This research paper represents a new, forward-looking indicator of responsibility for exceeding the 1.5°C target of the Paris Agreement, called “net-zero carbon debt”. The authors argue that exceeding this warming threshold requires new ways of understanding and allocating responsibility for climate action and the need to restore temperatures below this threshold.

Meaning and Concept of Net Carbon Debt

The paper highlights that current emission trends are likely to exhaust the 1.5°C-compatible carbon budget around 2030, leading to at least a temporary overshoot of the target. To clarify the responsibility for this overshoot, the authors define the net carbon debt as a measure of the extent to which a party is expected to exceed its “fair share” of the remaining carbon budget by the time it reaches net zero carbon emissions.

The calculation of net carbon debt takes into account three factors:

• Past carbon emissions (i)
• Future carbon emissions (ii)
• “Fair” allocations of the remaining carbon budget (RCB) (iii)

The net carbon debt (or credit) accumulates over time and peaks in the year a party reaches net zero carbon emissions. This study uses the same cumulative per capita RCB allocation with a base year of 1990 as an illustration. The total carbon budget consists of global CO2 emissions from fossil fuels and industry (CO2-FFI) from 1990 to 2022 and the estimated RCB for 1.5°C (50% probability) from 2023.

Methodology and Scenarios

The authors apply the net carbon debt measure to all validated climate change mitigation scenarios included in the IPCC Sixth Assessment Report (AR6 WGIII) and on two scenarios modeling current policies (CurPol) and commitments (CurPledge). Scenarios from the AR6 WGIII database are analyzed to see how regional net carbon debts and corresponding carbon sequestration commitments vary under different timescales for achieving regional carbon neutrality. The CurPol and CurPledge scenarios are used to compare regional carbon sequestration commitments with regional increases in lifetime exposure to extreme heat waves for different age cohorts.

Key Findings

• Regional accumulation of carbon debt: The IPCC AR6 scenario analysis revealed three broad groups of regions: those that continue to accumulate net carbon debt (regardless of the timing of carbon neutrality), those that may accumulate debt with a later date for carbon neutrality, and those that do not accumulate this debt this century. The regions that continue to accumulate debt include North America (NAM), Western Europe (EUR), developed Asia and the Pacific (APD), Eastern Europe and West and Central Asia (EEA), East Asia (EAS), and the Middle East (MEA).
• Impacts of current policies and commitments: Under the CurPledge scenario (fulfilling all pledges), global average temperature is expected to peak at around 1.8°C above pre-industrial levels by 2050. Under the Current Policies (CurPol) scenario, temperatures are expected to rise to around 3°C by 2100 and continue to rise. Regions that exhaust their allocations before 2030 (“early borrowers”) bear the primary responsibility for the budget overruns in both scenarios.
• Linking debt to impacts: The study links the responsibility for exceeding the temperature limit and the associated carbon sequestration commitments to the realised climate impacts, revealing a double intergenerational and interregional inequality. Comparing per capita carbon sequestration commitments with the increase in lifetime exposure to extreme heat waves shows how insufficient climate ambition exacerbates existing inequalities. Younger generations in regions with high carbon debt face significantly higher exposure to extreme heat while potentially bearing a greater burden of carbon sequestration.

Discussion and Conclusions

The authors emphasize that the net carbon debt can serve as a permanent tool to assess regional responsibility for exceeding the target and to quantify carbon removal commitmentsThey point out that even after the RCB for 1.5°C is exhausted, the need for a fair distribution of efforts to reduce temperatures remains.

The article highlights the importance international cooperation in achieving global climate goals, as no group of regions can address the expected overshoot on their own. In a reluctant world where significant net carbon debts persist, a serious long-term overshoot of 1.5°C is likely to occur, with the associated impacts felt particularly by younger generations. Conversely, if regional net carbon debts are rapidly eliminated through mutually beneficial cooperation, efforts can shift towards achieving long-term temperature reduction goals.

The authors suggest that a measure of net carbon debt may be useful in informing the creation and assessment of Nationally Determined Contributions (NDCs)For regions with high expected carbon debt, this means increasing the ambition and specificity of sectoral mitigation plans. For regions with low or no debt, this means carefully distinguishing between cost-effective measures with large co-benefits and costly measures.

The article also draws attention to the interconnectedness of the issues loss and damage and the growing need adaptation financingDiscussions based on a net carbon debt rate could guide international financial flows for climate-resilient development.

Overall, this study presents A new perspective on the distribution of responsibility for climate change in the context of the likely exceeding of the 1.5°C target, emphasizing interregional and intergenerational inequalities and the need fair and cooperative solutions. Spring

Glossary of key terms

• Net-zero carbon debt: A forward-looking measure of the extent to which a party is expected to exceed its “fair share” of the remaining carbon budget by the time it reaches net zero carbon emissions, in order to clarify commitments to restore temperatures below a set warming threshold.
• Overshoot: A situation where the global average temperature temporarily exceeds a certain target (e.g. the 1.5°C of the Paris Agreement) before returning to or below that target.
• Remaining carbon budget (RCB): The amount of additional carbon dioxide emissions that can be released into the atmosphere while still being likely to limit global warming to a certain level (e.g. 1.5°C).
• Fair share: A normative concept that seeks to determine an equitable distribution of efforts to reduce greenhouse gas emissions among countries or regions, often taking into account historical emissions, capabilities, and needs.
• Intergenerational equity: The principle of justice between present and future generations, which emphasizes that current activities should not disproportionately harm the living conditions of future generations.
• Interregional equity: The principle of fairness between different regions or countries, taking into account the uneven distribution of the impacts of and responsibility for climate change.
• Carbon drawdown/removal (CDR): The process of removing carbon dioxide from the atmosphere and storing it long-term in terrestrial, oceanic, or product reservoirs.
• Nationally Determined Contributions (NDCs): Self-defined targets and measures that countries submit under the Paris Agreement to reduce their greenhouse gas emissions.
• Global Stocktake: Periodic assessment of collective progress towards achieving the goals of the Paris Agreement.
• Common but differentiated responsibilities and respective capabilities (CBDR-RC): A principle enshrined in the United Nations Framework Convention on Climate Change (UNFCCC) that recognizes that all states have a shared responsibility for addressing global environmental problems, but their responsibilities differ depending on their contribution to the problem and their ability to address it.