Equitable cooperation deepens the solution space for high ambition pathways

Résumé fourni par la source

Abstract Integrated assessment model scenarios informing global climate ambition typically treat fairness and cooperation as external to the modelling process, reinforcing a perceived tension between fair effort-sharing and the urgent collective action needed to minimise exceedance of 1.5 °C. Here we introduce a generalisable framework that embeds principle-based fair-share considerations directly into the scenario generation process. This framework produces scenario variants that reconfigure transition pathways while preserving global peak and end of century climate outcomes. Exploring this space across allocation approaches, transfer limits, and cooperation mechanisms, we define a new lowest-feasible transfer scenario variant and examine the integrated system shifts it implies, contrasting it with the common exploration of fairness through ex-post assessments and unlimited financial transfers. We observe that the renewables deployment and electrification trajectories remain largely unchanged across all fair-share variants, while restricting transfers to lowest-feasible volumes accelerates global fossil decline, by up to 21% in 2040. This effect prevails even when interregional cooperation is focussed on financing carbon dioxide removal with geological storage, where higher-responsibility regions’ fair-share obligations remain met primarily through domestic gross emissions cuts. Against a globally cost-effective 2 °C (>67%) scenario that reduces consumption by approximately 0.8% relative to the no-new-policy baseline, we find that integrating fair-share transfers raises this to approximately 1.0% in total irrespective of approach and, where transfers are restricted to lowest-feasible volumes, to at most 1.5% in total. Notably, this aggregate is distributionally progressive, where lower-responsibility regions largely gain relative to the cost-effective source scenario. This deepened solution space persists even under a stringent 1.5 °C goal, though with differentiation increasingly settled through finance, and reveals dynamics that emerge only when scenario generation itself accounts for differentiated responsibilities and capabilities. Ultimately, we find that a ‘gross emissions reductions first’ strategy among higher-responsibility regions generates system-wide benefits, releasing pressure on lower-responsibility regions to collectively meet climate goals.

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