Todd Stern, the top U.S. climate negotiator for the Obama administration, also gave notice that, as expected, it will aim for a 17 percent reduction in emissions of carbon dioxide and other gases blamed for global warming by 2020, with 2005 as the base year.

A final emissions reduction target will be submitted, the U.S. said, once the U.S. Congress enacts domestic legislation requiring carbon pollution cuts. But such legislation has an uncertain fate in the Senate.