The world is on course to exceed the Paris Agreement’s 1.5°C warming limit within a decade. The increasing likelihood of at least a temporary temperature limit exceedance, or overshoot, raises critical questions about fairness and responsibility for climate change action, write Setu Pelz, Robin Lamboll and colleagues.
As we near the Paris Agreement’s 1.5°C warming limit, the question is not just about when we exceed it, but how we deal with the subsequent overshoot period fairly and collectively. In a study published in PNAS, we examine net-zero carbon debt – a measure for assessing which regions are responsible for climate overshoot. We demonstrate how this measure can be used to quantify regional carbon drawdown (carbon dioxide removal) obligations, and link this to regional responsibilities for local climate extremes during the overshoot period.
Defining responsibility for climate overshoot
In the study, we determine each region’s ‘net-zero carbon debt’, a measure that compares historical and projected CO2 emissions on the path to net zero, with the region’s fair share of the remaining carbon budget for staying below 1.5°C. Any emissions exceeding this fair share are defined as a region’s net-zero carbon debt.
The purpose of the net-zero carbon debt measure is straightforward: it is a forward-looking measure to track which regions will bear responsibility for climate overshoot while explicitly accounting for past inaction. This helps assess whether domestic and international mitigation efforts are on track or falling short, with the ultimate goal of avoiding net-zero carbon debt accrual altogether.
Applying this approach to the IPCC AR6 WGIII scenarios database reveals distinct regional patterns: some regions consistently accrue debt due to past inaction; others accumulate debt depending on the timing of their net-zero targets this century, and some only if they delay net zero beyond 2100.
NAM – North America; EUR – Europe; APD- Asia-Pacific Developed; EEA- Eastern Europe and West Central Asia; EAS- Eastern Asia; MEA – North Africa and Middle East; PAS – South-East Asia and developing Pacific; LAC – Latin America and Caribbean; AFR – Sub-saharan Africa; SAS – Southern Asia.
Figure 1. Assessing carbon debts accrued under varying regional net-zero CO2 from fossil fuel combustion and industry (FFI) timings. Density plots describe scenario distributions of regional net-zero carbon debt accrual grouped by regional net-zero CO2-FFI timings in the AR6 scenarios database, subset to reflect a maximum 2090 regional net-zero CO2-FFI timing group. The circular shapes indicate the median net-zero carbon debt within each timing group.
The figure shows that most regions will be in carbon debt under most scenarios, except for Sub-saharan Africa and Southern Asia.
Risk of reliance on future carbon removal
If a region is expected to accumulate net-zero carbon debt, it will need to compensate – either by supporting emissions reductions elsewhere or by removing additional carbon from the atmosphere. This raises critical questions about how to equate gross carbon emissions with net-negative carbon removals.
In theory, net-negative carbon removals can offset past emissions in terms of long-term temperature outcomes. However, there is uncertainty in how effective this may be. For example, the effectiveness of land-based carbon removal could decline over time, and most carbon removal projects do not work. Final temperature equivalence alone does not fully capture the lasting regional climate impacts. These issues underscore the importance of minimising net-zero carbon debt from the outset.
Who bears the burden?
Every ton of net-zero carbon debt not only adds to the emission reduction (and removal) burden younger generations must manage but also worsens climate impacts – such as extreme heatwaves – that they will have to endure. In the paper, we examine this dual burden under two real-world scenarios:
- A current policies scenario (CurPol), where global temperatures reach 3°C by 2100.
- An announced pledges and net-zero targets scenario (CurPledge), where warming is expected to peak at around 1.8°C in 2050.
We track how emissions and carbon debt evolve over time in each region and identify who is responsible for exceeding the 1.5°C limit in both scenarios, finding two distinct groups:
- Early debtors – regions with high historical emissions that typically accrue carbon debt by 2030.
- Later debtors – regions with rising future emissions, some of which may not accumulate carbon debt until after 2100.
We compare the amount of carbon debt each region needs to draw down – through both increased domestic and international mitigation efforts, as well as carbon dioxide removal (assuming these combined efforts begin in 2025) – with the projected lifetime exposure to extreme heatwaves that younger generations will face under these scenarios.
This comparison highlights stark disparities between regions and generations, underscoring the unequal burden of both carbon drawdown and climate impacts.
Increased extreme heatwave exposure and carbon drawdown obligations under current policies and pledges. Increase in years of life exposed to extreme (1-in-100-year) heatwaves (primary y-axis) relative to the 1.5 °C reference scenario (IMP-REN, AR6), for all cohorts born between 1960 and 2020 (x-axis). The shaded areas reflect the 33rd and 66th percentile across all global climate model runs and TRQs (Methods). The annual equal per capita drawdown obligation to address regional responsibility for scenario overshoot by the year 2100 (starting in 2025) is shown on the secondary y-axis. This refers to an equal cumulative per capita allocation of the total carbon budget from the year 1990, composed of global CO2 emissions from fossil fuel and industrial processes (CO2-FFI) between the years 1990 and 2023 and an estimated 1.5 °C RCB (50% chance) from the year 2023.
Under stated targets and pledges (CurPledge), younger generations in early debtor regions could face significantly higher lifetime exposure to extreme heatwaves – alongside larger carbon drawdown burdens due to their region’s past emissions – compared to a 1.5°C reference scenario. In later debtor regions, younger generations may also experience increased heatwave exposure, sometimes from a higher baseline. However, their drawdown obligations – and thus their regional responsibility for overshoot – remain relatively small. These estimates increase under the current policies (CurPol) scenario, exacerbating burdens transferred to younger generations.
Our findings highlight the urgent need for collective emissions cuts now. Net-zero carbon debt is not an accounting tool for defining future carbon dioxide removal targets – it serves as an early warning that every excess ton of CO₂ worsens future harms, makes achieving sufficient net-negative removals even more difficult, and can be tracked to preserve responsibilities.
Why this matters now
Using the net-zero carbon debt measure alongside considerations of domestic feasibility can help coordinate cost-effective and collective efforts.
Regions expected to accumulate large carbon debts should strengthen sectoral mitigation plans, establish international support mechanisms, and set clear targets for permanent carbon removal. Meanwhile, other regions should focus on distinguishing cost-effective domestic measures from those requiring international cooperation.
Embedding these considerations into national commitments can help all regions work together to minimize overshoot and achieve the Paris Agreement’s long-term temperature goals in a fair and effective way.
