CARBONDALE, Colo. — Deep inside Alaska’s six-million-acre Denali National Park and Preserve , I could see miles of space beneath my feet as I stood on the summit of the tallest mountain in North America. The startling view from the 20,310-foot Denali of rugged wilderness spreading out in all directions, plus the challenge of climbing it, were just two of the many wonders and adventures that I’ve experienced in America’s national parks.

I recently finished writing a book for National Geographic, “Atlas of the National Parks,” based on extensive research, a lifetime of exploring the parks and several years in the 1980s working as a ranger in two of them, Denali and Rocky Mountain in Colorado.

I meant the book as a celebration of the 103-year-old national park system, and it is. But what I also discovered was an operation in deep trouble, with some parks degraded by ruinous overcrowding; invasions of nonnative plants and animals that are upending delicate ecological balances; and a warming climate that is melting glaciers and withering away the rare yuccas that give their name to Joshua Tree National Park.

Adding to these woes, the system is badly underfunded and suffering from neglect. This is not a new problem, but it is getting worse, with deferred maintenance that mostly predates the Trump administration now topping $11 billion. But President Trump isn’t helping. He wants to cut the National Park Service’s budget by $481 million next year and is reportedly considering privatizing campgrounds and commercializing the parks in ways that contradict the agency’s goal of harmonizing with nature.