You can hear the climate changing. As the world warms, the soundtrack of the ocean is shifting.
In 2015, a US team of scientists and engineers reported that the loudest sound in some waters now comes from millions of tiny bubbles, which are released by melting glaciers and icebergs. In the fjords of Alaska and Antarctica, the average noise level is now over 100 decibels – louder than any ocean environment recorded before.
This is just one example of how Earth's natural soundscape is changing irreversibly, and human activity is driving the process.
Our natural spaces are now polluted with human-made noises. As we change forests into farms and drive species to extinction, we are fundamentally changing how our world sounds.
The phenomenon has inspired a new field of research, which aims to monitor the changing melody of our natural spaces. These acoustic studies could revolutionise the way we study whole ecosystems, from forests to coral reefs.
All the sounds of an ecosystem – from trickling streams to singing birds – add together to form a unique soundscape; a fingerprint of the habitat in its current state. Studying a soundscape is a quick and easy way to get an overview of the health of a habitat.
Shrimp snaps are one of the most characteristic sounds of marine ecosystems
But what scientists are hearing is not good. Some soundscapes are deteriorating in an alarming way.
Oceans in particular now sound very different. As the noise from melting glaciers rises, the sound produced by some marine species is falling.
For as long as biologists have been studying the seas, snapping shrimps have been creating a din. Each snapping shrimp has an asymmetrical, oversized claw, which it can snap shut at up to 60mph. This temporarily forms an air bubble in the water, and when this pops it forces water out in a high-pressure jet and produces a loud snapping sound.
These snaps are thought to be important for communication, as well as to stun prey and scare off predators.
Shrimp snaps are one of the most characteristic sounds of marine ecosystems like mangroves and coral reefs. They can travel up to a kilometre through the water. But a study published in March 2016 suggests the shrimp might soon fall silent.
As climate change continues to alter marine ecosystems, we might find our oceans go quiet
Biologists recorded sound at three hydrothermal vents, where carbon dioxide is naturally released into the water. Carbon dioxide forms a weak acid when it reacts with water, lowering ocean pH. This "ocean acidification" is now happening throughout the oceans because of the extra carbon dioxide in the atmosphere. The hydrothermal vents give scientists a chance to study what a more acidic ocean might be like.
They found it might sound a lot quieter. Shrimp around the vents snap less loudly and less often than expected, drastically changing the overall soundscape of the ocean. When tested in the lab under elevated carbon dioxide levels, individual snapping shrimp produced half as many snaps as those kept at current levels.
Although it remains unclear exactly why it silences snapping shrimp, widespread acidification could have a major impact on coral reef species that rely on sound for navigation. As climate change continues to alter marine ecosystems, we might find our oceans go quiet.
Scientists have divided the acoustic world into two groups. There is "geophony", which comes from natural processes like crashing waterfalls, the movement of the tides and the rumble of earthquakes – and then there is "biophony", which is produced by living things.
Human-made noises have become so ubiquitous on Earth that we have come up with a third category just for them: "anthropophony". In many landscapes, anthropophony dominates, drowning out the sounds of nature.
Studying a soundscape is a quick and easy way to get an overview of the health of a habitat
Human sounds are now found in almost every ecosystem on Earth, and it is rare to find an area remote enough to avoid human sounds entirely. But Michael Scherer-Lorenzen of the University of Freiberg in Germany says that our indirect effect on soundscapes has been even stronger. Sounds are slowly and subtly being lost as habitats become increasingly fragmented, species become extinct and urban landscapes expand.
This is an idea that Scherer-Lorenzen and his colleagues are exploring as part of one of the most ambitious studies in soundscape ecology undertaken to date. Beginning in September 2015, the researchers have left 300 microphones recording in the German countryside. The aim is to find out how different types of land use affect biodiversity, and how this is reflected in the soundscapes of those habitats.
Scherer-Lorenzen, who is lead researcher on the project, says that combining data from other research groups with their soundscape recordings allows the team to determine how the sounds of an ecosystem are influenced by human activities.
