Citing the “complex, urgent and global threat of climate change,” Secretary of State Hillary Rodham Clinton today appointed a special envoy for climate change, who will lead the United States in international climate negotiations.

Todd Stern, a climate negotiator during Bill Clinton’s presidency and more recently a fellow at the Center for American Progress, will assume the role.

“We have no shortage of evidence that our world is facing a climate crisis,” said Ms. Clinton, who went on to attribute a large part of the problem to the burning of fossil fuels.

The appointment caps off a day of aggressive environmental moves by the Obama administration. Together they signal a sharp break from the Bush administration, which took little action to mitigate climate change. Earlier in the day, President Obama called for swift action on California’s bid for stricter tailpipe emissions standards in automobiles, and to accelerate the timetable for raising national fuel-efficiency standards.

Mr. Stern called for a new multilateral agreement on climate change.

“A new day is dawning in the U.S. approach to climate change and clean energy,” he said.