Department of Earth and Environmental Sciences, University of Rochester, Rochester, New York 14627, USA.
Australian Nuclear Science and Technology Organisation (ANSTO), Locked Bag 2001, Kirrawee DC, New South Wales 2232, Australia.
Nature. 2017 Aug 23;548(7668):443-446. doi: 10.1038/nature23316.
Methane (CH) is a powerful greenhouse gas and plays a key part in global atmospheric chemistry. Natural geological emissions (fossil methane vented naturally from marine and terrestrial seeps and mud volcanoes) are thought to contribute around 52 teragrams of methane per year to the global methane source, about 10 per cent of the total, but both bottom-up methods (measuring emissions) and top-down approaches (measuring atmospheric mole fractions and isotopes) for constraining these geological emissions have been associated with large uncertainties. Here we use ice core measurements to quantify the absolute amount of radiocarbon-containing methane (CH) in the past atmosphere and show that geological methane emissions were no higher than 15.4 teragrams per year (95 per cent confidence), averaged over the abrupt warming event that occurred between the Younger Dryas and Preboreal intervals, approximately 11,600 years ago. Assuming that past geological methane emissions were no lower than today, our results indicate that current estimates of today's natural geological methane emissions (about 52 teragrams per year) are too high and, by extension, that current estimates of anthropogenic fossil methane emissions are too low. Our results also improve on and confirm earlier findings that the rapid increase of about 50 per cent in mole fraction of atmospheric methane at the Younger Dryas-Preboreal event was driven by contemporaneous methane from sources such as wetlands; our findings constrain the contribution from old carbon reservoirs (marine methane hydrates, permafrost and methane trapped under ice) to 19 per cent or less (95 per cent confidence). To the extent that the characteristics of the most recent deglaciation and the Younger Dryas-Preboreal warming are comparable to those of the current anthropogenic warming, our measurements suggest that large future atmospheric releases of methane from old carbon sources are unlikely to occur.
甲烷（CH）是一种强大的温室气体，在全球大气化学中起着关键作用。自然地质排放（海洋和陆地渗漏以及泥火山自然释放的化石甲烷）被认为每年向全球甲烷源贡献约 52 太克甲烷，约占总量的 10%，但用于约束这些地质排放的自下而上方法（测量排放）和自上而下方法（测量大气摩尔分数和同位素）都存在很大的不确定性。在这里，我们利用冰芯测量来量化过去大气中含放射性碳的甲烷（CH）的绝对数量，并表明地质甲烷排放不高于每年 15.4 太克（95%置信区间），平均分布在大约 11600 年前发生的新仙女木和北方森林期之间的突然变暖事件中。假设过去的地质甲烷排放不低于今天，我们的结果表明，目前对当今自然地质甲烷排放的估计（约 52 太克/年）过高，并且，由此推断，目前对人为化石甲烷排放的估计过低。我们的结果还改进并证实了早先的发现，即在新仙女木-北方森林事件期间，大气甲烷摩尔分数的快速增加约 50%是由湿地等来源的同时期甲烷驱动的；我们的研究结果将来自旧碳储层（海洋甲烷水合物、永久冻土和冰下甲烷）的贡献限制在 19%或以下（95%置信区间）。在最近的冰川消退和新仙女木-北方森林变暖与当前人为变暖的特征在一定程度上可比的情况下，我们的测量结果表明，从旧碳源向大气中大量释放甲烷的情况不太可能发生。
