[1] UCL, Inst Sustainable Resources, 14 Upper Woburn Pl, London WC1H 0NN, England
[2] Oxford Inst Energy Studies, 57 Woodstock Rd, Oxford OX2 6FA, England
[3] Stockholm Univ, 10 Univ Vagen, S-11418 Stockholm, Sweden
[4] Univ Kiel, 4 Christian Albrechts Platz, D-24118 Kiel, Germany
[5] Columbia Univ, Ctr Global Energy Policy, 1255 Amsterdam Ave, New York, NY 10027 USA
[6] Royal Inst Technol, Brinellvagen 8, S-11428 Stockholm, Sweden