1. Introduction

To achieve carbon neutrality by 2050, national efforts to reduce greenhouse gas (GHG) emissions must accelerate. The United States has committed to reducing GHG emissions by 50% by 2030 (relative to 2005 levels) and achieving net-zero emissions by 2050. In December 2023, the U.S. GHG Center was launched to implement the national GHG strategy. The center is hosted by four agencies: the National Aeronautics and Space Administration (NASA), the National Oceanic and Atmospheric Administration (NOAA), the National Institute of Standards and Technology (NIST), and the U.S. Environmental Protection Agency (EPA). The center will facilitate coordination across federal and nonfederal, domestic, and international entities to integrate and distribute actionable GHG data and to evaluate the effectiveness of emissions reduction efforts (https://earth.gov/ghgcenter).

Similarly to the United States, Japan has pledged to reduce GHG emissions by 46% by 2030 (relative to 2013 levels) and to achieve net-zero emissions by 2050. Japanese national laboratories have a long history of climate change–related research, including observations and modeling of GHG dynamics. Since 2009, Japan has operated the Greenhouse gas Observing Satellite (GOSAT) series, a program managed by the Ministry of the Environment, Japan (MOEJ), the National Institute for Environmental Studies (NIES), and the Japan Aerospace Exploration Agency (JAXA). In addition, long-term monitoring is conducted through ground-based stations, as well as platforms on research vessels and cargo ships, and commercial aircraft, operated by NIES, the Japan Meteorological Agency (JMA), the Meteorological Research Institute (MRI), and the Japan Agency for Marine-Earth Science and Technology (JAMSTEC). These efforts already contribute to numerous scientific applications, particularly GHG monitoring, reporting, and verification (MRV) (Janardanan et al. 2016, 2017).

Together with other research institutions and universities, NIES is expanding its efforts to focus on integrated analyses and assessments of GHG emissions and fluxes, aiming to provide best practices for reconciling bottom-up inventory emissions with top-down estimates using atmospheric data. The work supports policymaking for climate change mitigation, including the ambitious target of limiting warming to 1.5°C. As part of these efforts, NIES is developing an integrated information system for greenhouse gas emissions, observations, and modeling, which may evolve into the Japan GHG Center. As planning for the Japan GHG Center begins, there is substantial interest from scientists and policymakers, especially in the context of emerging/ongoing international climate initiatives, such as the Global Stocktake and the World Meteorological Organization’s (WMO) Global Greenhouse Gas Watch (GGGW). Also, Japan and the United States have a long history of scientific collaboration on satellite missions, including GOSAT, GOSAT-2, Orbiting Carbon Observatory (OCO)-2, and OCO-3, supported by the MOEJ, NIES, JAXA, NASA, Jet Propulsion Laboratory (JPL), and their respective science teams. During the hybrid meeting on 12 April 2024, participants exchanged ideas on the concept of the Japan GHG Center, shared the current status of GHG initiatives in both countries, and discussed future collaborations and potential joint efforts to contribute more effectively to climate change mitigation.

2. Meeting structure and overview

The meeting consisted of a single day of presentations and discussions. NIES opened the meeting with a brief introduction, presenting ongoing efforts in Japan and highlighting the growing need for integrated analyses and assessments of GHG emissions. Emphasis was placed on the importance of disseminating information in a unified manner to support climate change mitigation policies. The potential roles of the GHG Center were outlined, including serving as a “one-stop” information hub/platform, delivering strong and unified messaging to society, and expanding outreach in the policy arena. Three main objectives were set: 1) sharing information on GHG activities in the United States and Japan to identify similarities, 2) identifying potential areas of collaboration for mutual benefit and complementary roles, and 3) discussing the coordination of activities to achieve a greater impact. The meeting also encouraged discussions about international partnerships with the U.S. GHG Center.

The first session provided an overview of the U.S. GHG Center, highlighting its background, objectives, plans, and technical aspects, followed by detailed discussions. The second session focused on Japan’s ongoing research, policy activities, and perspectives regarding the concept of the Japan GHG Center. Presentations covered a range of scientific projects, and it was emphasized that, while numerous components are already in place, the key focus should be on integrating analyses and assessments of GHG emissions and fluxes, and disseminating information as a unified voice, while recognizing the contributions of various agencies. The third session focused on discussions of future Japan–U.S. collaborations, with participants brainstorming ideas for potential joint efforts within a collaborative context. Specific presentations and discussions are summarized below.

