[1] Columbia Univ, Lamont Doherty Earth Observ, Palisades, NY 10964 USA
[2] Schlumberger Doll Res Ctr, Cambridge, MA 02139 USA
[3] Barnard Coll, Dept Environm Sci, New York, NY 10025 USA
[4] NASA, Goddard Inst Space Studies, New York, NY 10025 USA
[5] Columbia Univ, Dept Earth & Environm Sci, New York, NY 10025 USA