Every thread of this volume gathers in the atmosphere. The fuel is bought, the journey made, the invoice settled; the carbon dioxide released along the way joins a stock whose consequences cross the boundaries of the bargain. People far from the sale, and people not yet born, have interests in what it permits. The familiar laundry could at least be found beside the mill. Here the missing party is dispersed through countries and generations, with neither a single representative nor a single set of circumstances. The difficulty is larger than collecting enough signatures. Someone must decide which consequences count, how the interests of absent people enter the decision, and what will make the resulting commitment hold. The atmosphere asks the theory of the externality to supply an institution as well as a price.
The persistence of the stock changes the meaning of correction. A smaller annual addition can still enlarge the accumulated problem. The IPCC's 2023 assessment relates warming to cumulative carbon dioxide emissions and finds that limiting human-caused warming requires net zero carbon dioxide emissions together with strong reductions in other greenhouse gases. Those gases differ in their lifetimes and effects; the claim is not that each ton of every gas has the same consequence. Nor does reaching net zero restore the previous climate on command. Some changes, including aspects of ocean warming and sea-level rise, continue over much longer timescales. The policy must therefore govern a path, not merely produce a satisfactory receipt for this year's emissions. Delay changes the world in which the next decision will be made.
The national account can omit much of the benefit of acting. Imagine four countries, each able to undertake one measure costing six units. Each measure brings three units of benefit to each country, including the country undertaking it. Acting alone therefore costs the actor six and returns three; acting together costs twenty-four and produces forty-eight in benefits. Every country would gain six if all participated, yet each would gain another three by withholding its own effort while the others continued. The favorable total does not make the promise self-enforcing. These invented countries have equal costs, equal benefits, one decision each and no other interactions. A sufficiently large domestic benefit would change their choice. Their value as an illustration lies in showing how an agreement can be desirable to everyone and still be vulnerable to each participant's temptation to leave its part undone.
A carbon price addresses a different part of the difficulty. It brings an assessed consequence into the emitter's calculation, making a cleaner process or a forgone journey worth considering when its cost is below the charge avoided. The payment can be collected where the fuel enters the economy rather than from everyone who eventually burns it; its effects then depend on how the charge passes through prices and choices. The payment itself is a transfer. It neither removes a molecule nor demonstrates that the people harmed have been compensated. The environmental gain comes through changes in emissions and the resulting physical consequences. Conflating those operations would allow a government to mistake a growing tax account for a diminishing disturbance.
A cap organizes the choice from the other direction. It limits the covered quantity and lets the price of permission move with the cost of meeting that limit. The comparison is developed in The Permit; here the additional difficulty is the boundary around the program. Emissions beyond that boundary still enter the atmosphere. Production may move, demand may change, and the technologies made cheaper by one country's effort may be used elsewhere. The net effect must be established rather than inferred from either a domestic reduction or the mere possibility of leakage. The atmosphere is indifferent to the location of the counted carbon dioxide. The institutions measuring and controlling it are not.
International agreement supplies a means of coordinating those institutions without first creating a world government. The Paris Agreement's 2015 architecture requires each party to prepare, communicate and maintain successive nationally determined contributions and to pursue domestic mitigation measures with the aim of achieving them. It couples those contributions to reporting, technical review and a recurring global stocktake. These are substantive obligations and procedures; they are not a single internationally imposed carbon price or a guarantee that every announced result will occur. Its compliance mechanism is expressly facilitative and non-punitive. The arrangement tries to make national undertakings visible, comparable and progressively more ambitious. Whether the mechanism produces enough action is a further question, answered by performance rather than by the existence of the agreement alone.
Other arrangements try to change the return to participation more directly. Nordhaus's climate-club model joined a common carbon-price target to trade penalties on countries remaining outside. The penalty made abstention costly in a way the shared climate benefit could not. Under some of the model's targets and assumptions, that changed the stability and size of the cooperating group. The qualifications are part of the proposal: the modeled regime permitted the penalties, excluded retaliation, and became less successful at sufficiently demanding price targets. A real arrangement would have to confront those questions, along with the distribution of its costs. The contribution of the model is to expose a missing term in the bargain. Asking a country to join and giving it a durable reason to remain are different tasks.
The terms of membership also have to recognize unequal means. A common marginal incentive can coexist with very different contributions to its financing. A country with limited access to capital may face a costly transition even where the cleaner investment would eventually save resources. Support can change what is feasible, while transfers can change whether a feasible agreement is acceptable. The Paris Agreement accordingly places financial support beside mitigation, adaptation and transparency, rather than treating it as a charitable supplement to a completed bargain. Neither the size of the common gain nor a uniform price decides the division. Historical contribution, present capacity, development needs and exposure to harm remain questions that the negotiating parties must answer.
Future people pose a further problem because a transfer among today's negotiators does not bring them into the room. Their interests must be represented through present institutions: through the time horizon of an assessment, the protection of an ecological threshold, the terms of a public commitment and the weight assigned to later losses. These choices can be made explicit and challenged. They cannot be eliminated by calling the calculation technical. The Shadow follows the valuation into the number; The Burden asks how the harm and the remedy are divided. The bargain becomes more defensible when its absent parties remain visible, including where no monetary estimate can carry their entire claim.
Domestic action still has a place within this larger arrangement. Cleaner air, a less vulnerable energy system or useful innovation can give a country reasons to act that extend beyond its own share of the climate benefit. At the same time, pricing cannot supply every missing transmission line, solve every financing constraint or make every institution capable of enforcing a rule. The IPCC's assessment treats economic instruments alongside regulation, investment, innovation and institutional capacity. A price can coordinate many dispersed choices because it does not require an official to prescribe each one. That economy of information gives the instrument its strength. It does not relieve the public authority of building conditions under which the choice can actually be made.
The atmosphere has no place at the bargain until an institution gives its consequences a voice.
The answer to who can strike the bargain is therefore neither nobody nor a single benevolent calculator. Governments can bind and coordinate their own conduct; agreements can connect their efforts; public institutions can give standing and weight to claims absent from the sale. Every part remains answerable to what it accomplishes and whom it leaves exposed. The price is one means by which the resulting decision reaches the chimney, the furnace and the journey. It becomes effective through measurement, alternatives, finance and credible continuation. What began as a cost outside an invoice has become a question about the reach of collective judgment. The account cannot close at the border of the transaction when the consequence does not.