The prospect of Donald Trump becoming the next president of the United States has been inching closer to reality as the billionaire reality TV star keeps winning crucial state primaries, locking up the popular vote to be the Republican Party's nominee.

The developments have worried environmentalists, both within the United States and globally - because Trump has said he does not believe in manmade global warming. He has called measures to limit emissions a "money-making industry" and a hoax. He is also no fan of other environmental restrictions.

More specifically, he has vowed to abolish the US agency that makes and enforces environment and climate laws. Were he to follow through with his promise to dismantle the Environmental Protection Agency (EPA), he could throw international efforts to reduce emissions into disarray.

Disaster for climate effort

"The US election is going to decide the extent to which the world can get to grips with the climate crisis," says Bert Wander, a campaign director with the climate action group Avaaz. A climate-change-denying US president would be a "disaster" for that global effort.

In Paris in December last year, the world's countries signed a historic agreement to limit emissions. But it must still be ratified by individual countries.

Richard Chatterton, an analyst with Bloomberg New Energy Finance, says that ratification before the end of President Barack Obama's term is impossible.

Trump has not said expressed a position on whether he would or would not veto ratification. But if he were to veto it, this could cause the whole agreement to unravel - because other big emitters like China might also pull out in response.

A Trump win would threaten the very existence of the US Environmental Protection Agency

"The overall agreement relies on continued momentum and good will," says Chatterton. "If all of a sudden the rhetoric turns and the Americans are saying, we’re not interested in this any more, then of course why would the Chinese make an effort? And America becomes the scapegoat."

"It would certainly poison the lake," Chatterton concludes.

Throwing out the tools

Even if the new president does not scupper the Paris accord, he could remove the tools available to get the US to its 26 percent emissions reduction commitment.

All of these tools are designed and implemented by the EPA, which Trump has accused of "making it impossible for our country to compete," because of alleged excessive environmental regulation.

Trump says he would abolish the agency, and instead give individual US states the authority to enact environmental protection laws - if they wish. Campaigners fear that such a policy would result in a "race to the bottom," with each state trying to have as little environmental restrictions as possible to attract businesses.

The EPA, set up in 1970 by Republican president Richard Nixon, was initially meant only as an enforcement body for laws passed by US Congress, specifically the Clear Air Act and the Clean Water Act.

But that role has changed significantly over the past decade.

Obama's Clean Power Plan is his linchpin for reducing US emissions

Legislation vs. regulation

Barack Obama, facing a Republican-controlled Congress hostile to environment and climate legislation, took the decision to use the EPA to enact new regulations by executive order, thereby bypassing the Congress.

Even under Obama's Republican predecessor George W. Bush, the EPA quietly advanced some environmental protection measures - also bypassing Congress. But Obama went much further by using the agency to put in place limits on power plant and vehicle emissions.

The restrictions on new power plant emissions and vehicle emissions are controversial, because the EPA only has a mandate to deal with pollution, and not specifically carbon emissions. To get around this, the EPA dubbed carbon a "pollutant" in 2009, on this basis putting in place the Clean Power Plan in 2015, limiting power plant emissions.

But last month the US Supreme Court issued a stay on enacting these new limits, while it examines the more than 30 lawsuits against it. That stay is likely to last until after Obama's presidency ends, leaving the next president in line to decide whether to defend the policy or abandon it.

"There’s a lot of risk not only due to the possibility of a Republican coming in to the White House, but also because of the Supreme Court decision," says Chatterton.

Coal, oil and gas companies run very powerful lobbies in the US

Republican climate skepticism

Wander says campaigners are worried about more than just Trump - anti-EPA rhetoric is prevalent among all of Trump's Republican rivals. "The most powerful industry on Earth [the fossil fuel industry] has a vested interest and is pouring millions of dollars into this election," he says.

Several Republican presidential candidates in 2012 vowed to abolish the EPA. During the present campaign season, candidate Marco Rubio branded it the "employment prevention agency," while candidate Ted Cruz called it "radical."

Even John Kasich, the more moderate contender who some believe is the most likely alternative nominee if the Republicans have a contested nominating convention this summer, has said the clean power plan "must be scrapped and not replaced." Last year he called the idea of manmade climate change "some theory that’s not proven."

All Republican candidates have said the agency’s move to limit carbon is illegal, and they would immediately reverse it.

But no candidate this year has gone as far as Trump in saying they would completely abolish the agency, and leave environmental policy to the states.

Though US carbon emission measures are vulnerable to a change in parties in the Oval Office, more standard EPA environmental protection measures addressing air pollution and water quality would be harder to undo because they are based on legislative acts of congress, and only another legislative act could scrap them.