In IIASA's fair-share pathways, limiting transfers lowers 2040 fossil fuel use by 3% to 21%

The temperature outcome holds across every variant because it is imposed. The global climate goal and each region's fair share both enter the optimisation as constraints, and the model solves for the least-cost mix of domestic abatement, carbon dioxide removal and interregional finance that satisfies them ^[5]. What varies between runs is the distribution of effort, and the study's claim is that this distribution has more freedom in it than a single cost-effective pathway suggests ^[2].

In the first of two contrasting cases, regions can transfer money without limit. The physical transition is then identical to the cost-effective pathway and the whole adjustment happens in finance: US$10.1 trillion to US$44.8 trillion in net present value moves between regions from 2026 to 2100, depending on which fairness principle sets the allocation ^[7]. The top of that band is about 4.4 times the bottom ^[1]. Spread across the 75 years to 2100 it averages roughly US$135 billion to US$600 billion a year in present-value terms, and since the figure is discounted, the undiscounted flows late in the century would be larger ^[2].

In the second case, transfers are pushed to the lowest feasible level and fall by more than half ^[8]. Halving the reported band puts them under about US$5 trillion at the low end and US$22 trillion at the high end ^[4]. Higher-responsibility regions make up the difference at home, and global fossil fuel use in 2040 comes in 3% to 21% lower, while the trajectories for renewables, electrification and cumulative emissions to 2100 are unchanged ^[8][9].

"Integrating fair shares into the scenario generation process changes the shape of the transition, while preserving the climate outcome. Such fair-share scenario variants provide new evidence that can inform the translation of global ambition to regional implementation," said lead author Setu Pelz, a researcher in the IIASA Energy, Climate, and Environment Program ^[12][13].

In aggregate the extra cost is small. Against a future with no new climate policy, global consumption falls by about 0.8% in the cost-effective pathway examined, rising to at most 1.3% once fair shares are integrated and transfers constrained ^[10]. The whole exercise costs half a percentage point of global consumption ^[3]. Regions with lower responsibility and capability end up better off on consumption than in the cost-effective pathway in all cases ^[11]. The allocation is made across world regions, so distribution within a region sits outside what these runs resolve ^[3].

How the money may be spent matters more than its volume. Restricting cooperation to financing carbon removal with geological storage costs roughly 10 times as much per tonne of mitigation transferred as when all mitigation options except land use are in scope, and even then higher-responsibility regions meet most of their obligations through domestic cuts ^[14][15].

"Fairness is often assessed after the most cost-effective global pathway has been identified. Our results show why it matters to bring it into the analysis from the beginning: doing so reveals different ways of sharing the effort while still achieving the same global climate outcome," said co-author Shonali Pachauri, who leads the Transformative Institutional and Social Solutions Research Group at IIASA ^[16][17].

This is one modelling framework, MESSAGEix-GLOBIOM-GAINS, and one scenario, consistent with about 2C at 67% likelihood with a temporary overshoot before returning to the limit ^[6]. The model takes a fairness principle as an input and reports what follows ^[3]. The authors conclude that fairness is a defining feature of feasible collective ambition ^[18].