Due to the changing climate, the underwater world is getting ever noisier. This is the main conclusion of a study recently published in the scientific journal PeerJ. “In some places, by the end of this century, the sound of ships, for example, will be five times as loud,” the article’s first author, NIOZ oceanographer Luca Possenti, said. “That will interfere with the behaviour of many species of fish and marine mammals.”
The study was based on mathematical modelling in collaboration with Utrecht University and TNO, using a UN climate panel IPCC moderate or extreme climate scenario. Both the temperature and acidity of the water affect how easily sound travels through the ocean. Because of ongoing emissions of greenhouse gases, seawater is becoming more acidic, and together with the rise in seawater temperature, the researchers anticipate that underwater sound will travel further in the future in most parts of the oceans.
Changing currents, increasing loudness
Because the supply of warmer surface water to the northern Atlantic Ocean will most likely decrease, the researchers foresee a change in temperature layers in this part of the ocean. Possenti: “As a result of this, a separated ‘sound channel’ in the upper part of the North Atlantic may be formed. This will act as a kind of tunnel, which will carry sounds much further. As a result, the underwater sound level in this part of the ocean will increase by 7dB by the end of this century, under a moderate climate scenario.”
An increase of ‘just’ 7dB corresponds to almost five times as much noise energy underwater. Therefore, sounds generated by marine traffic as well as other sources, such as air guns used for seismic surveys, will increase. Moreover, it is likely that the number of ships will also increase in the near future, adding to the total amount of noise in the oceans. Therefore, even under a moderate climate scenario, changes may be severe.
NIOZ oceanographer Possenti emphasized that this louder human noise will affect much of marine life. “In the absence of good visibility underwater, fish and marine mammals communicate mainly through sounds. If fish can no longer hear their predators, or if whales have a harder time communicating with each other, this will affect the entire ecosystem.”
In addition to this theoretical study, Possenti and collaborators at TNO and MARIN are also working on actual measurements of underwater sounds. Using breaking glass spheres, they generate sounds at a level that marine mammals use at great depth, which are then recorded from tens to hundreds of kilometres away. “Much is still unknown about the exact effects of underwater conditions on the speed of sound. But because of the potentially profound effects on the ecosystem, that knowledge is essential if we want to understand what the changing climate may do to marine life,” said Possenti.