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Abstract

In light of uncertainties regarding climate sensitivity and future anthropogenic greenhouse gas emissions, we explore the plausibility of global warming over the next millennium which is significantly higher than what is usually expected. Although efforts to decarbonize the global economy have significantly shifted global anthropogenic emissions away from the most extreme emission scenarios, intermediate emission scenarios are still plausible. Significant warming in these scenarios cannot be ruled out as uncertainties in equilibrium climate sensitivity (ECS) remain very large. Until now, long-term climate change projections and their uncertainties for such scenarios have not been investigated using Earth system models that account for all major carbon cycle feedbacks. Using the fast Earth system model CLIMBER-X with interactive CO2 and CH4 (the latter typically not included in most models), we performed simulations for the next millennium under extended SSP1-2.6, SSP4-3.4 and SSP2-4.5 scenarios. These scenarios are usually associated with peak global warming levels of 1.5, 2 and 3°C, respectively, for an ECS of ∼3°C, considered the best estimate in the latest IPCC report. As ECS values lower or higher than this estimate cannot be ruled out, we emulate a wide range of ECS from 2 to 5°C, defined as the "very likely" range by the IPCC. Our results show that achieving the Paris Agreement goal of a 2°C temperature increase is only feasible for low emission scenarios and if ECS is lower than 3.5°C. With an ECS of 5°C, peak warming in all considered scenarios more than doubles compared to an ECS of 3°C. Approximately 50% of this additional warming is attributed to positive climate–carbon cycle feedbacks with comparable contributions from CO2 and CH4. The interplay between potentially high ECS and carbon cycle feedbacks could drastically enhance future warming, demonstrating the importance of properly accounting for all major climate feedbacks and associated uncertainties in projecting future climate change.