UN says the 1.5°C global warming limit can no longer be avoided - critical summary review

The 1.5 degrees Celsius that keeps showing up in climate headlines is not a temperature anyone reads off a thermometer. It is how far the average temperature of the planet has risen compared with preindustrial levels, and holding the rise to that mark is the centrepiece of the Paris climate agreement.

The reason that abstract number travels so far is what sits on either side of it. The United Nations Environment Programme says every additional fraction of a degree above 1.5 raises the risk of irreversible changes in ice sheets, permafrost, ocean circulation and coral systems, "along with devastating human impacts". Even a world that peaks at 1.7 degrees, the agency says, would make heatwaves like the record ones of recent months frequent and common. This week, a UN report said that line will be crossed.

What "overshooting" 1.5 °C means

The report is the annual Emissions Gap assessment, written to inform governments heading into the next round of climate talks, COP31, in Turkey next month. Previous editions had expressed doubts about the 1.5 target. According to CNN, this is the first time its authors state that missing it is "imminent and unavoidable".

That changes what the report measures. Instead of calculating the distance between what countries promise and what it would take to stay below 1.5, it now models "overshoot" scenarios: how to keep peak warming to between 1.7 and 1.8 degrees and then bring the global average back down to 1.5 by the end of the century, through emissions cuts and carbon removal technologies that, the report notes, have not been shown to work at that scale.

"We are now saying this window has closed, but we can open it again by making the overshoot as brief as possible," said Inger Andersen, executive director of the programme, on a call with reporters. She urged governments to keep the 1.5 goal anyway. "It's not time to throw in the towel and give up."

The numbers the UN says are needed now

To follow the lowest overshoot path, global greenhouse gas emissions would have to fall by 18% by 2030 and 45% by 2035, measured against 2025 levels. That would also require tripling renewable electricity capacity by 2035 and phasing out coal by 2040.

Nothing close to that is on the table. If countries carried out the pledges they have already made, peak warming would land at about 2.6 degrees, according to the programme's projections. On current policies rather than promises, the figure is about 2.8. And the clock inside the arithmetic is unforgiving: every five years of emissions at today's level pushes projected peak warming up by around 0.1 degree.

Global emissions themselves appear to have flattened, with 2025 broadly similar to the year before, but analysts at the programme declined to call it a peak or a plateau given geopolitical uncertainty and continued reliance on fossil fuels.

How the world got here

The United States, the second largest emitter after China, has withdrawn from the Paris agreement and retracted its emissions reduction commitments, which the report cites as part of why its 2.8-degree estimate rose since last year. At COP30, held in Brazil last November, governments were supposed to arrive with stronger commitments; the programme's analysis found the pledges made there noticeably weak.

The shortfall is supposed to be corrected through the "global stocktake", the Paris mechanism that reviews countries' targets, next scheduled for 2028. That timetable now depends partly on a dispute inside the Intergovernmental Panel on Climate Change, the UN body that assesses climate science and whose reports normally appear every five to seven years. The sixth came out in 2021. At a meeting in Addis Ababa, members failed to agree on when to publish the seventh.

Many European and other countries wanted it in 2028, in time for the stocktake. Saudi Arabia, India and China are among those arguing that publishing in 2029 would give developing countries more time to absorb the findings. Ministers from more than 30 countries, including the chair of the group representing the 44 poorest nations and ministers from small island states, signed a letter arguing that separating the UN policy cycle from the panel's evidence would weaken international cooperation. The panel's chair, Jim Skea, asked members to end the stalemate, warning of the "organisational and reputational significance" of the decision.

For Johan Rockström, director of the Potsdam Institute for Climate Impact Research, the argument over dates should not hold up governments. "The science is already clear. Closing that gap is a question of political action, not a lack of scientific evidence," he said.

What this warming is already doing

While the timetable is debated, a separate annual review of the climate published in the journal BioScience found that seven of the 16 planetary vital signs it tracks have never been in a more extreme state, including record lows for the Greenland and Antarctic ice sheets and record highs for ocean heat. The same analysis found the pace of warming has quickened from about 0.05 degrees per decade between 1880 and 1970 to about 0.36 degrees per decade since 2010. "If Earth were a patient coming in for an annual physical, I would say its condition is serious and getting worse," said lead author Bill Ripple, of Oregon State University.

The effects are already reshaping ordinary things. In Longyearbyen, in the Norwegian archipelago of Svalbard, which is warming at five to seven times the global average rate, residents have been told they can no longer drink the tap water: thawing permafrost exposed bedrock that is leaching heavy metals into the lake that supplies the town. And a rapid study by the research teams World Weather Attribution and Climate Central estimated that global warming added about 6 mph to the winds of Hurricane Polo last month, the longest-lasting Category 5 storm recorded in the Pacific, with the current El Niño adding about 5 mph. The study has not been peer reviewed.

Failure, or a sign of how slow this is moving

Crossing 1.5 is a milestone rather than an absolute defeat for climate science and policy, argues Kate Marvel, a climate scientist at the nonprofit Project Drawdown, who was not involved in the UN report. "There's nothing special about any given temperature threshold," she said. "The right unit isn't degrees, it's tons, of carbon dioxide, methane and other greenhouse gases." Passing the mark, she added, "is a sign that we're not moving fast enough, but it doesn't mean we're doomed."

How that message is delivered may matter too. Jay Van Bavel, professor of psychology and neural science at New York University, described in the Guardian an experiment with 59,440 participants in 63 countries testing 11 different climate messages. The most frightening text was the one people were most likely to share online, and the worst at getting them to take concrete action such as planting trees. The message that simply stated the scientific consensus did better on every measure except sharing.

What to do with this information

The things worth watching are formal and dated, not declarations. The first is COP31 in Turkey next month, where the test is not whether governments restate long-term goals but whether the cuts they commit to for 2030 and 2035 move anywhere near the 18% and 45% the report describes. The second is whether the IPCC reconvenes and sets a publication date for its seventh report, since that determines whether the 2028 global stocktake happens with current science in hand or without it. The third is the stocktake itself, the moment the Paris agreement sets aside to measure promises against reality.

It also helps to keep two numbers separate. Pledges point to about 2.6 degrees of peak warming; actual policies point to about 2.8. The gap between those two figures is the distance between what has been announced and what is being implemented, and it is the one that moves least in headlines.

When the next round of commitments is announced, what will tell you whether anything really changed: the target a government sets for 2050, or the cut it signs up to for the next four years?