This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Who cuts emissions fastest, and who supports others in doing so, are central questions in global climate action. New IIASA-led research shows that building fairness into climate models from the start can inform deliberations without changing the overall climate goal.
The study, published in Environmental Research Letters, builds fair-share considerations directly into the process used to generate climate mitigation scenarios, rather than assessing fairness only afterward. The result is a wider range of possible pathways: The same global climate outcome can be reached with different combinations of emissions cuts and financial support across regions. The authors conclude that fairness is a defining feature of feasible collective ambition.
The researchers describe an approach that starts from an existing global mitigation pathway, allocating its emissions budget across world regions according to different principles of fairness including, for example, responsibility and economic capability. Regions that have already emitted, or are projected to emit, more than their share carry a carbon debt, which they can close by cutting emissions faster at home, removing CO2 from the atmosphere, or supporting a range of mitigation activities in other regions through interregional finance. The model then finds the least-cost way to meet both the global climate goal and every regional fair share.
Demonstrating this using the IIASA MESSAGEix-GLOBIOM-GAINS integrated assessment modeling framework, the researchers first examined a scenario consistent with limiting warming to approximately 2°C at 67% likelihood while allowing warming to temporarily exceed this limit before returning to it. The authors compared two contrasting ways of meeting each region's fair share. In the first, regions can make unlimited financial transfers to support mitigation elsewhere.
In this case, the physical transition remains the same as in the cost-effective pathway, while US$10.1 trillion to US$44.8 trillion (NPV) is transferred between regions from 2026 to 2100, depending on the fairness principle. They then asked what happens when regions cannot rely on such large financial transfers and must do more at home. The results show that when transfers are reduced to the lowest feasible level, they fall by more than half, and higher-responsibility regions cut emissions faster domestically.
As a result, global fossil fuel use in 2040 is 3% to 21% lower, while the overall trajectories for renewables, electrification and cumulative emissions by 2100 remain unchanged. The additional economic cost is relatively small and progressive: Compared with a future with no new climate policy, global consumption falls by about 0.8% in the cost-effective pathway examined, increasing to at most 1.3% when fair-share considerations are integrated and transfers are constrained. Regions with lower responsibility and capability see net improvements in consumption relative to the cost-effective pathway in all cases.
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