Integrated assessment models (IAMs) are increasingly used to evaluate carbon policy impacts on energy structure, but different models can yield considerably different results. This paper seeks to frame model results for policymakers and other consumers of model outputs. In this analysis we compare three models: GCAM, MERGE, and EPPA. We apply diagnostic carbon price scenarios, such as ramps and shocks, to identify key differences in model behavior that drive inter-model variability in projected policy impacts on the U.S. energy system. We report model results using several economic parameterizations and find that variation in carbon emissions across models results primarily from differences in carbon intensity of energy supply. These differences arise because models include different low-carbon energy technology options and vary widely in how flexible the electricity supply sector is at adapting to a change in policy.The timing of emissions abatement is also strongly influenced by whether the model is a simulation or an inter-temporal optimization scheme and the amount of foresight exhibited in the model. Our analysis demonstrates the usefulness of novel IAM diagnostic indicators and clarifies model features that are highly relevant for consumers of model results. field. Some recent examples: EMF29 assessed the role of border carbon adjustments in unilateral climate policy (Böhringer et al., 2012); EMF28 compared scenarios for transforming the European energy system (Weyant et al., 2013); EMF24 compared climate policy and technology scenarios for the U.S. (Fawcett et al., 2014); and EMF27 evaluated interactions between climate change policy architectures and advanced energy technology availabilities at global scale (Weyant and Kriegler, 2014). Each of these studies included more than a dozen models and organizations.
The Assessment of Climate Change Mitigation Pathways and Evaluation of the Robustness ofMitigation Cost Estimates (AMPERE) project included 22 institutions and 17 models (Kriegler, Riahi, et al., 2014). This three-year study assessed the potential costs of delaying climate policy, technology availability and path dependency of energy systems, and the implications of action in a fragmented international policy landscape. Others have addressed a wide range of timely topics (e.g., EMF, 2004;Weyant et al., 2006Weyant et al., , 2013Clarke et al., 2009;Fawcett et al., 2009). Large inter-model comparisons, such as EMF and AMPERE, typically harmonize the models by setting, at a minimum, the values of population and GDP to be similar (e.g., EMF, 2011;Kriegler, 2013).Depending on the focus of the study, other parameters can also be harmonized. This allows different models and methods to be more easily compared. Such model comparison efforts have made model behaviors and features more accessible to the community at large. These inter-model comparison studies are very useful for comparing model responses, but the models and scenarios generally remain under the control of the modelers. Some organizations have mad...
