Introduction

The global carbon budget refers to the cumulative amount of carbon dioxide (CO₂) that humanity can emit while retaining a specified probability of limiting global warming to a particular temperature level. Unlike an annual emissions target, a carbon budget focuses on cumulative emissions because the climate effect of CO₂ depends substantially on the total quantity released over time.

Global carbon-budget allocation raises a difficult legal and policy question: if the atmosphere represents a shared global resource, how should the remaining permissible emissions be divided among States, industries, generations and individuals? International climate law does not establish one universally accepted mathematical formula for allocating the remaining carbon budget. Instead, allocation is influenced by the principles of equity, common but differentiated responsibilities and respective capabilities, historical responsibility, sustainable development and national circumstances.

Scientific foundation

The concept of a carbon budget is closely associated with climate science. The Intergovernmental Panel on Climate Change (IPCC) uses cumulative CO₂ emissions as an important measure when assessing the probability of limiting warming to specified temperature levels.

A carbon budget is therefore not a fixed legal entitlement. It changes as scientific estimates of remaining allowable emissions are revised and as actual emissions occur.

Three concepts should be distinguished:

Global carbon budget: the remaining cumulative emissions compatible with a specified climate objective.

National carbon budget: a proposed share or pathway for a particular State.

Sectoral carbon budget: an allocation among sectors such as electricity, transport, industry and buildings.

The scientific budget provides the physical constraint; international law addresses how States should cooperate within that constraint.

International legal foundation

The United Nations Framework Convention on Climate Change (UNFCCC) provides the central legal framework for international climate cooperation.

Article 2 establishes the Convention's objective of stabilizing greenhouse-gas concentrations at a level that prevents dangerous anthropogenic interference with the climate system.

Article 3 contains important principles, including equity and the principle of common but differentiated responsibilities and respective capabilities (CBDR-RC).

Article 3 is particularly significant for carbon-budget allocation because it recognizes that climate responsibilities cannot necessarily be distributed identically among all States.

Principle of equity

Equity is central to the allocation debate.

A purely equal-per-capita allocation would divide the remaining global budget according to population. A historical-responsibility approach would give greater weight to past emissions. A capability-based approach would require States with greater economic and technological capacity to undertake deeper reductions or provide greater financial support.

No single formula has achieved universal international acceptance.

Consequently, carbon-budget allocation remains both a scientific and political question governed by broader principles of international climate law.

Common but differentiated responsibilities

CBDR-RC recognizes that States have common responsibility for addressing climate change but may have different responsibilities depending upon their circumstances and capabilities.

Developed countries have historically contributed a large proportion of cumulative greenhouse-gas emissions. Developing countries, however, continue to require economic development and may have lower historical per-capita emissions.

Carbon-budget allocation must therefore consider both the need for global emissions reduction and the principle of differentiated responsibility.

Paris Agreement

The Paris Agreement significantly influences contemporary carbon-budget governance.

Article 2 establishes the long-term temperature objective, while Article 4 requires Parties to undertake successive nationally determined contributions (NDCs) reflecting progression and the highest possible ambition.

The Paris Agreement does not assign a fixed quantity of the global carbon budget to each State.

Instead, States determine their own NDCs within a collective framework and are expected to increase ambition over time.

Nationally determined contributions

NDCs provide the principal mechanism through which States communicate their mitigation commitments.

A State's NDC may contain:

Absolute emissions reductions.

Reduction relative to a baseline.

Emissions-intensity targets.

Renewable-energy targets.

Sector-specific measures.

Net-zero objectives.

Because NDCs are nationally determined, they do not constitute a universally agreed mathematical allocation of the global carbon budget.

Historical responsibility

Historical responsibility is one of the most controversial allocation principles.

Under this approach, States that have contributed more to cumulative atmospheric CO₂ concentrations would bear a greater responsibility for reducing future emissions and supporting climate action.

The principle has particular relevance to debates involving industrialized countries whose economic development occurred during periods of high fossil-fuel consumption.

However, calculating historical responsibility requires choices concerning the starting date, gases included, territorial versus consumption-based emissions and population changes.

Equal per-capita allocation

An equal-per-capita approach would divide the global carbon budget among individuals and then aggregate national shares according to population.

Its principal justification is equality: every person would receive an equal claim to the atmosphere's remaining absorptive capacity.

However, this approach can produce substantial differences in national allocations because countries have very different populations.

It also does not by itself address historical emissions, technological capacity or developmental needs.

Capability-based allocation

A capability-based approach considers the economic and technological capacity of States.

Countries with greater financial resources and technological capabilities could be expected to undertake faster decarbonization and provide support to countries with fewer resources.

This approach is consistent with broader climate-finance and international-cooperation principles.

Developmental needs

Carbon-budget allocation must also account for legitimate development requirements.

Developing countries may require additional energy access, infrastructure and industrial development. A rigid uniform emissions ceiling could therefore impose disproportionate burdens on countries whose historical contribution to climate change has been comparatively small.

The legal challenge is to reconcile development rights with the physical limits imposed by the global carbon budget.

Consumption-based emissions

Traditional emissions inventories generally attribute emissions to the country where production occurs. This can produce difficulties where goods are manufactured in one country and consumed in another.

Consumption-based accounting attributes emissions associated with internationally traded goods and services to the final consumer.

This approach can alter assessments of national responsibility and therefore has relevance to carbon-budget allocation.

International carbon markets

Article 6 of the Paris Agreement provides mechanisms for voluntary cooperation between Parties in achieving their mitigation objectives.

