a new report produces a mixture of alarm and apathy

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 weatherand 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 ofthe Paris climate agreement—which commits them to keep warming“well below” 2°Cand 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 Science and technology section). How it was put together, the message it contains and the reaction it elicited all matter.

Aquestion ofdegree

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 reportmay 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.

The report’s message is also beyond doubt: the extra half a degree makes a big difference. Arctic summers could be icefree once a decade in a two-degree world, butonce a century in a one-and-a-half-degree one. Virtually all the ocean’s coral might be irreversibly wiped out in a two-degree world, rather than 70-90% if temperatures rise by less. Sea levels may rise an extra 10cm, washing away the livelihoods of millions more people. Permitting a rise oftwo degrees could also see an extra 420m people exposed to record heat. The 2°C target has been baked into climate policy for years—the number was first put forward by William Nordhaus, who shared the Nobel economics prize this week(see Free exchange). It is too lax.

Hitting either target would entail transforming economies at a breakneck pace. To achieve 1.5°C, the world would by 2050 need to eliminate all 42bn tonnes of carbon-dioxide in annual emissions. Renewables, including hydropower, would have at least to treble their share of electricity generation from today’s 25%. Internal-combustion engines, which power 499 out of 500 cars on the road today, would have to all but vanish. Progress is being made. The number of electric cars on the road is rising fast; green finance is gathering momentum; zero-carbon technologies are being developed. But the scale of the effort required is unprecedented.

That is why reaction to the IPCC’s report matters. Some European Union environment ministers want to adopt 1.5°C as a guide to policy before a UN summit in Poland in December. Their Australian counterpart called it “irresponsible” to phase out coal by 2050. Donald Trump, who plans to withdraw America from the Parisdeal, has not read it. Amixof alarm and apathy has both galvanised efforts to secure a 2°C future, and also bedevilled them. A target of 1.5°C is no more likely to be met, but may nonetheless encourage the world to try harder.