e177.
The “18 degrees” estimate is based on the conversion factor proposed by H. Damon Matthews and cited in Hatice Cullingford, “‘Matthews Measure’ Proposed New Yardstick for Climate Change”, Science 2.0, 10 September 2009: “In summary, independent of the timing of emissions or the atmospheric concentration of carbon dioxide, each tonne in carbon emissions means—as a best estimate—0.0000000000015 degrees of global temperature increase.” For a survey of the caveats that should be taken into account, see H. Damon Matthews, Nathan P. Gillett, Peter A. Stott and Kirsten Zickfeld, “The Proportionality of Global Warming to Cumulative Carbon Emissions”, letter in Nature, 459, 11 June 2009, pp 829–832.