在對抗氣候變遷時人類活動比自然界的回饋作用更加重要
土壤中的永凍層和深海中的甲烷水合物儲存了大量的古老碳質。當土壤和海洋的溫度升高，這些儲庫可能會分解並大量釋放出一種強力的溫室氣體――甲烷。但問題是這些甲烷真的會進到大氣當中嗎？
包括Michael
Dyonisius(左)的研究人員正在南極洲鑽取冰芯。研究人員可以利用冰芯來判斷氣候暖化時，古老的碳沉積物可能會把多少強力的溫室氣體――甲烷給釋放到大氣當中。圖片來源：Vasilii
Petrenko, University of Rochester
羅徹斯特大學的研究人員，包括地球與環境科學的教授Vasilii
Petrenko和他實驗室的研究生Michael
Dyonisius在內，聯合其他科學家探討了一段地球歷史，當時在某方面來說可以類比成目前的全球暖化。他們發表在《科學》(Science)上的研究結果指出，這些自然界儲有大量甲烷的地方就算會因為暖化而釋出，也只有一些真的會進到大氣裡面。
Dyonisius表示：「我們研究結果的重點之一是相較於自然界的回饋作用產生的甲烷，由人類活動排放出的甲烷才是我們需要更加擔憂的。」
甲烷水合物和永凍層是什麼？
植物死掉之後會在土壤中分解成以碳為主的有機物。在極為寒冷的環境中，有機物裡的碳會結凍，因此不會排到大氣當中而是封存在土壤裡。這會形成永凍層――一種經年累月，即使是在夏季都處於凍結狀態的土壤。永凍層大部分出現在陸地上，主要位於西伯利亞、阿拉斯加與加拿大北方。
永凍層中除了有機碳之外還有很多水結成的冰。當溫度升高使永凍層解凍，這些冰塊也會跟著融化，造成下方的土壤浸在水裡而有利於形成低氧狀態。這對土壤裡會消耗碳並產生甲烷的微生物來說是相當完美的環境。
另一方面，甲烷水合物則大都出現於堆積在大陸邊緣的海洋沉積物中。在甲烷水合物裡面由水分子組成的牢籠會把甲烷分子關住。由於甲烷水合物只能在高壓低溫的環境中形成，因此它們主要的發現地點為深海。如果海洋變得更加溫暖，那麼甲烷水合物所在的海洋沉積物溫度也會跟著提高。這會讓水合物變得不穩定，接著分解而釋放出甲烷氣體。
「由於甲烷是一種極為有力的溫室氣體，因此這些來源只要有一部份的穩定性迅速降低，使得甲烷跑到大氣裡面都會造成嚴重的溫室效應。」Petrenko表示。「隨著氣候持續暖化，我們確實應該擔憂這些儲藏地點會把極為大量的碳釋放到大氣裡面。」
從冰芯蒐集氣候變遷的數據
為了得知日益暖化的狀況下這些古老的碳沉積物可能會釋放多少甲烷到大氣當中，Dyonisius和同事向地球過往的行為模式尋求答案。他們前往南極的泰勒冰河鑽取冰芯。這些冰芯樣品就和時光膠囊一樣，其中含有的微小氣泡保存了過往的微量空氣。研究人員利用融化箱抽取出氣泡裡的古代空氣，接著研究它們的化學成分。
Dyonisius將研究重點放在測量八千到一萬五千年前，地球末次冰消期時的空氣成分。
「這段時期和目前有些類似，地球的環境都從寒冷變成較為溫暖的狀態。」Dyonisius表示。「但是末次冰消期的變化是自然作用，現今的變化則是由人類活動造成的，而且我們的暖化程度還在持續增加。」
分析樣品裡甲烷的同位素碳14之後，團隊發現這些古老碳庫只有排放出一些甲烷。因此Dyonisius總結說「目前來看這些儲存古老碳質的地點變得不穩定，造成會導致強烈暖化的正回饋作用的機率也不高。」
Dyonisius和同事的結論也指出甲烷釋放出來之後只有一小部分可以進到大氣裡面，他們認為這是因為有幾種自然作用可以達到緩衝的效果。
防止甲烷進入大氣的緩衝作用
以甲烷水合物來說，如果甲烷是從深海釋放出來，進到大氣之前大部分就會溶解到水裡並被海洋微生物氧化。羅徹斯特大學地球與環境科學教授John
Kessler的研究專長即為這類作用。另一方面，永凍層裡的甲烷如果釋放出來的地方夠深，可能就會被攝取甲烷的細菌給氧化掉；此外，永凍層裡的碳也有機會完全不會變成甲烷，而是以二氧化碳的形式釋放出來。
Petrenko表示：「看來不論是哪種自然的緩衝作用發揮效果，都能確保釋放出來的甲烷不會太多。」
數據同時顯示末次冰消期的氣候變遷發生時，濕地排放出來的甲烷也跟著增加。因此隨著目前全球持續暖化，未來濕地可能會排放出更多甲烷。
Petrenko表示即便如此「目前人類排放出來的甲烷是濕地排放出來的將近兩倍，同時我們的數據顯示我們不用太擔心大型碳庫會因為未來的暖化而排放出大量甲烷，因此我們應該更加顧慮由人類活動排放出來的甲烷才是。」
To combat climate change, human
activities more important than natural feedbacks
Permafrost in the soil and methane
hydrates deep in the ocean are large reservoirs of ancient carbon. As soil and
ocean temperatures rise, the reservoirs have the potential to break down,
releasing enormous quantities of the potent greenhouse gas methane. But would
this methane actually make it to the atmosphere?
