 A two-layer energy balance model has been developed to extend CMIP5 simulations of global mean surface temperature and thermal expansion to 2300 for RCP2.6, RCP4.5, and RCP8.5. The model was tuned to emulate the CO2 response of individual CMIP5 models and combined with model-specific radiative forcings to generate an ensemble to 2300. Errors in the emulated time series were quantified using a subset of CMIP5 models and factored into the ensemble uncertainty. The results show good agreement with 21st century ensemble projections reported in IPCCAR5 and also compare favorably with individual CMIP5 model simulations post-2100. The framework shows promise as a basis for extending process-based projections of global sea level rise beyond the 21st century time horizon that typifies CMIP5 simulations, and illustrates the differing responses of global mean surface temperature and Earth's energy imbalance to reductions in greenhouse gas emissions. This article was authored by Matthew D. Palmer, Glenn R. Harris, and Jonathan M. Gregory.