President Trump is reportedly poised to pull the United States out of the Paris Climate Agreement, fulfilling his campaign pledge but still sending shock waves through international and scientific circles.

While Mr. Trump has repeatedly criticized the agreement and promised to “cancel” it, many advisers in his inner circle – as well as top business leaders – had been urging him to remain in the accord, and there have been periodic reports that Trump was vacillating.

Now that the withdrawal seems imminent, the international community is trying to assess the impacts of such a major action. It would almost certainly have major ripple effects throughout the United States and around the globe, not just on climate action, but also on diplomatic relations and the US economy. At the same time, however, most observers say it is unlikely to derail the accord, or hamper more localized efforts currently being championed by communities, local leaders, and the private sector.

The diplomatic and economic repercussions for the US could be significant, but “we know that cities and states and businesses in the US and countries around the world are determined to move forward,” says David Waskow, director of the International Climate Initiative at the World Resources Institute. “Even with obstacles being thrown in their way, this is a course that many, many actors are intent on pursuing.”

When the Paris agreement was adopted at the end of 2015, it was hailed as a breakthrough by many and the best chance to limit greenhouse gas emissions and the global warming they cause. The goal – to be achieved through voluntary country-by-country targets for emission reduction – is to limit the increase in average global temperature to 3.6 degrees F (2 degrees Celsius).

The agreement was signed by 195 countries, and the US promised to reduce its own emissions by 26 to 28 percent of 2005 levels by 2025. It also pledged $3 billion to the Green Climate Fund to help poorer countries reduce their own emissions (and has fulfilled $1 billion of that pledge).

The US has the world’s largest economy and is the second-biggest polluter, behind China. Its withdrawal could clearly have a major impact on the chances of the Paris agreement succeeding, and the willingness of some other countries to live up to their promises.

Global implications

A decision to back out of a deal that has been ratified by almost every other nation could have both economic and diplomatic ramifications.

“Internationally, this will not be taken lightly in any sense,” says Mr. Waskow of WRI. “This will be seen as an attempt to blow up international cooperation on what the world sees as one of the most critical issues of our time.”

And that could affect relations on issues that extend well beyond environmental and energy policy.

“Climate change has become a top-level issue that could impede America’s ability to gain the cooperation of other countries over the typical top issues of security, trade, and the economy,” says Timmons Roberts, a professor of environmental studies at Brown University in Providence, R.I., and a nonresident senior fellow at the Brookings Institution.

But there are also important limits to potential impacts of US withdrawal. All other major signatories – including European nations, China, and India – have affirmed their commitment to climate action, even without US participation. And in the US, many of the efforts to lower emissions are coming from the private sector and from cities and states – efforts that may increase in the absence of federal climate leadership.

“The Paris agreement itself is going to move forward with or without the United States,” says Brian Deese, who served as President Obama’s senior adviser on climate change and who played a key role in negotiating the Paris agreement. “The question is really not about whether Trump can ‘cancel’ Paris, it’s how much can Trump set the United States back.”

Mr. Deese and others say the US risks harming itself both economically and diplomatically with the move, and is likely to see diplomatic repercussions down the road, as well as economic consequences as the rules around clean energy get decided without US involvement.

Recognition of that fact, Deese says, is a major reason why so many business leaders – including dozens of Fortune 500 CEOs – have lobbied Trump to stay in the agreement.

“It’s ironic that it took Donald Trump to highlight in such clear terms how unanimous the United States private sector is in supporting climate action,” Deese says.

What withdrawal might mean

Trump has not yet confirmed the reports of the withdrawal, and questions remain about the form it would take. A formal withdrawal would take several years. The quicker “nuclear option” of dropping out of the United Nations Framework Convention on Climate Change completely would leave the US as the only country not part of the convention.

“Withdrawal from Paris Climate Agreement puts the USA with Nicaragua and Syria,” says Robert Stavins, director of the Harvard Environmental Economics Program, referring to the two countries that have not signed the accord. “Withdrawal from UNFCCC would put the USA with North Korea.” Moreover, he says in an email, withdrawal “hands leadership in this realm and others to China.”

For the past month, Trump has been pushed hard in both directions on the Paris decision, even with the Republican Party. Within his administration, those wanting him to pull out reportedly included Trump’s chief strategist, Stephen Bannon, and EPA administrator Scott Pruitt. And a week ago, 20 GOP senators sent Trump a letter urging him to abandon the agreement. The pro-Paris lobbyers, according to reports, have included Secretary of State Rex Tillerson, chief economic adviser Gary Cohn, and Jared Kushner, a senior adviser to Trump and his son-in-law.

But regardless of whether Trump opted to keep “a seat at the table,” as Mr. Tillerson urged on Paris, it has become clear that the US will not be a leader on climate action under Trump. The president has voiced skepticism about climate change himself and has appointed climate-change skeptics to key roles. He has made it clear he’s not planning to take any actions to help the US meet its voluntary commitments or to fulfill its funding pledges. At least a few analysts have even argued that it may actually be better for the Paris agreement to have the US publicly back out, rather than remain a signatory in name but not in action.

Forging ahead, with or without federal leadership

Trump began to roll back federal leadership on climate action almost immediately after his inauguration in January. But significant action at the state and city level – as well as in the private sector – has continued undeterred and in concert with global efforts.

Combined, the cities in the C40 Cities Climate Leadership Group – more than 80 “megacities” around the world who have pledged to tackle climate change – represent more than 600 million people and a quarter of the global economy.

“That’s an example of mayors around the world coordinating with each other without waiting for their respective national governments to catch up, and I think that’s what you’re going to see,” says Pete Buttigieg, the mayor of South Bend, Ind., which has retrofitted many of its police cars to run on natural gas and also re-engineered its sewage system to handle increased rainwater. “If the American federal government abandons its role, American cities aren’t going to go along with it.”

And Trump’s actions are likely to increase calls for local action.

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“We all have our role to play in maintaining the overall commitment and pursuing that action to drive down emissions and make sure that President Trump isn’t the only measuring stick by which the world measures our contribution,” says Jeremy Symons, associate vice president on climate political affairs at the Environmental Defense Fund.

Staff writer Noelle Swan contributed to this report.