A short and non-technical version of this study was published in the New York Review of Books, December 5, 2002, and can be downloaded from that site, click here.
http://www.nybooks.com/articles/15850

The full study, to be published by the American Academy of Arts and Sciences, is available in preliminary form. Click here for a pdf version; click here for a MSWord version.
http://www.econ.yale.edu/~nordhaus/homepage/homepage.htm

William D. Nordhaus is Sterling Professor of Economics at Yale University, New Haven, Connecticut, USA.