This summer, an estimated 4 million visitors will trek through Yosemite National Park in California. They will circle through the valley, crane their necks at Half Dome, pause for the grazing wildlife and traverse a portion of the 800 miles of trails.

Along the way, they might notice the stark silhouette of burned-out pine trees, blackened and skeletal, a sign that the wildfire burn area across the Western United States, including the country’s third-oldest park, has doubled since 1984.

Similarly, visitors to Glacier Bay National Park in Alaska might notice the diminished size of the stately Muir Glacier, which lost the height of one World Trade Center. And those hiking through North Cascades National Park in Washington state will note the lowest snowpack in that region for the past eight centuries.

These wild places, the refuges we seek out for their fresh air, towering pines and crystalline glaciers, are bearing the brunt of the climate crisis, according to leading researchers.

“When people go to national parks, people who vacation will be witnessing the consequences of human-caused climate change,” said Patrick Gonzalez, a climate change scientist with the National Park Service. “It brings people closer to the real impact.”

Gonzalez, who is also a professor at the University of California, Berkeley, headed up a study that analyzed the climate in and near national parks since 1895. The results were stark: Human-caused climate change exposed the national parks to hotter and drier conditions than in the rest of the United States, Gonzalez said.

From 1895 to 2010, Gonzalez’s research found human-caused climate change increased the temperature in national parks by 2 degrees Fahrenheit, or double the rate of the United States as a whole.

Gonzalez’s research was included in a recent report by the National Parks Conservation Association, an advocacy organization for the NPS. The report, which relied on data from a number of other studies, was aimed at giving a comprehensive look at the harm climate change has on ecosystems.

“Overall, we identified that 96% of national parks are suffering from air pollution,” said Stephanie Kodish, senior director of clean air and climate with NPCA.

A recent review of ozone levels in 33 parks from 1990 to 2014 found that ozone was deemed unhealthy by the EPA on 35% of visitor days. Many people are visiting these parks when the pollution is particularly harmful, said David Keiser, an author of the study.

“We see parks as being these pristine areas you want to get away to,” Keiser said. But when he and his fellow researchers, Gabriel Lade and Ivan Rudik, studied the air quality, they found it was statistically indistinguishable from a major metropolitan area.

Their research found that ozone levels were improving in major metro areas in the early 1990s, thanks in large part to the Clean Air Act of 1990, which was one of the first air-quality laws. But the air pollution in national parks was deteriorating during this time, Keiser said.

“These places we think have fresh air, sometimes these areas can have high levels of air pollution,” he said.

If this rate of carbon output and air pollution continues, temperatures in national parks would increase by as much as 16 degrees Fahrenheit in a worst-case scenario, Gonzalez said.

In 2013, the United Nations warned the world it needed to cut carbon emissions by 2020. The Intergovernmental Panel on Climate Change echoed this call with its 2018 report.

“Cutting carbon pollution can save parks from the most extreme heat,” Gonzalez said.

While each visitor can take steps to minimize his or her carbon output, Kodish stressed the importance policy has in protecting public lands. “We encourage park-goers to engage in the rule-making process,” she said. “It’s really critical that park-goers express their love of these places.”

Kodish said the EPA under the Trump administration has taken steps to roll back air and climate regulations meant to curb greenhouse emissions.

The NPS, perhaps confronted with the realities of a changing climate long before others, has taken steps to lessen the negative consequences a high volume of human traffic could have. They routinely monitor air-quality information and provide some of the longest available data sets to researchers and scientists. In recent years, parks have created solar-energy programs, opted to use less fossil fuel, installed electric car charging stations and provided shuttle buses to visitors.

“The parks have been doing their part for years,” Gonzalez said.

Take Great Smoky Mountains National Park. Named for the natural blue mist that is common along the Appalachian mountain range, it is the most densely visited park in the country. In 2018, 11.4 million visitors passed through its scenic highways and hiking trails.

That’s almost double the 6.4 million people who visited the second-most popular park, Grand Canyon National Park.

Yet, in the 1980s and early 1990s, the park was violating conditions set by the EPA for human health. It didn’t meet the ozone public health standard for 100 days over a 10-year span, said Jim Renfro, air-quality specialist for the park.

When Renfro arrived in 1984, first as a park volunteer and later as a staff member, visibility averaged 9 miles during the summer months. The unhealthy air quality and potential public health risks got the attention of a lot of people, he said.

The park began to take steps to mitigate the pollution, first through the Clean Air Act and later through the 1999 Regional Haze Rule, which required states to take steps to improve air quality in designated areas, such as national parks.

“We’ve been seeing significant improvements in the Smokies,” Dana Soehn, a spokeswoman for Great Smoky Mountains National Park, said. “That’s not been without a lot of really hard work.”

Renfro credits a lot of that improvement to the reduction in nitrogen oxide emissions from power plants near the park and the implementation of cleaner fuel and engine standards. Combining data from the park’s air-quality monitoring system with collaboration at the local, state and federal levels, Renfro said, the park lowered the air pollution in about a decade. It’s a system he believes could work throughout the rest of the country.

“It’s a remarkable story of turning something around when people thought it could never change,” Renfro said.

If similar steps are taken on a national level to prioritize the use of renewable energy and other sustainable practices, it could reduce the potential heating in the national parks by two-thirds, Gonzalez said.

On any given day this summer, visitors will see the Smoky Mountains rolling along the horizon with at least 40 miles of visibility.

“Now, the norm is blue sky and green mountains,” Renfro said.