FOR decades scientists have warned that rising atmospheric concentrations of greenhouse gases from the burning of fossil fuels risk adversely affecting the climate, increasing ocean acidity, the frequency of freak weather and other symptoms of planetary ill-health. But it seemed that keeping the temperature within 2°C of pre-industrial levels, although disruptive, would probably leave Earth in a chronic but stable condition.

A report unveiled this week from the Intergovernmental Panel on Climate Change (IPCC), a UN-backed body that musters the science needed to inform policy, shows how optimistic that was. The survey was commissioned in 2015 by the then 195 signatories of the Paris climate agreement—which commits them to keep warming “well below” 2°C and to “pursue efforts towards 1.5°C”. The effects and technical feasibility of keeping warming within this tighter limit were the report’s focus (see article). How it was put together, the message it contains and the reaction it elicited all matter.

A question of degree

First, the way it was assembled. Although the report presents no new science of its own, its survey of more than 6,000 studies is meticulous. With every passing year scientists amass more data about how the climate has already changed. And, as many people battered in Florida this week by Hurricane Michael will attest, it is changing faster than anyone foresaw even two decades ago. This new knowledge, together with improved understanding of the complex climate system, makes projections like those the IPCC has compiled more compelling. Uncertainties remain; individual research contained within the report may yet be challenged. But in study after study, page after page, fact after fact, the evidence for anthropogenic climate change, long clear, is harder than ever to ignore.