President-elect Donald Trump could enact sweeping changes to environmental policy in the beginning days of his administration, with far-reaching effects both in the United States and around the world, experts say.

Even though the pro-oil Republican billionaire is buoyed by a partisan majority in Congress, he won't need the support of lawmakers for a host of changes he could make by presidential fiat.

But experts admit it is impossible to predict Trump's first steps after his inauguration on Friday.

Even though he declared climate change a "hoax" perpetrated by the Chinese during a speech in 2012, he has since said he was joking. He also told the New York Times that he would keep an open mind to the 2015 Paris climate deal, and that climate change might be influenced by human activity.

A key pledge of his during the campaign was to "cancel" the 2015 Paris accord, but during confirmation hearings this month, his nominee for Secretary of State, Rex Tillerson, said he felt it was important for the United States to stay at the table.

If Trump does decide to withdraw from the Paris climate accord, which saw more than 190 world leaders agree to lower emissions that lead to global warming, he could do that "on his own," said Michael Burger, executive director of the Sabin Center for Climate Change Law at Columbia University.

The process would still take several years.

The new US leader could go even further and pull out of a major international environmental treaty negotiated in 1992, the UN Framework Convention on Climate Change, "which would be a true disaster," Burger added.

"That framework has been the mechanism through which countries have been seeking to address climate change for over 25 years."

Trump is expected to stop making payments to the Green Climate Fund, which relies on US and foreign aid to help the world's poorest countries address climate change.

On Tuesday, the State Department announced its second $500 million payment on Tuesday to the fund, as part of a $3 billion pledge made in 2014.

It was the latest in a series of moves made by President Barack Obama's administration in its final weeks to preserve environmental protections, including blocking new leases for oil and gas drilling in sections of the Arctic and Atlantic.

- Friend to fossil fuel -

Another swift action that Trump could take would be granting State Department approval to move ahead with the long-contested transcontinental Keystone XL pipeline, carrying crude oil from Canada to US refineries on the Gulf Coast.

"The only hold-up for the Keystone pipeline was the Obama administration's denial," said Brendan Collins, an environmental lawyer who represents clients in the electric power sector and the oil and gas industry.

Trump's nomination of former ExxonMobil chief Tillerson as secretary of state sent a message that the new administration will be "sympathetic on a fundamental level" to fossil fuel interests, Collins told AFP.

In recent days, Tillerson and other nominees have been grilled by lawmakers about their stances on climate change.

Oklahoma attorney general Scott Pruitt, tapped to lead the Environmental Protection Agency, acknowledged Wednesday that human activity affects climate change, but insisted the extent of that impact remains subject to debate.

Montana Congressman Ryan Zinke, Trump's nominee for Secretary of the Interior, also said climate change is not a hoax.

But their views on global warming may not be the most relevant question, experts say.

"Although Zinke believes in the reality of climate change as a matter of science, he also feels there is room for continued development of fossil fuels on federal lands," Collins said, signaling Trump could end Obama's limits on extraction of minerals, oil and gas.

And Pruitt has sued the very agency he is being asked to lead, in an attempt to undo the "endangerment finding," which states that greenhouse gasses endanger public health and welfare.

"That finding set in motion and authorized a whole slew of regulations around power plants, oil and gas, methane emissions," said Burger.

Pruitt lost that lawsuit, and "President Trump's EPA will be unable to undo that finding. The science doesn't back it up," he said.

- A 'do-nothing approach -

Trump has also vowed to undo "needless and job-killing regulations."

On the whole, environmental regulations are difficult to undo, because they take years of work and public comment to become law.

But Trump may weaken regulations by simply doing nothing, said Burger.

"Doing nothing in this context would mean not enforcing regulations that are put on the books, not requiring state or private entities to do anything about climate change," he said, describing this approach as "a huge risk."

In cases where regulations are caught up in courts, with oil and gas interests suing the US government, the new administration could win if "it simply stopped defending them," added Collins.

One Trump pledge in particular has alarmed environmentalists -- his vow to remove two existing regulations for every new rule put in place.

"If you want to be protected from lead you have to reduce protection from mercury?" asked Alden Meyer, director of policy and strategy at the Union of Concerned Scientists, in a conference call with reporters this week.

"I mean, it is just insane."