"We have a tremendous amount of information about many aspects of those ecosystems, including observations of bats, birds, insects, frogs, mammals, and the vegetation composition and structure," he says.
Ultimately, they hope the project will allow them to develop an early-warning system for changes in ecosystems due to human exploitation or climate change.
Already it is clear from studies elsewhere that daily and seasonal rhythms in the soundscape can become disrupted when human sounds begin to encroach. For instance, one study in Brazil found that animal calls were loudest during the day in forests close to opencast mines, whereas wildlife further from mines preferred to call at night.
The constant noise from the mines, which is mostly due to the 700 or so trucks that can visit daily, increased sound levels by up to 22dB. This forced species that would usually call at night to become more active in the day.
There is often a link between the richness of a soundscape and the diversity and abundance of wildlife
Fewer species were recorded at sites closer to the mine, suggesting that, for some species, this noise pollution is too much to bear.
Another study, published in June 2016, monitored the annual timing of bird migrations in the soundscape of Glacier Bay National Park in Alaska, and found that it had shifted significantly during the three-year study. For instance, the migration of the varied thrush (Ixoreus naevius) peaked five days earlier in 2014 compared to 2012. Such changes in the timing of events, such as plants flowering and birds migrating, have caused widespread concern because of their potential to disrupt key ecological relationships.
Many organisms use sound to navigate, forage and communicate. As a result, there is often a link between the richness of a soundscape and the diversity and abundance of wildlife in the area.
For example, in some Costa Rican forests, 85% of variation in the diversity of sounds was explained by the complexity of the forest structure, and the presence of swamps. In eucalypt forests in Australia, the volume of the soundscape is correlated with habitat fragmentation and overall ecological condition.
Unsurprisingly, anthropophony dominates our urban environments, and the impact of this extra man-made noise on wildlife has been enormous.
Biophony like bird song has been shown to decline as urbanisation increases, linked to lower species richness in urban environments. Even those species that remain may have fundamentally changed the nature of the sounds they make.
Killer whales have been found to call 1dB louder for every 1dB increase in background noise
For instance, one study comparing great tit (Parus major) songs in city and forest environments across Europe, found that the birds tend to sing shorter, faster, higher-pitched songs in urban environments.
These behavioural responses have been found to track fluctuating noise levels during a single day. This suggests that behavioural flexibility, rather than genetic changes, are enabling some species to adapt better to increasing anthropophony in their habitats.
However, not all animals have the option to change their calls and adapt to noise pollution. One study found that while ash-throated flycatchers (Myiarchus cinerascens) are able to increase their call frequency as background noise increases, the closely-related grey flycatchers (Empidonax wrightii) do not change their calls, while their numbers decline dramatically in urban environments.
In marine ecosystems, anthropophony may be having a major impact, too. Shipping noises measured in Falmouth Bay in the UK for a July 2016 study fall within the hearing range of several species of marine mammal such as whales and seals, as well as many fish species, and may be affecting communication and navigation for marine animals.
Previous work has shown that right whales can adapt to noise pollution by making higher-pitched calls, and killer whales have been found to call 1dB louder for every 1dB increase in background noise. However, these adaptations come at a cost – increasing noise pollution in our oceans has also been linked to increased stress in marine animals.
The effect of global warming and resulting drought has created the first completely silent spring I've ever experienced
But while some environments are getting louder, the global picture is that our remaining natural spaces are losing sounds – possibly forever. Bernie Krause, one of the pioneers of soundscape ecology, has been recording habitats around the world since 1968.
During his 50-year career he has recorded over 5,000 hours of soundscapes, on land and underwater, in 25 different countries. He estimates his archive contains nearly 15,000 different species and over 3,000 distinct habitats, making it one of the largest natural soundscape collections of its kind.
