Global efforts to address climate change, notably through the Paris Agreement, face significant challenges. A recent assessment has shown that current national targets (NDCs) are insufficient to meet the goals of the Paris Agreement, with global warming reaching 2.5 to 3°C by 2100 instead of the set targets. With the NDC update planned for 2025, it is crucial to inform policymakers about fair emissions targets aligned with the Paris Agreement.

The concept of "fair shares" is complex and relies on principles of division of effort, which lead to different outcomes. These variations complicate policymaking and create what the authors call "black box" of fair sharesTo increase transparency and understanding of the impact of various factors on these shares, the study systematically examines three key dimensions of variability:

• Scientific uncertainties (physical and social): These include uncertainty in the Earth's temperature response to emissions, which fundamentally affects the remaining carbon budget. They also include social uncertainties associated with gross domestic product (GDP) and population projections, captured in so-called shared socio-economic scenarios (SSPs).
• Global strategies to meet the goals of the Paris Agreement: This dimension includes temperature targets (e.g. 1.5°C or 2.0°C), the timing of mitigation actions, and assumptions about negative and non-CO2 emissions.
• Normative considerations (justice): These are about how to fairly distribute the effort to achieve climate goals. They are based on three main principles of justice:
  • Responsibility (Equal Cumulative Per Capita – ECPC): It takes into account the historical contribution to global warming, favoring countries with low historical emissions such as India and Nigeria.
  • Ability to Pay (AP): Based on a country's ability to reduce emissions, i.e. their GDP per capita, it favors countries with lower GDP per capita and high baseline emissions, such as China, South Africa and Brazil.
  • Equality (Per Capita Convergence – PCC): It assumes that every individual has an equal right to emission allowances, with convergence from current levels to the same per capita allocation over time. For countries like Nigeria, this means moderately high allocations close to their NDCs.

The study highlights that normative considerations have a significant impact in the short term on fair emission quotas, while global discussions on temperature targets and non-CO2 emissions are gaining importance in the long term. The impact of all three dimensions varies for different countries. For China and the US, the short-term impact of fairness considerations is parameter-dependent (e.g. historical emissions), while for Mexico, fair share is almost independent.

Implications for National Climate Action and Financing

The study identifies many countries whose current NDCs are insufficient with respect to justiceCountries such as Russia, Saudi Arabia, Mexico and Argentina are aiming for more than double their fair share under the standard setting. For many countries in the Global South, NDCs are more ambitious than their least stringent rule.

These findings have direct implications for domestic mitigation actions and international financing of mitigation. The gap between cost-optimal emissions and fair allowances highlights the need for international financing. It is estimated that the total international financial flow could reach $0.5-7.4 trillion by 2030For countries like the US and the EU, international financing is the cheapest way to close the gap with NDCs. Conversely, for countries like Saudi Arabia, Russia and China, it is optimal combination of domestic measures and international fundingIndia can either accept funding (ECPC) or finance domestically (PCC, AP), while many countries in the Global South like Nigeria should accept funding.

This work serves as an important A guide to the "black box" of fair shares, providing a comprehensive database of emissions, quantifying the impact of individual factors, and highlighting implications for the 2025 NDC revision and international financing. Spring

the article was published in the journal nature.com

Glossary of key terms

• Paris Agreement: An international agreement on climate change adopted in 2015, which aims to limit global warming to well below 2°C above pre-industrial levels, and to pursue efforts to limit it to 1.5°C.
• Nationally Determined Contributions (NDC): Each country's commitments to reduce national greenhouse gas emissions and to take adaptation measures under the Paris Agreement.
• Fair Emission Quotas / Fair Shares: The amount of greenhouse gas emissions that should be allocated to individual countries based on various principles of fairness in order to jointly achieve global climate goals.
• Principles of Justice: Ability to Pay (AP): The principle that countries with higher economic capacity (e.g. higher GDP per capita) should contribute more to reducing emissions.
• Responsibility (Equal Cumulative Per Capita, ECPC): A principle that takes into account countries' historical contributions to emissions (historical debt) and allocates allowances based on cumulative per capita emissions.
• Equality (Per Capita Convergence, PCC): A principle that assumes convergence to the same per capita emissions for all countries over a certain period of time.
• Grandfathering (GF): An allocation rule that preserves the current distribution of emissions, meaning that all countries reduce their emission allowances proportionally to their current levels, regardless of principles of fairness.
• Remaining Carbon Budget (RCB): The total amount of CO2 that can be released into the atmosphere to keep global warming within specific temperature targets (e.g. 1.5°C or 2.0°C) with some probability.
• Global Stocktake: A process under the Paris Agreement that assesses collective progress towards achieving the agreement's long-term goals every five years.
• Integrated Assessment Models (IAMs): Comprehensive models that integrate economic, energy, land and climate systems to analyze possible scenarios of future developments and climate change policies.
• Sobol's decomposition: A sensitivity analysis method that quantifies how different input factors (or groups of them) contribute to the variance of a model's output. It is used in the study to determine the impact of different dimensions on fair shares of emissions.
• Shared Socioeconomic Pathways (SSPs): Scenarios of future socio-economic development (e.g. population growth, GDP, technology) that are used to assess future emissions and adaptation needs.
• International financing for emissions mitigation: Funds provided by one country (or group of countries) to another country (or group of countries) to support projects and policies aimed at reducing greenhouse gas emissions.
• GHG (Greenhouse Gas): Greenhouse gas, e.g. CO2 (carbon dioxide), CH4 (methane), N2O (nitrous oxide). The study also includes LULUCF emissions (land use, land use change and forestry).
• Cost-optimal scenarios: Scenarios that describe paths to achieving climate goals with minimal global costs, often without explicit consideration of fair burden sharing between countries.