Using net-zero carbon debt to track climate overshoot responsibility
A forward-looking measure of regional responsibility for breaching a fair share of the 1.5 °C budget
Read the published paper ↗doi.org/10.1073/pnas.2409316122
Regional responsibility once overshoot begins
Recent estimates place the remaining carbon budget at approximately 200 GtCO2 for a 50% chance of staying below 1.5 °C from 2024. Current emissions trends will likely deplete it around 2030, resulting in at least a temporary overshoot of the 1.5 °C limit.
This creates a problem for effort-sharing. Most 'fair share' measures divide a 'remaining' quantity, so they lose their footing once that quantity is gone, just as the question of responsibility becomes most contested. We ask whether a forward-looking measure of responsibility can persist beyond budget depletion, and what it implies for the drawdown obligations that follow. We contend that relaxing the temperature limit in response is inherently inequitable, as it disproportionately harms vulnerable populations and increases the likelihood of irreversible climate impacts. The measure therefore holds the limit fixed and describes the return to it.
Measuring the breach of a fair-share budget
We define 'net-zero carbon debt' as the extent to which a region breaches its fair share of the remaining 1.5 °C budget by the time it reaches net-zero CO2 from fossil fuels and industry (CO2-FFI). We apply it to the vetted scenarios in the IPCC AR6 WGIII database in which all regions reach net-zero CO2-FFI by 2100, grouped by each region's decade of net-zero. We add two scenarios: CurPol, reflecting current policies, and CurPledge, reflecting full implementation of conditional and unconditional pledges and all net-zero targets.
We use equal cumulative per capita allocation from 1990 as an illustration and also test an 1850 start year. The total budget comprises global CO2-FFI emissions from 1990 to 2022 plus a remaining 1.5 °C budget of approximately 247 GtCO2 from 2023, distributed equally across global population over 1990–2050. Results are reported for ten world regions. We use the simple climate model FaIR to derive temperature outcomes. FaIR experiments also test whether gross emissions before net-zero and permanent net-negative removals afterwards have equal and opposite effects on long-term temperature. We find an approximate 1:1 equivalence, with a 25% uncertainty in either direction.
Debts persist and fall unevenly across regions
We find that regional debts persist across pathways, with stark interregional disparities. Under median regional pathways reaching net-zero CO2-FFI by 2050, net-zero carbon debt reaches 201.4 GtCO2 in North America and 101.2 GtCO2 in Eastern Asia. We then group regions by when they exhaust their allocation. 'Earlier debtors' do so before 2030 and hold approximately 42% of the 2022 global population. 'Later debtors' do so afterwards and hold approximately 58%.
Under CurPledge, later debtors initially account for only 3% of total budget exceedance in the year of peak temperature increase (2050). This indicates minimal responsibility for the immediate impacts of overshoot in a world where stated targets are met. Under CurPol, prolonged late-century emissions raise that share to 40% by 2100. Global mean temperature peaks at approximately 1.8 °C (median) by 2050 under CurPledge, and rises to approximately 3 °C (median) by 2100 under CurPol.
A measure that persists after the budget is gone
These debts map onto an intergenerational burden. In the Asia-Pacific Developed region, the cohort born in 2020 faces an approximate 50% increase in lifetime extreme heatwave exposure under CurPledge, double that of the region's 1980 cohort. Under CurPol the increase is approximately 220%, over four times that of the 1980 cohort. The same region's above-average drawdown obligations reach 5.1 tCO2 per capita per year under CurPledge and 7.3 under CurPol, against its per capita CO2-FFI emissions of 9.5 tCO2 in 2022.
We argue that a persistent measure of responsibility robust to budget depletion is of value, as it draws a line at the acceptable level of warming and guides a fair return to that level. Two futures present themselves. In an uncooperative world where substantial net-zero carbon debts persist, we will likely witness severe long-term exceedance of the 1.5 °C limit, with impacts felt primarily by younger generations. Where regional debts are rapidly eliminated through mutually beneficial cooperation, efforts can instead pivot towards identifying and striving for long-term temperature reduction targets.
@article{pelz_using_2025,
title = {Using net-zero carbon debt to track climate overshoot responsibility},
volume = {122},
copyright = {All rights reserved},
issn = {0027-8424, 1091-6490},
url = {https://pnas.org/doi/10.1073/pnas.2409316122},
doi = {10.1073/pnas.2409316122},
abstract = {Current emissions trends will likely deplete a 1.5 °C consistent carbon budget around the year 2030, resulting in at least a temporary exceedance, or overshoot. To clarify responsibilities for this budget exceedance, we consider “net-zero carbon debt,” a forward-looking measure of the extent to which a party is expected to breach its “fair share” of the remaining budget by the time it achieves net-zero carbon emissions. We apply this measure to all vetted mitigation scenarios assessed in the Intergovernmental Panel on Climate Change’s Sixth Assessment Report and two scenarios that model current policies and pledges, using an illustrative equal per capita allocation of a remaining 1.5 °C carbon budget starting in 1990. The resulting regional carbon debt estimates inform i) the scale and pace of regional carbon drawdown obligations necessary to address budget exceedance and ii) relative regional responsibilities for increased lifetime exposure to extreme heatwaves across age cohorts due to budget exceedance. Our work strengthens intergenerational equity considerations within an international climate equity discourse and informs the implementation of effort-sharing mechanisms that persist beyond the exhaustion of a rapidly dwindling remaining carbon budget.},
language = {en},
number = {13},
urldate = {2025-04-03},
journal = {Proceedings of the National Academy of Sciences},
author = {Pelz, Setu and Ganti, Gaurav and Lamboll, Robin and Grant, Luke and Smith, Chris and Pachauri, Shonali and Rogelj, Joeri and Riahi, Keywan and Thiery, Wim and Gidden, Matthew J.},
month = apr,
year = {2025},
pages = {e2409316122},
}