"Thousands of tired, nerve-shaken, over-civilized people are beginning to find out that going to the mountains is going home; that wildness is a necessity," naturalist John Muir wrote in 1901.
Today, Muir would be dismayed to learn that in a majority of the United States' protected areas, there's no escape from human-made noise pollution.
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A new study by researchers at Colorado State University and the National Park Service assessed noise levels from parks, designated wilderness areas, Forest Service lands and other protected areas of the United States and found that noise pollution doubled sound levels in more than half of all U.S. protected areas. In 21 percent of these sanctuaries, the grind, buzz and hum of human-made commotion raised noise levels tenfold.
"All of us were quite surprised at how high noise levels were in these protected areas," says Rachel Buxton, post-doctoral researcher in Colorado State University's Department of Fish, Wildlife, and Conservation Biology and lead author of the study appearing today in the journal Science. "We found a massive increase in sound energy."
A tenfold increase in sound levels, Buxton explains, means that what could be heard at 100 feet (30 meters) away in natural conditions can only be heard from 10 feet (3 meters) away. The natural background of bird songs, rushing rivers and scurrying animals becomes muted by the din of, as Muir describes, "over-civilized people."
To assess the extent of noise pollution, Buxton and her colleagues analyzed millions of hours of audio recordings captured by National Park Service employees at 492 sites across the country. With the help of machine-learning algorithms that incorporated natural and unnatural features at each site, they extrapolated the data to estimate existing sound levels, natural sound levels and the amount that human noise elevated natural background noise.
"Noise pollution is not easy to convey," says George Wittemyer, a conservation biologist at Colorado State University and senior author of the study. "We can't take a photo of sound at a place today and compare it to a snapshot taken 20 years ago."
The new research, he says, was an effort to convey "the increasing creep of noise pollution."
In 63 percent of protected areas, sound levels were elevated by more than 3 decibels, according to the study. And in 21 percent of the areas, sound levels were amplified by 10 decibels. The culprit, for the most part, says Buxton, was traffic on nearby roads, aircraft flying overhead and noise from extraction operations, such as logging or mining.
While noise may not be the first thing that most people think of when considering all the ways people pollute environments, Buxton says that it can have a profound impact on wildlife and ecosystems.
Noise pollution "masks" natural sounds, she explains, inhibiting critical functions. For example, a predator like a fox may no longer be able to detect the rustle of its prey in the woods. The fox may also become spooked by the sounds of, say rushing cars, and simply leave an area. This can affect an entire ecosystem since prey such as rabbits may then increase in numbers and the plants they eat may, in turn, become depleted.
"Even if sound levels affect just one kind of animal, it can trickle down and have a cascading effect on an entire ecosystem," Buxton says.
Noise can also force a change in birds' calls. Research has shown that some birds shift their calls into higher pitches to avoid overlapping with the chiefly lower tones of human-made noise.
"If a bird is shifting its song to a higher pitch, we don't know if a potential mate is less interested," says Buxton. "We just don't know the consequences."
There is also the impact on our own species to consider. Research has shown that noise pollution can inhibit human memory, raise stress-related cortisol levels and hamper learning. Quiet places in nature, the authors argue, offer critical respites from the din of everyday life.
The good news, says Wittemyer, is that large parks in the United States remain mostly quiet sanctuaries.
"A lot of the smaller protected areas are loud, but many of the larger, marquis national parks remain quiet," says Wittemyer. "Given the pervasiveness of roads and the amount of aircraft crisscrossing our skies, the fact that we still have so many of these natural intact soundscapes is pretty encouraging."
One reason why the bigger parks have retained their natural soundscapes, says Wittemyer, is simply due to their size and relative isolation from development. But he said it's also thanks to efforts underway at several state and national parks to minimize humans' noisy imprint.
For example, park service officials at Yosemite, Grand Canyon National Park and elsewhere have confined vehicle traffic to limited areas of the park and supply shuttle bus service for access to less-traveled areas. The National Parks Air Tour Management Act of 2000 requires the park service to work with the Federal Aviation Administration to restrict air traffic to already noisy corridors, leaving swathes of overhead sky plane-free.
At Death Valley National Park, officials have installed what's known as quiet pavement, which features a porous surface that absorbs sound from a vehicle's wheels and engine.
Combined, these innovations can make a difference, says Buxton, who adds that it's important to remind ourselves why untainted wilderness is, as Muir wrote, "a necessity."
"Whether it's a rushing river or a bird chorus or bugling elks," Buxton says, "the sounds we hear in nature can be just as magnificent as the sites we see."
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