As of latest THP publication:
Rob Williams is a professor at the University of Maryland, the chief economist for the Climate Leadership Council, a university fellow with Resources for the Future, and a research associate of the National Bureau of Economic Research.
His research looks at taxation and environmental regulation, and especially on the intersection of those two topics (environmental taxation). It covers broad theoretical questions such as how to measure the effects of taxes and regulations on economic efficiency as well as more applied work on specific policy issues such as carbon pricing.
