<div><p>The world's oceans are becoming noisier thanks to pollution, with potentially harmful effects for whales, dolphins and other marine life, US scientists said in a study published Sunday.</p><p>Low-frequency sound in the ocean is produced by natural phenomena such as rain, waves and marine life, and by human activities such as sonar systems, shipping and construction.</p><p>The sound is absorbed mainly through the viscosity of the water and the presence of certain dissolved chemicals, said the report published in the science journal Nature.</p><p>But the concentration of chemicals that absorb sound in the oceans has declined as a result of ocean acidification, in turn caused by rising concentrations of carbon dioxide.</p><p>Rising levels of carbon dioxide come from human activity such as shipping, with the number of ships roughly doubling over the past 40 years, the scientists said.</p><p>This was in turn increasing the acidity of the ocean, shown by a lowering of its pH levels, they said.</p><p>Using model simulations, the scientists found that increases in acidity could reduce seawater sound absorption by as much as 60 percent by 2100 in high latitude oceans.</p><p>Concern about the negative effect of the sea's increased acidity had previously been concentrated on the reduced rate of calcification, such as in coral reefs.</p><p>"However, a less anticipated consequence of ocean acidification is its effect on underwater sound absorption," the authors said.</p><p>"A decrease in seawater pH lowers sound absorption in the low-frequency range and, as a result, leads to increasing sound transmission."</p><p>Future global warming due to an accumulation of greenhouse gases may further decrease the ocean's sound absorption capacity at certain frequencies, the study said.</p><p>"High levels of low-frequency sound have a number of behavioural and biological effects on marine life," it added.</p><p>This included tissue damage, mass stranding of mammals such as whales and temporary loss of hearing in dolphins associated with military tests using intense mid-frequency sonar, the report said.</p><p>Marine species had adapted to varying levels of noise but the consequences of the sea's decreased ability to absorb sound were uncertain and required further research, the scientists said.</p><img src="http://admatch-syndication.mochila.com/images/ad.gif?aid=65819552&bid=informcom" /></div><div id="copyright"><div>

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