北極的甲烷冷泉能減緩全球暖化？

原文網址：http://www.sciencemag.org/news/2017/05/methane-slowing-global-warming-arctic

Are methane seeps in the Arctic slowing global warming?

北極的甲烷冷泉能減緩全球暖化？

By Randall Hyman

Good news about climate change is especially rare in the Arctic. But now comes news that increases in one greenhouse gas—methane—lead to the dramatic decline of another. Research off the coast of Norway’s Svalbard archipelago suggests that where methane gas bubbles up from seafloor seeps, surface waters directly above absorb twice as much carbon dioxide (CO₂) as surrounding waters. The findings suggest that—in isolated spots in the Arctic—methane seeps in isolated spots in the Arctic could lessen the impact of climate change.

在北極，跟氣候變遷有關的好消息總是特別稀少。但現在有則新聞指出一種溫室氣體――甲烷的增加會導致另一種溫室氣體的含量劇烈降低。在挪威斯瓦巴群島外海進行的研究指出，從海底冷泉(seep)產生的甲烷氣泡冒出海面之處，表層海水吸收的二氧化碳量是周遭海水的兩倍。這項發現顯示在北極零散分布的甲烷冷泉，能減輕氣候變遷帶來的衝擊。

“This is … totally unexpected,” says Brett Thornton, a geochemist at Stockholm University who was not involved in the research. These new findings challenge the popular assumption that methane seeps inevitably increase the global greenhouse gas burden.

未參與此研究，瑞典斯德哥爾摩大學的地球化學家Brett Thornton表示：「這真的是……完全出乎意料。」這項新發現挑戰了廣為接受的假設中，認為甲烷冷泉必然會增加全球溫室氣體的總量。

Methane is a potent greenhouse gas. Molecule for molecule, it traps nearly 30 times as much heat in the atmosphere as CO₂. But scientists know relatively little about its role in the global carbon cycle. Most atmospheric methane comes from biological sources—belching bovines and bacteria feasting on decomposing litter—or from the burning of fossil fuels. In the ocean, methane bubbles up from deep seeps, where it is often stored in icelike crystal lattices of water called hydrates. When those hydrates “melt,” because of changing temperatures and pressures, the methane is released, and it can percolate into the atmosphere above.

甲烷是種強力的溫室氣體。從單一分子的比較來說，空氣中甲烷吸收熱量的能力將近是二氧化碳的30倍。然而，科學家對它在全球碳循環中的地位，相對而言卻知道得不多。大氣中多數甲烷是來自生物――牛隻排氣和細菌分解枯枝落葉的產物――或者是燃燒化石燃料。在海洋，深海冷泉會冒出甲烷氣泡，此處的甲烷通常會儲存在由水分子組成的晶格中，形成像冰塊一樣的水合物。當這些水合物因為溫度和壓力改變而「融化」，甲烷便會從中釋出，可能還會滲入上方的大氣層。

To find out just how much methane the Arctic Ocean was contributing to the global balance, biogeochemist John Pohlman of the U.S. Geological Survey in Woods Hole, Massachusetts, set out to measure the gas close to the ocean surface above known methane seeps near Svalbard during the Arctic summer. He and his team were surprised by how little methane they found. But the bigger surprise was that surface water CO₂ levels dropped whenever their ship crossed a seep. “[The CO₂ data] became the most important part of the story,” Pohlman says.

任職於美國麻塞諸塞州伍茲霍爾海洋研究所，美國地質調查局的地球化學家John Pohlman，為了找出北極海對全球甲烷的收支平衡貢獻了多少，於是在北極夏季出發前往斯瓦巴附近的甲烷冷泉，測量其上方接近海洋表層的氣體性質。他們發現的甲烷含量之少時常令他和他的團隊感到十分驚訝。但最大的驚喜是每次他們的船隻經過冷泉時，表層水中的二氧化碳含量便會下降。Pohlman說：「[二氧化碳的數據]成為整個故事中最重要的一章。」

When combined with other data—sudden drops in water temperature, along with increases in dissolved oxygen and pH at the surface—the lower CO₂ levels were telltale signs of bottom water upwelling and photosynthesis, Pohlman says. Pohlman and his team conclude that the same physical forces that are pushing the methane bubbles up are also pumping nutrient-rich cold waters from the sea bed to the surface, fertilizing phytoplankton blooms that soak up CO₂, they write today in the Proceedings of the National Academy of Sciences.

