In wintertime, the sounds of nature are so subtle they're almost imperceptible: The whistling of the wind though craggy mountaintops, the whispering branches of the trees; the soft, delicate patter of an unseen animal's paws across snowy ground.
"It's a really quiet experience," said Rachel Buxton, recalling a recent winter hike in southwest Colorado's La Garita Wilderness. "You're almost hearing your own heartbeat."
But every 30 minutes, a jet flew overhead, shattering the fragile calm. "It's shocking, right?" she said. "You're in the middle of nowhere, yet you still can't escape the sounds of humans."
That's the trouble with noise pollution, continued Buxton, an acoustic ecologist at Colorado State University: "It really doesn't have any boundaries. There's no way of holding it in."
This problem pervades wilderness areas across the United States, Buxton and her colleagues reported Thursday in the journal Science. Using a model based on sound measurements taken by the National Park Service, they found that human noises at least double the background sound levels at the majority of protected areas in the contiguous states. This noise pollution doesn't just disrupt hikers; it can also frighten, distract or harm animals that inhabit the wilderness, setting off changes that cascade through the entire ecosystem.
"When we think about wilderness, we think about dark skies, going to see outstanding scenery," said Megan McKenna, a scientist with the National Park Service's Natural Sounds and Night Skies division and a co-author on the report. "We really should think about soundscapes, too."
Measuring noise pollution is a tricky task. Unlike smog or light, sound can't be detected from a satellite. To take stock of the soundscape of a specific site, Park Service scientists need to hike into the wilderness and set up a listening station by hand.
Each station includes a sound level meter and a recorder that runs for 30 days, collecting every birdsong, thunderclap and rumble of cars on the road. The Park Service has taken these measurements at hundreds of sites – ranging from the remote Hoh Rain Forest of Olympic National Park to the crowded running trails of Washington's Rock Creek Park.
The resulting recordings were then analyzed by acoustic specialists, who can pick out each sound in an audio clip and categorize its source. McKenna said that some of her colleagues are perceptive enough to distinguish between different types of jet engines.
Using data from more than 400 sites across the country, the researchers figured out which sounds are associated with a range of geographic features – elevation, annual rainfall, proximity to cities, highways and flight paths. These associations were then built into a model that can predict noise levels at any given spot in the country. By subtracting out the natural sound sources at sites, the scientists found the expected amount of noise pollution for the wilderness areas they studied.
The findings were mixed. Buxton said that protected areas had much lower levels of human-caused sound than the adjacent"buffer zones" of unprotected land – suggesting that these buffer zones really do insulate parks from unnatural sounds.
But 63 percent of protected areas experienced at least a three-decibel increase in sound levels caused by noise pollution (because decibels are logarithmic, this has the effect of doubling the level of background noise).
More than a fifth of protected areas experienced 10 extra decibels of human noise – a tenfold increase in the level of sound. The majority of areas considered "critical habitat" for endangered species were among the regions that dealt with at least an extra three decibels of sound, and 14 percent of critical habitats were in the 10-decibel category.
The noise can come from a wide array of sources – visitor center HVAC systems, air traffic overhead, growling car engines, children shrieking nearby, mining and drilling taking place miles away.
And the effects of this racket can be far reaching, Buxton said. Animals rely heavily on their ability to hear minute natural noises – the movement of predators, the trickle of a stream. Noise pollution may cover up those sounds, putting wild creatures at risk. Noise from human activity is also frightening and distracting; it can change animals' behavior with consequences for the entire ecosystem. A recent paper in the Proceedings of the Royal Society of London B found that noise pollution makes it more difficult for plants to reproduce because it scares away the rodents that help distribute seeds.
Even life that lacks ears may be affected. Spiders don't "hear" sound, but they can feel its vibrations, and research suggests that they act differently when bombarded with human noise. Likewise, plants have been found to extend their roots in the direction of acoustic vibrations from running water. Though a recent study found that garden peas can distinguish between real nature sounds and a recording, scientists don't know whether plants may be confused by the rumble of a passing car.
"We're realizing more and more just how delicate sound is, and how essential it is to things you wouldn't expect," Buxton said.
McKenna said that parks are taking steps to alleviate the impact of human sounds. Some implement shuttle systems to reduce the number of cars within their boundaries. Muir Woods National Monument, a forest of cathedral-like old-growth redwoods on the California coast, took the simple step of posting library-style "quiet" signs and reported a dramatic reduction in noise pollution.
The most problematic type of noise pollution – traffic sounds from cars and planes – is not so easily mitigated. But Buxton said that parks can look into "quiet pavement," which muffles the sounds of tires rolling down a road, and establish "noise corridors" that align flight paths with highways on the ground.
These efforts aren't just for the animals' sake, the researchers say. "We have all this research about how important it is to our human health and well-being," Buxton said, referencing studies that link listening to nature sounds with reductions in stress, improvements in mood and other markers of good health.
"Also it enhances our experiences in protected areas," Buxton continued. "Imagine walking in Yellowstone, seeing beautiful vistas. You've got bird songs filling the landscape. You might hear a pack of wolves howling on your way home at night. All these things are really magnificent. That's something that deserves protection."