Turner claims that to keep the temperature rise close to two degrees, the world’s greenhouse gas emissions must peak in 2016 then fall by either three or four per cent a year. A 3% rate of decline is most likely to deliver a temperature rise of 2.2 degrees this century; a 4% annual cut would produce about 2.1%(3). That’s more or less consistent with his 2050 targets.
So far so good. But a recent paper in the Philosophical Transactions of the Royal Society, using the same sources, comes to completely different conclusions(4). It agrees that to deliver a reasonable chance of preventing more than two degrees of warming, greenhouse gases in the atmosphere need to stabilise at a maximum of 450 parts per million, carbon dioxide equivalent (ppmCO2e). But it shows that to achieve this, global emissions of greenhouse gases from the parts of the system we can control need to peak by 2015, then fall by 6-8% a year between 2020 and 2040, leading to “full decarbonization sometime soon after 2050.” Even this, it shows, relies on an optimistic reading of the current data. Turner’s suggested cuts are more likely to produce four degrees of warming than two degrees.
The difference between the two reports comes down to this: Turner assumes that greenhouse gases can rise to 500ppmCO2e before falling back to 450(5). The other paper shows that this is a dangerous assumption. Not only does this mean that the cut comes too late, but far from falling back, the enhanced levels in the atmosphere are likely to trigger more emissions, as the biosphere starts producing more greenhouse gases than it absorbs. We cannot afford to overshoot(6).