As extreme weather events and other impacts of climate change continue to intensify, there is an urgent need to drastically reduce emissions of greenhouse gases (GHGs), exemplified by carbon dioxide (CO₂), and to achieve a decarbonized society with net-zero anthropogenic GHG emissions. Despite the Paris Agreement and other ongoing efforts to this end, there are evidences that the GHGs concentrations continuously rise or even accelerate in the most recent decade. Recent analyses point to the continuing unreliability of GHG emission values reported by individual countries and the inadequacy of scientific bases for evaluating emissions reduction efforts as critical challenges.

Nevertheless, our understanding of climate change has advanced, resulting in widespread recognition that emissions of not only CO₂ but all types of GHGs, including methane (CH₄) and nitrous oxide (N₂O), need to be reduced if ambitious goals of Paris Agreement for limiting global surface air temperature increase below 1.5°C is to be achieved. Reducing short-lived climate forcers (SLCFs)—atmospheric constituents related to air quality, such as nitrogen oxides (NOₓ); black carbon (soot originating from the burning of biomass); and some hydrofluorocarbons (HFCs), a class of alternative fluorocarbon compounds—must also be accounted in mitigation strategies of climate change.

Comprehensive monitoring and budget analyses of these climate-relevant atmospheric substances is critical to providing scientific underpinnings for wide-ranging measures and policies aimed at mitigation. A new research project was launched to observe/estimate the emissions and removals of climate-relevant substances based on precise monitoring in the atmosphere and the ocean, supported by model-based synthesis. This project will be executed over a five-year period from fiscal years 2024 to 2028 as a high-priority strategic research project (S-22) under the Environment Research and Technology Development Fund (JPMEERF24S12200).

The most well-known example of atmospheric GHG monitoring is the continuous observations of CO₂ concentrations conducted at Mauna Loa, Hawaii. There is no question regarding the unparalleled contribution of the Mauna Loa data, which has documented changes in atmospheric CO₂ in a globally representative location since the 1950s. Modern atmospheric monitoring, however, requires the measurement of a wide range of gases with high precision and greater spatial and temporal resolution. To enable the implementation of more finely tuned climate mitigation measures, it is becoming increasingly important to provide not just total global emissions or average atmospheric concentrations but, rather, more detailed information on the distributions of different GHGs and other climate-relevant substances (SLCF etc.) at the scale of individual countries, regions, and even major urban areas.

The S-22 project is structured to address these emerging needs comprehensively through the integration of observations and modeling and by fostering collaborative research between the natural and social sciences. As can be seen in Figure 1, S-22 has four thematic areas that address precise characterization of the current state of the atmosphere based on observations, elucidation of the mechanisms driving climate change using global models, estimation of surface-level emissions and removals, and activities aimed at supporting policy development (described in detail in the following section). The project leverages Japan’s extensive research resources, including observational data from the Greenhouse Gases Observing Satellites (GOSAT) series, regional monitoring networks deployed across the Asia-Pacific, and advanced simulation models. Furthermore, the project aims to respond to societal needs rapidly and flexibly by fostering close collaboration among observation, modeling, and policy-oriented research groups.

One of the key policy contributions targeted by the S-22 project is to the Global Stocktake (GST) effort under the Paris Agreement. To provide context, the S-22 project was preceded by the SII-8 project, whose purpose was to contribute to the GST process for confirming progress toward the goals set out in the Paris Agreement. The first GST was conducted from 2021 to 2023, with the resulting document being adopted at COP28 in 2023. Participating countries are expected to raise their emission-reduction targets based on the outcome of this stocktaking. The importance of reliable scientific data to this process is evident; SII-8 was able to contribute by leveraging Japan’s scientific expertise to provide comprehensive datasets on greenhouse gas (GHG) emissions and removals. It is anticipated that S-22, many of whose members also participated in SII-8, will contribute further to this effort in the second GST, scheduled for 2026–2028.

Various international initiatives are proceeding at a rapid pace. One such initiative is the Global Greenhouse Gas Watch (G3W), led by the World Meteorological Organization (WMO). This initiative seeks to develop a system for observing atmospheric GHGs in the same manner as weather observation and for performing numerical analyses and generating forecasts for GHGs in a manner similar to weather predictions. The program requires international cooperation among participating countries. Accordingly, it is anticipated that Japan, which possesses one of the world’s most advanced observation networks and robust model development and operation capabilities, will play an increasingly significant role in the G3W through the S-22 project.

Another important policy contribution targeted by the project is the provision of input to the IPCC, which compiles and publishes assessment reports that synthesize the state of scientific knowledge on climate change and serves as the most reliable information source for international climate policy (including the GST). Thus, the achievement of outcomes that warrant inclusion in these reports is highly significant. In addition to generating new scientific knowledge through observation and modeling, S-22 seeks to contribute to the development and refinement of methodologies for assessing GHG and SLCF emissions that are defined in the IPCC guidelines. Various other related international initiatives are also underway, including the Global Carbon Project (GCP), Integrated Land–Ecosystem–Atmosphere Process Study (iLEAPS), and the International Global Atmospheric Chemistry (IGAC) project. Many members of S-22 play leading roles and make meaningful contributions to these related initiatives on the global stage.