What does a U.S. president do when he has concluded a groundbreaking climate agreement but doesn’t think he can get it through the Senate?

If that president is Barack Obama, he considers the treaty to be an executive agreement and announces that the United States has ratified it, with the stroke of his pen. Interestingly, Obama made his announcement jointly with China, providing political heft to the move, coming as it did from the globe’s two biggest polluters.

That’s not to say the president did not have legal grounds to take that action, but it certainly open himself to criticism of his tendency toward unilateralism, especially by those who opposed the climate agreement in the first place.

The Paris climate agreement was concluded under United Nations auspices last December. The objective of the agreement is to limit global warming well below 2°C and as close to 1.5°C as possible, to increase economic and social ability to adapt to extreme climate, and to direct the scale and speed of global financial flows to match the required path to very low-emission, climate-resilient development.

“I would like today to thank China and the United States for ratifying this landmark agreement—an agreement on which rests the opportunity for a sustainable future for every nation and every person,” said Patricia Espinosa, Executive Secretary of the UN Framework Convention on Climate Change (UNFCCC). “The earlier that Paris is ratified and implemented in full, the more secure that future will become.”

The Paris Agreement enters into force on the 30th day after the date on which at least 55 parties to the Convention accounting in total for at least an estimated 55 percent of total global emissions have deposited their instruments of ratification with the UN.

The announcement by Obama, together with China’s President Xi Jinping, puts the balance at just over 39 percent of the global total.

Obama’s legal grounds for acceding to the agreement is to characterize it as an executive agreement, rather than a treaty, thereby bypassing the Senate’s advise-and-consent role under Article II of the Constitution.

The State Department’s Foreign Affairs Manual says that presidents may conclude executive agreements in three cases: pursuant to a treaty already authorized by the Senate; on the basis of existing legislation; and pursuant to his authority as Chief Executive when such an agreement is not inconsistent with legislation enacted by the Congress.

Obama could assert that he has the authority to bind the U.S. to take on international obligations under the Paris climate agreement because it is pursuant to the already authorized UN Framework Convention on Climate Change and is consistent with existing federal environmental legislation.

Steven Groves, senior research fellow, at the conservative think tank The Heritage Foundation, disagrees, calling Obama’s announcement “a shameful, unilateral power grab.”

“The Paris Agreement is a treaty by every definition of the word and should be submitted to the Senate for its consideration,” he added. This action by the president is an intentional end-run around the American people and their elected representatives, done to advance an ineffective, expensive climate agreement. The U.S. should stay out of the Paris Agreement.”