John Kerry

Kerry heads the Senate Foreign Relations Committee and has been involved in both domestic and international climate policy going back to the 1980s and '90s, sponsoring legislation at home to cut greenhouse gases while frequenting U.N. climate change conferences.

At times, he has been the only senator and highest-ranking U.S. official to attend the talks, noted Andrew Light, a senior fellow at the Center for American Progress. In those years, the Massachusetts senator took on the task of reassuring a skeptical world that the United States would eventually act on climate change.

Light remembered Kerry's appearance in Bali, Indonesia, in 2007, nearly seven years into the George W. Bush administration, during which he promised the global community that the United States would be able to take a more proactive role after the 2008 presidential election because both Obama and 2008 GOP nominee John McCain believed in climate change.