This paper analyzes the climate projections of 11 Earth System Models (ESMs) that performed both emission-driven and concentration-driven Representative Concentration Pathway 8.5 (RCP8.5) simulations. The models simulate a wide range of atmospheric CO₂ concentrations, with projected concentrations by 2100 ranging from 795 to 1145 ppm. Seven out of the 11 ESMs simulate higher CO₂ concentrations and radiative forcing when driven by CO₂ emissions compared to concentration-driven scenarios. The uncertainty in CO₂ projections is primarily due to uncertainties in the response of the land carbon cycle. As a result, temperature projections are generally higher when ESMs are driven with CO₂ emissions, with a multimodel average increase of 3.9°C ± 0.9°C by 2100 compared to 3.7°C ± 0.7°C in concentration-driven simulations. The study highlights the importance of considering carbon cycle feedbacks in climate projections and suggests that the standard CMIP5 climate simulations might have resulted in significantly different climate projection ranges if they were emission-driven rather than concentration-driven.This paper analyzes the climate projections of 11 Earth System Models (ESMs) that performed both emission-driven and concentration-driven Representative Concentration Pathway 8.5 (RCP8.5) simulations. The models simulate a wide range of atmospheric CO₂ concentrations, with projected concentrations by 2100 ranging from 795 to 1145 ppm. Seven out of the 11 ESMs simulate higher CO₂ concentrations and radiative forcing when driven by CO₂ emissions compared to concentration-driven scenarios. The uncertainty in CO₂ projections is primarily due to uncertainties in the response of the land carbon cycle. As a result, temperature projections are generally higher when ESMs are driven with CO₂ emissions, with a multimodel average increase of 3.9°C ± 0.9°C by 2100 compared to 3.7°C ± 0.7°C in concentration-driven simulations. The study highlights the importance of considering carbon cycle feedbacks in climate projections and suggests that the standard CMIP5 climate simulations might have resulted in significantly different climate projection ranges if they were emission-driven rather than concentration-driven.