Table of Contents

Sonic… Silence? How Climate Change is Making The Ocean Go Deathly Quiet – And Why That’s A Problem

Above: Man-made carbon dioxide affects ocean acoustic sounds. Credit: University of Hawaii

From above, the deep ocean seems intuitively silent – but underwater, it is full of sound. Marine organisms depend on sound in the ocean to survive. In the murky, deep ocean, sound is valued over sight. However, as climate change warms the oceans, it changes how sound travels through water. Rising ocean temperatures, melting sea ice, increased storms and expanding industrial activity are making the oceans noisier, altering the marine soundscapes that countless animals depend on and sending industrial sounds further and further out. As climate change and human activity disrupts underwater acoustics, scientists are increasingly finding evidence that marine ecosystems are being stressed in ways most people have never heard of.

Although the ocean appears silent, underwater it is filled with sound^[¹^,²^]. For marine species, sound is one of the most important tools of survival^[³^,¹^]. Sound is used to communicate, navigate, and locate food by countless marine species. Because light is quickly absorbed with depth, sound can travel farther than light in the ocean^[⁴^,⁵^]. Whales, dolphins, fish, crustaceans, and even some plankton producing ecosystems rely heavily on underwater acoustics for communication, navigation, hunting, migration, and reproduction^[³^,⁶^]. Sound is a critical resource in the ocean when visibility is often limited. In fact, it is so important that most marine animals depend on sound as much as terrestrial animals depend on sight^[⁵^,¹^]. When this sound is disrupted, it can have serious consequences to ecosystems and life everywhere in the ocean. For instance, large whales use low-frequency vocalizations, capable of travelling hundreds or even thousands of kilometers through water^[⁷^,⁸^]; dolphins use echolocation clicks to map their surroundings and locate prey^[⁹^x], and many fish species produce sounds during mating and territorial behavior. Not only do individual species depend on acoustic communication, but entire ecosystems are built from sound. Coral reefs create complex “reef soundscapes” which help floating fish larvae locate suitable habitats by listening to the sounds of a healthy reef: snapping shrimp, fish calls, and other biological activity attract organisms to healthy reefs, allowing these ecosystems to continue to thrive^[¹⁴^,¹⁵^]. Scientists found that degraded or bleached reefs are acoustically quieter, making it harder for young fish and larvae to locate suitable habitats^[¹⁶^,¹⁴^]. Since climate change is driving widespread coral bleaching through ocean warming, the loss of reef soundscapes may be contributing to declining reef biodiversity.

Acoustic sounds cause whales and dolphins to beach and perish. Image Credit: Scripps News @ Youtube

However, climate change is disrupting this underwater acoustic environment in a few major ways. One of the biggest threats comes from ocean warming and acidification. As greenhouse gas emissions warm the planet, oceans absorb over 90% of the excess heat trapped by climate change, storing it^[¹⁷^,¹⁸^]. Warming seawater changes physical properties of the ocean, including how sound waves can travel through the water. Sound moves faster in warmer water because of the changes to water density and pressure. Lower pH (acidification) also reduces sound absorption, allowing low-frequency noises to travel farther underwater^[¹⁹^,²⁰^]. Scientists found that warmer, more acidic ocean water allows certain low-frequency sounds to travel farther than they previously could^[²⁰^,²¹^]. This means that industrial noise from ships, drilling operations, military sonar, and offshore construction spreads across much larger distances, exposing more and more marine animals to chronic noise pollution over much wider areas^[²²^,²³^].

At the same time, climate change is melting Arctic and Antarctic sea ice at accelerating rates, warming the Arctic faster than any other part of the planet^[²⁴^,²⁵^]. Sea ice naturally acts as a sound barrier that dampens wave energy and limits noise movement^[²⁶^,²⁷^]. As ice disappears, oceans become acoustically louder. As more ice melts and water opens, there is increased shipping traffic and greater industrial access into previously isolated ecosystems^[²⁸^,²⁷^]. This sends industrial noise at high volume through ecosystems unadapted to almost any industrial noise whatsoever, creating a massive problem. In the Arctic particularly, scientists are observing major increases in acoustic noise levels as sea ice retreats and shipping routes expand. This is particularly concerning in these areas, as Arctic marine mammals, such as narwhals, belugas, bowhead whales, and seals are highly dependent on sound to survive in the dark, ice-covered waters. Noise pollution interferes dramatically with communication, mother-calf contact, mating calls, navigation, migration routes, predator detection, and hunting behavior^[²²^,²⁹^]. This phenomenon is known as Acoustic Masking^[³⁰^,³¹^]. Acoustic masking occurs when human-generated noise overlaps with the frequencies animals use to communicate, drowning out important biological signals. Scientists found that more animals were forced to call louder, repeat calls more often, and in many cases simply abandon important habitats entirely because communication becomes too difficult^[³²^,²³^]. Research has linked chronic underwater noise exposure to increased stress hormones, disrupted feeding patterns, hearing damage, disorientation, and changes in migration behavior among marine species^[³³^,³⁴^]. High industrial acoustic sound causes problems for marine life everywhere – such as mass whale strandings as a result of intense military sonar activity^[³⁵^,³⁶^]. As sea ice continues to retreat, it not only attracts more industrial traffic, but additionally amplifies the sound of the traffic in areas not used to coping with intense sound.

Not only are these acoustic changes worsening the impact already felt by human activity, but activity has actually increased in the past years. Global shipping traffic has increased dramatically over recent decades^[³⁷^,²³^], and large cargo ships produce continuous low-frequency noises capable of traveling vast distances underwater^[³⁸^,³⁹^]. Offshore oil and gas exploration, deep-sea mining proposals, wind-farm construction, and seismic airgun surveys further contribute to underwater noise pollution. The expansion of Arctic shipping due to melting ice is particularly alarming because many Arctic ecosystems have evolved in relatively quiet conditions. However, scientists and conservation groups are beginning to explore solutions. Some researchers are studying quieter ship technologies by improving propeller designs and exploring speed reductions that can significantly lower underwater noise pollution^[⁴⁰^,¹⁴^]. Certain marine protected areas are also experimenting with acoustic conservation, where noise levels are monitored similarly to chemical pollution. Scientists are increasingly recognizing that protecting ocean ecosystems may require preserving not only water quality and biodiversity, but also the natural soundscape itself^[³¹^]. Climate change is not only transforming temperatures and sea levels, but also fundamentally altering the sensory environment marine life depends on to survive.