US Interior Secretary Ryan Zinke, who grew up just 30 minutes away from Montana's Glacier National Park, claims he once watched a glacier in his home state melt so quickly that it visibly receded before his eyes during the course of a single lunch.

Now, with authority over 500 million acres of public land, a fifth of US territory, Zinke will play a key role in determining the amount of fossil fuels that get pulled from the ground and burned, which contributes to glacier melt like that near Zinke's hometown of Whitefish, Montana.

Environmentalists and the fossil fuel industry are watching closely to see how Zinke resolves the tension between development and conservation of public lands, and the diligence he'll bring to carrying out US President Donald Trump's promise to boost economic growth through deregulation and exploitation of the sizable oil, gas, and coal reserves located on federal property.

"A lot of people across the political spectrum are going to hold Zinke accountable to preserve our public lands and waters," said Sharon Buccino, director of the Land & Wildlife Program at the Natural Resources Defense Council. "Those lands are a big part of what it means to be an American."

Much of the drama will play out in the American West and in coastal waters, where the reach of the Interior Department is vast.

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The federal government controls 45 percent of land in California, 85 percent in Nevada, 37 percent in Colorado, and 69 percent in Alaska. Back east, Zinke's reach is more limited, with only 3 percent of land managed by the federal government in Pennsylvania and just 2 percent in Ohio.

His department manages 35,000 miles of coastline and 1.7 billion acres of offshore territory in what is known as the Outer Continental Shelf, or OCS, three nautical miles away from the shore.

A key question will be how Zinke handles offshore oil development in the waters north of Alaska known as the Chukchi and Beaufort Seas, both marginal seas of the Arctic Ocean that are thought to be rich in fossil fuel reserves.

In December, after Trump won the election, Obama moved to permanently block new energy leases in both seas, along with about 3.8 million acres in the Atlantic, by invoking a little-known provision of a law called the Outer Continental Shelf Land Act, which allowed him to pull those areas out of circulation for future development.

Zinke may now attempt to put those areas back into play. Pressure is already mounting on him to try.

During Zinke's confirmation hearing, Alaskan Senator Lisa Murkowski railed against Obama's move, saying the White House deprived her state of its resource wealth and pushed too hard to preserve Alaska's pristine environment.

The Secretary of the Interior controls so much of her state, Murkowski said, that Zinke would be "Alaska's landlord."

"To state that Alaska has had a difficult or a tenuous relationship with the outgoing administration is probably more than an understatement," Murkowski said at the hearing, days before Trump was sworn into office. "Instead of seeing us as a state of Alaska, our current president and secretary seem to see us as Alaska, the national park and wildlife refuge."

Zinke promised Murkowski he would undertake a formal review of Obama's order.

"I can guarantee you it is better to produce energy domestically under reasonable regulation than watch it be produced overseas with no regulation," Zinke told her.

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Yet production in Arctic waters is expensive and difficult — and even if new opportunities were to be served up by the Trump administration, oil companies might hold out for higher global energy prices to make those projects more attractive. Firms like Royal Dutch Shell and ConocoPhillips have walked away from plans to develop US Arctic reserves after the price of oil fell in 2014, despite spending $2.5 billion for drilling rights, according to data from Bloomberg.

Another hot area could be the Alaskan National Wildlife Refuge, or ANWR, the largest national wildlife refuge in the United States, which lies on Alaska's northeastern coast. Debate over drilling in ANWR has roiled for decades — but opening up the area to energy firms would require an act of Congress.

Trump and Zinke could potentially seek to open up new leasing areas along the coast of California, where oil production has been present but heavily restricted for years.

However, the state itself would likely throw up significant obstacles, and environmentalists would probably file lawsuits to stop them, said Michael Lynch, president of Strategic Energy & Economic Research in Amherst, Massachusetts.

In offshore reserves, "the one that really matters is California," said Lynch said. "That would be a big battle."

Here, too, Zinke would need to overcome barriers imposed by Obama.

Before leaving office, Obama released a federal plan banning new offshore drilling in federal waters off the coasts of California, Oregon, and Washington until the year 2022. Any attempt to undo that plan would require public hearings and analysis — and likely face an enormous pushback, including lawsuits.

On Monday, Zinke announced that 73 million acres of offshore territory in the Gulf of Mexico will soon be offered up for oil and gas exploration and development, and hailed the move as "a pillar of President Trump's plan to make the United States energy independent."

However, that sale had already been set in motion by the Obama administration.

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Another place where Zinke will contend with the Obama legacy is in coal country.

Some 40 percent of coal produced in the United States is mined on land owned by the federal government, primarily in Wyoming and Montana.

Zinke's predecessor, Obama's Interior Secretary Sally Jewell, issued a moratorium on new coal mining leases on federal lands in January 2016, citing a need to review how companies pay for development rights as well as concerns about coal's contribution to climate change.

That move, along with a policy of restricting greenhouse gas emissions from existing power plants, were two reasons why Republicans denounced the Obama administration for waging a "war on coal" — a war Trump has vowed to end.

Zinke has promised to review the coal leasing moratorium.

"The war on coal, I believe, is real," he told the Senate during his confirmation hearing. "We should be leading the world on clean energy technology and I'm pretty confident that coal can be a part of that."

In his testimony, Zinke sought to present a balanced approach between development and environmental protections.

Zinke told the senators he does believe that the climate is changing, and that humanity is having an "indisputable" influence on that change.

But pressed by Democrat Bernie Sanders, Zinke said, "I think where there's debate on it is what that influence is; what can we do about it."

Then he told the story about the glacier.

"I've seen glaciers over the period of my time recede," he said. "As a matter of fact, when my family and I have eaten lunch on Greenough Glacier, the glacier has receded during lunch."

Trump campaigned hard on opening up public land to fossil fuel interests, and there is little doubt he will push Zinke to follow through. What remains to be seen is how successful Trump and Zinke will be, and how much resistance they'll face from states, environmentalists, and the rules Obama put in place before leaving office.

"Trump is going to run into the politics of not-in-my-back-yard," said John Freemuth, a public policy professor at Boise State University in Idaho. "There's a growing number of people in the West who are opposed to oil and gas leasing in their favored areas. And that acreage seems to be getting bigger and bigger. It's not going to be as easy as he thinks.

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