This study gives a synthesis of a model comparison assessing the technological feasibility and economic consequences of achieving greenhouse gas concentration targets that are sufficiently low to keep the increase in global mean temperature below 2 degrees Celsius above pre-industrial levels. All five global energy-environment-economy models show that achieving low greenhouse gas concentration targets is technically feasible and economically viable. The ranking of the importance of individual technology options is robust across models. For the lowest stabilization target (400 ppm CO 2 eq), the use of bio-energy in combination with CCS plays a crucial role, and biomass potential dominates the cost of reaching this target. Without CCS or the considerable extension of renewables the 400 ppm CO 2 eq target is not achievable. Across the models, estimated aggregate costs up to 2100 are below 0.8% global GDP for 550 ppm CO 2 eq stabilization and below 2.5% for the 400 ppm CO 2 eq pathway.

Standard-Nutzungsbedingungen:Die Dokumente auf EconStor dürfen zu eigenen wissenschaftlichen Zwecken und zum Privatgebrauch gespeichert und kopiert werden.Sie dürfen die Dokumente nicht für öffentliche oder kommerzielle Zwecke vervielfältigen, öffentlich ausstellen, öffentlich zugänglich machen, vertreiben oder anderweitig nutzen.Sofern die Verfasser die Dokumente unter Open-Content-Lizenzen (insbesondere CC-Lizenzen) zur Verfügung gestellt haben sollten, gelten abweichend von diesen Nutzungsbedingungen die in der dort genannten Lizenz gewährten Nutzungsrechte. We investigate conditions that aggravate market failures in energy innovations, and suggest optimal policy instruments to address them. Using an intertemporal general equilibrium model we show that "small" market imperfections may trigger a several decades lasting dominance of an incumbent energy technology over a dynamically more efficient competitor, given that the technologies are very good substitutes. Such a "lock-in" into an inferior technology causes significantly higher welfare losses than market failure alone, notably under ambitious mitigation targets. More than other innovative industries, energy markets are prone to these lock-ins because electricity from different technologies is an almost perfect substitute. To guide government intervention, we compare welfare-maximizing technology policies in addition to carbon pricing with regard to their efficiency, effectivity, and robustness. Technology quotas and feed-in-tariffs turn out to be only insignificantly less efficient than first-best subsidies and seem to be more robust against small perturbations. Terms of use: Documents inJEL-Code: O380, Q400, Q540, Q550.

This paper summarizes results from ten global economy-energyenvironment models implementing mechanisms of endogenous technological change (ETC

SummaryThis paper evaluates the consequences of renewable energy policies on welfare, resource rents and energy costs in a world where carbon pricing is imperfect and the regulator seeks to limit emissions to a (cumulative) target. We use a global general equilibrium model with an intertemporal fossil resource sector. We calculate the optimal second-best renewable energy subsidy and compare the resulting welfare level with an efficient first-best carbon pricing policy. If carbon pricing is permanently missing, mitigation costs increase by a multiple (compared to the optimal carbon pricing policy) for a wide range of parameters describing extraction costs, renewable energy costs, substitution possibilities and normative attitudes. Furthermore, we show that small deviations from the second-best subsidy can lead to strong increases in emissions and consumption losses. This confirms the rising concerns about the occurrence of unintended side effects of climate policy { a new version of the green paradox. We extend our second-best analysis by considering two further types of policy instruments: (1) temporary subsidies that are displaced by carbon pricing in the long run and (2) revenue-neutral instruments like a carbon trust and a feed-in-tariff scheme. Although these instruments cause small welfare losses, they have the potential to ease distributional conflicts as they lead to lower energy prices and higher fossil resource rents than the optimal carbon pricing policy. Abstract This paper evaluates the consequences of renewable energy policies on welfare, resource rents and energy costs in a world where carbon pricing is imperfect and the regulator seeks to limit emissions to a (cumulative) target. We use a global general equilibrium model with an intertemporal fossil resource sector. We calculate the optimal second-best renewable energy subsidy and compare the resulting welfare level with an efficient first-best carbon pricing policy. If carbon pricing is permanently missing, mitigation costs increase by a multiple (compared to the optimal carbon pricing policy) for a wide range of parameters describing extraction costs, renewable energy costs, substitution possibilities and normative attitudes. Furthermore, we show that small deviations from the second-best subsidy can lead to strong increases in emissions and consumption losses. This confirms the rising concerns about the occurrence of unintended side effects of climate policy -a new version of the green paradox. We extend our second-best analysis by considering two further types of policy instruments: (1) temporary subsidies that are displaced by carbon pricing in the long run and (2) revenue-neutral instruments like a carbon trust and a feed-in-tariff scheme. Although these instruments cause small welfare losses, they have the potential to ease distributional conflicts as they lead to lower energy prices and higher fossil resource rents than the optimal carbon pricing policy. Keywords