1 Department of Environmental Systems Science, Swiss Federal Institute of Technology (ETH Zürich), Schmelzbergstrasse 9, CH-8092 Zürich, Switzerland. Electronic address: mescher@usys.ethz.ch.
2 Department of Environmental Systems Science, Swiss Federal Institute of Technology (ETH Zürich), Schmelzbergstrasse 9, CH-8092 Zürich, Switzerland. Electronic address: consuelo.demoraes@usys.ethz.ch.