IN 1996 the European Union became the first significant political body to suggest that the goal of preventing “dangerous anthropogenic interference in the climate”, to which the world had signed on at the Rio Earth summit of 1992, meant, in practical terms, keeping global warming below 2°C relative to the late 1800s. This two-degree limit had been an informal measure of the point where climate change gets serious since the 1970s. William Nordhaus, a pioneer of climate economics who this week shared the Nobel prize for his efforts (see Free exchange) seems to have been the first to use it as such. But between 1996 and the Copenhagen climate summit of 2009 it was transformed from one possible interpretation of the Rio goal to the target on which the world agreed.

At the Paris climate summit of 2015, though, this changed. In light of both new evidence and new concerns, notably those of low lying countries that might not survive the amount of sea level rise two degrees would bring, the nations of the world agreed a new target: keeping warming “well below” 2°C above pre-industrial temperatures. Indeed, they urged themselves to “pursue efforts towards 1.5°C”.

This lower target would presumably be better for all, not just the likes of Kiribati. But exactly how much better has been far from obvious. So the Paris agreement also gave to a body called the Intergovernmental Panel on Climate Change (IPCC) the task of finding out. Given that the world is actually on track for a rise of more than 3°C, regardless of the pieties of Paris, it was also charged with finding out whether limiting the rise to 1.5°C is in any way feasible.

The judgment on Paris

On October 8th, nearly three years, several drafts and some 40,000 reviewer comments later, the panel unveiled the fruit of its labours at a gathering in Incheon, South Korea. The 1,200-page report, written by 91 researchers from 44 countries, presents no truly new science. The panel’s brief was to survey all relevant literature—more than 6,000 studies, many spurred by the report’s commissioning—and to synthesise the results. It makes for sobering reading, both in terms of what the half-degree difference between the two targets may mean for the planet, and regarding the effort needed to meet the tougher goal.

The authors profess “high confidence” of a “robust difference” between 1.5°C and 2°C worlds. At 1.5°C, 6% of insect species, 8% of plants and 4% of vertebrates would lose more than half their habitat. The figures for 2°C are 18%, 16% and 8%, respectively.

At that temperature rise, ecosystems covering between a twelfth and a fifth of Earth’s land mass can be expected to undergo transformation to another type—savannah to desert, say. That is 50% more than would happen with a rise of 1.5°C. Most dramatically, the IPCC finds it almost certain that a 2°C rise would wipe out more than 99% of corals. By contrast, a rise of 1.5°C would leave 10-30% of them alive, and with them the hope of regeneration if temperatures subsequently stabilised.

Permitting a rise of 2°C rather than 1.5°C could also see 420m more people exposed regularly to record heat. “Several hundred million” more would have to contend with climate-induced poverty. Food security would decline and water scarcity increase, especially in poor and already-fragile areas such as the Sahel region of Africa, just south of the Sahara desert. And an additional 10cm of sea-level rise could hurt the livelihoods of more than 10m people living on the coast.

The report also nods towards the chance of dangerous feedback loops. A two-degree temperature rise could lead to the thawing of 1.5m-2.5m km2 of permafrost—about the area of Mexico. That, in turn, would release methane, a potent greenhouse gas which would lead to further warming, thawing and so on.

The IPCC does not quantify the effects of such feedback. But work which appeared in August, after the deadline for consideration in the report, attempts to do so. This study, led by Will Steffen of the Stockholm Resilience Centre and published in the Proceedings of the National Academy of Sciences, concludes that five feedback loops unleashed by a rise of 2°C are likely to be important. These involve the permafrost, natural carbon sinks such as the ocean, increased methane emissions from marine bacteria, and the dying of Amazonian and boreal forests. Together these could add between 0.24°C and 0.66°C of extra warming.

Such alarming conclusions are necessarily subject to the huge uncertainties inherent in climate science. Though they have survived scrutiny by peer review in the journals in which they appeared, and then again by the IPCC’s authors, individual studies may yet be challenged. Taken together, however, they paint a picture that looks bleak. There is, remarks Glen Peters of the Centre for International Climate Research in Oslo, who was not involved in the report, perhaps one-tenth of the material where there might be disagreements, but scientists agree 100% about the remaining nine-tenths.

