The ocean is getting louder

Put your head underwater in a swimming pool and the world changes. Voices become a muffle, the filter hums, and the splash of a distant swimmer seems to come from every direction at once. That is a small taste of how animals in the sea experience their surroundings, except that for them hearing is no hobby. Light fades within a few hundred metres of the surface and smell is useless in open water, so many species rely on sound to find food, keep in touch and avoid danger. Sound also travels four times faster in water than in air.

That makes the sea an unusually good place to talk and an unusually bad place to be noisy. Since 1950 the number of large ships has multiplied, and each one leaves a long trail of low-frequency sound from its engine and propeller. Along busy routes, background noise rose steadily for decades. Add the pulses of seismic surveys for oil and gas, the hammering of pile drivers and the sonar of navies, and the sea begins to resemble a city centre at rush hour.

The effects are easiest to see in whales. A blue whale's call can in principle be heard across an ocean basin, but in a noisy sea the distance at which it can be understood shrinks sharply. Some animals respond by calling louder or at a higher pitch; others simply stop feeding. A rare natural experiment in 2001 showed what is at stake. Ship traffic in the Bay of Fundy fell after the attacks of 11 September. Researchers found that the stress hormones of North Atlantic right whales dropped as the water grew quieter.

The encouraging part of the story is that noise, unlike plastic, does not linger. It vanishes the moment the source is switched off, and several fixes are already within reach. Ships can be slowed down: a voluntary scheme off the coast of Vancouver asked captains to ease their speed near feeding orcas, and measurements recorded a clear drop in underwater noise. Propellers can be redesigned to produce fewer of the tiny bubbles that cause much of the racket. Wind-farm builders can surround their piles with a curtain of rising bubbles that soaks up part of the hammering.

None of this will happen quickly without rules, and shipping is a global business that no single government controls. The International Maritime Organization published guidelines for quieter ships in 2014, but they are voluntary, and few owners have gone far beyond them. Cost is part of the difficulty: a newer engine or a slower voyage means money, and the benefit goes to animals that cannot lodge a complaint. Anyone who has heard a recording of a whale singing over the drone of a tanker knows which sound is the intruder.