U.S. leaders usually only attend world gatherings when they know they can return home with firm achievements. But as he arrived at the U.N. summit on climate change, the President warned, "The world's ability to take collective action on global warming is in doubt."
Mr. Obama said the will to tackle the issue "hangs in the balance." Top White House officials who have been involved in tough negotiations for days admit it is impossible to predict the outcome.
There are vexing political and scientific issues. In a speech to leaders gathered in Copenhagen, Mr. Obama said no country will win everything it wants if an agreement is to be reached. He bluntly warned that the summit must decide "whether it will move forward or split apart."
The president said the U.S. has already acted boldly by promising to reduce heat-trapping pollution. He also pointed to America's pledge to help poorer nations finance similar actions.
But the president arrived at the meeting with no new U.S. initiatives. Like many of the other leaders at the summit table, Mr. Obama faces limits at home. Congress has yet to pass legislation to cut pollution blamed for global warming.
As the day wore on, there were glimmers of hope for progress following President Obama's nearly hour-long meeting with Chinese Premier Wen Jiabao. A U.S. official said the discussion moved the two big polluting countries "a step forward" to a possible agreement.
The two leaders discussed emissions targets, financing of efforts to cut global warming pollution and transparency (China has been criticized for rejecting international monitoring of its pollution-cutting agenda).
Mr. Obama arrived in snowy Copenhagen following an overnight flight from Washington, which is bracing for a major snowstorm. Aides insist that he'll return home later today — with or without progress on the global warming issue.
By CBS News White House correspondent Peter Maer