His repeated recordings in the same location, year after year, offer an acoustic record of the ecological impacts that climate change brings. From the temperate forests of North America to Alaskan tundra, tropical forests in Brazil and coral reefs in Fiji, Krause has heard dramatic changes to Earth's soundscapes over the last 50 years.
"Well over 50% of this archive comes from sites now either altogether silent, or so transformed by human endeavour they can no longer be heard in any of their original form," he says.
Krause describes his long-term recordings in Sugarloaf Ridge State Park; "a site not far from our home in Northern California, where the effect of global warming and resulting drought has created the first completely silent spring I've ever experienced".
The biophony, the density and diversity of that biophony, has not yet returned to anything like it was before the operation
His first recordings in the park, made in 2004, included a rich chorus of bird song over the background of a trickling stream. But by 2014, repeated droughts and rising temperatures had stripped the soundscape of almost all biophony, and by 2015 only the stream could be heard.
Another example from Krause's collection shows how soundscape ecology has the power to expose environmental destruction that we cannot even see. In 1988 a logging company was given permission to try a new "low-impact" method, known as selective logging, in Lincoln Meadow in the Sierra Nevada Mountains.
The belief was that by removing the odd tree here and there, rather than cutting down a whole area of forest, damage to the ecosystem would be avoided. And to the human eye, the post-logging forest appeared superficially the same, but Krause's recordings tell a different tale.
"I've returned to Lincoln Meadow 15 times in the last 25 years, and I can tell you that the biophony, the density and diversity of that biophony, has not yet returned to anything like it was before the operation," Krause explained in a 2013 TED Talk.
Many ecosystems would be totally unrecognisable to a person living just a century ago
Sound is a source of data that scientists are only just starting to exploit to its full potential. The key challenge facing soundscape ecologists is big data, Scherer-Lorenzen says. His Biodiversity Exploratories project in Germany will produce over 15.5 million minutes of sound recordings. That is 125 terabytes of data that need to be stored, processed and analysed.
"To listen to all these files, it would take 30 years of uninterrupted listening," he says. Instead, measures of acoustic complexity and diversity are calculated by a computer. Developing better computer algorithms to process and interpret this data is a fast-developing field of research.
The cost of technology can be prohibitive to large-scale soundscape studies, but projects like this show that by sharing resources, scientists can make high-resolution studies of soundscape ecology more affordable.
We have already changed Earth's soundscape so fundamentally that many ecosystems would be totally unrecognisable to a person living just a century ago. "There are few other critters whose impact on the environment has been so deeply and profoundly destructive [as humans]," says Krause.
One mangrove restoration project was able to restore a healthy soundscape in just three years
And, of course, the rise in anthropophony is as bad for human health as it is for wildlife.
Noise pollution can cause hearing loss in extreme cases. More commonly, lower levels of anthropophony can lead to increased stress, disturbed sleep, cognitive impairment and even behavioural changes such as increased aggression. Hearing more natural sounds has also been linked to better mood and improved cognitive abilities in the workplace.
However, although extinction means we risk losing some sounds forever, there may still be cause for hope. A study published in March 2016 reported that one mangrove restoration project was able to restore a healthy soundscape in just three years.
Mangroves are home to a rich sponge community, many of which share a cooperative relationship with snapping shrimp. Snapping shrimp are therefore a strong indicator of a healthy mangrove. But when sponges die off due to toxic algal blooms, which are becoming more common due to climate change, the snapping shrimp go too.
It may not be too late to save some of the most unique sounds of our planet
To restore heavily-degraded mangrove ecosystems in the Florida Keys, in 2010 scientists cloned sponges from healthy mangrove ecosystems and transported them into the degraded ones. By 2013, the soundscapes of restored mangroves were indistinguishable from those recorded in healthy, untouched mangroves.
Soundscapes offer a powerful way to measure the success of restoration projects like this, and can provide a view of the health of an ecosystem not visible to the naked eye.
The Earth sounds nothing like it did a century ago, and some of those changes may be irreversible. But there is also hope that it may not be too late to save some of the most unique sounds of our planet.
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