| Summary: | In light of uncertainties regarding climate sensitivity and future greenhouse gas emissions, we explore the plausibility of sustained global warming over the next millennium higher than what is usually expected. Using an Earth system model with interactive CO2 and CH4, we performed simulations for the next millennium under extended SSP1-2.6, SSP4-3.4 and SSP2-4.5 scenarios. These scenarios are associated with peak global warming levels of 1.5, 2, and 3°C, respectively, for an equilibrium climate sensitivity (ECS) of ∼3K; considered the best estimate in the latest IPCC report. With ECS=5K, which is within the likely range given by the IPCC, peak warming in all these scenarios more than doubles. Approximately 50% of this additional warming is attributed to positive carbon cycle feedbacks with similar contributions from CO2 and CH4. Residual CO2 emissions of 1 PgC yr−1 (<10% of present-day) for the duration of the entire millennium would allow high atmospheric CO2 concentration levels to remain constant, keeping global temperatures climbing until 3000 CE. Under the highest ECS considered, warming in SSP2-4.5 can be sustained around 8°C, which would have serious consequences for strongly nonlinear Earth system elements such as the Greenland and Antarctic ice sheets. This study shows that a hothouse Earth trajectory cannot be excluded, even without potentially significant nonlinear positive feedbacks not included in the model. It demonstrates the importance of properly accounting for all major climate-related feedbacks and associated uncertainties, to correctly assess of the probability of irreversibly locking the planet on a path to a warm state.
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