He was Chairman of the Global Environment Facility's Scientific and Technical Advisory Panel from 1991 to 1994, Chair of the Intergovernmental Panel on Climate Change (IPCC) from 1997 to 2002 and Board co-chair for the Millennium Ecosystem Assessment from 2000 to 2005. He was then Director of the International Assessment of Agricultural Science and Technology for Development which ran from 2005 to 2007, and had previously been co-chair of the International Scientific Assessment of Stratospheric Ozone for their reports from 1994[9] to 2006.[10] He has been Chair or co-chair of other international scientific assessments, including the IPCC Working Group II, the United Nations Environment Programme/World Meteorological Organization (UNEP/WMO), and the UNEP Global Biodiversity Assessment.