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NPS research has shown that, in some cases, an increase in sound in the ambient environment of just three decibels can be substantial. Such a change can slash the so-called "listening area" for birds and mammals by half — meaning that wildlife can no longer hear many of the natural sounds in the surrounding area. (A listening area represents the size of a geographic area in which wildlife can hear sounds. An increase of six decibels — a decibel is a unit of sound intensity — is often considered a doubling of loudness.) "That has implications for all animals that depend on being hidden or being heard," Rocchio said.
Prolonged exposure to noise, Fristrup wrote in a recent study, can cause wildlife to avoid certain areas altogether, thus further minimizing the already limited potential habitats for some species. For example, studies of songbirds near oil and gas developments have found major reductions in bird density, bird species diversity, and pairing success.
The NPS has also conducted extensive surveys on the way unnatural sounds impact visitors' experiences, reaching unsurprising conclusions — that park-goers value the quiet and that noise intrusions are particularly disruptive to hikers who have journeyed into the wilderness. One study found that 72 percent of Americans believe that one of the most important reasons for preserving national parks is to protect the opportunities to experience natural peace and nature sounds, according to the NPS. Fristrup has often talked about how natural quiet has a documented calming effect on people, while vehicle noise can have the opposite impact.
As Cicely Muldoon, superintendent of Point Reyes National Seashore, explained in an interview: "For people coming out for a nice wilderness hike ... you can be out there by yourself, a ways down the trails in the park, and not see anybody." Given that this kind of park experience exists so close to a major metropolitan area, she added, "Point Reyes is really pretty extraordinary. ... The quality of the quiet is really profound."
To fulfill its mandate to protect natural sounds, the NPS and individual parks have limited vehicle traffic, used quieter mechanical equipment for maintenance and infrastructure projects, and educated visitors about ways they can help preserve "quiet zones."
Compared to significantly more complicated environmental threats, such as climate change and invasive plants, restoring the quiet in federally protected lands is relatively simple and achievable, said Daphne Hatch, NPS chief of natural resources management for Golden Gate National Recreation Area (a series of parks that extends from Marin County, just south of Point Reyes, to the Peninsula). "With all the other pressures we have, this is one that could be controlled," she said. "There are incredibly important resources of all kinds ... and visitors come here from all over the world to appreciate these resources."
When it comes to the biggest source of noise pollution for parks — aircraft buzzing overhead — officials throughout the federal government have long acknowledged the problem. In 1994, the NPS produced a lengthy report for Congress that investigated the effects of aircraft flights on federal parks. That report noted that biologists believe flight noise disturbances might be causing animals to abandon their habitats, injure themselves while in panic mode, and suffer a loss of energy due to the stress of chronic exposure.
The NPS's subsequent acoustical monitoring projects — along with local airports' official noise reports — have documented that, for some parks, the jet noise is, in fact, chronic. In 2000, SFO's Aircraft Noise Abatement Office reported an average of 105 flights per day over Point Reyes. And in 2011, the FAA and the NPS collaborated on a sound report for the park, which concluded that aircraft noise was audible for as much as 27 percent of the day in certain parts of Point Reyes.
A similar 2005–2006 NPS report for Yosemite National Park, has long experienced frequent flights overhead, also determined significant noise impacts from jets at sensitive habitats and popular backcountry hiking areas. The NPS found that aircrafts were audible 55 percent of the day at Granite Lake near Tioga Pass; 58 percent of the time at Tuolumne Meadows; and between 41 and 49 percent at locations along the Tioga Road corridor. "A lot of high-altitude jets ... get funneled over Yosemite," said Vicki Ward, NPS overflights program manager, who is also based in Colorado. "It's just constant."
Given the extensive research demonstrating that jet noise produces negative consequences, NPS officials were hopeful that the FAA's NorCal project would provide a unique opportunity to preserve and restore the natural quiet. So when the FAA's noise data suggested that the NextGen-based flight routes would actually increase jet sounds over certain sensitive habitats, NPS officials were stunned.
The data was especially troubling considering that the FAA was claiming that its project would have no significant impact on the environment.
When I first met Rocchio at the NPS offices in downtown San Francisco, she handed me a large stack of papers from her NorCal Metroplex research. The records included several years worth of letters between the FAA and the NPS, along with detailed analyses of the noise data that the FAA released in its environmental assessment. The documents provide insight into the challenging process that the NPS faced in deciphering NextGen's impacts on wilderness areas — and park officials' ensuing struggle to convince the FAA to take their concerns seriously.