Existing technologies, institutions, and behavioral norms together act to constrain the rate and magnitude of carbon emissions reductions in the coming decades. The inertia of carbon emissions due to such mutually reinforcing physical, economic, and social constraints is referred to as carbon lock-in. Carbon lock-in is a special case of path dependency, which is common in the evolution of complex systems. However, carbon lock-in is particularly prone to entrenchment given the large capital costs, long infrastructure lifetimes, and interrelationships between the socioeconomic and technical systems involved. Further, the urgency of efforts to avoid dangerous climate change exacerbates the liability of even small lock-in risks. Although carbon lock-in has been recognized for years, efforts to characterize the types and causes of carbon lock-in, or to quantitatively assess and evaluate its policy implications, have been limited and scattered across a number of different disciplines. This systematic review of the literature synthesizes what is known about the types and causes of carbon lock-in, including the scale, magnitude, and longevity of the effects, and policy implications. We identify three main types of carbon lock-in and describe how they coevolve: (a) infrastructural and technological, (b) institutional, and (c) behavioral. Although each type of lock-in has its own set of processes, all three are tightly intertwined and contribute to the inertia of carbon emissions. We outline the conditions, opportunities, and strategies for fostering transitions toward less-carbon-intensive emissions trajectories. We conclude by proposing a carbon lock-in research agenda that can help bridge the gaps between science, knowledge, and policy-making.

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International audienceCo-benefits rarely enter quantitative decision-support frameworks, often because the methodologies for their integration are lacking or not known. This review fills in this gap by providing comprehensive methodological guidance on the quantification of co-impacts and their integration into climate-related decision making based on the literature. The article first clarifies the confusion in the literature about related terms and makes a proposal for a more consistent terminological framework, then emphasizes the importance of working in a multiple-objective–multiple-impact framework. It creates a taxonomy of co-impacts and uses this to propose a methodological framework for the identification of the key co-impacts to be assessed for a given climate policy and to avoid double counting. It reviews the different methods available to quantify and monetize different co-impacts and introduces three methodological frameworks that can be used to integrate these results into decision making. On the basis of an initial assessment of selected studies, it also demonstrates that the incorporation of co-impacts can significantly change the outcome of economic assessments. Finally, the review calls for major new research and innovation toward simplified evaluation methods and streamlined tools for more widely applicable appraisals of co-impacts for decision making

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Achieving a truly sustainable energy transition requires progress across multiple dimensions beyond climate change mitigation goals. This article reviews and synthesizes results from disparate strands of literature on the coeffects of mitigation to inform climate policy choices at different governance levels. The literature documents many potential cobenefits of mitigation for nonclimate objectives, such as human health and energy security, but little is known about their overall welfare implications. Integrated model studies highlight that climate policies as part of well-designed policy packages reduce the overall cost of achieving multiple sustainability objectives. The incommensurability and uncertainties around the quantification of coeffects become, however, increasingly pervasive the more the perspective shifts from sectoral and local to economy wide and global, the more objectives are analyzed, and the more the results are expressed in economic rather than nonmonetary terms. Different strings of evidence highlight the role and importance of energy demand reductions for realizing synergies across multiple sustainability objectives.

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