The IPCC uses 'reference' scenarios of future emissions that assume no policy interventions directed towards reducing greenhouse-gas emissions (notably carbon dioxide) to determine the magnitude of additional emissions reductions ('mitigation') needed to stabilize atmospheric carbon-dioxide concentrations at various levels. It is on these additional reductions that policy-makers have focused most attention.

Here we show that two-thirds or more of all the energy efficiency improvements and decarbonization of energy supply required to stabilize greenhouse gases is already built into the IPCC reference scenarios. This is because the scenarios assume a certain amount of spontaneous technological change and related decarbonization. Thus, the IPCC implicitly assumes that the bulk of the challenge of reducing future emissions will occur in the absence of climate policies. We believe that these assumptions are optimistic at best and unachievable at worst, potentially seriously underestimating the scale of the technological challenge associated with stabilizing greenhouse-gas concentrations.

A spring gale is lashing orthodox climate policy. This week, an article was published in Nature that should shake the certainty of anyone who assumes that the Kyoto protocol approach is the sensible way to go, and that signing the accord is a responsible step for the United States to take....