Equitable cooperation deepens the solution space for high ambition pathways: explorer

About the paper

Equitable cooperation deepens the solution space for high ambition pathways (Pelz et al., 2026, Environmental Research Letters) asks what changes when fairness is a design constraint during scenario generation. Starting from a cost-optimal pathway to 2 °C or 1.5 °C, each region must meet a principle-based fair share of the carbon budget and may cooperate with others to do so. Two corners bound the result: with unlimited transfers the physical pathway is unchanged and the fair share is settled by financial transfers; with transfers held to the lowest level the model tolerates, higher-responsibility regions cut faster at home and the global fossil exit accelerates, at a modest and progressive cost. The paper page has the question, the approach and the findings in full.

Indicators of climate action for every pathway in the paper. Every line reaches the same carbon budget. The lines differ in who does what: the cost-optimal source pathway, the same pathway once each region must meet a fair share of the budget with unlimited transfers between regions, and the same again with transfers held to the lowest level the model tolerates. Pick a region to see where the pathways part ways; the Paper figures tab redraws the paper's own figures.

How to read these figures

What is on this page. The cards cover the energy-system and emissions indicators that the paper's scenarios resolve, in the order a reader of the 2023 global stocktake outcome would expect: capacity and efficiency, coal power, emissions and their sectors, fossil supply, low-emission electricity and storage, then transport. Indicators that only exist as historical series, such as vehicle sales, fossil fuel subsidies and deforestation, are outside what a scenario ensemble reports.

Fair shares and carbon debt. Each region receives a share of the carbon budget under a fairness principle. A region that has already used more than its share carries a debt. It can pay the debt down by cutting emissions faster at home, by removing emissions at home, or by financing action in other regions.

The two corners. Under unlimited transfers (U, up-pointing triangle in the paper figures) finance between regions is unlimited, the physical transition matches the cost-optimal source pathway, and the fair share is settled by financial transfers. Under lowest transfers (L, down-pointing triangle) finance is held to the lowest level the model tolerates, the higher-responsibility regions cut fossil fuels faster at home, and the regions with less historical responsibility gain room relative to the source pathway. A cross marks the source pathway.

The fair-share variants. ECPC shares the budget by equal cumulative emissions per person, CAPC adjusts that share for capability, and the year is when responsibility starts to count (1990, 2015 or 2025). ECPC 2015 on the SSP2 narrative, the paper's reference case, is selected at first. ECPC 2015* is ECPC 2015 with a ten-year delay before transfers begin. Further pairs change the narrative (SSP1), the cooperation channel (novel carbon removal only) or the discount rate (1%). Each pair has both corners.

The carbon budgets. Two budgets, both counted from 2020 and both allowing overshoot: one consistent with about 2 °C at a two-in-three chance and one with 1.5 °C at an even chance. The budget binds the emissions the fair-share rules cover. Land use, international shipping and aviation sit outside it, so total CO₂ over 2020 to 2100 lands within a few percent of the budget for every line.

Transfers, investment and consumption. Transfers and investment are present values at 2025 of flows from 2026, in market exchange rate dollars. Consumption changes are the same present value against the no-new-policy baseline or against the source pathway.

Separate comparison run, for orientation only. The Low marker pathway of the ScenarioMIP-CMIP7 scenario set is a MESSAGEix-GLOBIOM-GAINS run (SSP2, Low Emissions) and the nearest public MESSAGEix pathway to the 2 °C source. It is shown so that the paper's scenarios can be placed against a published CMIP7 pathway from the same model. Switch it on to see the same indicators from that run for the World and for every region. * The series drawn here are not the ScenarioMIP series as reported: the harmonised release starts in 2023, so for 2020 to 2025 the cumulative CO2 figure uses our source scenario, which follows the same history before the first model timestep in 2030; regional series are the model's native reporting rather than the harmonised release. For the scenario data itself, use the IIASA ScenarioMIP explorer at scenariomip.apps.ece.iiasa.ac.at. Cite: Riahi, K., van Vuuren, D.P., et al. (in preparation), Overview of the Socioeconomic, Emissions and Land Use Futures of the ScenarioMIP-CMIP7 Pathways; scenario data Riahi, K., van Vuuren, D.P., et al., Socioeconomic, Emissions and Land Use Futures, the ScenarioMIP Pathways for CMIP7, doi:10.5281/zenodo.22296051; model documentation Fricko et al. (forthcoming), full list at scenariomip.apps.ece.iiasa.ac.at/documentation.