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Research progress on China’s emissions of greenhouse gas methane by Yuzhong Zhang’s group at Westlake University


14, 2022

PRESS INQUIRIES Xueping Dai
Email: daixueping@westlake.edu.cn
Phone: +86-(0)571-86889887
School of Engineering

Methane is an important greenhouse gas. In 2022, the atmospheric concentrations of methane surpassed 1900 ppbv, a value three times the preindustrial level. During the past decade, the increase in methane concentrations shows a sign of acceleration with 2020 and 2021 being the years with largest growth rates since 1984. If methane concentrations continue to increase at this rate, it will offset any benefits from decarbonization in the coming decades, posing further challenges to the Paris Accord climate goal. As part of the “Carbon Neutrality” strategy, China plans to actively reduce methane emissions in the 2020s. A better understanding of the status quo not only provides useful input to the plan making, but also establishes a reference point necessary for tracking future progress.


Methane in the atmosphere comes from a series of human activities, including fossil fuel production and consumption (coal, oil, ang natural gas), agricultural activities (rice cultivation and livestock), waste treatment (landfills, wastewater treatment). Compared to other major economies (e.g., the USA, India, Russia, Brazil), coal mining and rice cultivation stands out as two major sectors in China, which combined accounts for more than 50%. In addition to methane emissions, coal and rice production are also crucially related with major issues such as energy supply, food security, and environmental protection.


A recent study by Yuzhong Zhang's group at Westlake University and Shuangxi Fang’s group at Zhejiang University of Technology has quantified methane emissions from China during 2010-2017 using an inverse analysis of long-term surface and satellite observations, which revealed the spatial distribution and temporal trends of China's methane emissions during the period and, more importantly, their interactions with recent energy, environmental, and agricultural policies. The study is published in Proceedings of the National Academy of Sciences with a title "Observed changes in China's methane emissions linked to policy drivers".

The spatial gradient of atmospheric methane concentrations within a region is related to the methane emission intensity of this region, but is also influenced by complicated atmospheric transport processes, resulting in a complex relationship between concentration gradients and emission fluxes. The Zhang and Fang groups have built a high-resolution inverse analysis system covering East Asia to interpret surface and satellite observations, which used an atmospheric transport model to quantitatively resolve the concentration gradients and emission fluxes relationship. The derived dataset of their inverse analysis is accessible through a public data repository https://doi.org/10.57760/sciencedb.02269 (Figure 1). Their result shows that the annual methane emissions of China during 2010-2017 amounts to 54 Tg/a with 50 Tg/a from anthropogenic sources and that China’s annual methane emissions increase by 0.73 Tg/a.


Figure 1. Spatial distribution and temporal trends of China's methane emissions during 2010-2017 derived from an inversion of satellite and surface observations.

This study used new data from a surface observation network across China, which provides long-term, high-quality methane measurements, and combined surface data with satellite data. These observations greatly improve the ability of the inversion to constrain methane emissions, making it feasible for researchers to analyze at the sub-national level (Figure 2). The researchers then attributed the derived methane fluxes to multiple source sectors (e.g., rice and coal) based on spatial information, which allows for interpretation of sectoral emission trends.


Figure 2. Prior and posterior methane emissions and emission trends during 2010-2017 from China and varied sub-national regions within China.

The study found that coal methane emissions show spatially varying trends in different regions. For example, coal methane emissions from Southwest have decreased, while those from North China increased. But within North China, the Qinshui Basin in southern Shanxi showed decreases (Figure 3). These results are generally consistent with China's energy policy of close-up of small coal mines (concentrated in Southwest China) and consolidating large coal mines (mainly in North China). The decreases in the Qinshui Basin are thought to benefit from the “first extracting coal bed methane and then coal” strategy in the region, which is the largest coal bed methane production region of China.


Figure 3 Inferred 2010-2017 trends for methane emissions from coal mining.


The study also showed that methane emissions over the rice cultivation regions (i.e., East, Central, and Northeast China) have increased during 2010-2017 (Figure 4). This result is inconsistent with the general understanding that the rice production over these regions have been stable in the past decade. The authors explained this unexpected increase with the increase in straw retention practices during the period (Figure 4). Straw retention, a practice promoted by current policy because it increases crop yields and reduced air pollution, can enhance methane emissions owing to increased soil organic carbon. The work demonstrates that this side effect of straw retention requires further evaluation.


Figure 4. Analysis of factors driving 2010-2017 trends for methane emissions from rice paddies.

Summary

This study established a high-resolution inverse system that can assimilate long-term surface and satellite observations to quantify methane emissions from China. Using this system, the authors investigated changes in methane emissions from coal mining and rice cultivation, two major sectors in China, and revealed region-dependent responses of methane emissions to energy, environmental, and agricultural policies. These findings may provide useful input to methane policy making.

Yuzhong Zhang Lab at Westlake University study atmospheric chemistry that underlying the environmental problems such as air pollution and climate change. The group thrive to understand the changes of key atmospheric components across varied scales and evaluate the impact of these changes on human and our environment. This is done through development of process and statistical tools to interpret big data from satellite, airplanes, and surface networks.


The lab is now having opening for assistant researchers and post-doctors. We welcome applications from those with training in atmospheric chemistry, environmental sciences, ecology, and related disciplines.

Emailzhangyuzhong@westlake.edu.cn

Websiteatmosphere-westlake.com