 Complete the following phrase:  After his graduation Tietenberg started his academic career in 1971 as assistant professor in economics at Williams College. In 1977 he started at Colby College as Associate Professor in economics, got promoted to Professor in Economics in 1984, and chaired the department of economics from 1985 to 1988 and from 1993 to 1995. Tietenberg was also principal consultant in the field of environmental policy for national and international government agencies and organizations, such as the World Bank, the Inter-American Development Bank and the United States Environmental Protection Agency. He was one of the speakers at the first 