Energy system transformations for the phase-out of fossil fuels towards 1.5°C future

The final decision of the 2023 United Nations Conference of the Parties (COP28) called for transitioning away from fossil fuels, sparking a growing interest in the full phase-out of fossil fuels1. Integrated assessment and energy system models have outlined energy system transformation pathways to limit global warming to 1.5°C2,3. However, pathways towards a full phase-out of fossil fuels, which may require additional efforts beyond those for the 1.5°C goal, remain unclear4. Here, we employ two global energy system models to explore energy system transformations, and the challenges and opportunities related to attaining zero-fossil (ZF) energy systems. Our results showed that reaching a ZF goal by 2050 would accelerate direct and indirect electrification, involving 1.6–1.8-fold increases in total power generation compared to the cost-optimal 1.5°C pathways. This transformation would inevitably increase cumulative energy supply investments within this century by up to 30% and require the rapid scaling of technologies such as solar and wind power, as well as electrolysers in the near term. Despite opportunities including reduced climate impacts and lower reliance on carbon dioxide removal from the energy and land use sectors, these challenges imply that international society must approach the transition towards ZF energy systems with strong determination.