1. IIASA - International Institute for Applied Systems Analysis, Schlossplatz 1, 2361 Laxenburg, Austria, Department of Geosciences, University of Oslo, P.O. Box 1047, Blindern, 0316 Oslo, Norway, and CICERO - Center for International Climate and Environmental Research - Oslo, P.O. Box 1129 Blindern, 0318 Oslo, Norway