Emissions Scenarios

Emissions scenarios are hypothetical trajectories of future greenhouse gas emissions used to drive Earth System Models and produce climate projections. They are not predictions; they are boundary conditions — what-if assumptions about economic development, technological change, population growth, and policy ambition over the coming century. The scenarios produced by the IPCC and the broader research community have become the default language for framing climate risk, and that framing deserves scrutiny.

The most widely used scenarios, the Shared Socioeconomic Pathways (SSPs), span a range from SSP1-1.9 (sustainability-oriented development with rapid emissions reductions) to SSP5-8.5 (fossil-fueled development with high emissions). The intermediate scenarios — SSP2-4.5 and SSP3-7.0 — receive the most attention in policy discussions. But the selection of which scenarios to emphasize is itself a political choice. SSP5-8.5, the highest-emissions scenario, has been criticized as increasingly implausible given recent renewable energy cost reductions, yet it remains the benchmark for worst-case assessments. Conversely, SSP1-1.9 requires assumptions about global cooperation that appear politically unrealistic.

The deeper problem is that scenarios treat emissions as exogenous inputs to the climate system, decoupled from the Earth system dynamics they perturb. In reality, emissions and climate are coupled: carbon cycle feedbacks may reduce the effectiveness of natural sinks, making the same emission trajectory produce higher atmospheric concentrations than models assume. The scenario framework does not capture this coupling. It treats the human system and the Earth system as separate modules that interact only at the interface of emission totals, not as a single coupled dynamics.

The scenario framework is a necessary fiction — necessary because policy requires quantified inputs, fiction because it pretends that the future of human civilization can be captured in five narrative variants. The real future will not follow any scenario. It will be messier, more contingent, and more surprising than the smoothed trajectories suggest. The question is whether treating the fiction as real helps or harms our capacity to respond.