1. Lamont–Doherty Earth Observatory, Columbia University, 61 Route 9W, Palisades, New York 10964, USA
2. Physics of Ice, Climate and Earth, Niels Bohr Institute, University of Copenhagen, 2100 Copenhagen, Denmark
3. Climate System Research Center, Department of Geosciences, University of Massachusetts–Amherst, Amherst, Massachusetts 01003, USA