Abstract
In recent studies, proxy XCH4 retrievals from the Japanese Greenhouse gases Observing SATellite (GOSAT) have been used to constrain top-down estimation of CH4 emissions. Still, the resulting interannual variations often show significant discrepancies over some of the most important CH4 source regions, such as China and Tropical South America, by causes yet to be determined. This study compares monthly CH4 flux estimates from two parallel assimilations of GOSAT XCH4 retrievals from 2010 to 2019 based on the same Ensemble Kalman Filter (EnKF) framework but with the global chemistry transport model (GEOS-Chem v12.5) being run at two different spatial resolutions of 4° × 5° (R4, lon × lat) and 2° × 2.5° (R2, lon × lat) to investigate the effects of resolution-related model errors on the derived long-term global and regional CH4 emission trends. We found that the mean annual global methane emission for the 2010s is 573.04 Tg yr−1 for the inversion using the R4 model, which becomes about 4.4 Tg yr−1 less (568.63 Tg yr−1) when a finer R2 model is used, though both are well within the ensemble range of the 22 top-down results (2008–17) included in the current Global Carbon Project (from 550 Tg yr−1 to 594 Tg yr−1). Compared to the R2 model, the inversion based on the R4 tends to overestimate tropical emissions (by 13.3 Tg yr which is accompanied by a general underestimation (by 8.9 Tg yr−1) in the extratropics. Such a dipole reflects differences in tropical-mid-latitude air exchange in relation to the model’s convective and advective schemes at different resolutions. The two inversions show a rather consistent long-term CH4 emission trend at the global scale and over most of the continents, suggesting that the observed rapid increase in atmospheric methane can largely be attributed to the emission growth from North Africa (1.79 Tg yr−2 for R4 and 1.29 Tg yr−2 for R2) and South America Temperate (1.08 Tg yr−2 for R4 and 1.21 Tg yr−2 for R2) during the first half of the 2010s, and from Eurasia Boreal (1.46 Tg yr−2 for R4 and 1.63 Tg yr−2 for R2) and Tropical South America (1.72 Tg yr−2 for R4 and 1.43 Tg yr−2 for R2) over 2015–19. In the meantime, emissions in Europe have shown a consistent decrease over the past decade. However, the growth rates by the two parallel inversions show significant discrepancies over Eurasia Temperate, South America Temperate, and South Africa, which are also the places where recent GOSAT inversions usually disagree with one other.
摘要
大量研究基于“自上而下”方法同化温室气体观测卫星(GOSAT)的proxy XCH4产品来优化反演全球甲烷(CH4)排放,然而各研究对甲烷重要源地(如中国和热带南美洲)的排放年际变化的评估存在显著差异,原因尚待明确。因此,本研究在基于集合卡尔曼滤波的甲烷排放同化系统中分别使用4°(经向) × 5°(纬向)(R4)和2°(经向) × 2.5°(纬向)(R2)两种空间分辨率的大气化学传输模型,同化GOSAT 2010-19年XCH4观测结果探究模式分辨率对全球和区域CH4排放反演结果及其长期趋势的影响。研究发现R4(573.04 Tg yr-1)和R2(568.63 Tg yr-1)同化反演的全球甲烷年排放均在全球碳计划(Global Carbon Project, GCP)最新报告中给出同化结果范围内(576[550~594] Tg yr-1)。相比于R2,较粗分辨率R4的反演结果会高估热带排放量(13.3 Tg yr-1)而低估热带以外地区的排放(8.9 Tg yr-1),这种偶极子分布差异来源于粗分辨率模式对中高纬度对流层顶的垂直传输以及平流层赤道-极地水平交换的不精确模拟。在大部分区域,两组同化结果反演的甲烷排放的变化趋势相近,表明近十年大气甲烷的增长主要来源于2010-14年北非地区(R4为1.79 Tg yr-2,1.29 Tg yr-2)和南美洲温带地区(R4为1.08 Tg yr-2,R2为1.21 Tg yr-2)的排放增加,以及欧亚寒温带地区(R4为1.46 Tg yr-2,R2为1.63 Tg yr-2)和南美洲热带地区(R4为1.72 Tg yr-2,R2为1.43 Tg yr-2)在2015-19年期间的排放。欧洲的甲烷排放量在过去十年则呈现出下降趋势。然而,两组同化结果在欧亚温带、南美温带和南非地区表现出显著差异,可能的原因包括传输模型的误差和GOSAT探测点较稀疏的空间覆盖。
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Acknowledgements
This work is supported by the National Key R&D Plan of China (Grant No. SQ2019YFE013078), the Key Research Program of the Chinese Academy of Sciences (Grant No. ZDRW-ZS-2019-1), and the National Key R&D Program of China (Grant No. 2017YFB0504000). The support provided by the China Scholarship Council (CSC) and the University of Chinese Academy of Sciences during a visit of Sihong ZHU to the University of Edinburgh are also acknowledged. We thank Professor Paul Palmer for his useful comments. We thank the individual investigators who collected XCO2 and XCH4 data as part of the Total Carbon Column Observing Network (TCCON). We thank the broader GOSAT team, who provided their L1 data to develop the proxy CH4 data product. We also thank the GEOS-Chem community, particularly the team at Harvard who helped maintain the GEOS-Chem model and the NASA Global Modeling and Assimilation Office (GMAO), who provided the MERRA 2 data product.
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Article Highlights
• Inversion modeling systems using CTMs with coarse horizontal resolutions can reliably estimate global total methane emissions and give a rather credible long-term trend in all TransCom-3 regions except for Eurasia Temperate, South America Temperate, and South Africa.
• Emission increases in North Africa and South American Temperate contributed the most strongly to global emission growth from 2010 to 2014. During the second half of the 2010s, accelerated methane increases in the atmosphere were mainly driven by Eurasia Boreal and Tropical South America emissions.
• There are large uncertainties and debates in methane emission from Eurasia Temperate. We discuss possible causes for different emission estimates, particularly over China, to highlight the adverse effects of the model transport error over regions that are poorly constrained by observations or a priori estimates.
This paper is a contribution to the special issue on Carbon Neutrality: Important Roles of Renewable Energies, Carbon Sinks, NETs, and non-CO2 GHGs.
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Zhu, S., Feng, L., Liu, Y. et al. Decadal Methane Emission Trend Inferred from Proxy GOSAT XCH4 Retrievals: Impacts of Transport Model Spatial Resolution. Adv. Atmos. Sci. 39, 1343–1359 (2022). https://doi.org/10.1007/s00376-022-1434-6
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DOI: https://doi.org/10.1007/s00376-022-1434-6