UNEP Bets the World Can Overshoot 1.5 Degrees

The UN Environment Programme said on 2 September 2026 that exceeding 1.5°C of warming is now unavoidable, likely within the next few years. Even its optimistic path peaks at 1.8°C (3.24°F), then tries to come back down with deep cuts and carbon removal.

UNEP’s Limiting Overshoot report, issued in Nairobi, is the first time the programme has treated staying below the Paris limit as impossible rather than unlikely. It asks governments to hit 1.5°C (2.7°F) from above, and to treat the next decades as a managed hump rather than a wall they can still avoid.

The UN Now Prices 1.5 as a Round Trip

Inger Andersen, UNEP’s executive director, wrote in the foreword that the 1.5°C goal “must now be approached from above.” The report’s chosen route is an overshoot, peak, and decline pathway, which it calls the best remaining option, and which it also calls neither acceptable nor preferred.

There are no good outcomes if we remain above 1.5°C. Across the globe, extreme heatwaves are already proving that climate impacts will strike faster, hit harder, and last longer.

Inger Andersen, Executive Director, UNEP, Limiting Overshoot foreword

UN Secretary-General António Guterres, in the Nairobi release, said the northern summer’s heat, fires and floods were a warning, and that the overshoot above 1.5 degrees must be made “as small and short as possible.” He called that demand “an overshoot of ambition.”

Paris never defined 1.5°C as a single hot year. It is a long average. The calendar has already printed figures above that line. What UNEP is now conceding is the multi-year climate, the one written into the 2015 pact and later treated by the International Court of Justice as the parties’ primary temperature goal.

WHERE THE MERCURY ALREADY STANDS

• 2024: Copernicus ERA5 put the year at 1.60°C above the 1850-1900 average, the warmest in the record and the first to sit clearly over 1.5°C.
• 2025: The same dataset put the year at 1.47°C, third warmest, 0.13°C cooler than 2024.
• Three-year mean: 2023 through 2025 averaged 1.52°C in ERA5, the first three-year stretch above 1.5°C.

A UNEP explainer published with the report walks through five things to understand about overshoot, starting with the claim that it is a curve spread over decades, not a single day when a siren goes off. The flatter and shorter that curve, the explainer says, the better, and that shape is still a political choice.

COP30 Closed With 1.5 Still Within Reach

Nine months before Nairobi, the UN climate secretariat was still selling the old line. On 22 November 2025, as COP30 closed in Belém, Executive Secretary Simon Stiell told delegates they had shown “a firm resolve to keep 1.5C within reach.”

I’m not saying we’re winning the climate fight. But we are undeniably still in it, and we are fighting back.

Simon Stiell, Executive Secretary, UN Climate Change, COP30 closing plenary, Belém, 22 November 2025

He counted 194 countries still in the pact after “one country stepping back,” and he announced a new deal to triple adaptation finance, plus a trillion dollars pledged for clean grids. The speech was written for a room that needed 1.5°C alive as a diplomatic fact.

Sixteen days earlier, Guterres had already told leaders at the Belém climate summit that “we have failed to ensure we remain below 1.5 degrees,” and that a temporary overshoot starting at the latest in the early 2030s was inevitable. UNEP has now pulled that private scientific consensus into a public planning document, and moved the crossing from “early 2030s” to “the next few years.”

HOW THE 1.5 LINE MOVED

1. 12 December 2015: The Paris Agreement commits parties to hold warming well below 2°C and to pursue 1.5°C.
2. 2024: The first calendar year sits clearly above 1.5°C in Copernicus ERA5.
3. 20 January 2025: Washington orders an exit from the Paris pact on inauguration day.
4. 6 November 2025: Guterres tells Belém that a temporary overshoot is inevitable by the early 2030s.
5. 22 November 2025: Stiell closes COP30 still pledging to keep 1.5°C within reach.
6. 27 January 2026: The U.S. withdrawal takes effect under Article 28.
7. 2 September 2026: UNEP publishes Limiting Overshoot and calls exceedance unavoidable.

Debra Roberts, a report author at the University of KwaZulu-Natal, said the rules of the game are now changing and that “none of our plans are prepared” for a world that has already gone through 1.5°C. She also said some losses will be irreversible, “which means returning to 1.5 doesn’t mean we return back to the same world we left.”

Halving Emissions by 2035 Is the Entry Fee

The optimistic peak of 1.8°C is not a forecast of what governments are doing. It is the price of doing almost everything they have already promised, then going further, into net-negative emissions. Joeri Rogelj, a report author at Imperial College London, said holding the peak near that level requires a halving of global emissions by 2035. Emissions are still rising.

THE WAGER UNEP IS ASKING GOVERNMENTS TO TAKE

The removal number is the part of the bet that does not yet have a market or a ministry behind it. Current energy and industry emissions run about 40 gigatonnes of CO2 a year, so the report’s return path is asking for a yearly drawdown equal to about a quarter of what the world still puts up. Reto Knutti at ETH Zurich, who was not an author, said cheap options such as planting forests cannot be scaled that far because the land is not there, and that direct air capture could be scaled but is expensive.

Knutti put the required scale-up of carbon removal at a factor of around 100,000 from today’s dedicated effort. “We should be honest when we talk to policy-makers and the public that this is not the thing to plan for,” he said. “I think the plan should be to adapt for at least 2 degrees, if not more.”

The report itself warns that just holding net-zero CO2 could exhaust estimated storage in sedimentary basins by 2200. James Dyke at the University of Exeter said there are still no credible plans for removal at the scale the return path needs, and that societies should prepare for a warmer world that may last centuries. Oliver Geden at the German Institute for International and Security Affairs said the overshoot idea “may turn out to be no more than a way to mask ongoing failure.”

