Reduced methane growth rate explained by decreased Northern Hemisphere microbial sources

摘要

甲烷是对气候有显著变暖效应的一种温室气体rn(只有水蒸气和二氧化碳比它更重要)，然而rn人们对影响其在大气中浓度的因素仍然不是很rn了解。尤其是，20世纪中期其浓度的迅速上升rn在世纪之交却逐渐地(但暂时性地)慢了下来，rn造成其增长速度的这一降低的原因目前仍在争rn论中。两项新的研究工作为这个谜团提供了线rn索，但其得出的结论却是相互冲突的。Fuurn Mirlg Kai等人对北半球和南半球甲烷浓度及同rn位素特征之间的差别进行了测量，并且得出结rn论认为，湿地微生物活动程度的降低是主要原rn因。不断改变的水稻耕作方式似乎能对北半球rn趋势的大约一半做出解释。与这结论形成对rn比的是，Murat Aydin等人将对南极洲冰层中rn束缚的甲烷的测量与一个简单的大气模型结合rn了起来，得出结论认为，大气中甲烷浓度上升rn速度的减慢是由化石燃料生产所排放的甲烷减rn少引起的。在News and Views文章中，MartinrnHeimann对这两项研究得出的不同发现进行了rn讨论。%Atmospheric methane (CH_4) increased through much of the twentieth century, but this trend gradually weakened until a stable state was temporarily reached around the turn of the millennium~_(1,2), after which levels increased once more3. The reasons for the slowdown are incompletely understood, with past work identifying changes in fossil fuel, wetland and agricultural sources and hydroxyl (OH) sinks as important causal factors~(1,4-8). Here we show that the late-twentieth-century changes in the CH_4 growth rates are best explained by reduced microbial sources in the Northern Hemisphere. Our results, based on synchronous time series of atmospheric CH_4 mixing and ~(13)C/~(12)C ratios and a two-box atmospheric model, indicate that the evolution of the mixing ratio requires no significant change in Southern Hemisphere sources between 1984 and 2005. Observed changes in the interhemispheric difference of ~(13)C effectively exclude reduced fossil fuel emissions as the primary cause of the slowdown. The ~(13)C observations are consistent with long-term reductions in agricultural emissions or another microbial source within the Northern Hemisphere. Approximately half (51 ± 18%) of the decrease in Northern Hemisphere CH_4 emissions can be explained by reduced emissions from rice agriculture in Asia over the past three decades associated with increases in fertilizer application9 and reductions in water use~(10-11).