The Global Energy Assessment (forthcoming) has estimated that the required share of zero carbon energy in 2030 would need to be about 22 per cent, in order for the target of staying below a 2° C increase from pre-industrial levels to be achieved with a probability of at least 50 per cent. Only the most ambitious technology-optimistic scenarios achieve such a high share, as illustrated by a literature review of renewable energy scenarios (Hamrin, Hummel and Canapa, 2007). The most technology-optimistic of the IEA scenarios (IEA ETP tech plus) barely reaches this level, the others include the EU World Energy Technology Outlook-2050 (WETO-H2) scenario with C02 constraint, and the Greenpeace “revolution” scenario. Assumptions in these scenarios are heroic indeed, requiring unprecedented technological progress, international cooperation and transfers.