The study, led by researchers from Kyoto University, Hokkaido University, and IIASA, provides one of the first comprehensive multi-model assessments of what a “zero-fossil” global energy system would require.
The findings arrive during an increasingly consequential phase of international fossil fuel transition discussions. Since governments agreed in 2023 at COP28 to “transition away from fossil fuels in energy systems,” international discussions have increasingly shifted from whether fossil fuel phase-out is necessary to how it could be achieved in practice. The study reports ongoing policy discussions emerging from last year's COP30 in Belém, Brazil, as well as through the recent Santa Marta Conference process and the Transitioning Away from Fossil Fuels initiative, both of which are exploring pathways for accelerating the global transition away from coal, oil, and gas.
“Decarbonization and complete fossil fuel phase-out are often treated as if they are the same thing, but our study shows they can imply very different transformation pathways,” explains lead author Shotaro Mori, Assistant Professor at the Graduate School of Engineering, Kyoto University and an alumnus of the IIASA Young Scientists Summer Program (YSSP). “A full fossil fuel phase-out is technically possible, but it requires much faster deployment of renewable electricity, hydrogen systems, and end-use transformation than conventional 1.5°C pathways.”
Using two globally recognized energy system models – the Asia-Pacific Integrated Model-Technology model (AIM-Technology) and the Model for Energy Supply Strategy Alternatives and their General Environmental Impact combined with the Global Biosphere Management Model (MESSAGEix-GLOBIOM) – the researchers compared conventional 1.5°C compliant mitigation pathways with scenarios in which coal, oil, and natural gas are fully phased out between 2050 and 2100.
They found that pathways achieving full fossil fuel phase-out by mid-century rely on rapid direct electrification alongside large-scale deployment of hydrogen-based energy carriers such as hydrogen, ammonia, and synthetic fuels in industries and transportation sectors that are difficult to electrify.
Compared to conventional 1.5°C pathways, the zero-fossil scenarios required:
At the same time, the scenarios also delivered important benefits. Full fossil fuel phase-out substantially reduced residual fossil CO2 emissions and lowered dependence on carbon capture and storage (CCS) and CO2 removal technologies such as bioenergy with carbon capture and storage (BECCS) and direct air carbon capture and storage (DACCS). The study also found that earlier fossil fuel phase-out increased the likelihood of returning global temperatures to 1.5°C after temporary overshoot.
“The results show that phasing out fossil fuels is not simply about replacing one fuel source with another,” says coauthor Volker Krey, who leads the Integrated Assessment and Climate Change Research Group at IIASA. "It implies a profound restructuring of global energy systems, industrial processes, infrastructure investment, and international trade patterns. For example, compared to today we are talking about a 2.5 to 3-fold increase of average annual investments in non-fossil electricity generation in the period 2026 to 2050."
The authors emphasize that the study does not conclude that a complete fossil fuel phase-out is the only pathway consistent with the Paris Agreement's 1.5°C goal. Instead, it highlights the trade-offs between cost-optimal decarbonization pathways that continue limited fossil fuel use with carbon capture and removal, and more ambitious zero-fossil pathways that minimize long-term dependence on these technologies.
Importantly, the researchers note that pathways toward full fossil fuel phase-out must also address questions of equity and just transition, especially for countries that are highly dependent on fossil fuel production and exports. The study points to the need for international cooperation, coordinated transition planning, and complementary policies that support affected regions and workers.
"From a strictly techno-economic standpoint, mitigation pathways that retain limited fossil fuel consumption might be more cost-effective. Yet, full defossilization serves as a vital safeguard against climate uncertainty," highlights Siddharth Joshi, a study coauthor and Research Scholar at IIASA. "By committing the substantial capital required to entirely transform our energy supply, we achieve more than baseline emissions reductions; we engineer a fundamentally lower-risk energy system."
Several of the study's findings also align with themes emerging in the Santa Marta process and the Transitioning Away from Fossil Fuels initiative, including the need to rapidly scale up renewable electricity, hydrogen production, and enabling infrastructure; reduces dependence on CO 2 removal; and address the equity and just transition dimensions of fossil fuel phase-out, particularly for fossil fuel-producing economies.
“This work contributes directly to ongoing international discussions on what 'transitioning away from fossil fuels' means in practice,” notes coauthor Shinichiro Fujimori, a senior guest research scholar in the IIASA Energy, Climate, and Environment Program, who is also a Professor at Kyoto University and associated with the National Institute for Environmental Studies (NIES) in Japan. “The analysis helps clarify both the opportunities and the scale of the transformation challenge facing governments as they prepare the next generation of climate strategies and nationally determined contributions.”
The authors hope the findings will support ongoing international discussions on implementing the COP28 and COP30 outcomes on transitioning away from fossil fuels, including through the Santa Marta Conference process, the Transitioning Away from Fossil Fuels initiative, and future assessments by the Intergovernmental Panel on Climate Change (IPCC).
The foundational research and analysis were developed by Shotaro Mori during his participation in the prestigious IIASA Young Scientists Summer Program (YSSP) in collaboration with IIASA researchers, Siddharth Joshi, Oliver Fricko, Takuya Hara, and Volker Krey, highlighting the role of international scientific collaboration in advancing evidence-based climate policy.
Reference
Mori, S., Joshi, S., Krey, V., Oshiro, K., Fricko, O., Hara, T., & Fujimori, S. (2026). Challenges and opportunities of the full phase-out of fossil fuels under the 1.5°C goal. Nature Communications DOI: 10.1038/s41467-026-72841-7
