Scenarios are among the most visible and widely used products of the IPCC. Many kinds of scenarios are used in climate research, but emissions scenarios and the socio-economic assumptions that underpin them have a distinct status because the IPCC orchestrated their development. They have evolved from assessment cycle to assessment cycle and serve as ‘boundary objects’ across Working Groups and as instruments of policy relevance. The field of Integrated Assessment Modelling has emerged to produce these scenarios, thereby taking centre stage within the IPCC assessment process. Because these scenarios harmonise assumptions about the future across disciplines, they are essential tools for the IPCC’s production of a shared assessment of climate research and for ensuring the policy relevance of this assessment. Yet, the reliance on a relatively small set of complex models to generate scenarios spurs concerns about transparency, black-boxed assumptions, and the power of IAMs to define the ‘possibility space’.

Two preliminaries: US experience points to the unprecedented challenge of the rapid, drastic energy transition we face, and the labor needed to accomplish it should regarded as a cost, not a benefit. This chapter thus encourages skepticism regarding claims that the cost of meeting carbon goals will be minimal and proposes questions that should be asked of optimistic studies. The most reliable analyses draw on integrated assessment models, which show we can achieve a two-thirds chance of limiting warming to 2° at moderate cost only by greatly overshooting corresponding carbon budgets, followed by decades of negative emissions using yet-to-be developed carbon removal technologies. Not predicating policy on these technologies, however, results in very high costs. Even so, these models don’t include the likelihood that much of the existing capital stock will be uneconomic to operate at high carbon prices, a potential source of enormous disruption. The closest analogy we have is the post-1989 shock in Eastern Europe when economic opening caused widespread shutdowns. It is important to be honest about costs: rosy forecasts are unconvincing, fail to prepare us for the problems we’ll need to solve and obscure the political economy of policy change.