Development of a near-real-time estimation method for China's CO2 emissions from atmospheric observations —CO2/CH4 variability ratios observed on Hateruma and Yonaguni Islands reflect the emission ratio in China —

　In the Paris Agreement adopted in December 2015, it was agreed to reach a balance between anthropogenic greenhouse gas emissions and removals by sinks in the second half of this century and to achieve net zero emissions. The degree of achievement of each country's reduction targets is usually validated based on an inventory obtained by aggregating various statistical data on greenhouse gas emissions. However, it is crucial to develop independent estimation methods for the emissions for tracking policy implementation.
　Analyzing atmospheric carbon dioxide (CO₂) and methane (CH₄) concentrations observed on Hateruma Island (HAT) and Yonaguni Island (YON)¹⁾, our research team, led by Yasunori Tohjima in the Earth System Division of the National Institute for Environmental Studies, had found that the variability ratio (ΔCO₂/ΔCH₄) in winter reflected the variations in the fossil fuel-related CO₂ (FFCO₂) emissions from China^{2, 3)}. The ΔCO₂/ΔCH₄ ratios showed a gradual increase during 2000-2010 and a marked decrease in February 2020 (Fig. 1). The former change reflected an unprecedented increase in FFCO₂ emissions associated with the rapid economic growth in China, and the latter changes corresponded to the period of a severe lockdown implemented in China. Therefore, we revisited the ΔCO₂/ΔCH₄ ratios observed on both islands to develop a near-real-time estimation method for the temporal change in the FFCO₂ emissions from China.

　We simulated atmospheric CO₂ and CH₄ using an atmospheric transport model (NICAM-TM) and a full set of surface CO₂ and CH₄ fluxes and found a linear relationship between simulated ΔCO₂/ΔCH₄ ratios and FFCO₂/CH₄ emission ratios in China during January, February, and March (Fig. 2). Therefore, we could use this linear relationship to convert the observed ΔCO₂/ΔCH₄ ratios to FFCO₂/CH₄ emission ratios in China. Then, we calculated the change rate of the FFCO₂/CH₄ emission ratios compared to the corresponding averages for the preceding 9-year period (2011–2019), during which relatively stable ΔCO₂/ΔCH₄ ratios were observed. Under the additional assumption that there were no interannual variations in biospheric CO₂ and CH₄ fluxes, we can interpret the rate of change in the FFCO₂/CH₄ emission ratio as the rate of change in FFCO₂ emissions from China. Finally, we computed the weighted averages of the estimated change rates for HAT and YON.

　Using the method described above, we estimated the change in FFCO₂ emissions from China for 2020, 2021, and 2022 compared to the preceding 9-year average (Fig. 3)⁴⁾. The resulting changes in FFCO₂ emissions for January, February, and March (JFM) were 17 ± 8%, −36 ± 7%, and −12 ± 8%, respectively, in 2020 (−10 ± 9% for JFM overall), 18 ± 8%, −2 ± 10%, and 29 ± 12%, respectively, in 2021 (15 ± 10% for JFM overall), and 20 ± 9%, −3 ± 10%, and −10 ± 9%, respectively, in 2022 (2 ± 9% for JFM overall). Our estimation results suggest that FFCO₂ emissions from China, which decreased considerably during February 2020, rebounded with the recovery of socioeconomic activity in China after the COVID lockdown. However, the slight decrease in our estimated FFCO₂ change in March 2022 suggests that the FFCO₂ emissions from China were still being affected in 2022 by the COVID-19 infection status in China.

　The estimation method developed in this study has some drawbacks: the estimated changes do not uniformly cover the emissions from all over China, the estimation period is limited to three months in winter, and the interannual variations in the CO₂ emissions from the terrestrial biosphere and CH₄ emissions would cause uncertainty. Nevertheless, this method is highly useful in the sense that changes in emissions can be estimated in quasi-real time with extremely simple calculations without collecting global observation data and performing large-scale atmospheric transport simulation analyses. Therefore, continuously adapting this method to the observed data at HAT and YON we will monitor future changes in FFCO₂ emissions from China to validate the pledged reduction targets.

This study was supported by funds provided by the Environment Research and Technology Development Fund (JPMEERF21S20800) and the Global Environmental Research Coordinate System from the Ministry of the Environment, Japan (grant no. E1451).

【Article title】
　Near-real-time estimation of fossil fuel CO₂ emissions from China based on atmospheric observations on Hateruma and Yonaguni Islands, Japan 【Authors】
　Yasunori Tohjima, Yosuke Niwa, Prabir K. Patra, Hitoshi Mukai, Toshinobu Machida, Motoki Sasakawa, Kazuhiro Tsuboi, Kazuyuki Saito, Akihiko Ito 【Journal】Progress in Earth and Planetary Science
【URL】https://progearthplanetsci.springeropen.com/articles/10.1186/s40645-023-00542-6 【DOI】10.1186/s40645-023-00542-6

National Institute for Environmental Studies
Earth System Division
Environmental Chemodynamics Section
　Head　Yasunori Tohjima
Biogeochemical Cycle Modeling and Analysis Section
　Head　Akihiko Ito
　Senior Researcher　Yosuke Niwa
Office for Atmospheric and Oceanic Monitoring
　Head　Toshinobu Machida
　Chief Senior Researcher　Motoki Sasakawa
Center for Climate Change Adaptation
　Director　Hitoshi Mukai
Japan Agency for Marine-Earth Science and Technology
Environmental Geochemical Cycle Research Group
　Deputy Group Leader　Prabir K. Patra

Earth System Division of the National Institute for Environmental Studies
Yasunori Tohjima