In dark ocean waters, marine mammals such as whales and dolphins rely on sound to communicate with each other, locate prey and find their way over long distances. All these activities - critical to their survival - are being interfered with, experts say, by the increasing levels of noise from ocean-going ship engines, sonar devices and seismic exploration. Climate change could make the noise problems for marine mammals even worse.

With only 300-400 animals remaining, North Atlantic right whales are among the most endangered whales in the world. Ship strikes kill as many as are born each year. Click to listen to the calls of the North American right whales (Eubalaena glacialis)

Christopher Clark is the director of the Bioacoustics Research Program at the Cornell Laboratory of Ornithology in New York state. He says that part of his personal discovery over the last 15 years has been the incredible richness of the ocean in terms of the songs and the voices of whales.

"As our world is vision, their world is sound," he says.

With their groans, clicks, booms and shrieks, marine mammals send warnings, point out good sources of food, and attract mates. But over the past four decades, says Clark, more and more man-made noises have been invading their acoustic universe. The noise produced by ocean-going humans falls into the same low-to-mid frequency range as many whale calls. Clark believes this acoustic competition has drastically reduced the distance over which whales can communicate and may be drowning them out altogether.

Clark draws an analogy to trying to have a conversation with someone across the kitchen table when a truck goes by on the street.

Propeller-driven ships are the dominant source of man-made, low-frequency noise in the world's oceans. Click to listen to the noise produced by a marine ship (30x speed)

"The noise from the truck might be mildly annoying. We may not even pay attention to it. It doesn't interrupt our conversation," he says.

But, he continues, the truck will have a much bigger impact if we're talking with one another across the street.

"Suddenly, I can't hear you anymore," says Clark.

Although scientists don't know for sure, Clark says there is strong evidence that whales normally communicate with each other across large areas. The call of a blue whale - the largest animal on the planet - can be heard by another blue whale 1,000 kilometers away.

IFAW report: "Ocean Noise: Turn it down"

A report released December 3 by the International Fund for Animal Welfare - or IFAW - warns of the potential adverse effects of increasing noise pollution. In addition to impeding marine mammal communication and masking sounds produced by predators and prey, ocean noise may cause animals to lose their way, or it could interfere with essential behaviors like feeding and breeding.

The report also contends that ocean noise has resulted in injury and even death by driving stressed animals to become entangled in fishing nets or to strand themselves on shore.

U.S. Supreme Court considers effects of Navy sonar

In November, the U.S. Supreme Court turned its attention to this issue when it considered a case brought by an environmental advocacy group, the Natural Resources Defense Council, against the U.S. Navy. IFAW was a co-plaintiff in the lawsuit, which charged that the Navy needs to do more to protect marine mammals from harm caused by its use of sonar in training exercises.

In a 5-4 decision, the court stated the Navy does not have to mitigate even likely harm to marine mammals because by executive order President George W. Bush has declared naval training essential to national security.

Although the court decided in favor of the military, the case did bring nationwide attention to the complex problem of ocean noise and its potential implications for marine life.

Climate change increases ocean noise

Climate change may make a complicated problem even more so.

Male humpback whales sing complex "songs" which typically last 10-20 minutes and may be repeated for hours. Click to listen to the calls of a humpback whale (Megaptera novaeangliae)

Peter Brewer, a senior scientist at the Monterey Bay Aquarium Research Institute in California, says we're putting carbon dioxide in the ocean from the atmosphere at a rate of just over one million tons per hour. His work focuses on how these growing levels of carbon dioxide are changing the chemistry of the ocean.

One well-understood effect of all this additional carbon dioxide is to make ocean water more acidic - and that, says Brewer, is having a surprising impact on underwater acoustics.

"Some of the molecules which absorb sound are decreasing in quantity, and so sound levels are going up," he says.

Specifically, low-frequency sounds like the ones produced by ships, sonar, oil and gas exploration - and whales.

Brewer says addressing the global problem of increasing ocean noise will require a change in communication strategy - not by the mammals in the ocean, but by their land-based human counterparts. He believes that improved dialogue among scientists and the world's environmental, commercial and military interests is needed to understand the impact of human activities on the marine environment and its inhabitants.

Audio recordings courtesy of the Cornell Bioacoustics Research Program

"Lethal Sound," was produced by the nongovernmental organization the Natural Resources Defense Council. This advocacy video targets noise from Navy sonar and oil and gas exploration for harming marine mammals and contains graphic images of dead whales and dolphins.