3. Overview of the U.S. GHG Center

The U.S. GHG Center presented an overview that included the motivation, vision, mission, and strategic goals of the center. Its primary objectives are to accelerate GHG monitoring, measurement, and reporting, foster collaborations with networks, promote scientific innovation and transparency, develop products and services to meet user needs, and integrate diversity, equity, and inclusion. The initial focus areas include gridded anthropogenic GHG emissions, natural GHG sources and sinks, point-source emission observations, and near-term strategies for refining these areas. The center also aims to expand collaborations with other federal agencies, establish criteria for data inclusion, deliver best estimate products, develop decision support tools, and engage with the business and private sectors on GHG data.

NASA highlighted their substantial efforts to connect observations, models, applications, and decision-making processes. They highlighted the importance of the U.S. GHG Center as a centralized platform where decision-makers, policymakers, and the public can access a unified picture of both atmospheric and Earth observations, including satellite data. Previously, different agencies presented the public with different numbers on their websites, and the current challenge lies in integrating top-down approaches and bottom-up approaches. NASA stressed the importance of partnerships and collaborations, particularly international efforts, to amplify the impact and extend the use of this information at both national and international levels.

NOAA explained their collaboration with focus groups from the GHG Center to see how they could integrate their data on GHG emissions estimated using NOAA satellite data. They also contributed through modeling and in situ measurements, with the goal of providing sustained service delivery that meets the needs of end users.

4. Japan’s research and policy activities

This session showcased seven research activities in Japan. Since 2021, NIES, through the Earth System Division (ESD) and its Climate Change and Air Quality Research Program, alongside the MOEJ/Environmental Restoration and Conservation Agency (ERCA) Strategic Project (SII-8, S-22), has published the “GHG Budget Report—Bulletin of Multi-scale Estimation of Greenhouse Gas Budgets” in collaboration with JAMSTEC, the MRI, and Chiba University. This report contributes to the Global Stocktake processes, specifically “1st Global Stocktake Technical Dialogue Synthesis Report” (https://www.nies.go.jp/sii8_project/en/index.html). The report integrates major research activities and provides the latest updates related to climate change mitigation policies implemented by the MOEJ. Particular attention was given to how data from the GOSAT-series satellites contributed to the verification of GHG emissions at the national and subcontinental scales. Looking ahead, the upcoming Global Observing Satellite for Greenhouse Gases and Water Cycle (GOSAT-GW) satellite, scheduled for launch in early 2025, is expected to play an important role in the pursuit of carbon neutrality by the 2050s (Tanimoto et al. 2025). The importance of increasing efforts to reduce GHG emissions from developing countries was also emphasized.

To improve top-down estimates and verification, the development of inverse modeling capabilities is a key priority. The 4D-Var inversion system, Nonhydrostatic Icosahedral Atmospheric Model (NICAM)-based Inverse Simulation for Monitoring CO₂ and CH₄ (NISMON-CO₂ and CH₄), built upon the NICAM-based Transport Model (NICAM-TM), has demonstrated its utility in successfully estimating fluxes on a global scale using Japan’s atmospheric observations from ground-, ship-, and aircraft-based platforms, as well as global databases. This suggests the potential for near-real-time flux estimates in the future (Niwa et al. 2022). In addition, using global CO₂ concentration fields optimized by NISMON-CO₂, the high-resolution NICAM-TM with tagged tracers can estimate finer-scale fluxes, such as those from the Tokyo megacity (Yamada et al. 2025).

JAMSTEC’s efforts over the decades have resulted in the development of an inverse modeling system for CO₂, CH₄, and N₂O based on the MIROC4-atmospheric chemistry transport model (MIROC4-ACTM), which has been well tested for tropospheric and stratospheric transport and characterized for hydroxyl radicals in CH₄ modeling (Patra et al. 2021). This system has advanced our understanding of regional CO₂ sources and sinks, enabling the evaluation of regional CO₂ fluxes through inversions using ecosystem model simulations and CO₂ observations (Chandra et al. 2022). The CH₄ and N₂O inversion fluxes are evaluated individually and as part of the Global Carbon Project (GCP) budgets (Chandra et al. 2024). For the flux estimates of CO₂ and NO₂ observed by the GOSAT-GW satellite, new approaches are under development, including the WRF-GHG/Chem modeling system, which uses the Vegetation Photosynthesis and Respiration Model (VPRM) and full-chemistry modules at fine resolutions of up to 1 × 1 km² (J. Bisht et al. 2024, unpublished manuscript CO₂ high-resolution simulation using WRF-GHG over the Kanto region in Japan). A model and observation-based divergence method is also being tested to optimize the use of satellite observations with measurement pixels of a few square kilometers. In addition, JAMSTEC’s long-term contributions to ocean–atmosphere pCO₂ measurements aboard the R/V Mirai, which feeds into the Surface Ocean CO₂ Atlas (SOCAT) database, were discussed, along with the estimation of net annual Arctic Ocean CO₂ uptake (Yasunaka et al. 2018) and the evaluation of an Earth system model with atmospheric inversion (Patra et al. 2022). Future plans may include harmonized atmosphere–ocean CO₂ observations from the Arctic R/V Mirai II and the development of remote sensing and internet of things (IoT) sensor–based observation systems.

