Joint Submission to the first Global Stocktake:
The JAXA/GOSAT GHG product for tracking city-level emission
changes
The Japan Aerospace Exploration Agency (JAXA) and the Institute for the Global Environmental
Strategies (IGES) are pleased to submit an input to the Global Stocktake of the Paris Agreement inresponse to the mandate of Decision 19/CMA.1, paragraph 19, 36 and 37. Our previous joint
submission responded to a mitigation guiding question 3 by presenting the decadal monitoring ofglobal GHG concentration trends using the Greenhouse gases Observing SATellite (GOSAT)1. Thissubmission responds to mitigation guiding question 1 2 by giving GHG emissions estimates for world
megacities using GOSAT data.

Summary・ Cities are major sources of GHG emissions, consuming about 75% of the world’s energy andproducing 70% of global carbon emissions 3 . Monitoring trends of emissions can help cities
identify emission sources to more effectively achieve their pledges.
・ JAXA developed the space-based surface GHG Emission Indicator (GEI) for tracking emission
changes at key subnational policy-relevant levels, such as cities. JAXA’s GEI is based on thedifference between the upper and lower-tropospheric CO2 and CH4 concentrations.
・ Of the six-year CO2 emission trends for the Greater Tokyo Area (Figure 1), the GEI indicates alarge emission reduction in 2020, attributed to the significantly reduced economic activities
during the COVID-19 pandemic.
・ GEI is available from the JAXA/GOSAT GHG product. JAXA also plans to release a CH4 version
of GEI to contribute toward local climate mitigation pledges.

1GOSAT is a joint project of the Japan Aerospace Exploration Agency (JAXA), the Ministry of the Environment (MOE), and theNational Institute for Environmental Studies (NIES).
2
What are the past and present trends of greenhouse gas (GHG) emissions by sources and removals by sinks -and their underlying
drivers- and mitigation efforts undertaken by Parties -and their impacts on emissions and removals, including based on theinformation referred to in Article 13, paragraph 7(a), and Article 4, paragraphs 7, 15 and 19, of the Paris Agreement (para 36(a))?
3
United Nations Human Settlements Programme (UN HABITAT) (2020). World Cities Report 2020.
https://unhabitat.org/sites/default/files/2020/10/wcr_2020_report.pdf

1Figure 1. The six-year CO2 emission trends in the Greater Tokyo area derived from GEI.
GEI is calculated with JAXA/GOSAT GHG product from January to April to assess the local
concentration enhancement due to local emissions. The average of GEI between 2016 and 2019 isplotted with a dotted red line.

1. Methodology
1.1 Overview of GHG observations by Japan’s GOSAT satellite
Japan’s Greenhouse gases Observing SATellite (GOSAT) was launched as the first GHG-dedicatedsatellite on the 23rd of January 2009. Since then, GOSAT has monitored changes in atmosphericcarbon dioxide (CO2) and methane (CH4) to advance our understanding of the global carbon cycle andsupport climate monitoring.
The GOSAT observation concept is illustrated in the right image of Fig. 3. GOSAT observations
are based on remote sensing and measure the solar light reflected from the Earth’s surface forcollecting atmospheric CO2 and CH4 data. GOSAT carries a Thermal and Near Infrared Sensor for
Carbon Observation Fourier-Transform Spectrometer (TANSO-FTS). The spectrometer detects gas
absorption spectra of solar radiation in the ShortWave-InfraRed (SWIR) region reflected from theEarth’s surface and the Thermal Infrared Radiation (TIR) emitted from the ground and the atmosphere.
TANSO-FTS simultaneously measures the spectra at the oxygen A band (0.76 μm), weak and strong
CO2 bands (1.6 and 2.0 μm), weak CH4 bands (1.6 μm) with orthogonal polarizations, and a wide TIR
band (5.5–14.3 μm) with 0.2 cm−1 spectral sampling intervals. GOSAT has been collecting global
CO2 and CH4 data continuously every three days with a circular observation footprint of 10.5 km
diameter from an altitude of 666 km.
GOSAT pioneered the two-axis agile pointing mechanism for implementing special target mode
observations for megacities and large power plants. This mechanism has been adopted by other state-2

of-the-art satellites, such as NASA’s OCO-3 and GOSAT-2. The pointing system allows us to
implement multiple observation patterns tailored for target emissions and removals, such as nominal
grid observations over land, glint observations over oceans, and target observations over megacities
and large point sources. The technical details of the GOSAT satellite are described in Kuze et al., 2009,2016.

