The success or failure of the talks in Copenhagen could hinge on whether the United States offers a concrete plan. Failure would mean a loss of momentum toward a treaty to reduce carbon emissions that includes all countries, which already has been delayed to next year. Moreover, other countries are unlikely to move forward to cut emissions if the United States doesn't pledge to make mandatory reductions.

Todd Stern, the U.S. negotiator, said that the U.S. hasn't decided whether it will say how much it intends to reduce emissions.

"If we do, it will be contingent on the enactment of our legislation," he said in an interview with McClatchy Newspapers.

All other industrialized nations have unveiled their emissions reduction plans. Some other large emerging markets have done so, as well, including South Korea and Brazil. Many other countries, however, are waiting to see the United States' long-term plans before they agree to make firm offers and commit to them under the force of an international accord.