The Overshoot COP

The world will cross 1.5°C. COP31's job is no longer only the target; it is also the height, the length, and the end of the overshoot.

On the first of September, in Ankara, Türkiye's COP31 President Murat Kurum stood before the assembled diplomatic missions and made a promise. Antalya, he said, would go down in history as the "implementation COP"; the summit that finally converts decisions into action, starting with the eighty-plus decisions the process has adopted and never carried out.

One day later, on the second of September, the UN Environment Programme published a report called Limiting Overshoot, and with it, the United Nations formally conceded what its own process has spent a decade treating as unsayable; the world will cross 1.5°C. Not might. Will. And most likely within the next few years.

These two should be read together. When roughly forty thousand people gather in Antalya this November, they will attend the first Conference of the Parties convened after the UN officially moved 1.5°C from the column of targets to the column of trajectories. COP31 will also be the Overshoot COP.

From a boundary to a window

It is worth being precise about what the UNEP report changes, because the number itself changes nothing. Anyone reading thermometers knew where the trend was pointing; this summer's heatwaves, in Inger Andersen's words, showed impacts that strike faster, hit harder, and last longer.

What changes is the official geometry of ambition. For ten years, 1.5°C functioned as a boundary; a line whose purpose was not to be crossed. Limiting Overshoot replaces the boundary with a window. The new pathway has a name that would not be out of place in a bond prospectus; overshoot, peak and decline. In the best case examined, warming peaks near 1.8°C and is then brought back down. The task, in Antonio Guterres's formulation, is to make the overshoot as small and short as possible.

A window, unlike a boundary, has parameters. Three of them, to be exact. How high does the temperature go; the height of the overshoot? How long does it stay above the line; the duration? And the third parameter, the one the report's scenario language is quietest about; who is obliged to close it?

Because peak and decline is not a physical process. It is a repayment schedule. Every tonne emitted above the carbon budget consistent with 1.5°C becomes, in this framing, a liability to be redeemed later through emission cuts nearly halved by 2035 and, crucially, through the removal of billions of tonnes of carbon dioxide from the atmosphere before the century ends. The report is careful and serious about this; it calls for governance of carbon dioxide removal, for methane cuts, for the protection of the sinks that still work. But read it as an accountant rather than a diplomat and the structure is unmistakable. The world has just booked a debt with no named debtor, no maturity date, and no enforcement mechanism. The creditor, inconveniently, is the atmosphere, which does not restructure.

What implementation now means

This is where the Ankara and the Nairobi report collide. An implementation COP, in the ordinary sense, means executing what was already promised; the eighty decisions, the finance pledges, the transition language agreed in Dubai and carried through Belém. That is necessary work, and the host's instinct to elevate it is sound. But the overshoot framing quietly redefines the job. Once the target is officially a window, implementation can no longer mean only doing what we said. It must mean assigning what nobody has yet said; writing the repayment schedule for the overshoot itself.

Put concretely, an implementation COP that deserves the name would have to answer three questions that no COP has ever been forced to answer. At what height and on what date does warming peak; not as a scenario, but as the sum of the NDCs on the table? Whose national plans carry the net-negative obligations, and in what proportion? Is carbon removal a line item in the pledges of those who emitted most, or a commons everyone gestures at and no one budgets? And what is owed, in adaptation finance, to those who must live inside the window while it stands open; the populations for whom small and short as possible describes decades of their lives?

Notice what these questions have in common. Each one converts an aspiration into a liability with a name attached. That is the real test of the implementation rhetoric, not whether Antalya produces action plans, but whether it can say who closes the window, and on what calendar. A summit that cannot is not implementing; it is performing temporal arbitrage, drawing down the credibility of a target in the present while booking the repayment to a future that holds no seat at the table. The commitment debt of the climate regime, thirty years of adopted-and-unimplemented decisions, would then simply be rolled over into its largest tranche yet; the overshoot itself.

The honest objection

There is a serious argument against everything I have just written, and it deserves the floor. Declaring overshoot inevitable, critics of this framing have long warned, is itself a moral hazard. It converts failure into a planning assumption, and planning assumptions are comfortable. As small and short as possible is elastic language; every deferred coal phase-out fits inside it. It is no accident that the Pacific island states, meeting in Palau this same week, refused the obituary and reaffirmed 1.5°C as a living target for the countries whose existence is priced in fractions of a degree. The boundary is not a negotiating position but a property line.

The objection is right about the risk and wrong about the remedy. The moral hazard of admitting overshoot is real; the moral hazard of pretending otherwise is larger, and we have been living inside it. A target everyone privately treats as dead but publicly treats as alive disciplines no one. It becomes a credential, cited in every framework and binding in none. The window framing is dangerous only if the window's parameters stay unwritten. Height, duration, obligation; named, dated, and assigned, they are a harder constraint than the boundary ever was. Unnamed, they are a license.

Which is why the Pacific's insistence and UNEP's concession are not, in the end, opposed. One defends the line; the other prices the crossing. Antalya's task is to hold both at once.

The host has chosen its epitaph, "the implementation COP." History will apply a simpler test. The first summit of the overshoot era will be remembered not for the number of decisions it implements, but for whether it was the moment the world's largest unsecured debt finally got a repayment schedule or the moment it was quietly rolled over again, at a floating rate, in someone else's name.