We study the drivers behind the global atmospheric methane (CH₄) increase observed after 2006. Candidate emission and sink scenarios are constructed based on proposed hypotheses in the literature. These scenarios are simulated in the TM5 tracer transport model for 1984–2016 to produce three-dimensional fields of CH₄ and δ¹³C-CH₄, which are compared with observations to test the competing hypotheses in the literature in one common model framework. We find that the fossil fuel (FF) CH₄ emission trend from the Emissions Database for Global Atmospheric Research 4.3.2 inventory does not agree with observed δ¹³C-CH₄. Increased FF CH₄ emissions are unlikely to be the dominant driver for the post-2006 global CH₄ increase despite the possibility for a small FF emission increase. We also find that a significant decrease in the abundance of hydroxyl radicals (OH) cannot explain the post-2006 global CH₄ increase since it does not track the observed decrease in global mean δ¹³C-CH₄. Different CH₄ sinks have different fractionation factors for δ¹³C-CH₄, thus we can investigate the uncertainty introduced by the reaction of CH₄ with tropospheric chlorine (Cl), a CH₄ sink whose abundance, spatial distribution, and temporal changes remain uncertain. Our results show that including or excluding tropospheric Cl as a 13 Tg/year CH₄ sink in our model changes the magnitude of estimated fossil emissions by ∼20%. We also found that by using different wetland emissions based on a static versus a dynamic wetland area map, the partitioning between FF and microbial sources differs by 20 Tg/year, ∼12% of estimated fossil emissions.

中文翻译：

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使用 δ13C-CH4 改进对全球甲烷排放和汇的限制


我们研究了 2006 年之后观察到的全球大气甲烷 (CH ₄ ) 增加背后的驱动因素。候选排放和汇情景是根据文献中提出的假设构建的。这些场景在 1984-2016 年的 TM5 示踪剂输运模型中进行模拟，以产生 CH ₄和δ ¹³ C-CH ₄的三维场，并将其与观测结果进行比较，以在一个通用模型框架中测试文献中的竞争假设。我们发现全球大气研究排放数据库4.3.2清单中的化石燃料(FF) CH ₄排放趋势与观测到的δ ¹³ C-CH ₄不一致。尽管 FF 排放量有可能小幅增加，但 FF CH ₄排放量的增加不太可能成为 2006 年后全球 CH ₄增加的主要驱动力。我们还发现，羟基自由基 (OH) 丰度的显着下降无法解释 2006 年后全球 CH _{4 的}增加，因为它没有跟踪观察到的全球平均值δ ¹³ C-CH ₄的下降。不同的CH ₄汇具有不同的δ ¹³ C-CH ₄分馏因子，因此我们可以研究CH ₄与对流层氯（Cl）反应引入的不确定性，CH ₄汇的丰度、空间分布和时间变化仍然存在不确定。我们的结果表明，在我们的模型中包含或排除对流层 Cl 作为 13 Tg/年 CH ₄汇，会使估计的化石排放量改变约 20%。 我们还发现，通过基于静态湿地面积图和动态湿地面积图使用不同的湿地排放量，FF 和微生物源之间的划分相差 20 Tg/年，约占估计化石排放量的 12%。