1.2 JAXA/GOSAT GHG products
Several research teams have utilized GOSAT data to retrieve the column concentration of both
CO2 and CH4. While there are several retrieval methods, most focus on using the reflected sunlight
spectra in the SWIR region for retrieving the total column information. In contrast to conventional
methods, JAXA developed a retrieval algorithm to derive vertical concentration information by usingreflected sunlight with two orthogonal polarizations and thermal emissions observed by GOSAT. Theproduct allows us to use concentration information for the lower and upper troposphere to assess
surface emissions better. The two-layer vertical concentration information, especially the one near thesurface, will enhance our ability to examine surface emissions and removals and advance ourunderstanding of the carbon cycle.
Figure 2 shows a schematic diagram of the satellite GHG observation concept and the difference
between conventional total-column GHG retrieval data and the JAXA/GOSAT GHG product (withtwo-layer vertical information). Due to CO2 emissions and removals at the surface, the lower
troposphere concentration should reflect surface emissions more directly than the total columnaveraged concentration. The JAXA/GOSAT GHG product includes both total and two-layer verticalconcentration to assess anthropogenic emissions better. A detailed technical overview is described inKikuchi et al., 2017.

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Figure 2. A schematic diagram of the JAXA/GOSAT GHG product1.3 Space-based surface GHG Emission Indicator (GEI)
Estimations of GHG emissions from local sources often start with calculating CO2 and CH4
concentration enhancements solely due to the emission sources of interest. One of the biggest
challenges in calculating these enhancements is to define a background that can be used for a reference
regardless of the presence of many different CO2 sources. JAXA developed the space-based surface
GHG Emission Indicator (GEI) to utilize the two-layer vertical information in the JAXA/GOSAT
GHG product. GEI defines CO2 and CH4 concentration enhancement as the difference between theupper and lower-tropospheric CO2 and CH4 concentrations. The upper-tropospheric concentrations areused as the background in the GEI calculation. As illustrated in Fig. 2, the lower tropospheric CO2
concentration should capture changes in the surface emissions better than the total column average
concentration. Then, we assumed that the upper troposphere is less affected by the local emission. The
technical details are described in Kuze et al., 2021.
2. Results
In addition to the global grid observation, GOSAT has implemented target observations at specificlocations where climate mitigation impacts are expected, such as cities. Figure 3 presents target
observations over the Greater Tokyo Area on March 17, 2015. As shown in Fig. 3, the JAXA/GOSAT

4GHG product provides both the total column concentration of GHG and the two-layer verticalinformation (lower troposphere and upper troposphere concentration). The lower-troposphericconcentration data showed a higher CO2 concentration than the upper-tropospheric concentration inthe winter. While CO2 contributions from the local terrestrial biosphere could be nonnegligible, themore elevated CO2 is attributed primarily to the local emissions from human activities in the GreaterTokyo Area. This result demonstrated that the lower-tropospheric concentration data are more
susceptible to local CO2 emission than the conventional total column concentration. Target
observations have been implemented over global megacities to collect more GHG data to track theemission changes. Our results suggested a potential emission tracking skill of the JAXA/GOSAT GHG
product.
Our GEI analysis also suggests that GEI could inform us of timely emission changes due to humanactivity changes in megacities. Figure 1 compares the GEI-based CO2 emission estimation changes
over the Greater Tokyo Area from 2016 through 2021. GEI indicated significant emission reductionsin 2020 compared to the previous years. The timing of the significant emission reductions in 2020 isconsistent with that reported in the literature 4. The emission reduction is attributed to the reduced
economic activities during the COVID-19 pandemic.
These results suggest that the JAXA/GOSAT GHG product has a unique potential to track
emissions changes over megacities, accelerating our understanding of the carbon cycle and climateactions toward the Global Stocktake (GST).

Figure 3. An example of target observations over the Greater Tokyo Area and retrieved two-layer
CO2 concentration data.

4e. g., https://www.nies.go.jp/whatsnew/20211210/20211210-e.html

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3. Contact Persons
Hiroshi Suto, Japan Aerospace Exploration Agency, suto.hiroshi@jaxa.jp
Akibi Tsukui, Institute for Global Environmental Strategies, tsukui@iges.or.jp

4. Reference
Kuze, A., Suto, H., Nakajima, M., and Hamazaki, T.: Thermal and near infrared sensor for carbon
observation Fourier-transform spectrometer on the Greenhouse Gases Observing Satellite forgreenhousegases

monitoring,Appl.

Optics,

48,6716–6733,

https://doi.org/10.1364/AO.48.006716, 2009.
Kuze, A., Suto, H., Shiomi, K., Kawakami, S., Tanaka, M., Ueda, Y., Deguchi, A., Yoshida, J.,
Yamamoto, Y., Kataoka, F., Tay- lor, T. E., and Buijs, H. L.: Update on GOSAT TANSO- FTS
performance, operations, and data products after more than 6 years in space, Atmos. Meas. Tech.,9, 2445–2461, https://doi.org/10.5194/amt-9-2445-2016, 2016.
Kuze, A., Nakamura, T., Oda, T., Yoshida, J., Kikuchi, N., Kataoka, F., Suto, H., SHiomi., K,“Examining partial-column density retrieval of lower-tropospheric CO2 from GOSAT target
observations over global megacities. (Accepted)
Kikuchi, N.; Kuze, A.; Kataoka, F.; Shiomi, K.; Hashimoto, M.; Suto, H.; Knuteson, R.; Iraci, L.;
Yates, E.; Gore, W.; Tanaka T. Three-dimensional distribution of greenhouse gas concentrations
over megacities observed by GOSAT, AGU fall meeting. 2017.

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