In 1998, he was elected a member of the German Bundestag for the Social Democratic Party (SPD) and re-elected in 2002. As a legislator, he chaired the select committee on globalization (2000-2002) and the environment committee (2002-2005). After his decision not to run in the 2005 election, he became Dean of the Bren School of Environmental Science & Management at the University of California, Santa Barbara. He returned to Germany in 2009 where is his an honorary professor at Freiburg University.