Uncertainties in CMIP5 Climate Projections due to Carbon Cycle Feedbacks

Abstract

In the context of phase 5 of the Coupled Model Intercomparison Project, most climate simulations use prescribed atmospheric CO ₂ concentration and therefore do not interactively include the effect of carbon cycle feedbacks. However, the representative concentration pathway 8.5 (RCP8.5) scenario has additionally been run by earth system models with prescribed CO ₂ emissions. This paper analyzes the climate projections of 11 earth system models (ESMs) that performed both emission-driven and concentration-driven RCP8.5 simulations. When forced by RCP8.5 CO ₂ emissions, models simulate a large spread in atmospheric CO ₂; the simulated 2100 concentrations range between 795 and 1145 ppm. Seven out of the 11 ESMs simulate a larger CO ₂ (on average by 44 ppm, 985 ± 97 ppm by 2100) and hence higher radiative forcing (by 0.25 W m ⁻²) when driven by CO ₂ emissions than for the concentration-driven scenarios (941 ppm). However, most of these models already overestimate the present-day CO ₂, with the present-day biases reasonably well correlated with future atmospheric concentrations’ departure from the prescribed concentration. The uncertainty in CO ₂ projections is mainly attributable to uncertainties in the response of the land carbon cycle. As a result of simulated higher CO ₂ concentrations than in the concentration-driven simulations, temperature projections are generally higher when ESMs are driven with CO ₂ emissions. Global surface temperature change by 2100 (relative to present day) increased by 3.9° ± 0.9°C for the emission-driven simulations compared to 3.7° ± 0.7°C in the concentration-driven simulations. Although the lower ends are comparable in both sets of simulations, the highest climate projections are significantly warmer in the emission-driven simulations because of stronger carbon cycle feedbacks.