Equitable cooperation deepens the solution space for high ambition pathways

Pelz, S. ORCID: https://orcid.org/0000-0002-3528-8679, Fricko, O. ORCID: https://orcid.org/0000-0002-6835-9883, Riahi, K. ORCID: https://orcid.org/0000-0001-7193-3498, Pachauri, S. ORCID: https://orcid.org/0000-0001-8138-3178, Brutschin, E. ORCID: https://orcid.org/0000-0001-7040-3057, Rogelj, J. ORCID: https://orcid.org/0000-0003-2056-9061, Krey, V. ORCID: https://orcid.org/0000-0003-0307-3515, Johnstone, I., Vinca, A. ORCID: https://orcid.org/0000-0002-3051-178X, Schleussner, C.-F. ORCID: https://orcid.org/0000-0001-8471-848X, Kikstra, J. ORCID: https://orcid.org/0000-0001-9405-1228, & Gidden, M. ORCID: https://orcid.org/0000-0003-0687-414X (2026). Equitable cooperation deepens the solution space for high ambition pathways. Environmental Research Letters 21 (18) e184019. 10.1088/1748-9326/aea34d.

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Abstract

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. We explore this space across allocation approaches, transfer limits, and cooperation mechanisms, and propose a new lowest-feasible transfer scenario variant to bound it. We examine the integrated system shifts this new variant implies, and contrast 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. 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 reductions. Against a globally cost-effective 2 °C (>67%) scenario that reduces discounted consumption by approximately 0.8% relative to the no-new-policy reference scenario, integrating fair-share considerations raises this to at most 1.3% when transfers are restricted to lowest-feasible volumes. Notably, this aggregate is distributionally progressive, where lower-responsibility regions 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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