Pohlman說結合其他數據――表層水溫突然下降，以及溶氧量和pH值上升――便能看出二氧化碳含量下降是底層水上湧和光合作用在該處發生的跡象。Pohlman和他的團隊今日在期刊《美國國家科學院院刊》(Proceedings of the National Academy of Sciences)中撰寫的結論認為，推動甲烷氣泡往上冒出的物理作用力，也將海底富含養分的冷水往上泵至海洋表面，帶給會吸收二氧化碳的光合浮游生物養分並使其大量增長。

Such a “fertilization effect” would be “really surprising,” says Thornton, who has studied methane emissions above seeps in the Laptev and East Siberian seas. “There are lots of nutrients in bottom water and bringing that to the surface could certainly [result in] draw down of CO₂.”

這種「肥料效應」帶來的結果「相當令人驚嘆」。研究拉普捷夫海和西伯利亞東部海域的冷泉上方，甲烷氣體排放量的Thornton表示。「底層水中有許多營養鹽，把它們帶到海洋表層勢必會造成二氧化碳被吸收。」

In fact, the study finds that in such zones, nearly 1900 times more CO₂ is being absorbed than methane emitted. That’s a small but real consolation for those concerned about global warming, Pohlman says: In these limited zones, the atmospheric benefit from CO₂ sequestration is about 230 times greater than the warming effect from methane emissions.

事實上，研究發現這些區域的二氧化碳吸收量是甲烷排出量的1900倍左右。雖然微小，但對全球暖化來說確實是令人安慰的消息。Pohlman表示：「在這些範圍有限的區域中，二氧化碳封存給大氣帶來的益處，大約是甲烷排放造成暖化效應的230倍。」

But whether the findings apply to ocean seeps in other parts of the world is still a big question. Svalbard is in many ways a bellwether. Some methane seeps occur because the hydrates there are barely stable, and can be upset by slight changes in temperature and pressure. Globally, methane hydrate reservoirs may hold as much as one-third the carbon content of all fossil fuels. And with similar seeps along continental margins worldwide, there has been growing concern that methane emissions will dramatically increase as oceans warm.

斯瓦巴在許多方面都扮演領頭羊的腳色，但此發現是否適用於世上其他地區的冷泉，仍然是個大問題。有些甲烷冷泉的形成是因為該處的水合物處於不穩定狀態，容易受到溫度壓力的輕微改變而發生擾動。全球來看，甲烷水合物儲存的碳含量可能多達化石燃料總和的三倍。由於世界各處的大陸邊緣都有類似的冷泉，有越來越多人憂慮隨著海洋暖化，甲烷排放量會急遽增加。

But Pohlman says one can’t count on the methane fertilizing effect being the same everywhere. Even in his study area, it’s apt to change with the seasons. He notes that his team’s data were collected in the constant sunlight of Arctic summer. During the dark polar night, photosynthesis would drop to nearly nothing, and methane emissions wouldn’t be offset by declining CO₂. 

然而，Pohlman表示我們不能指望在所有地方都有相同的肥料效應。即使在他的研究區域，肥料效應也有隨著季節而變動的傾向。他指出他的團隊收集數據時，北極正好是有陽光不停照射的夏季。在極區的永夜期間，光合作用會下降到近乎停止，而甲烷的排放也不會被二氧化碳的減少抵銷。

原始論文：John W. Pohlman, Jens Greinert, Carolyn Ruppel, Anna Silyakova, Lisa Vielstädte, Michael Casso, Jürgen Mienert, and Stefan Bünz. Enhanced CO₂ uptake at a shallow Arctic Ocean seep field overwhelms the positive warming potential of emitted methane. PNAS, 2017 Doi:10.1073/pnas.1618926114