He stopped short of withdrawing the US from the landmark Paris accord on climate change negotiated by his predecessor. Aides say that is still being discussed, even as the steps he took would make it hard for the nation to keep its commitment.

Some changes will happen immediately, such as the repeal of a 2016 policy that encouraged federal regulators to consider climate change in environmental reviews as well as directives from Obama that compelled government agencies and the military to factor the phenomenon into their planning. The Interior Department also will swiftly rescind a moratorium on the sale of new rights to extract coal on federal land.

And the Trump administration also is tossing an Obama-era “social cost of carbon” metric that estimated the potential economic damage from climate change and was used to justify a slew of environmental actions, from efficiency standards for microwave ovens to the revamp of government buildings.

Instead, the government will return to an earlier 2003 approach for estimating the potential costs of any regulations governing greenhouse gas emissions.

Other policy pivots will take years of work, such as reversing the Clean Power Plan that forced states to cut greenhouse gas emissions from electricity. An Interior Department rule setting requirements for hydraulic fracturing on federal land will be rescinded. And a pair of regulations governing potent methane emissions from oil and gas wells also will be reviewed at the Environmental Protection Agency and the Interior Department -- with possible changes or a reversal years away.

Most -- if not all -- of the changes will face legal challenges from the same environmentalists who already are fighting to defend Obama’s Clean Power Plan in federal court.

“In taking a sledgehammer to US climate action, the administration will push the country backward, making it harder and more expensive to reduce emissions,” Andrew Steer, president of the World Resources Institute, said in an emailed statement. “Climate science is clear and unwavering: mounting greenhouse gas emissions are warming our planet, putting people and business in harm’s way.”

Agencies will have to justify their regulatory reversals and submit the proposed changes to public comment. And saying climate change isn’t real won’t cut it, said Gina McCarthy, the former EPA administrator who drove many of the mandates now on the chopping block.

“They are going to have to figure out a lot better argument than that,” McCarthy said in a phone interview.

Unwinding some mandates will be tougher than others, notes Victor Flatt, co-director of the Centre for Climate, Energy, Environment and Economics at the University of North Carolina School of Law.

“According to the Supreme Court, all actions by federal agencies, including rulemakings, must be made on the basis of evidence, and may not be arbitrary and capricious or an abuse of discretion,” Flatt said by email. “Having established a deep base of evidence in support of these rules, the EPA can’t simply say it changed its mind. It’ll need new evidence, new analysis, new public comments, a new cost-benefit analysis, and in all likelihood, new litigation.”

Trump’s order was cheered by some conservatives who have been pushing the president to go even further in stripping climate regulations from the rule books, including by undoing the EPA’s landmark declaration that greenhouse gas emissions jeopardise the public health and welfare. That 2009 endangerment finding served as the underpinning for later EPA carbon rules.

Myron Ebell, director of the Competitive Enterprise Institute’s Centre for Energy and Environment, cast Trump’s executive order as a good start.

“It takes the necessary first steps in undoing President Obama’s energy-rationing agenda,” Ebell said by email. “Of course, there is more work to be done down the road, most importantly withdrawing from the Paris climate treaty and reopening the endangerment finding.”

The Trump administration hasn’t said if the US will remain a part of that 2015 Paris climate accord -- despite the president’s campaign pledges to rip up the deal that set broad, non-binding carbon-cutting targets for the US and nearly 200 other countries.

Secretary of State Rex Tillerson has advocated the US keep its seat at the table by sticking with the plan. Even without formally pulling out of the Paris pact, the US is abandoning its pledge to pay US$3 billion into a United Nations fund to help countries on the front lines of climate change.

Trump’s actions are politically significant, following months of promises to reverse the fortunes of struggling coal miners -- campaign vows that helped propel him to victory in industrial strongholds like West Virginia and Pennsylvania. Although the changes he is setting in motion will make it cheaper to extract coal and use it to generate electricity, they are unlikely to dramatically boost domestic demand for coal, which faces stiff competition from cheap natural gas and is affected by other pollution regulations untouched by Trump’s order.

And mining jobs have been in decline for decades as automated equipment increasingly unearths coal, doing the work that once required pick axes and mules.

Sierra Club Executive Director Michael Brune called Trump’s order “the single biggest attack on climate action in US history” and said it was a misguided attempt to help workers displaced by the global shift to cleaner energy sources.

“The best way to protect workers and the environment is to invest in growing the clean energy economy that is already outpacing fossil fuels and ensuring no one is left behind,” Brune said in an emailed statement.

“At a time when we can declare independence from dirty fuels by embracing clean energy, this action could only deepen our dependence on fuels that pollute our air, water and climate while making our kids sicker.”