President Obama will nominate Sen. John Kerry, D-Mass., to be secretary of State, sources tell ABC News and CNN.

Kerry, the Massachusetts Democrat who chairs the Senate Foreign Relations Committee, has long been viewed as a likely candidate to replace Secretary of State Hillary Rodham Clinton.

While United Nations Ambassador Susan Rice more recently had been viewed as the top contender for the post, she withdrew her name from consideration on Thursday after months of criticism for her response to the terrorist attack in Benghazi.

 If Kerry becomes the next secretary of State, he will likely raise climate change to a top-tier priority.

“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 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.

And in an impassioned Senate floor speech in August, Kerry compared the threat of climate change to the threat of war. “I believe that the situation we face [with climate change] is as dangerous as any of the sort of real crises that we talk about” in Iran, Syria, and other world trouble spots, he said.

While many lawmakers speak passionately about climate change, Kerry has also logged years doing the thankless behind-the-scenes work of climate diplomacy and in the process has earned the respect of the rest of the world on the issue.

He was the only U.S. senator to attend key U.N. climate-change negotiations in Bali, Indonesia, in 2007, and Poznan, Poland, in 2008. At a major climate summit in Copenhagen, Denmark, in 2009, Kerry arrived before President Obama and Secretary Clinton and spent time in back-room meetings with ministers from China, India, and several European countries to help pave the way for final negotiations.

More recently, in his position as Foreign Relations chairman, Kerry has prioritized action on a Law of the Sea treaty that addresses melting polar ice caps.

“Kerry’s been one of the most active champions on the Arctic. He’s out there saying, ‘The Arctic ice is melting, we’ve got to do something,’ ” said Charles Ebinger, director of the Foreign Policy and Energy Security Initiative at the Brookings Institution.

Asked if Kerry would make climate change a top priority in a Cabinet post, Ebinger said, “There’s no doubt—100 percent. He is impassioned on the issue and would bring it front and center.”

While Obama has indicated he’d like to tackle climate change in his second term, energy and environment policy experts remain skeptical that the president is willing to put in the passion and commitment necessary for action on the issue.

That could change if his top diplomat or Pentagon chief makes the case that the issue is a driving foreign-policy and national-security concern.

“I think he could be quite powerful with the president,” said Brooks Yeager, who led the State Department’s global environmental negotiations during the Clinton administration and now serves as vice president for policy at the advocacy group Clean Air-Cool Planet.

“The president respects intelligence and competence, and he’s got both on these issues. He’d have to listen to Kerry.”

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