair Rosenthal of Rutgers University in New Brunswick and colleagues from the Lamont-Doherty Earth Observatory at Columbia University in New York, and at the Woods Hole Oceanographic Institution in Massachusetts, report in the journal Science that deep ocean warming could right now be taking much of the heat that meteorologists had expected to find in the atmosphere.

????In the last few years, even though greenhouse gas levels in the atmosphere have gone up, the rate of increase in global average temperatures has slowed and there is evidence that much of the expected heat is being absorbed by the oceans and carried beneath the surface.

????Record in shells

????But records of ocean temperatures are patchy, and in any case date back only half a century. Rosenthal and his colleagues decided that they could reliably calculate a pattern of temperature changes by looking at a record of deposition through time.

????One little single-celled organism called Hyalinea balthica has evolved to live only at depths of 500 to 1,000 metres. H.balthica makes a microscopic shell, and when it dies, this shell falls to the ocean bottom. It takes the ingredients for the shell from the elements dissolved in the water around it, and the chemical mix available varies with temperature: the warmer the water, the greater the ratio of magnesium to calcium – and this difference is then recorded in the surviving shell.

????So the marine sediments around Indonesia preserve a thermal record of changes with time. The scientists studied ocean cores to “read” a pattern of climate change over the last 10,000 years, since the end of the Ice Age. The readings from the sediments mirror a series of already-known climate shifts – a very warm spell at the end of the Ice Age, a “medieval warm period” when vineyards flourished in Britain, and a “Little Ice Age” when rivers like the Thames of London routinely froze.

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