To enhance the integration of observations and modeling, and to address a broader range of climatically important trace gases, a national strategic research project was funded by the MOEJ. This 5-yr project titled, “Comprehensive Study and Monitoring of Long-lived Greenhouse Gases and Short-lived Climate Forcers toward Mitigation of Climate Change (S-22 Project)” was launched in April 2024. The project aims to advance the comparison of top-down and bottom-up emissions estimates and to produce global, regional, and country-level GHG budgets that will support the Global Stocktake and other scientific and policy-relevant activities.

Ongoing efforts in data sharing and outreach activities were also highlighted. The Center for Global Environmental Research (CGER) at NIES operates a Global Environmental Database (GED), which provides access to a wide range of data, including observations, modeling, inventory, and other scientific products (https://db.cger.nies.go.jp/ged/en/). In addition, the team is also developing a Global Environmental Research Data Management System (GERDaMS), which provides key functions such as metadata creation, versioning, DOI minting, and licensing, to help scientists make their data publicly available. CGER also hosts the GCP Tsukuba International Office (https://www.cger.nies.go.jp/gcp/), which promotes domestic and international research collaborations and engages in public outreach activities aligned with the GCP’s goal of achieving a comprehensive understanding of the global carbon cycle by integrating scientific knowledge of the Earth system and its interactions with human activities to support policymaking. Both the GED and the GCP Tsukuba International Office are expected to play important roles in the future Japan GHG Center.

Regional sustainability studies will complement the national-level project. A notable example is the Seto Inland Sea Carbon-Neutral Research Center (S-CNC) (https://en.s-cnc.hiroshima-u.ac.jp), a new initiative led by Hiroshima University. The center comprises three divisions with the goal of producing innovations for GHG reduction and testing their applicability for societal use. It promotes interdisciplinary research across areas such as green and blue innovations, global carbon dynamics, and social implementation.

5. Discussion of joint Japan–U.S. efforts/collaborations

The discussion then turned to brainstorming potential areas for international collaboration. A lively discussion followed on the integration of top-down and bottom-up methods, focusing on how both countries can contribute to the Global Stocktake. The U.S. GHG Center sought to demonstrate how top-down estimations can be used to improve bottom-up methods, with a few examples to illustrate this approach. Participants agreed that science should provide the best practices for comparing bottom-up and top-down inventories to support climate change mitigation policies. For Japan, the MOEJ is responsible for the national contribution to the Global Stocktake. The MOEJ highlighted the differences in the policy implications of the Global Stocktake compared to emission inventory reporting to the United Nations Framework Convention on Climate Change (UNFCCC), emphasizing the importance of using top-down estimates to refine bottom-up inventories and establish best practices based on the latest scientific understanding.

The discussion on identifying potential areas of collaboration was met with great interest, and six specific areas were suggested.

6. Final remarks

Japan’s climate change–related research has long included extensive observations of GHGs from satellite, aircraft, ship, and ground-based platforms in the Asia–Pacific region, along with global and regional forward and inverse model development for GHGs. These efforts will continue to enhance the ability to make top-down estimates of sources and sinks, contributing to the formulation and verification of climate change mitigation policies. Given the urgency of achieving the 1.5°C goal through the implementation of nationally determined contributions (NDCs) with more ambitious targets, Japan is expected to leverage its research capacity to build a consolidated inter-agency platform. This platform will focus on improving the accuracy of top-down estimates, increasing the transparency of independent bottom-up inventories, and contributing more effectively to the Global Stocktake. In addition to interagency collaborations, international partnerships with the U.S. GHG Center are expected to amplify the impact and provide greater benefits to society as it faces the challenges of achieving carbon neutrality by 2050.

This workshop marks the first step in Japan’s national process toward establishing the Japan GHG Center by engaging key stakeholders, ranging from policymakers to scientists at government and national laboratories, in exploring potential areas of collaboration for mutual interests and complementary roles, with the aim of better serving the global community. The next step involves continuing interagency discussions to define the vision, mission, and objectives of the prospective Japan GHG Center, while maintaining ongoing communication with the U.S. GHG Center. The U.S. GHG Center has assured its continued support and collaboration.