The atmosphere is a finite sink, and every tonne of carbon dioxide emitted narrows what remains for everyone else. Faced with this scarcity, negotiators and philosophers alike have been drawn to an appealingly simple answer: divide the remaining carbon budget equally among the world's people. Per capita allocation, often framed through Aubrey Meyer's contraction and convergence model, treats the atmosphere as a global commons to which each human being holds an equal claim.

The intuition is powerful because it imports a familiar domestic principle, equal moral standing, into a global domain where states have traditionally bargained as sovereign equals rather than as representatives of persons. Yet the move that makes per capita frameworks attractive also exposes their limits. Equality of shares at a single moment says nothing about how we arrived here, what the shares are for, or who exactly bears the obligation to adjust.

This is a recurring pattern in global political theory. Concepts forged within bounded political communities rarely transfer cleanly to transnational relationships, and distributive justice is no exception. Climate burden-sharing forces us to confront three complications that a snapshot of equal entitlement obscures. The first is the historical accumulation of emissions. The second is the moral difference between emissions that sustain life and those that sustain affluence. The third is whether present persons can inherit responsibility for what the dead have done. Each one reshapes what a just allocation might look like.

Per capita frameworks typically begin counting from the present and treat the remaining carbon budget as the pie to be shared. But much of the pie has already been consumed. Industrialised states in Europe and North America produced the bulk of cumulative carbon dioxide during two centuries of fossil-fuelled growth. Because carbon dioxide persists in the atmosphere for centuries, those historical emissions continue to drive warming today. The United States alone accounts for roughly a quarter of cumulative emissions since the mid-eighteenth century.

The polluter pays principle yields a direct conclusion: those who caused a harm should bear the costs of remedying it. Applied historically, this generates what Global South negotiators and scholars have called an ecological debt or carbon debt. On this view, equal per capita shares going forward would ratify an unequal prior appropriation. It resembles dividing an estate equally among heirs after one of them has already spent half of it.

The argument faces serious objections. The first concerns excusable ignorance. Before scientific understanding consolidated, often dated to the IPCC's First Assessment Report in 1990, emitters could not reasonably have foreseen the harms they were causing. Culpability, many argue, requires foreseeability. The second objection concerns agency. The individuals who burned coal in 1880 are long dead, and it is unclear why their compatriots today should answer for choices they never made.

Simon Caney and others respond by shifting the grounds of obligation. The beneficiary pays principle holds that those who enjoy the fruits of historical emissions owe something to those who bear their costs, even without fault. Those fruits include industrial infrastructure, accumulated wealth, and institutional capacity. One need not have caused an injustice to be obligated to relinquish its benefits. Combined with an ability-to-pay principle, this hybrid roughly tracks the UNFCCC's language of common but differentiated responsibilities and respective capabilities.

The institutional record shows how contested this remains. The decision accompanying the Paris Agreement states explicitly that its loss and damage provisions provide no basis for liability or compensation. A dedicated loss and damage fund was later operationalised, but it was framed as solidarity rather than restitution. That framing is itself revealing. States recognise that the vocabulary of debt carries normative weight the vocabulary of assistance does not. Choosing between the two determines whether transfers are owed or merely offered.

Takeaway

Equal shares of what remains can be deeply unequal when what remains is only what others left behind. The baseline from which we measure fairness is itself a moral choice.

Per capita allocation treats a tonne of carbon as morally interchangeable with any other. In 1991, Anil Agarwal and Sunita Narain of India's Centre for Science and Environment challenged this assumption. They distinguished the survival emissions of the global poor from the luxury emissions of the affluent. Methane from a smallholder's rice paddy and carbon from a second home's climate control are not, they argued, equivalent contributions to a shared problem.

Henry Shue gave this distinction rigorous philosophical form. Drawing on his theory of basic rights, Shue argued that emissions necessary for subsistence should be protected and never traded against the convenience of others. A person cooking over a fire or heating a single room is exercising a claim grounded in survival. Someone emitting for leisure travel is exercising a preference. Justice, on this account, cannot weigh these as equal entries on a common ledger.

