1.Climate uncertainty impacts on optimal mitigation pathways and social cost of carbon

Authors:Christopher J. Smith, Alaa Al Khourdajie, Pu Yang, Doris Folini

Abstract: Emissions pathways used in climate policy analysis are often derived from integrated assessment models (IAMs). However, such emissions pathways do not typically include climate feedbacks on socioeconomic systems and by extension do not consider climate uncertainty in their construction. Here we show that climate uncertainty alone significantly changes the cost-benefit optimal CO$_2$ emissions, varying from -14 to +12 GtCO$_2$ yr$^{-1}$ in 2050 (5-95% range) for an ensemble of scenarios that limit warming to 1.5{\deg}C with low overshoot. Climate uncertainty is also responsible for a factor of five range in the social cost of carbon (SCC) in this scenario ensemble. Equilibrium climate sensitivity (ECS) and the strength of present-day aerosol radiative forcing are strong determinants of SCC and optimal mid-century CO$_2$ emissions. This confirms that reducing climate uncertainty can refine cost-optimal emissions projections, and points to a missing feedback between climate and emissions in scenario construction.