An ambitious global pact among almost 200 nations to cut greenhouse gases may stall in the Senate, where members are likely to be skeptical of supporting an international climate change treaty.

The agreement to phase out the use of hydrofluorocarbons (HFCs), powerful greenhouse gases commonly used in refrigeration and air conditioning, was struck on Oct. 15 in Rwanda with the goal of averting a half-degree Celsius of warming this century.

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“Under the Vienna Convention — the treaty about treaties — when you amend a treaty, you need to treat the amendment just like you would treat a new treaty,” said Mike Wara, an environmental law professor at the Stanford Law School.

“So you’ve got the ratify the amendment,” he added.

Although the EPA may already have the authority to mandate HFC cuts, Wara said, the Montreal Protocol — originally struck to reduce ozone-depleting pollutants — requires that richer nations pay to help poorer nations comply.

“Unless they have a novel theory about why this doesn’t have to be ratified, and how they’re going to pay for it in the absence of ratification and Senate support, it’s potentially a risky move,” Wara said.

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But Wara said that historically, such amendments have gone through Senate review.

“The past practice has been to ratify amendments to the Montreal Protocol,” Wara said.