Park ranger Matt Holly stands atop the bald, rounded top of Cadillac Mountain as a small knot of tourists huddles around him. At just over 1,500 feet, Cadillac is the highest point along the US Atlantic coastline. On this late summer day, the vista is spectacular: Lush green forests and azure ocean spread from the Porcupine Islands in Frenchman Bay to the east to the Cranberry Islands in the south and Seal Cove to the west on the Gulf of Maine. Most of the panorama is part of Acadia National Park, the oldest national park east of the Mississippi, which is made up entirely of lands donated by private citizens.

But the view isn’t always this pristine, explains the slender young ranger from under his Smokey Bear hat. He holds up two charts: One shows high levels of air pollution in the park when the winds are from the west. Another shows sharply lower levels when winds are from the north or south. Ozone levels at Acadia sometimes rise to what are considered unhealthful levels, and the polluted air is suspected of causing the mercury found in fish caught in the park’s shimmering freshwater lakes. Without air pollution, a visitor atop Cadillac should be able to see 110 miles. Today, 33 miles is more like it.

In many ways, Acadia typifies the state of US national parks today. It possesses extraordinary scenic beauty that remains largely intact. Americans visit it in impressive numbers.

But less visible forces are at work that could undermine its future.

This fall may prove to be a watershed moment for the parks, which historian and conservationist Wallace Stegner called “the best idea America has ever had.” They will earn a rare moment in the national spotlight, largely because of a six-part, 12-hour series by award-winning filmmaker Ken Burns that began Sept. 27. The documentary, which Mr. Burns calls “the most beautiful film we have ever made,” took 10 years to research and six years to film.

In addition, later this year a 28-member bipartisan board called the Second Century Commission, led by former Sens. Howard Baker Jr. and J. Bennett Johnston, is expected to issue its report on the challenges facing the parks as the National Park Service approaches its centennial in 2016. Many park advocates are also eagerly awaiting the new direction they hope the arrival of Jon Jarvis as the next director of the NPS will bring. His appointment, awaiting approval by the US Senate, could come at any time.

While chronic underfunding of the NPS has caused an estimated $8 billion to $9 billion backlog of sorely needed maintenance in the parks, for many that threat has been overshadowed by a greater concern: climate change.

“How people are changing the climate is the greatest threat the parks have ever faced,” says Stephen Saunders, president of the Rocky Mountain Climate Change Organization and a former deputy assistant secretary of the Department of the Interior, which oversees the NPS. Low-lying park properties, such as Dry Tortugas off Key West, Fla., and Ellis Island in New York Harbor, could disappear underwater later this century as sea levels rise an expected 2.5 to 4 feet. Large areas of the Everglades and Big Cypress National Preserve in south Florida could be inundated as well, he says.

As climate zones shift northward, “Joshua Tree National Park may lose all of its Joshua trees,” Mr. Saunders says. “Saguaro National Park may lose all of its saguaro [cactus].” Glacier National Park is expected to no longer have any glaciers by 2030, possibly sooner. Rocky Mountain and Yellowstone parks may lose their vast tracts of lodgepole and whitebark pines to mountain pine beetles, which could survive at higher latitudes and elevations.

“The parks aren’t going to continue to provide these wonderful refuges for Americans, places where we can learn about our natural systems and wildlife if we don’t act in significant ways to protect them from climate-change impacts,” adds Mark Wenzler, a clean-air expert at the National Parks Conservation Association (NPCA), a nonprofit park advocacy group. “We’re at this kind of crossroads. What are we going to do?”

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