Internationally transferred mitigation outcomes can allow emissions reductions to be transferred between participating States subject to applicable rules.

Article 6 therefore creates a legal mechanism for cooperation but does not itself establish an overall global allocation formula.

Carbon-budget integrity

International carbon markets create an important issue of environmental integrity. If the same emissions reduction is counted toward more than one country's target, global mitigation could be overstated.

Article 6 therefore includes accounting requirements intended to reduce double counting.

The broader legal principle is that transferred mitigation should represent genuine additional reductions rather than merely changing the accounting location of emissions.

Intergenerational equity

Carbon-budget allocation also raises questions concerning future generations.

A large proportion of the remaining carbon budget can be consumed by current generations, leaving future generations with fewer options.

The principle of intergenerational equity therefore supports consideration of long-term climate consequences when establishing emissions pathways.

The comparative decision Minors Oposa v. Secretary of the Department of Environment and Natural Resources, G.R. No. 101083 (Philippines, 1993) is frequently discussed in environmental-law scholarship for recognizing the interests of future generations. Although it is not an international climate judgment or binding outside the Philippines, it provides useful comparative support for intergenerational environmental responsibility.

Human rights dimension

Climate change can affect rights relating to life, health, housing, food, water and culture.

This creates an additional argument for equitable carbon-budget allocation because excessive use of the global carbon budget can increase climate-related risks affecting vulnerable populations.

The Urgenda Foundation v. State of the Netherlands, ECLI:NL:HR:2019:2007 decision is particularly significant in comparative climate law. The Dutch Supreme Court upheld obligations concerning greenhouse-gas reduction by reference to human-rights protections under the European Convention on Human Rights.

The case does not establish a universal carbon-budget allocation formula, but it demonstrates how domestic courts can connect emissions reduction with fundamental rights.

Climate litigation and carbon budgets

Courts increasingly encounter arguments based on scientific carbon budgets.

The Milieudefensie v. Royal Dutch Shell litigation in the Netherlands illustrates another dimension of carbon-budget governance: the responsibilities of private corporations.

The case demonstrates that carbon-budget considerations are no longer limited to relationships between States. Corporations, investors and other private actors can also become subject to legal obligations concerning climate-related conduct.

Precautionary principle

Climate science involves uncertainty concerning the precise scale and timing of future impacts. The precautionary principle therefore supports action despite scientific uncertainty where potentially serious environmental harm exists.

The comparative case Vellore Citizens Welfare Forum v. Union of India, (1996) 5 SCC 647 recognized precautionary and sustainable-development principles in environmental law.

Although the decision is not binding internationally, it provides useful comparative guidance concerning precautionary environmental governance.

Loss and damage

Carbon-budget allocation is also connected with loss and damage. States and communities that have contributed relatively little to historical emissions may nevertheless experience severe climate impacts.

The international climate regime has developed mechanisms addressing loss and damage, but international law has not established a universal liability formula under which historical emitters must compensate all climate-related losses.

This distinction is important because responsibility for mitigation, financial support and legal liability are not necessarily identical concepts.

Role of climate finance

An equitable carbon-budget system cannot rely exclusively on emissions restrictions. Financial and technological support can help developing countries transition to lower-carbon development pathways.

The international climate regime therefore combines mitigation obligations with provisions concerning finance, technology transfer and capacity-building.

Climate finance can help reconcile development needs with the finite global carbon budget.

Net-zero pathways

Net-zero strategies provide a long-term framework for managing the remaining carbon budget.

A net-zero pathway generally involves reducing emissions substantially and balancing residual emissions through removals or other permitted mechanisms.

However, reliance on large-scale carbon removal raises legal and environmental questions concerning land use, biodiversity, permanence and measurement.

Carbon removal should therefore not automatically be treated as equivalent to avoiding emissions.

Governance and transparency

Carbon-budget allocation requires reliable emissions accounting.

Governance systems should establish:

Transparent emissions inventories.

Consistent accounting methodologies.

Independent verification.

Monitoring and reporting.

Clear treatment of carbon removals.

Rules for international transfers.

Mechanisms to prevent double counting.

Without reliable accounting, allocation of a global carbon budget becomes legally and scientifically difficult to enforce.

Conclusion

Global carbon-budget allocation is fundamentally an issue of climate justice, international cooperation and scientific constraint. The IPCC's carbon-budget assessments establish the physical limits associated with particular temperature objectives, but international law does not prescribe a single formula dividing that budget among States.

The UNFCCC, particularly its principles of equity and CBDR-RC, provides the foundational legal framework. The Paris Agreement establishes a cooperative architecture through NDCs, successive increases in ambition and Article 6 mechanisms, but leaves the precise distribution of mitigation responsibilities largely to national determination and international negotiation.

Possible allocation approaches include equal per-capita distribution, historical responsibility, economic capability, developmental need and combinations of these principles. Each method has advantages and limitations, and no universally accepted formula currently determines national carbon entitlements.

Comparative cases such as Urgenda, Milieudefensie v. Royal Dutch Shell, Minors Oposa and Vellore Citizens Welfare Forum demonstrate how climate and environmental law increasingly incorporates human rights, intergenerational equity, corporate responsibility, precaution and sustainable development. These cases are not binding universally and should be treated as comparative authorities.

Ultimately, a legally credible global carbon-budget framework requires more than numerical allocation. It requires equitable mitigation, transparent accounting, climate finance, technology cooperation and protection of vulnerable communities. The central challenge is to ensure that the remaining atmospheric capacity is managed as a shared global constraint while recognizing differences in historical emissions, economic capability, developmental needs and national circumstances.