Quantifying the effect of carbon cycle feedback uncertainty in concentration and warming projections under representative concentration pathway emissions scenarios

Abstract:

Carbon-cycle feedbacks are an important and often under-appreciated factor in determining future warming. While the IPCC's primary future warming projections account for an best-estimate of carbon-cycle feedbacks in the Representative Concentration Pathway (RCP) scenarios, they do not include the large uncertainties in carbon-cycle feedback estimates. Here we use both the CMIP5 C4MIP results and HadCM3 Perturbed Physics Ensemble (PPE) to estimate the range of possible carbon cycle feedbacks within each emission scenario associated with the different RCPs. We find that it is possible that carbon-cycle feedbacks could allow a more moderate emissions scenario – such as that conventionally associated with RCP6.0 – to reach concentrations found in the very-high-end RCP8.5 scenario, but only in the highest of all available carbon-cycle feedback estimates. Projections of warming in 2100 relative to preindustrial in CMIP5 could be up to 25% higher – or 10% lower – across the range of carbon cycle feedback estimate examined.