He serves the Montreal Protocol as a Co-Chair of its Scientific Assessment Panel which produces the quadrennial assessments of stratospheric ozone depletion. He has authored other major assessments on the topics of climate change, black carbon and aviation to address their roles in the climate system and was a lead author of the 2017 Climate Science Special Report of the National Climate Assessment. He has received distinctions from the U. S. Department of Commerce, the Environmental Protection Agency, the American Meteorological Society, the American Geophysical Union, and the United Nations Environment Program.