Shortly after taking office on Jan. 20, President Donald Trump announced that the United States is withdrawing from the Paris Agreement. This is the second time he has pulled out of the international climate treaty: The U.S. formally dropped out at the very end of Trump’s first term in 2020, only for President Joe Biden to rejoin it upon taking office a few months later.

Quitting the Paris Agreement again reaffirms the incoming president’s claim that the deal is unfair to the United States. As a practical matter, however, such a move is unnecessary: Under the agreement, if countries are unhappy with its terms, they’re free to change them.

As Bård Harstad, a professor of political economy at Stanford Graduate School of Business, explains, the 2015 pact allows the 196 countries that signed it to set their own carbon emissions targets. If they don’t achieve those targets, they may revise their pledges without penalties. “The emission cuts are not legally binding, and a country has the freedom to revise the pledge,” says Harstad, also a professor at the Stanford Doerr School of Sustainability. “It’s a bit puzzling that a country like the U.S. would consider leaving an agreement where they can do almost whatever they want.”

That flexibility made the Paris Agreement possible, convincing nearly every country in the world to pursue efforts to keep global temperatures from rising more than 1.5 degrees Celsius. In a recent paper, Harstad explores the mechanism behind Paris and its counterintuitive approach to rallying collective action on climate change. How can an agreement that puts so few constraints on its parties achieve anything?

The Paris Agreement marked a significant shift from its predecessor, the 1997 Kyoto Protocol. Under Kyoto, 37 countries negotiated binding emissions targets. Paris took a different approach: Countries set their own non-binding goals, known as nationally determined contributions, that could be reviewed and revised every five years.

At first glance, this “bottom-up” approach may seem naïve or counterproductive. Without mandatory targets or outside incentives, wouldn’t most countries lowball their pledges so they don’t have to make bigger cuts than anyone else? Traditional game theoretical models suggested that’s precisely what negotiators looking over their shoulders would do. “Many economists, especially game theorists, have criticized this procedure because one cannot expect the cuts to be very ambitious if they’re nationally determined,” Harstad says.

Yet Harstad’s modeling, which he details in another paper, shows why Paris’ pledge-and-review approach worked. Even though their pledges were lower than they might have been under a more stringent deal, countries were willing to “stretch” their pledges beyond the bare minimum. The added cost of a slightly more ambitious target was preferable to the cost of the agreement collapsing. “The difference in bargaining procedure means that the cost of participating in the agreement is much smaller than for the top-down procedure used for the Kyoto Protocol,” Harstad says.

The voluntary nature of the Paris Agreement also makes it more likely to be “self-enforcing,” according to Harstad. Since countries set their own targets, “the temptation to defect is weakened simply because their contribution is less ambitious. Countries are willing to comply even without having a stick.”

Will we always have Paris?

By reducing the pressure to hit tough targets, the agreement was more palatable to more countries than a Kyoto-style framework. And with more countries on board, its total target for emission reductions was much higher than it would have been if fewer countries had signed onto a more ambitious plan.

That’s not to say that the Kyoto Accord took the wrong approach. While most countries signed on, only the most developed economies committed to emissions targets. That approach made sense since many developing countries did not have large carbon footprints at the time, Harstad says. The switch to a big-tent approach in Paris “can be explained by the necessity to include more countries in 2015 than in 1997 because many of the developing countries have become emerging economies that are richer and have larger pollution levels.” More flexibility allowed for more buy-in. “The fact that the Paris Agreement is basically requiring all countries to contribute and that all countries are participating is its main claim of success,” Harstad says.

Getting the world on board in Paris was intended as a first step toward more global cooperation to slow climate change. “The hope is that countries will stay within the Paris Agreement while moving toward a more ambitious treaty over time without losing members,” Harstad says. “Of course, that possibility is under threat when countries, especially big, important countries, consider leaving.”

When Trump abandoned Paris the first time, no other leaders followed suit. This time, however, there are concerns it could kick off a domino effect. (Argentine President Javier Milei is also reportedly thinking about leaving the agreement.) For now, the agreement’s future is uncertain. And any hope for another, more comprehensive agreement, Harstad says, “is basically postponed.”