Cooking in gas

The same uncertainties apply to the report’s outline of possible pathways to a 1.5°C future. On the bright side, the IPCC concludes that such a future remains geophysically within reach, thanks to what remains of the Earth’s “carbon budget” for 1.5°C—the cumulative sum of emissions at which the climate system stands a good chance of remaining below a particular temperature. The panel’s Assessment Report, a septennial compendium of the latest climate science, most recently published in 2013-14, warned that an eventual minimum rise of 1.5°C, though it would not manifest itself until mid-century, would be “baked” irreversibly into the climate system by 2020 if economic activity continued to belch carbon dioxide at the present rate. In the past few years climate modellers have, controversially in the eyes of some, revised the Earth’s remaining budget to around 12 years’ worth of current emissions, thus pushing back the date of bake-in.

Even with a bigger carbon kitty though, keeping the temperature rise below 1.5°C would take an epic effort. Of 90 published models purporting to chart the most economically efficient way to achieve this goal, the IPCC considers that just nine stay below the threshold throughout this century. The rest overshoot it, and so require removal of carbon dioxide from the atmosphere to offset the excess emissions.

These “negative emissions” could come from planting more forests, which draw in carbon dioxide as they grow. Planting “energy crops” such as fast-growing grasses, which could be burned instead of fossil fuels (with the carbon dioxide thus generated captured and stored underground), is also possible. Either approach, though, would mean converting to that purpose an area of agricultural land somewhere in size between India and Canada. An alternative is “direct air capture”—artificial devices that retrieve carbon dioxide directly from the atmosphere. These exist but they, too, would need to be deployed at a gargantuan scale. (Solar geoengineering, a controversial idea to disperse particles of matter into the atmosphere to reflect heat back into space, was not considered in detail.)

Negative emissions or solar geoengineering might ease the need to decarbonise economies quickly—but not eliminate it. As the charts show, even with negative emissions carbon-dioxide release still needs to fall by 45% or thereabouts by 2030. To have any hope of achieving this, two-thirds of coal use must be phased out in little more than a decade. By the middle of the century virtually all electricity must come from carbon-free sources (up from a quarter today), and all cars will need to run on electric motors (up from one in 500), as will trains and most ships.

Some of the technology needed to achieve this (solar panels, nuclear-power plants, electric cars and so on) is around, but not all of it. For aeroplanes to keep flying, either novel aviation biofuel will need to be developed or negative emissions used to offset those from aircraft. Because cows produce lots of methane people will either have to switch to laboratory-grown burgers or change diets (see Briefing). Even when appropriate technology does exist, market forces alone will not improve it and spread it fast enough to have the necessary climatic effect.

Were any of this actually to happen, it would transform economies beyond recognition. And it would cost money. How much, the IPCC has resisted predicting, blaming limited economic research in the area. But, for the same reason, it does not attempt to value the flip side—the damage caused by delay.

Another paper that missed the deadline, by Simon Dietz of the London School of Economics and his colleagues (one of whom worked on the IPCC report), tries to fill the first of those gaps. It estimates that keeping temperature rises to 1.5°C would cost 150% more than keeping them to 2°C, though it gives no absolute figures. Like the IPCC, Dr Dietz stops short of comparing this to averted losses. But earlier work by others suggests that a rise of 1.5°C would shave 8% from global GDP per person by 2100, relative to a world with no more warming. A rise of 2°C, by contrast, would cause a discrepancy of 13%.

Third-degree treatment?

The world’s press reacted to the IPCC’s tome with alarm sometimes verging on hysteria. News bulletins, front pages and op-eds harangued governments to get their act together and ratchet up climate action—especially since all of them signed off on the report’s 30-page précis. That included the government of America, which President Donald Trump plans to yank out of the Paris agreement. (Mr Trump has since expressed doubts about the précis’s legitimacy.)

On October 9th, a day after the volume’s release and ahead of an important UN climate summit in Poland this December, environment ministers from 15 of the EU’s 28 members pressed the bloc to revise its climate targets in line with the 1.5°C target. This is welcome. But in a world where even the existing target looks likely to be missed by a mile, how much difference it will make is open to doubt. In climate change, as in so many other areas, words are cheap. It is actions that are eloquent.