Reefs and Ice Do Not Get a Refund

UNEP’s own harm list is the reason the 1.8°C peak is not a rounding error. Heat waves get worse. Seas rise further. Floods spread. Coral reefs can collapse. Glaciers, permafrost and other cold systems go through what the authors call profound change. Small island states and low-lying cities, Andersen wrote, could be partly or fully submerged. Some of that damage does not unhappen if the global average later falls.

WHAT A LATER DECLINE DOES NOT REWIND

• Warm-water reefs: IPCC’s 1.5°C special report found 70 to 90 percent of coral reefs would be lost at 1.5°C, and more than 99 percent at 2°C.
• Ice and frozen ground: The new UNEP report flags deep change in glaciers and permafrost, systems that keep moving toward melt even if air temperature later eases.
• Adaptation limits: UNEP’s explainer says weak cuts can push risks past the point where coping is still an option, which is a planning failure, not a wording one.
• Food and water: Roberts listed greater food and water insecurity, harm to physical and mental health, and damage to homes and infrastructure, with lost livelihoods and lives.

That is the hidden ledger of the wager. A government can write “return to 1.5 by 2100” on a chart. A reef that blanched in the 2030s does not reappear because a direct-air-capture plant ran hard in the 2070s. Roberts’s line is the one the report cannot soften: coming back down is not the same as going home.

Konstantin Weber, also at ETH Zurich, noted that among the latest emissions scenarios being prepared for the next round of climate models, only two imply a return to 1.5°C before 2150. The scenario based on current policies, he said, still points toward about 3°C by 2100. He still argued for trying. “We have to give it a shot. There’s no real alternative.”

Current Policies Still Point Toward 2.8 Degrees

UNEP already priced the do-nothing-new path in November 2025. Its Emissions Gap Report found that if today’s policies merely continue, warming this century stays below 2.8°C with a 66 percent chance, inside a range of 2.1°C to 3.9°C. Full delivery of unconditional national plans would trim that to about 2.5°C. Neither figure is the 1.8°C peak that Limiting Overshoot needs if a return this century is to stay plausible.

The new report is therefore not describing the world as it is. It is describing the world if cuts arrive fast enough to cap the hump, methane is slashed, net zero is hit around mid-century, and then the balance goes negative. Andersen told journalists the day before publication that even after the crossing, 1.5°C “is still the goal,” approached from above, with “rapid and deep cuts” and a push beyond net zero.

Glen Peters, a senior researcher at the CICERO Center for International Climate Research in Oslo, pointed to land fights if reforestation is asked to do too much of the drawdown, and to wildfire and climate stress that can kill the trees supposed to store the carbon. The report still treats carefully governed removal as essential. Without it, the authors say, the odds rise of locking in ice-sheet change and a slowdown of the Atlantic overturning circulation.

The U.S. Withdrawal Took Effect in January

The largest historical emitter is not in the room where this wager is supposed to be placed. On 20 January 2025 the White House issued an order withdrawing from the Paris Agreement and told the U.N. ambassador to notify the depositary at once, to drop related climate-finance pledges, and to treat the exit as immediate as a matter of U.S. policy.

Under Article 28 of the pact, a withdrawal takes effect one year after the depositary receives notice. The U.N. recorded the U.S. action on 27 January 2025 and set the effective date at 27 January 2026. Stiell’s Belém speech danced around that fact without naming Washington. UNEP’s September report now asks for the steepest cuts of the Paris era from a coalition that no longer includes the United States as a party.

That hole does not make the physics worse on its own. It makes the political arithmetic of a 2035 halving harder, because the return path UNEP drew assumes national plans are delivered and then exceeded. A party that has left is not updating a plan. Other large emitters can still cut. They will be asked to do so while the original architect of much of the historical carbon stock sits outside the treaty.

The Next Climate Talks Inherit an Overshoot Script

UNEP put the new frame on the record the morning of 2 September.

Global warming is set to cross 1.5°C, pushing climate risks and impacts to increasingly dangerous heights, a new UNEP report warns.  

Predicted impacts include extreme weather and threats to human health and economies.  

Action now can still limit the magnitude and duration of… pic.twitter.com/YGrAUaoEuT

— UN Environment Programme (@UNEP) September 2, 2026

The next U.N. climate conference will not reopen a fight over whether 1.5°C can still be held from below. It will be asked to bargain over how high the peak goes and how many years the world spends above the line. That is already the language of the report, and it is the language delegates will carry in.

The first physical test of that script is already in the Pacific. Berkeley Earth put the Niño 3.4 index near 2.7°C in July 2026 and projected a peak near 4.0°C around November, among the strongest El Niño events in the instrumental record. Leon Hermanson, lead author of the World Meteorological Organization’s annual-to-decadal update in May, said an El Niño at the end of 2026 raises the chance that 2027 is the next record year.

A hotter 2027 would not, by itself, be the Paris breach. The pact still lives on a long average. It would be the first full year of UNEP’s new doctrine meeting the weather, with reefs, ice and food systems taking the pulse in real time while ministers argue about the length of the overshoot.

Andersen wrote that stronger political will is required, and that “decisions taken now and in the next few years will determine whether viable options remain to reverse warming towards 1.5°C.” Every fraction of a degree avoided, she said, and every year by which overshoot is shortened, will save lives, protect ecosystems and cut economic losses. The report leaves the purchase of that return ticket to the same governments that let the outbound journey become unavoidable.