Uncertainty in Terrestrial Carbon Cycle Challenges the Assessment of Energy Transition Pathways

The Paris Agreement seeks to combat climate change by limiting global temperature rise to well below 2°C, with aspirations of restricting it to 1.5°C by the end of the century. However, substantial uncertainties persist regarding the pace and direction of energy transitions required to achieve these goals, particularly in reducing fossil fuel dependence and determining the social cost of carbon dioxide (CO2). This study examines the impact of uncertainties in the terrestrial carbon cycle on mitigation strategies and energy transition pathways. Using simulations from 11 models within the TRENDY Model-Intercomparison Project, we incorporated these results into the Hector simple climate model, which is coupled with the Global Change Analysis Model (GCAM), a multisector integrated assessment model. Focusing on scenarios limiting warming to 1.5°C, we evaluated energy trajectories and carbon price projections. Our findings indicate that uncertainties in terrestrial carbon cycle projections significantly influence the pace of global energy transitions. Specifically, the phase-out of unabated coal power generation is projected to occur by 2050 ± 7 years, while the ensemble simulations estimate a carbon price of $170.25 ± 38.84 per tCO2e (2010$) by 2100. These results underscore the critical need to enhance terrestrial carbon cycle representations in integrated assessment models to improve projection reliability. Addressing these uncertainties is essential for guiding effective climate policy, enabling informed decision-making, and supporting the implementation of strategies to meet long-term climate objectives.