The United Nations Environment Programme published a report on 2 September stating that global temperature will likely cross 1.5°C above pre-industrial levels within the next few years. The report — “Limiting Overshoot — Navigating Exceedance of 1.5°C and Pathways Towards Return” — finds that even its most favourable scenario has warming peaking at around 1.8°C, and that most other scenarios peak higher still.
The significance is less in the numbers than in the change of question. For a decade the 1.5°C figure has been discussed as a limit to be held. UNEP now treats crossing it as expected and sets out what it calls an overshoot, peak and decline pathway — the temperature rises past the threshold, tops out, and is brought back down again. That framing asks something different of policy. Holding a line requires cuts. Returning from beyond it requires cuts and then, on top of them, sustained removal of carbon dioxide already in the atmosphere, over decades, at a scale that would need its own governance.
The report’s requirements are correspondingly layered: immediate and sustained reductions in greenhouse gases including methane, moving toward net zero; long-term net-negative emissions through carbon dioxide removal; adaptation running in parallel to reduce the damage done while temperatures are above the line; and governance arrangements for removal itself.
“Now is the time we must double down on climate action. We can — and must — get on the right path,” said Inger Andersen, the programme’s executive director.
A threshold reframed as a round trip is a harder thing to communicate than a threshold defended, and harder still to legislate. The report’s own argument is that the difference between a peak at 1.8°C and a higher one is decided by what happens now, while the crossing is still ahead.