1. U.S. Geological Survey, Florence Bascom Geoscience Center, Reston, VA 20192, USA
2. Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, RI 02912, USA
3. School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK
4. NORCE Norwegian Research Centre, Bjerknes Centre for Climate Research, N-5007 Bergen, Norway