NOTICE: The project that is the subject of this report was approved by the Governing Board of the U.S. National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The Board of the Academia Mexicana de Ciencias also approved this project. The members of the working group responsible for the report were chosen for their special competencies and with regard for appropriate balance.

This report was supported by contracts from the National Oceanic and Atmospheric Administration and the National Science Foundation in the United States and through funding provided by the National Research Council and the Academia Mexicana de Ciencias. The views expressed herein are those of the authors and do not necessarily reflect the views of the sponsors.

Library of Congress Catalog Number 97-68979