There’s no ambiguity about the process by which the United States can enter into a treaty. The Constitution, in Article II, Section 2, states that a president “shall have power, by and with the advice and consent of the Senate, to make treaties, provided two thirds of the Senators present concur.”
The ratification process is a very specific limitation on presidential power, one that provides a legislative check on the executive branch. But President Barack Obama can’t be bothered by the constitutional process. The New York Times reports that, in his latest move to get around Congress, President Obama’s State Department is negotiating a climate deal at the United Nations to update a 1992 treaty with new emission reduction targets (emphasis added):
Lawmakers in both parties on Capitol Hill say there is no chance that the currently gridlocked Senate will ratify a climate change treaty in the near future, especially in a political environment where many Republican lawmakers remain skeptical of the established science of human-caused global warming.
American negotiators are instead homing in on a hybrid agreement — a proposal to blend legally binding conditions from an existing 1992 treaty with new voluntary pledges. The mix would create a deal that would update the treaty, and thus, negotiators say, not require a new vote of ratification.