1. Hoffman, P. F., Kaufman. A. J., Halverson, G. P. & Schrag, D. P. A Neoproterozoic snowball earth. Science 281, 1342–1346 (1998).
2. Hoffman, P. F. & Schrag, D. P. The snowball Earth hypothesis: testing the limits of global change. Terra Nova (in the press).
3. Held, I. M. & Soden, B. J. Water vapor feedback and global warming. Annu. Rev. Energ. Environ. 25, 441–475 (2000).
4. Pierrehumbert, R. T. in Mechanisms of Global Change at Millennial Time Scales (eds Clark, P. U., Webb, R. S. & Keigwin, L. D.) Geophys. Monogr. Ser. 112 (American Geophysical Union, Washington DC, 1999).
5. Dahl-Jehnsen, D. et al. Past temperatures directly from the Greenland Ice Sheet. Science 282, 268–271 (1998).