THE Intergovernmental Panel on Climate Change (IPCC), a gathering of scientists who advise governments, describes itself as “policy-relevant and yet policy-neutral”. Its latest report, the third in six months, ignores that fine distinction. Pressure from governments forced it to strip out of its deliberations a table showing the link between greenhouse gases and national income, presumably because this made clear that middle-income countries such as China are the biggest contributors to new emissions. It also got rid of references to historical contributions, which show that rich countries bear a disproportionate responsibility. That seems more like policy-based evidence than evidence-based policy and bodes ill for talks on a new climate-change treaty, planned to take place in Paris next year.

The new report is intended to measure how far governments have met their promises, formalised in 2010, to keep the global rise in mean surface temperatures compared with pre-industrial times to less than 2°C. It says they are miles from achieving that goal and are falling further behind.

Between 2000 and 2010, it says, greenhouse-gas emissions grew at 2.2% a year—almost twice as fast as in the previous 30 years—as more and more fossil fuels were burned (especially coal, see article). Indeed, for the first time since the early 1970s, the amount of carbon dioxide released per unit of energy consumed actually rose. At this rate, the report says, the world will pass a 2°C temperature rise by 2030 and the increase will reach 3.7-4.8°C by 2100, a level at which damage, in the form of inundated coastal cities, lost species and crop failures, becomes catastrophic.

The report looks at what would be needed to rein back the rise in temperatures, so that it would not exceed 2°C. This, it says, would mean cutting greenhouse-gas emissions in 2050 to between 30% and 60% of their levels in 2010. Unfortunately, emissions are still rising and are likely to increase by around 10% by 2030, at which point, the IPCC suggests, there will be only a 33-66% chance of hitting the 2°C target. By 2100, moreover, the burning of fossil fuel would have to cease altogether unless all the carbon dioxide thus generated is captured and stored.

The panel puts enormous weight on carbon capture and storage (CCS): in some versions of its calculations, doing without it raises the cost of reducing greenhouse-gas emissions by between 30% and 300%. But CCS remains unproven at a large scale.

The IPCC still thinks it might be possible to hit the emissions target by tripling, to 80%, the share of low-carbon energy sources, such as solar, wind and nuclear power, used in electricity generation. It reckons this would require investment in such energy to go up by $147 billion a year until 2030 (and for investment in conventional carbon-producing power generation to be cut by $30 billion a year). In total, the panel says, the world could keep carbon concentrations to the requisite level by actions that would reduce annual economic growth by a mere 0.06 percentage points in 2100.

These numbers look preposterous. Germany and Spain have gone further than most in using public subsidies to boost the share of renewable energy (though to nothing like 80%) and their bills have been enormous: 0.6% of GDP a year in Germany and 0.8% in Spain. The costs of emission-reduction measures have routinely proved much higher than expected.

Moreover, the assumptions used to calculate long-term costs in the models are, as Robert Pindyck of the National Bureau of Economic Research, in Cambridge, Massachusetts, put it, “completely made up”. In such circumstances, estimates of the costs and benefits of climate change in 2100 are next to useless. Of the IPCC’s three recent reports, the first two (on the natural science and on adapting to global warming) were valuable. This one isn’t.