Common but differentiated convergence (CDC): a new conceptual approach to long-term climate policy
This article describes a new concept for an international climate regime for differentiation of future commitments: the 'common but differentiated convergence' approach (CDC). Under CDC, Annex-I countries' per-capita emission allowances converge within a convergence period to a low level. Individual non-Annex-I countries' allowances converge to the same level also within the same period ('common convergence'), but starting when their per-capita emissions are a certain percentage above global average ('differentiated'). Until then they may voluntarily take on 'positively binding' targets. This approach eliminates two concerns often voiced in relation to gradually converging per-capita emissions: (i) advanced developing countries have their commitment to reduce emissions delayed and their targets are not the same as Annex-I countries with equal per-capita emissions; (ii) CDC does not provide excess emission allowances to the least developing countries. Under CDC, stabilizing greenhouse gas concentrations at 550 and 650 ppm CO<sub>2</sub>-equivalent can be reached with participation at roughly 0% and 50% above global average and convergence to around 3 and 4.5 tCO<sub>2</sub>-eq/cap within 40 years. Even if the CDC approach is not implemented in its entirety, it is possible that the step-by-step decisions on the international climate regime can be guided by the principles provided in the CDC approach.
