Ebell is the director of global warming and international environmental policy at the Competitive Enterprise Institute, a U.S. conservative think tank, and helped to guide the EPA’s transition after Trump was elected in November until he was sworn in on Jan. 20.

Trump, a climate skeptic, campaigned on a pledge to boost the U.S. oil and gas drilling and coal mining industries by reducing regulation.

He alarmed nations that backed the 2015 Paris agreement to cut greenhouse gases by pledging to pull the United States out of the global deal agreed by nearly 200 countries. However, Trump told the New York Times in November that he had an “open mind” on the agreement.

Trump’s administration has asked the EPA to halt all contracts, grants and interagency agreements pending a review, sources said.

“The U.S. will clearly change its course on climate policy. Trump has made it clear he will withdraw from the Paris Agreement. He could do it by executive order tomorrow or he could do it as part of a larger package,” Ebell told reporters in London on Monday.

The top energy official for the European Union, meanwhile, said he hoped that Trump would stick to the Paris deal.

‘Anxiety’

“I wouldn’t hide that in discussion with our partners; there is a lot of anxiety over future U.S. policies,” said European Commission Vice President Maros Sefcovic, who was a key broker in clinching Paris agreement.

“If the (U.S) administration decides to go in a different direction, I think sooner or later they will have to come back to the realization that climate change is happening.”

The timing of any action by Trump is difficult to predict because government departments are still in transition, Ebell said, adding that he had not met Trump in person.

Any country wanting to pull out of the Paris agreement after ratifying it has to wait four years.

A source on Trump’s transition team said last year that there were speedier alternatives, such as sending a letter withdrawing from the 1992 international framework accord that is the parent treaty of the agreement; voiding U.S. involvement in both in a year’s time; or issuing a presidential order simply deleting the U.S. signature from the Paris accord.

Ebell said the “cleanest way” would be to withdraw from the United Nations Framework Convention on Climate Change itself.

“Whether the U.N. secretariat wants the U.S. to continue to have a seat at the table is up to them. I don’t think Trump cares about that. The people who elected him would prefer not to have a seat at the table,” Ebell said.

Trump appointed Oklahoma Attorney General Scott Pruitt, who has led 14 lawsuits against the EPA, as the agency’s administrator, though a vote on his nomination has not been scheduled.

The President has also drawn heavily from the energy industry lobby and pro-drilling think tanks to build its landing team for the EPA, according to a list of the newly introduced 10-member team seen by Reuters on Monday.

Ebell also said he thought the political make-up of the Federal Energy Regulatory Commission, which reviews applications for the construction and operation of natural gas pipelines, will change dramatically under Trump.

“Given the way the campaign went, I think you will see very quick executive action to expedite LNG (liquefied natural gas) terminals and pipelines,” he said.