Introducing FLEX: a simplified framework for future scenario exploration

Climate scenarios for impact assessment and policy targets are generally drawn from Integrated Assessment Model databases, which explore diverse but ad-hoc futures, making it difficult to inform the effectiveness of individual policy measures. Pre-defined climate target objectives also tend to cluster scenarios around common thresholds, such as 1.5 or 2 degrees, failing to sample the full space of Paris-compatible climate futures. Finally, some scenario exercises provide only near-term futures, making them difficult to reconcile with end-of-century warming targets.

To address these issues, we present FLEX (Framework for Long-term EXtensions), a toolkit that allows scenarios to be indefinitely extended by defining a concise list of properties (e.g. net-zero timing, methane policy and carbon removal assumptions), using storylines to generate self-consistent, harmonised emissions trajectories. We show how FLEX can be used to explore trade-offs and uncertainties in near-term policy outcomes, varying net-zero timing, non-CO2 contributions, and CDR deployment.

We have used FLEX to define the extensions for CMIP7's ScenarioMIP experiment, to explore long-term (post-2100) policy-relevant questions where IAM-based projections are unavailable. The design explores long-term commitments to policies and provides boundary conditions for slow-responding processes such as ice-sheets and permafrost loss. FLEX is used to produce extensions that continue the narratives defined in each of the ScenarioMIP members, exploring a range of climate stabilisation levels, reversibility, and tipping point risks. We provide FLEX as open-source software compatible with existing scenario processing tools.