The role of climate impacts in Transition Pathways in an Integrated Assessment Model: interdependencies and nonlinearities

The coupled human-Earth system is shaped by complex feedbacks between human society and climate. These bidirectional interactions – where human activities alter the climate system, and climate change, in turn, reshapes socioeconomic systems – play a pivotal role in determining long-term adaptation or mitigation strategies. However, scenario-generating Integrated Assessment Models (IAMs), when used to provide emissions scenarios within the framework of the Shared Socioeconomic Pathways (SSPs), do not represent these coupled feedbacks between climate and human society. While such a distinction between climate change, as modelled in Earth System Models, and its impacts on societal sectors, as modelled in impact models, prevents any double counting of the climate impacts in the emissions pathways, it limits the understanding of the coupled effects and is disadvantageous for resilient decision-making. 

The newly developed Feedback-based knowledge Repository for Integrated Assessments-version 2.1 (FRIDA v2.1) endogenously incorporates several climate-society feedbacks at the structural level in the form of global impact functions of climate variables. The coupled effect of all climate impacts, which are part of the endogenous model behavior, diverges from their simple additive contribution, indicating a non-linear model response. These non-linearities arise primarily when individual impact channels generate opposing feedbacks of differing magnitudes, which drive the system beyond certain thresholds in the coupled setting that would remain untriggered under additive aggregation, reflecting the multiplicative nature of system feedbacks.

This study further investigates the cascading effects and relative strength of each of the impact channels in the context of associated feedback loops. Indirect economic impacts—representing climate-driven effects on investment and bank assets—exert a strong, system-wide influence and play a central role in shaping the model’s endogenous behaviour, owing to their cumulative effects. Climate impacts on labour productivity, government expenditure, and energy demand have less influence across the system. In contrast, impact channels related to mortality, human behaviour, concrete production, and land-use generate important localised effects, but do not significantly alter system-wide dynamics.