Under the United Nations Framework Convention on Climate Change (UNFCCC), the international community has set a goal of limiting the rise in global average temperature since the Industrial Revolution to below 1.5 degrees Celsius. However, 2024 is expected to become the hottest year on record, following 2023. For the first time on a single-year basis, the global average temperature increase exceeded 1.5 degrees, reaching 1.55 degrees Celsius. United Nations Secretary-General António Guterres has stated that the world is facing a “climate crisis” and has called on all countries to take more “ambitious” actions [1].

Japan and the U.S. have cooperated on environmental issues since the U.S.-Japan Agreement on Cooperation in Environmental Protection was signed in 1975. In the area of climate change, the two countries launched the Japan–U.S. Climate Partnership in 2021, through which they have collaborated on clean energy development and climate policy. Nonetheless, neither Japan nor the United States is currently on track to achieve net-zero greenhouse gas (GHG) emissions by 2050, the level considered necessary to meet the 1.5-degree target. According to the Climate Change Performance Index (CCPI), published by the think tank Germanwatch and covering 63 countries plus the European Union, the United States ranks 57th and Japan 58th [2]. Furthermore, the Climate Action Tracker (CAT), a consortium of independent scientific research organizations, rates the overall efforts of both Japan and the United States as “insufficient” [3].

The United States has historically taken contrasting positions toward international climate change frameworks, withdrawing from them under Republican administrations and supporting them under Democratic ones. This pattern has been evident in its responses to the Kyoto Protocol, adopted at COP3 under the UNFCCC, and the Paris Agreement, adopted at COP21. In recent years, however, partisan divisions over climate change have intensified, with the issue increasingly taking on the characteristics of a “culture war”. Immediately after taking office in January 2025, President Trump not only announced the withdrawal from the Paris Agreement but also declared a “national energy emergency”, promoting the development of fossil fuels such as oil and coal. At the same time, by excluding solar and wind from the definition of “energy” and “energy resources”, his administration effectively abandoned the previous “all of the above” energy strategy, which would have included renewables like wind and solar. Furthermore, the downsizing of the federal government, led by the Department of Government Efficiency (DOGE), has extended to agencies such as the Environmental Protection Agency (EPA) and the National Oceanic and Atmospheric Administration (NOAA). This has raised concerns not only about the suspension of subsidies but also about the potential weakening or elimination of environmental standards and the loss of scientific data.

Japan is one of the 194 countries plus the European Union that participate in the Paris Agreement. It has promoted a Green Transformation (GX) strategy aimed at achieving economic growth through decarbonization, and in 2023 enacted the GX Promotion Act [4]. However, despite the fact that the Intergovernmental Panel on Climate Change (IPCC) had proposed a “60% reduction compared to 2019 levels” for the voluntary GHG emission reduction target (NDC) by 2035 under the Paris Agreement, Japan’s proposed NDC is “60% compared to 2013 levels” and “49% compared to 2019 levels”. Note that the CAT mentioned above states that Japan, as a developed country, should reduce its emissions by 81% if compared to 2013 levels [5].

While overall evaluations of decarbonization efforts, including policy implementation, remain relatively low in both Japan and the United States, the deployment of renewable energy has shown a consistent upward trend in both countries. In the United States in particular, 95% of newly installed electricity generation capacity in 2024 came from clean energy sources, including battery storage and nuclear power. Solar energy accounted for 62% and wind power for 10%, meaning that these two sources alone made up over 70% of the total [6]. Notably, the states with the highest levels of solar power deployment include Texas, Florida, and Arkansas, all of which are traditionally strongholds of the Republican Party. In addition, more than 80% of the investment driven by the Inflation Reduction Act, the largest climate policy initiative in U.S. history, has been directed toward congressional districts represented by Republican lawmakers [7]. Although political polarization has made renewable energy a flashpoint in America’s culture wars, actual adoption and investment do not necessarily align with party affiliation. This dynamic deserves close attention.

Following the U.S. withdrawal from the Paris Agreement under the first Trump administration, new domestic initiatives have been established to promote decarbonization in accordance with the agreement. America’s Pledge, founded by former New York City Mayor Bloomberg and California Governor Brown, has reported to the UNFCCC Secretariat on its members’ climate change actions as an alternative to the federal government. Bloomberg also donated $45 million from Bloomberg Philanthropies in 2018 to shoulder the U.S. government’s mandatory contributions and has indicated that he intends to do the same for the 2025 withdrawal. The “We Are Still In” declaration initiative, which promotes actions in line with the Paris Agreement, has been signed by state governors, mayors, companies, investors, and education and research institutions, similar to America’s Pledge. These two initiatives have since been integrated under “America Is All In” initiative. In addition, the bipartisan U.S. Climate Alliance, which advances decarbonization efforts led by state governors, now includes the governors of 23 states as well as Guam and Puerto Rico.

A recent study by Harvard Business School researchers focusing on publicly listed companies in the United States revealed that 45% of these firms in key industries such as energy, agriculture, and transportation are engaged in climate solutions [8]. As most countries around the world have joined the Paris Agreement, global financial markets are showing heightened interest in decarbonization. Concepts such as sustainable finance and ESG investment, which emphasize environmental, social, and governance considerations, have gained increasing attention. However, ESG investments have become the subject of culture wars, with anti-ESG investment bills introduced in many state legislatures with the support of conservative think tanks, claiming that ESG investments are radical “woke capitalism”. On the other hand, an international framework called the Task Force on Climate-related Financial Disclosures (TCFD) has been formed to address corporate climate change initiatives, which has now been taken over by International Financial Reporting Standards (IFRS), and many Japanese and U.S. companies, mainly multinationals, are participating. Similarly, many Japanese and U.S. companies are participating in the RE100 initiative in which members pledge to use 100% renewable electricity in their business operations. In this way, a decarbonized society is being constructed by diverse actors, especially in the United States, where initiatives are emerging to replace the role of government. In addition to these points, given the gradual expansion of renewable energy sectors such as solar and wind power in both Japan and the U.S., the cooperation between Japan and the U.S. toward the 1.5°C target is expected to be multidimensional, centered on various non-state actors that are actually implementing decarbonization.