1. Environmental Physics, Institute of Biogeochemistry and Pollutant Dynamics, ETH ZurichZurich Switzerland
2. Center for Climate Systems Modeling, ETH ZurichZurich Switzerland
3. Now at Program in Atmospheric and Oceanic SciencesPrinceton UniversityPrinceton New Jersey USA
4. Climate Physics, Institute of Atmospheric and Climate Science, ETH ZurichZurich Switzerland