WASHINGTON — Secretary of State Hillary Rodham Clinton today appointed a special envoy for climate change, vowing that the Obama administration would restore America's credentials and leadership in shaping environmental policy.

Pledging that the United States would play a primary role in international efforts to stem global, Clinton named Todd Stern, a former White House assistant who was the chief U.S. negotiator at the Kyoto Protocol talks in her husband's administration, to the post.

"American leadership is essential to meeting the challenges of the 21st century, and chief among those is the complex, urgent and global threat of climate change," she said at a State Department ceremony held shortly after President Barack Obama announced new policies to allow states greater latitude in limiting greenhouse gas emissions.

Sterns' appointment, she said, sends "an unequivocal message that the United States will be energetic, focused, strategic and serious about addressing global climate change and the corollary issue of clean energy."

"The urgency of the global climate crisis must not be underestimated," Clinton said. "Nor should the science behind it or the facts on the ground be ignored or dismissed. The time for realism and action is now."

The Bush administration had come under fire for failing to act on climate change, walking away from the Kyoto Protocol on grounds it favored large developing nations like China and India, and refusing to allow states to bolster some environmental regulations.

"The time for denial, delay and dispute is over," Stern said in accepting the job. "The time for the United States to take up its rightful place at the negotiating table is here."

Stern will serve as the chief U.S. negotiator at United Nations talks on climate change including an upcoming session in Copenhagen as well as with individual nations and groups.