In Australia, a critically endangered bird is losing its song. Among the regent honeyeaters, adult males teach the young their complex mating tune. But as their population dwindles and the adults disappear, this song culture is breaking down, forcing the young to change their tune. What was once a soft warble of the honeyeater has, in several places, become a mimicry of other birds; they are singing borrowed songs in an attempt to attract mates and establish breeding grounds, but with little success. It’s a development that scientists say is further exacerbating their population decline.

The honeyeater is not alone. As our world gets louder with the din of human activity, ecologists say there’s an eerie silence spreading across our natural soundscapes. Humpback whales, for instance, stop singing when ships are nearby. Healthy coral reefs used to be one of the loudest in the depths of the ocean due to the variety of marine life that inhabited these ecosystems. But with the increasing stress placed upon them through pollution, rising temperatures and environmental degradation, coral reefs are damaged and bleached, falling silent and attracting less fish that rely on reef sounds to find their way home.

At the same time, technological advancements are allowing us to hear sounds that lie outside our range of perception, such as the damage wrought by climate change-induced events. Listen closely and you can hear a tree dying. Its slow death as it is seared by oppressive heat in drought-like conditions sounds like “popcorn popping,” researchers say.

Our planet is loud. It’s meant to be. There’s the raucous cacophony of vocalizing species that sound ecologist and musician, Bernie Krause, dubbed “biophony.” Then there is the background score of whistling wind, cascading water, rustling leaves and splitting glaciers that all make up the “geophony.” Added to that is the mechanical noise – “anthrophony” – that is a product of human activity​​. This indelible imprint of humans on the planet’s soundscape – that ranges from chemical explosions to car horns, from industrial sounds to urban noise – is only growing louder.

When Krause began recording soundscapes across the world in 1968, he would only have to record for 15 hours to get one hour of audio. “Now it takes nearly 2,000 hours to obtain one hour of untainted natural sound,” he said in a speech delivered at the San Francisco World Affairs Council in 2001. Many of his recordings are now archival footage of decimated habitats and species long gone while others may be on the same path as extinction plays out before us in real time. Krause estimates that “Almost 70% of my archive comes from habitats that have disappeared.”

What do we stand to lose as our natural soundscapes become quieter?

Sound is a crucial sense that helps us connect with our environment and gives us a deeper understanding of place. But it is also far more than that. For one, it’s a key indicator of ecological health as well. “One of the single most important resources of the natural world is its voice – or natural soundscape,” said Krause, who has been recording these soundscapes for over 55 years. A healthy ecosystem is a diverse one. But as species die off, modern soundscapes are becoming less varied. Krause’s recordings from Lincoln Meadow in the Sierra Nevada mountains are proof. A logging company had assured residents that the local wildlife would not be impacted if they would be allowed to cut down a few selective trees. Krause recorded the sounds before and after logging and found that the diversity of the calls had not only diminished afterwards, but had not returned to its original state even after 25 years.

This rapid decline is undoubtedly troubling from a conservation perspective. Natural soundscapes are rich in information – as they are silenced, a crucial source of insights into the natural world diminishes too. It is by listening to these sounds – through small recording devices hidden in tree canopies, submerged in the ocean, or disguised in the hollows of tree trunks – that scientists have been able to not only monitor individual species and identify new ones but also unearth the complex inter-species relationships that make up an ecosystem.

“We’re such a visual species, and we put such a high priority on it that we’ve discounted sound as a means to assess change and measure change,” said Bryan Pijanowski, an avian ecologist at Purdue University in Indiana, who – along with his colleagues – established the relatively recent field of “soundscape ecology.” This is the world of bioacoustics, where recorded soundscapes are being decoded using artificial intelligence. What we find is that sound is used in surprising ways in the natural world – we now know that Amazonian sea turtles make sounds while still within their eggshells in order to time the very moment they are born. Meanwhile, female Atlantic right whales lower their voices, almost “whispering” to their calves to avoid predators taking notice.

Climate change also has a part to play in changing the nature of soundscapes. As temperatures rise and weather patterns change, so does “the range of communication between individuals,” noted researchers in a 2019 paper. These variations in the air, water, and land through which sound travels not only change the quality of the sound but also affect the production of sound itself, adding additional pressure upon species to adapt. Climate change is also making the underwater world far noisier. A recent paper, published in the journal PeerJ, found that "In some places, by the end of this century, the sound of ships, for example, will be five times as loud." Such changes have the potential to wreak havoc upon underwater species that depend upon sound more than sight to not only communicate, but also to forage, mate, feed, find their young; in short, to survive.

The same changes disrupting our natural soundscapes and the complex rhythm of life it represents affects humans too. “We are just now beginning to understand that many of these sounds create stress in both [human and non-human] worlds, even though the victims may not seem conscious of the effect,” said Krause. The mechanical noise pollution that makes up a majority of our modern soundscapes today is a health concern – exposure to which has been found to increase stress levels, disturb sleep, exacerbate cardiovascular disease and even cause cognitive impairment in children. But noise pollution, like several other pollutants, is unequally distributed and those most exposed and thus most at risk are the poor and marginalized populations.

Several studies now exist that connect natural sounds with human health and wellbeing. Soundscapes intersect with public health; where jarring noise can lead to a deterioration in mental and physical health, natural sounds have been found to be restorative and calming with several psychological benefits.

But as our natural soundscapes dwindle, scientists are concerned we may also lose a powerful tether to the natural world. “My great worry is that we would potentially become disconnected from nature… Sound is a way to reconnect us because it’s an emotional trigger. It’s an emotional bridge to place,” Pijanowski said. Take birdsong for instance, a sound that is a “defining component of our relationship with nature,” according to Simon Butler. Butler is the lead author of a study that found the loss of bird species had changed the sound of spring across North America and Europe. "These results suggest that the soundtrack of spring is getting quieter and less varied and that one of the fundamental pathways through which humans engage with nature is in chronic decline, with potentially widespread implications for human health and wellbeing,” Butler said.

Sound can also be an important tool to mobilize environmental action. But a weakening link with nature may hamper this – with the soundscapes, we risk losing not only a holistic understanding of our ecological surroundings but also our place in it. “We live embedded in systems designed to turn our attentions away from the voices of the living Earth, inward toward the human,” wrote David George Haskell. Changing soundscapes are then records of our changing relationship with our natural surroundings. “The sonic crisis extends from the global scale of continents and ocean basins, right down to the individual scale of singing birds and human city dwellers. At all these scales we have a crisis of inattention. “Not listening” is a form of sonic loss.” What we’re witnessing is an “extinction of experience” in Butler’s worlds, where not only the quantity of time spent in nature is reducing but also the quality of the natural soundscape itself.

Now, scientists are using sounds to monitor restoration efforts, even employing the sound of healthy coral reefs to rebuild the reef community and bring it back to life. Machine learning is being used to detect illegal deforestation and logging by listening for unnatural sounds in forest environments, bioacoustics is helping scientists identify new species such as the Himalayan thrush found in Arunachal Pradesh in 2016, while large-scale mapping projects – such as Sounding Nature – are creating archives of natural soundscapes that are then being reimagined by artists across the globe in their work. Scientists like Butler are also reconstructing soundscapes of the past and predicting future changes in order to determine the conservation action needed to prevent further species decline and restore the health of our ecosystems.

As Krause told Grist, “[W]ithout these ambiences to lure us outside, to calm us and restore our flagging spirits, human culture becomes dystopic and pathological.”