This reframing aligns closely with Martha Nussbaum's capabilities approach. If justice requires securing a threshold of central human capabilities, such as health, bodily integrity, shelter, and practical reason, then emissions necessary to reach that threshold acquire a special priority. The relevant question shifts from how much each person may emit to what emissions human dignity requires. Equal shares may be too little for some and far more than enough for others.

The implications reach below the level of the state. National per capita averages obscure enormous internal inequality. Affluent households in Delhi or São Paulo may emit more than poor households in Ohio. The Greenhouse Development Rights framework responds by setting a development threshold. Only income and emissions above that line count toward a country's responsibility and capacity. Obligations thereby track wealthy individuals across borders rather than assuming that nationality is a reliable proxy for affluence.

Difficulties remain, chiefly in defining sufficiency. Refrigeration, motorised mobility, and air conditioning during lethal heatwaves are increasingly preconditions of basic functioning rather than luxuries. The line shifts with technology, climate, and social context. Yet the capabilities framework offers a principled way to draw it. It anchors the threshold in what people need in order to function as full human beings, rather than in habitual consumption or market demand.

Takeaway

A just allocation might begin not with equal shares but with a protected floor. That means asking what emissions human dignity requires before asking how to divide the rest.

Climate change stretches responsibility across time in ways that strain ordinary moral concepts. Emissions from the 1850s shape conditions in the 2050s. The agents responsible, the beneficiaries, and the victims are often separated by many generations. This raises two distinct questions. Can present generations bear responsibility for their ancestors' emissions? And can present emitters wrong future persons who do not yet exist?

Derek Parfit's non-identity problem complicates both questions. Large-scale policy choices, such as industrialising on coal, alter who meets whom and when, and therefore which people come into existence. A future person suffering from climate harms might not exist at all had history unfolded differently. If their life is still worth living, the standard comparative account of harm fails. That account holds that someone is harmed when made worse off than they would otherwise have been.

The puzzle cuts backward as well. Present citizens of wealthy states owe their existence to the very industrial trajectory that produced historical emissions. It seems odd to say they are burdened by a history that constituted them. Yet it seems equally odd to conclude that no one can be wronged by climate change because everyone affected owes their existence to its causes. Parfit himself treated such conclusions as signs that our theories needed revision.

Two responses gain traction here. The first adopts a threshold rather than comparative account of harm. On this account, bringing people into conditions below a sufficiency line wrongs them regardless of counterfactuals. This connects naturally with capabilities theory. The second, developed by Janna Thompson among others, locates responsibility in transgenerational collective agents. States persist while their members change, much as states honour treaties and sovereign debts contracted by previous governments.

The collective-agent view sidesteps the problem of individual guilt but raises its own questions. Should recent immigrants share the historical obligations of their adopted states? The inheritance analogy suggests an answer. Those who accept the benefits of membership, including public infrastructure, accumulated wealth, and stable institutions, also accept its encumbrances. Political membership becomes a form of inheritance, carrying both the assets and the liabilities of a shared past.

Takeaway

Responsibility across generations may not run through bloodlines or individual guilt but through institutions that persist while their members change. Asking who owes is partly asking which collective agents endure.

Per capita equality is a useful starting heuristic, but it cannot serve as a complete theory of climate justice. It captures the intuition that the atmosphere belongs to no one in particular. It misses how unevenly that commons has already been used, what emissions are for, and how obligations travel across time.

A more adequate framework would be layered. It would first protect a subsistence floor grounded in central human capabilities. It would then allocate remaining burdens according to beneficiary-pays and post-awareness polluter-pays principles, adjusted for capacity. Finally, it would assign obligations to persisting collective agents rather than to aggregated individuals.

Such a framework demands more of political philosophy than the extension of domestic principles to a larger map. It requires concepts that operate across borders and generations at once. Whether our institutions can carry that weight remains an open question, and the atmosphere will continue to keep its own accounts.