Why the IPCC’s report on global warming matters
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 2C 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 agreementwhich commits them to keep warming well below 2C and to pursue efforts towards 1.5C. The effects and technical feasibility of keeping warming within this tighter limit were the reports 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. The reports 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 oceans 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 2C target has been baked into climate policy for yearsthe 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 1.5C, 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 todays 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 IPCCs report matters. Some European Union environment ministers want to adopt 1.5C 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 2C future, and also bedevilled them. A target of 1.5C is no more likely to be met, but may nonetheless encourage the world to try harder.