Celebrations marking the National Park Service's centennial year are winding down. If predictions hold true, the nation's 410 national parks, monuments and other sites will receive well more than the record 307 million visitors recorded in 2015.

Such enthusiasm, however, has its downsides. Heavy visitation, coupled with an array of environmental threats, has landed a growing number of parks on the endangered list.

Climate change is a major factor, causing sea level rise, coastal erosion, increased flooding and more frequent large wildfires — all of which are damaging archaeological resources, historic buildings and cultural landscapes across the nation. The natural beauty of America's national parks is further endangered by air pollution, oil drilling, mining, encroaching development and irresponsible recreation.

Activists and conservation groups are warning that without drastic action, these destructive elements could threaten the very existence of many of our national treasures. The following nine parks are facing some of the biggest threats:

1. Statue of Liberty & Ellis Island National Monument, New York

In 2012, Superstorm Sandy revealed the peril facing these two famous American icons sitting in New York Harbor. The storm left 75 percent of Liberty Island and all of Ellis Island underwater in some areas as much as eight feet.

Both remain as sitting ducks in a harbor vulnerable to future storms and rising sea levels.

2. Biscayne National Park, Florida

This park near Miami protects nearly all of Biscayne Bay, one the country's top scuba diving destinations. Overfishing, pollution and irresponsible recreation threaten coral and fish populations in the bay. The entire park also is at risk of being lost to rising sea levels tied to global warming.

Ongoing coastal development and encroaching sprawl from Miami also threaten the park. Biscayne has lost more than 40 percent of its coral reef since 1990.

3. Everglades National Park, Florida

A true natural wonder, the Everglades covers nearly 4,000 square miles and is the largest fresh water wetland on the North American continent. Thanks to more than a century of human mismanagement, hurricanes, floods, drought, wildfires and pollution resulting from urban and agricultural runoff, the 'Glades, sad to say, is a big mess.

The latest in a series of restoration projects the largest, most expensive and comprehensive environmental repair attempt in history does show promise. Given sufficient funding and public support, it has at least some chance of success.

4. Great Smoky Mountains National Park, North Carolina and Tennessee

Air pollution is the major culprit endangering the nation's most visited national park (10.7 million visitors last year). Coal-fired power plants and vehicle exhaust threaten the health of visitors and park employees, while acid rain and ozone wreak havoc on the park's flora and fauna.

Conservationists say the first step toward reducing pollution would be for the Tennessee Valley Authority to clean up its power plants.

5. Indiana Dunes National Lakeshore, Indiana

A favorite weekend getaway spot for some 2 million Chicago-area residents each year, this 15-mile strip of sandy lakeshore between Gary and Michigan City provides year-round recreational opportunities, including fishing, hiking, camping, horseback riding and cross-country skiing.

Problems here are wide-ranging. Higher temperatures are impacting the winter recreation season. Cross-country skiing and snowshoeing seasons have been shortened. Warmer waters are reducing Lake Michigan's salmon and trout populations. And irresponsible recreation, due largely to overcrowding, is damaging delicate dunes.

6. Big Thicket National Preserve, Texas

This sprawling 112,000-acre preserve in southeastern Texas boasts some of the richest biological diversity in North America. It features 10 distinct ecosystems ranging from scrub forests to sand hills to swamps, all supporting an astonishing array of wildlife.

The preserve, however, suffers from a number of unwanted guests. Invasive plants, animals and insects have become a serious threat to native species. Invaders include Norway rats, feral hogs, cats and dogs, and a long list of aggressive terrestrial plants and highly invasive aquatic plants such as water hyacinth. Management plans to deal with these issue have had only limited success.

7. Mesa Verde National Park, Colorado

Home to nearly 5,000 known archaeological sites, including 600 cliff dwellings, Mesa Verde offers a spectacular look into the lives of ancestral Pueblo people who made it their home for more than 700 years, from A.D. 600 to 1300.

Like many parks, this one is under siege by exotic or invasive plant species, declining air quality due to emissions from nearby coal power plants, and wildfires that have damaged ancient Pueblo masonry and destroyed 36,000 acres of old-growth forests. Other issues include physical wear-and-tear as a result of heavy visitation and overgrazing by horses that stray into the park from the adjoining Ute Mountain Indian Reservation.

8. Grand Canyon National Park, Arizona

When President Theodore Roosevelt first laid eyes on it in 1903, he said, "The Grand Canyon fills me with awe." Today, the nation's 15th national park (designated in 1919) and arguably its most iconic landmark has the same effect on nearly 6 million visitors a year.

But during its long history, humans have dramatically impacted the park's natural resources. Non-native plant and animal species have been introduced. Air pollution routinely drifts into the Canyon from metropolitan areas and nearby coal-fired power plants. River and stream waters have been tainted with fecal coliform from free-ranging cattle and human waste. Construction of the Glen Canyon Dam in 1963 altered riparian and aquatic ecosystems within the park, and forest landscapes have been disturbed by decades of wildland fire suppression.

The National Park Service, Native American tribes and conservation organizations are doing what they can to mitigate these threats, but they face a long, uphill battle.

9. Yellowstone National Park, Wyoming and Montana

America's first national park designated by President Ulysses S. Grant in 1872 is sadly one of its most threatened. Nearly 4 million visitors crowd the park each year, making it the nation's third-most visited national park. On a typical summer day, it seems as though the majority of them are jammed together around Old Faithful Geyser.

Most of the park's problems, however, stem from rising temperatures, i.e., global warming. The winter recreation season is starting later and ending earlier. Wildfires have ravaged portions of the park in recent years, and pine beetles are infesting trees at higher elevations. Warming waters have reduced fish populations and encouraged parasites that have further decimated native trout and whitefish stocks. This problem has forced intermittent bans on recreational fishing in the park.

On the brighter side, years of intense wrangling between snowmobilers and environmentalists over noise and pollution issues have been more or less resolved by new regulations that limit the number and noise levels of machines admitted to the park.