Often called "America's best idea," the nation's 400-some national parks, monuments, and refuges are more popular than ever, but they face crumbling buildings and lean budgets, Interior Secretary Sally Jewell warned at National Geographic headquarters Tuesday, during National Parks Week.

"At a time when our public lands face serious threats, from land grabs to climate change, we can't afford to turn our back on them," Jewell said at an event celebrating the hundredth anniversary of the U.S. National Park Service. "We need to use this centennial to set a new path for conservation."

Jewell said the national parks maintenance backlog has reached nearly $12 billion, the worst in the system's history. That has meant roads out of service and shuttered bathrooms, as well as cuts in staffing and programs.

At the same time, the national parks had 307 million visits last year and are on track to experience even more growth this year. The visits add up to a major economic benefit for surrounding communities. (Learn more about the parks in National Geographic's yearlong series.)

Citing a study soon to be released, Jewell said U.S. national parks generate $32 billion in annual economic activity, while they cost federal taxpayers $3 billion. "That's a 10-to-1 return on investment," she said.

Jewell urged Congress to allocate more funding to safeguard the natural treasures and provide a better experience to the millions who visit from all over the world. One possible funding source could be higher fees for mining, logging, and other industries that use public lands, Senator Maria Cantwell (D-Washington) has urged.

Jewell also advocated for more partnerships between conservationists and business interests to protect more lands and to balance economic growth and sustainability. She pointed to a recent agreement with western states to manage lands to save the greater sage grouse, while still allowing oil and gas development in some areas.

Asked about the seizing of a wildlife refuge in eastern Oregon by armed protesters last year, Jewell said, "there are a thousand or more ranchers that work with [the federal government] for every one who doesn't like the way we work."

The stakes are high, because the American West loses a football-field-size amount of land to development every 2.5 minutes, she noted. At the same time, scientists warn that the world is teetering on the edge of a great extinction of plants and animals.

The issues are front and center in Yellowstone, where bears and wolves have returned in such high numbers that they have begun spilling out of the park boundaries, angering some local landowners. (Read about the issues confronting Yellowstone.)

Jewell, who was appointed by President Barack Obama in 2013, grew up in Seattle and was previously the president and CEO of retailer REI. An engineer by training, Jewell also worked in the banking and oil industries. A new secretary will be appointed by the next president in January.

Fighting Nature Deficit Disorder

Another critical goal should be to "give everyone an equal chance to get outdoors," the secretary said.

That was an idea echoed by National Geographic Society President and CEO Gary Knell, who told the audience, "increasingly we see a world that is more urban and less connected to nature."

The solution may be to make national parks cool again, suggested Susan Goldberg, editorial director of National Geographic Partners and editor in chief of National Geographic magazine.

Toward that end, Jewell said she would like to see the Park Service update its outreach to visitors, including geotagging locations with phone apps. Multimedia content, such as hearing voices from historic events, would also help. She also said park service staff should work harder to include more diversity in hiring, so rangers look more like the increasingly diverse visiting public.

Recently new parks have honored a more inclusive history, from Cesar Chavez to Harriet Tubman to woman suffrage, but a lot more could be done, the secretary said.

The challenges aren't insurmountable, she said. After World War II, veterans returned to parks that were in disrepair and understaffed. The country poured more than a billion dollars into the system, unveiling major upgrades by the 50th anniversary in 1966.

It's also important to remember the things that can't be quantified, she said. "It's feeling the first rays of sun on Cadillac Mountain in Acadia or the child trying in vain to wrap their arms around an old-growth tree."

Admittance to all national parks is free this week, April 16 to 24, and many are hosting special programs.