Researchers at the University of Rochester—including
Michael Dyonisius, a graduate student in the lab of Vasilii Petrenko, professor
of earth and environmental sciences—and their collaborators studied methane
emissions from a period in Earth’s history partly analogous to the warming of
Earth today. Their research, published in Science,
indicates that even if methane is released from these large natural stores in
response to warming, very little actually reaches the atmosphere.
“One of our take-home points is that we need to be
more concerned about the anthropogenic emissions—those originating from human
activities—than the natural feedbacks,” Dyonisius says.
What are methane
hydrates and permafrost?
When plants die, they decompose into carbon-based
organic matter in the soil. In extremely cold conditions, the carbon in the
organic matter freezes and becomes trapped instead of being emitted into the atmosphere.
This forms permafrost, soil that has been continuously frozen—even during the
summer—for more than one year. Permafrost is mostly found on land, mainly in
Siberia, Alaska, and Northern Canada.
Along with organic carbon, there is also an abundance
of water ice in permafrost. When the permafrost thaws in rising temperatures,
the ice melts and the underlying soil becomes waterlogged, helping to create
low-oxygen conditions—the perfect environment for microbes in the soil to
consume the carbon and produce methane.
Methane hydrates, on the other hand, are mostly found
in ocean sediments along the continental margins. In methane hydrates, cages of
water molecules trap methane molecules inside. Methane hydrates can only form
under high pressures and low temperatures, so they are mainly found deep in the
ocean. If ocean temperatures rise, so will the temperature of the ocean
sediments where the methane hydrates are located. The hydrates will then
destabilize, fall apart, and release the methane gas.
“If even a fraction of that destabilizes rapidly and
that methane is transferred to the atmosphere, we would have a huge greenhouse
impact because methane is such a potent greenhouse gas,” Petrenko says. “The
concern really has to do with releasing a truly massive amount of carbon from
these stocks into the atmosphere as the climate continues to warm.”
Gathering climate
change data from ice cores
In order to determine how much methane from ancient
carbon deposits might be released to the atmosphere in warming conditions,
Dyonisius and his colleagues turned to patterns in Earth’s past. They drilled
and collected ice cores from Taylor Glacier in Antarctica. The ice core samples
act like time capsules: they contain tiny air bubbles with small quantities of
ancient air trapped inside. The researchers use a melting chamber to extract
the ancient air from the bubbles and then study its chemical composition.
Dyonisius’s research focused on measuring the
composition of air from the time of Earth’s last deglaciation, 8,000-15,000
years ago.
“The time period is a partial analog to today, when
Earth went from a cold state to a warmer state,” Dyonisius says. “But during
the last deglaciation, the change was natural. Now the change is driven by
human activity, and we’re going from a warm state to an even warmer state.”
Analyzing the carbon-14 isotope of methane in the
samples, the group found that methane emissions from the ancient carbon
reservoirs were small. Thus, Dyonisius concludes, “the likelihood of these old
carbon reservoirs destabilizing and creating a large positive warming feedback
in the present day is also low.”
Dyonisius and his collaborators also concluded that
the methane released does not reach the atmosphere in large quantities. The
researchers believe this is due to several natural “buffers.”
Buffers protect
against release to the atmosphere
In the case of methane hydrates, if the methane is
released in the deep ocean, most of it is dissolved and oxidized by ocean
microbes before it ever reaches the atmosphere. Rochester earth and
environmental science professor John Kessler studies these processes. If the
methane in permafrost forms deep enough in the soil, it may be oxidized by
bacteria that eat the methane, or the carbon in the permafrost may never turn
into methane and may instead be released as carbon dioxide.
“It seems like whatever natural buffers are in place
are ensuring there’s not much methane that gets released,” Petrenko says.
The data also shows that methane emissions from
wetlands increased in response to climate change during the last deglaciation,
and it is likely wetland emissions will increase as the world continues to warm
today.
Even so, Petrenko says, “anthropogenic methane
emissions currently are larger than wetland emissions by a factor of about two,
and our data shows we don’t need to be as concerned about large methane
releases from large carbon reservoirs in response to future warming; we should
be more concerned about methane released from human activities.”
原始論文：M. N. Dyonisius, V. V. Petrenko, A. M. Smith,
Q. Hua, B. Yang, J. Schmitt, J. Beck, B. Seth, M. Bock, B. Hmiel, I. Vimont, J.
A. Menking, S. A. Shackleton, D. Baggenstos, T. K. Bauska, R. H. Rhodes, P.
Sperlich, R. Beaudette, C. Harth, M. Kalk, E. J. Brook, H. Fischer, J. P.
Severinghaus, R. F. Weiss. Old
carbon reservoirs were not important in the deglacial methane budget. Science, 2020; 367 (6480):
907 DOI: 10.1126/science.aax0504
引用自：University of Rochester. " To combat climate change, human activities more
important than natural feedbacks"
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