Uncertainties in CMIP5 Climate Projections due to Carbon Cycle Feedbacks

TLDR

In CMIP5 climate simulations, atmospheric CO₂ is typically prescribed, omitting interactive carbon‑cycle feedbacks, though RCP8.5 has also been run with prescribed emissions. The study examines climate projections from 11 earth system models under both emission‑driven and concentration‑driven RCP8.5 scenarios. It compares the results of emission‑driven simulations with those of concentration‑driven runs to assess the impact of carbon‑cycle feedbacks. Emission‑driven RCP8.5 simulations produce a larger spread in atmospheric CO₂ (795–1145 ppm) and higher radiative forcing, with temperature increases of 3.9 ± 0.9 °C by 2100 versus 3.7 ± 0.7 °C in concentration‑driven runs, largely because of greater land‑carbon‑cycle feedbacks.

Abstract

Abstract In the context of phase 5 of the Coupled Model Intercomparison Project, most climate simulations use prescribed atmospheric CO2 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 CO2 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 CO2 emissions, models simulate a large spread in atmospheric CO2; the simulated 2100 concentrations range between 795 and 1145 ppm. Seven out of the 11 ESMs simulate a larger CO2 (on average by 44 ppm, 985 ± 97 ppm by 2100) and hence higher radiative forcing (by 0.25 W m−2) when driven by CO2 emissions than for the concentration-driven scenarios (941 ppm). However, most of these models already overestimate the present-day CO2, with the present-day biases reasonably well correlated with future atmospheric concentrations’ departure from the prescribed concentration. The uncertainty in CO2 projections is mainly attributable to uncertainties in the response of the land carbon cycle. As a result of simulated higher CO2 concentrations than in the concentration-driven simulations, temperature projections are generally higher when ESMs are driven with CO2 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.