One of the most sensitive unresolved questions facing negotiators of a new global climate treaty is how binding it will be.

If the treaty expected to be reached in Paris this year is without enforceable strict rules, some argue it will not be credible, while others say if nations are bound too rigidly, they won't agree to anything ambitious enough to matter. And the key to the whole conundrum is likely the United States.

Behind the scenes, a consensus may be emerging that the treaty's only truly enforceable rules will be procedural ones under which nations state their emissions goals and describe their progress. What will not be binding are exact numerical targets for how many tons of greenhouse gases each may emit.

That's the view of two experienced negotiators who issued a report on Wednesday after co-chairing a months-long dialogue among climate treaty experts from 22 countries under the auspices of the Center for Climate and Energy Solutions (C2ES), an environmental think tank in Washington. Valli Moosa of South Africa and Harald Dovland of Norway described their conclusions to journalists in a conference call.

Participants in those informal talks came from China, the United States, European nations and a variety of larger and smaller states with diverse perspectives and sometimes countervailing self interests. They spent a lot of time discussing the question of how a treaty might bind the parties, and on which specific points—emission goals, financial obligations, or timetables and deadlines.

"At times the discussions seemed almost philosophical," said Elliot Diringer of C2ES.

Perhaps that is why the emerging answer seems to resemble the old koan in which a Zen master declared that the best way to keep your horses in a pasture is to remove the fences.

Whether that's the outcome in the formal talks could have profound consequences not just for whether a treaty text can satisfy all 190 states that it would bind, but for the concrete results of global emissions of carbon dioxide and global warming itself. After two rounds of bargaining so far this year, the question remains open in the draft negotiating text.

All involved understand the legal form is crucial, as no treaty can succeed without the signature of the United States. While President Barack Obama is committed to the cause, the Republican majority in the Senate is deeply opposed to phasing out fossil fuels.

The treaty's overarching goal is to keep warming within a relatively safe limit of no more than 2 degrees Celsius. And scientists say that will require reducing carbon dioxide emissions practically to zero by late in this century, demanding a wholesale, worldwide conversion to carbon-free energy.

Legal scholars say that Obama may have legal grounds to unilaterally commit the nation to such a global compact if it is loosely structured around procedures like reporting requirements. But he would probably have to seek Senate approval for one that included strictly binding provisions such as capping emissions. (The complex issues of international treaty law and U.S. constitutional law, and other legal considerations facing the treaty negotiators, are described in depth in two recent C2ES reports.)

A spirited debate at a recent Senate hearing on the international climate agenda made clear that Obama would face an argument, whatever his legal rationale for signing a Paris accord, however strict or lax its legal fine print.

"The Senate has to look hard at that and ask itself, going forward, does it really want to let the president make these kind of commitments?" said Jeremy Rabkin of the George Mason Law School.

Jeff Holmstead, a partner at the law firm of Bracewell and Giuliani, questioned whether the Obama administration can achieve its own pledge to the United Nations that the U.S. would achieve by 2025 a 26 to 28 percent reduction in greenhouse gas emissions below the levels of 2005. "I guess I'm puzzled by the assertion that the president shows leadership by making promises that he has no way of keeping," he said.

David Bookbinder, a consultant and former Sierra Club climate official who has been called as a witness by Democrats and Republicans alike, said he accepts the scientific consensus on climate change and agrees that action is urgent, but also doubts that the White House can honor its UN pledge without Congress on board.

"I promise you the rest of the world can look at the same regulatory measures and can do the numbers just as well as we can," he said. "The Chinese, the E.U., the Indians, the developing countries, they all have very sophisticated people who understand U.S. regulatory measures. And they're all going to come up with the same answer."

Karl Hausker of the World Resources Institute said he was confident that the U.S. could meet its target by 2025 "using existing federal laws combined with actions by the states." (C2ES, for its part, has concluded that measures adopted or proposed so far will get the U.S. most of the way to its goal, but more is needed to fully achieve it.)

So the question about the Paris treaty is, what would happen if the U.S., or any other country, fell short of its target?

Those who favor binding procedures but not binding targets say nations would hold each other's feet to the fire – but not penalize them or kick them out of the compact.

"Our view is that, is that in the end, the strength of the agreement rests on the transparency and accountability it provides, and on the parties' political will to implement it," said Moosa.

Republican naysayers in the Senate, especially, are vowing to block the Paris treaty, whatever its finesse.

"The president may have creative legal arguments to sign on to a legally non-binding international agreement," said Senator James Inhofe at the hearing on July 8 of the environment committee, which he chairs. "But he does not have the backing of the United States Senate, which significantly limits such an agreement's domestic application." A Republican from Oklahoma, Inhofe calls manmade climate change a hoax.

The legal enforcement of any Paris accord doesn't affect only the U.S., of course. China's Politburo is every bit as jealous of its prerogatives as the United States Senate. So are other governments, whether they run oil-rich monarchies, tin-pot dictatorships or tin-cup democracies. The treaty has to gain the assent of nearly 200 signatories to the United Nations Framework Convention on Climate Change to meaningfully address the global climate crisis. The failure of the Kyoto Protocol under the UNFCCC is widely blamed on how it governed only the biggest and most developed countries, many of whom never met their emission control obligations, which were supposedly binding.

"It would be a gross oversimplification for anyone to come to the conclusion that the peculiar legal form and structure of the agreement is one that has been designed to accommodate the United States or any one particular country," said Moosa.

So far, countries accounting for nearly 60 percent of world emissions have put forth their pledges, which are formally known as INDCs, short for intended nationally determined contributions. Some like the Europeans, who have made some of the most ambitious promises, are perfectly happy to have their pledges made binding, so confident are they of success.

And although it looks like the pledges presented before the December climax of the talks in Paris will fall short of what scientists say is needed, optimists say the binding procedures of the new treaty would commit all nations to come back every few years with even tighter emissions limits, eventually controlling runaway warming.