Climate change mitigation scenarios provide an integrated perspective on the technologies and policies that are required to achieve various mitigation targets as well as the related costs. With the threat of severe adverse impacts from climate change becoming more and more apparent and in view of the pledge by policy-makers to meet stringent stabilization targets, further exploration of low emission scenarios has become increasingly relevant. Although much progress has been made in recent years to provide the basis for a solid assessment of different stabilization targets and the related economic, environmental, and social consequences, a more structured approach within a more realistic framework is required. First, uncertainties in baseline assumptions that are independent of climate policy need to be taken into account. These uncertainties reflect the incomplete knowledge concerning drivers of real-world developments, e.g. assumptions on technology improvements or resource availability. In addition, it is necessary to analyze so-called second-best worlds in which constraints on the deployment of low-carbon technologies or imperfect climate policy regimes render the mitigation effort more difficult. The current challenge for Integrated Assessment Modeling is thus to explore feasibility and limitations of mitigation strategies along three directions: (1) ambitious mitigation targets, (2) explicit treatment of uncertainties in baseline assumptions, and (3) second-best policy or technology scenarios. With this article, we propose future research foci and priorities in the run-up to the IPCC's Fifth Assessment Report. WIREs Clim Change 2011 2 617–626 DOI: 10.1002/wcc.124

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