“No senator since Al Gore knows as much about the science and diplomacy of climate change as Kerry,” said David Goldwyn, an international energy consultant who served as Clinton’s special envoy and coordinator for international energy affairs. “He would not only put climate change in the top five issues he raises with every country, but he would probably rethink our entire diplomatic approach to the issue.”

Kerry could also have a strong impact on climate policy as Defense secretary given the Pentagon’s emergence as a leading force in the Obama administration on energy and climate issues.

“He has a lot of gravitas on national security. He’s made that a touchstone of his career,” said Paul Clarke, a retired Air Force lieutenant colonel who served on the National Security Council staff in both the George H.W. Bush and Clinton administrations, and is now a senior adviser at the Truman National Security Project. “If he talks about the issue of climate change as a national-security issue, he will be taken seriously.”

Kerry has been engaged with climate policy since he attended the first major U.N. climate summit in Rio de Janeiro in 1992. He was coauthor, along with Sens. Lindsey Graham, R-S.C., and Joe Lieberman, ID-Conn., of sweeping legislation that would have capped U.S. emissions of greenhouse gases, although the bill fell apart before making it to the Senate floor. In 2007 he and his wife, Teresa Heinz Kerry, coauthored a book, This Moment on Earth: Today’s New Environmentalists and Their Vision for the Future.