Scott Pruitt Attorney General of Oklahoma arrives to meet with U.S. President-elect Donald Trump at Trump Tower in Manhattan, New York City, U.S., December 7, 2016. Brendan McDermid/Reuters WASHINGTON (Reuters) - Scott Pruitt, Donald Trump's pick to head the U.S. Environmental Protection Agency, has fought President Barack Obama’s measures to curb climate change at every turn as attorney general of Oklahoma. Now he is hoping to take apart Obama's environmental legacy from the inside out, a task that could prove tougher than it sounds.

Legal experts and former EPA officials said Pruitt could score some early easy wins in January, killing regulations the Obama administration rushed through during his final months in office, such as the agency's rule to curb methane emissions from the oil and gas industry.

That is because a little-used law called the Congressional Review Act allows Congress to erase such 'midnight' rules with a simple majority vote, something that should come easily in the Republican-controlled Congress.

But regulations that have been on the books for longer, most of those Obama ushered through during his two four-year terms, will be more difficult to reverse, experts on both sides of the political divide said.

For these regulations, which include the Clean Power Plan that requires states to cut carbon output, along with vehicle emissions standards, Pruitt will have just a handful of options, none of them easy, and nearly all of them triggering drawn out legal battles against well-funded environmental groups and attorneys general from Democratic Party-controlled states.

The outcomes of these battles will have broad impacts on American industry, air and water quality, and the country's role in global climate change, which an overwhelming majority of scientists say is causing sea level rise, increased droughts, and more frequent violent storms.

Republican Trump campaigned on a promise to slash regulation to free up drilling and coal mining, something he said was possible without compromising air and water quality. He vowed that within his first 100 days in office he would rescind Obama's Clean Power Plan, eliminate "unwarranted restrictions" on hydraulic fracturing oil drilling technology, cut "outdated" regulations, and pull the country out of a global pact to curb warming of the planet.

Trump's transition team named Pruitt as his pick to lead the EPA on Thursday, cheering industry and enraging green groups – both sides citing the 48-year-old lawyer's repeated lawsuits against the agency he now seems destined to lead.

Pruitt did not respond to requests for comment.

U.S. President-elect Donald Trump speaks at a Victory Tour Rally, on December 8, 2016 in Des Moines, Iowa. Getty/ Steve Pope

Rounds of litigation

One of the first categories of established EPA regulation that Pruitt may target are those that are already being challenged in court: the Clean Power Plan and the Waters of the United States Act, which expands the number of waterways that are federally protected.

As state prosecutor in oil- and gas-producing Oklahoma, Pruitt joined coordinated legal efforts by states to block these rules, calling them examples of federal overreach. As head of the EPA, Pruitt will likely try to take the simple step of halting the EPA's defense of these regulations.

Jody Freeman, a law professor at Harvard University, said such a move could create complications for the EPA, however, as it may be required legally to explain and support the change in direction.

And even if the EPA was able to withdraw its defense for the rules, other interested parties, including state governments and businesses, could intervene to defend them - raising the specter of a lengthy court battle.

New York Attorney General Eric Schneiderman said he is "leading a coalition of states that is already aggressively fighting back against efforts to reverse the progress this country has made in combating climate change over the past eight years." California Governor Jerry Brown's top aide Nancy McFadden said California will "stand our ground" against attempts to gut EPA rules.

Another option Pruitt could take would be to challenge the very basis upon which Obama's EPA created many of its climate change focused regulations: its finding that carbon dioxide endangers public health. Successfully overturning the so-called "endangerment finding" would remove the foundation upon which most of the administration’s clean air regulations were based, experts said.

But doing so would be hard given the volumes of scientific research that support it, and the requirement to build up a new case that shows carbon dioxide is innocuous.

The effort would likely also trigger lawsuits. The DC Circuit Court of Appeals, which hears all cases challenging federal clean air rules, has been supportive of the scientific evidence for manmade climate change.

"I do think that the new administration will have to expend substantial effort for a rule that rolls back or repeals the endangerment finding to withstand legal challenges," said Megan Berge, a lawyer for Baker Botts who represents power companies.

David Doniger, a lawyer for the Natural Resources Defense Council, said he believed "there is no chance you could get either the DC Circuit, or Supreme Court, to find that CO2 and other pollutants don't endanger public welfare and health."

Bob Perciasepe, former Deputy EPA Administrator under Obama, said challenging the endangerment finding would have to be done through the Administrative Procedures Act, which outlines a long process of gathering and considering public feedback.

Pruitt could also try to undo the existing regulations using the same steps Obama's EPA used for adopting them, legal experts and former government officials said. But that is a months-long process of proposal, public engagement and public comment that would lead to heated debate and potential litigation.

Perhaps easiest would be to slash staff and reduce the agency's funding, making it powerless to enforce its own rules. In this case, as with the others, Pruitt seems likely to be taken to court.

"I stand ready to use the full power of my office to compel their enforcement by the agency," said New York's Schneiderman.

Susan Dudley, a top regulatory official under President George W. Bush, who also attempted to shrink EPA regulation, said campaign promises to gut the government bureaucracy can be tough to fulfill.

"None of those initiatives has succeeded at accomplishing more than minor changes at the margin," she said.

(Reporting by Valerie Volcovici and David Shepardson; Editing by Richard Valdmanis and Grant McCool)