The report’s message is also beyond doubt: the extra half a degree makes a big difference. Arctic summers could be ice-free once a decade in a two-degree world, but once 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 of two 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 Paris deal, has not read it. A mix of 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.