Emissions targets for CO₂ stabilization as modified by carbon cycle feedbacks

Abstract

Carbon cycle feedbacks will have a direct effect on anthropogenic emissions required to stabilize CO₂ in the atmosphere. In this study, I used an intermediate complexity coupled climate-carbon model to quantify allowable CO₂ emissions for a series of scenarios leading to CO₂ stabilization at levels between 350 and 1000 ppmv. For all scenarios, global temperature did not stabilize over the next several centuries, but rather continued to increase well beyond the point of CO₂ stabilization. Furthermore, neither long-term climate change, nor total allowable CO₂ emissions, were sensitive to the shape of the CO₂ stabilization profile, but only to the final stabilization level. For the 550-stabilization scenario, positive carbon cycle-climate feedbacks required a reduction of annual CO₂ emissions throughout the simulation, with a maximum reduction of 2.3 GtC/yr occurring at 2050. Total emissions over the 21st century were 20% lower than those derived from an equivalent simulation without feedbacks. In two additional runs with varied climate sensitivities, emissions consistent with 550-stabilization were reduced by between 190 and 540 GtC over the next 400 yr relative to the no-feedbacks run. Allowable emissions were further reduced in all cases if CO₂ increases did not affect future vegetation productivity, as this removed an otherwise important negative feedback on atmospheric CO₂.