Abstract in HTML and technical report in PDF available on the Massachusetts Institute of Technology Joint Program on the Science and Policy of Global Change website (http://mit.edu/globalchange/www/)Includes bibliographical references (p. 31).Supported in part by the Univ. of Washington's Joint Institute for the Study of the Atmosphere and Ocean (JISAO) under NOAA Cooperative Agreement (NA67RJ0155 841). Also supported in part by MIT's Joint Program on the Science and Policy of Global Change, with support from the U.S. Dept. of Energy's Office of Biological and Environmental Research to (DE-FG02-93ER61677).