海洋深層吸納暖化熱度 與海平面上升相關 | 台灣環境資訊協會-環境資訊中心

海洋深層吸納暖化熱度 與海平面上升相關

2010年09月24日
摘譯自2010年9月20日ENS美國,西雅圖報導;謝雯凱編譯;莫聞審校

科學家分析20年來採集自世界各地深層海洋的測量數據,發現有暖化的趨勢存在,並造成海平面上升,特別是在南極洲周圍。

南極洲陸地邊緣的南極底層水流向圖示(圖片來源:維基百科。)

華盛頓大學海洋學家,亦是此研究主要作者沙拉‧波基(Sarah Purkey)表示:「過去的研究便已顯示海洋上層正在暖化,我們的分析結果則確定了海洋深層儲存了多少來自於暖化的額外熱度。根據觀察,暖化現象一直到海底都存在。」

波基與共同作者葛瑞格利‧強森 (Gregory Johnson)解釋,諸如二氧化碳與甲烷這些溫室氣體存在於大氣中,會使得地球變熱。在過去數十年之間,這樣的熱能至少有80%進到海洋中,並在此過程中使海洋暖化。強森是太平洋海洋環境研究室的海洋學家,該研究室位於西雅圖,是美國國家海洋暨大氣管理局旗下的研究機構。

此研究名為「全球深海與南冰洋於1990年代及2000年代之間的暖化現象:對全球熱度與海平面上升之貢獻程度」,將在下一期的《氣候》期刊發表。

這項研究顯示出海平面下1000公尺的深海,大約承接上層海洋吸收熱度的16%。

作者指出造成此深海暖化的數個可能因素──南冰洋的風向轉向、所謂的南極底層水(Antarctic Bottom Water)在密度上的改變,或是底層水以多快的速度在南極大陸周圍形成、在何處沉入海底並填補在這塊世界各大洋中最深且最冷的地方。

科學家在南極洲周圍發現到最為顯著的深海暖化,全球其他地方的深海暖化則隨著與南極洲的距離而逐漸減弱。

研究報告指出,深海中溫度增加的幅度並不多(於南冰洋中約為每10年增加攝氏0.03度,其他地方則較少),但海洋體積龐大,加上龐大的海水容積吸收了可觀的熱度,意味這樣的暖化有著巨大的能源儲存量。

報告中推估,假使熱度不進入深海而進入大氣中(實際上不可能發生),可能使得地球以每10年升高攝氏3度的速度暖化。

「暖化的地球會以兩個方式造成海平面上升」強森説明:「暖化增加海洋溫度,造成海水膨脹,也讓陸冰融化,使更多水流入海洋。膨脹與水量增加都會使得海水侵入陸地。」

海平面已經在上升,其速度自1993年以來,每年平均約為3毫米。其中約有半數來自海洋熱膨脹,另一半則由於進入海洋中的水量增加,後者大部分來自融化的陸冰。

波基與強森指出,過去數十年間,在南極洲周圍海域海平面上升的幅度之中,南冰洋的深海暖化每年便佔有1.2毫米的上升值。

Sea Level Rise Linked to 20 Years of Deep Ocean Warming
SEATTLE, Washington, September 20, 2010 (ENS)

Scientists analyzing 20 years of measurements taken in the deep oceans of the world find a warming trend that is contributing to sea level rise, especially around Antarctica.

"Previous studies have shown that the upper ocean is warming, but our analysis determines how much additional heat the deep ocean is storing from warming observed all the way to the ocean floor," said Sarah Purkey, an oceanographer at the University of Washington and lead author of the new study.

Greenhouse gases in the atmosphere, such as carbon dioxide and methane, cause heating of the Earth. Over the past few decades, at least 80 percent of this heat energy has gone into the ocean, warming it in the process, explain Purkey and her co-author Gregory Johnson, an oceanographer at the Pacific Marine Environmental Laboratory in Seattle, a National Oceanic and Atmospheric Administration research facility.

Their study, "Warming of Global Abyssal and Deep Southern Ocean Waters between the 1990s and 2000s: Contributions to Global Heat and Sea Level Rise Budgets," will be published in an upcoming edition of the "Journal of Climate."

The study shows that the deep ocean, below about 3,300 feet, is taking up about 16 percent of the heat that the upper ocean is absorbing.

The authors note several possible causes for this deep warming - a shift in Southern Ocean winds; a change in the density of what is called Antarctic Bottom Water; or how quickly that bottom water is formed near Antarctica, where it sinks to fill the deepest, coldest portions of the ocean around much of the planet.

The scientists found the strongest deep ocean warming around Antarctica, weakening with distance from its source as it spreads around the globe.

While the temperature increases in the deep ocean are small - about 0.03 degrees Celsius per decade in the deep Southern Ocean, less elsewhere - the large volume of the ocean over which they are found and the high capacity of water to absorb heat means that this warming accounts for a huge amount of energy storage, the authors say.

If this deep ocean heating were going into the atmosphere instead - a physical impossibility - it would be warming at a rate of about three degrees Celsius (over five degrees Fahrenheit) per decade, the authors say.

"A warming Earth causes sea level rise in two ways," explained Johnson. "The warming heats the ocean, causing it to expand, and melts continental ice, adding water to the ocean. The expansion and added water both cause the sea to encroach on the land."

Sea level has been rising at around three millimeters (1/8 of a inch) per year on average since 1993, with about half of that caused by ocean thermal expansion and the other half because of additional water added to the ocean, mostly from melting continental ice.

Purkey and Johnson note that deep warming of the Southern Ocean accounts for about 1.2 mm (about 1/20th of an inch) per year of the sea level rise around Antarctica in the past few decades.

全文及圖片詳見:ENS報導