THE big blue ocean is about to get noisier, because sound can now travel further than it did a century ago 鈥� thanks to carbon emissions making oceans more acidic.
It has been known for some time that acidity can influence how far sound travels in seawater. In the 1970s, acoustic measurements showed that the reach of low-frequency sounds varies between oceans. A whale鈥檚 call, for example, travels further in the north Pacific than in the north Atlantic, due to differences in pH.
Exactly how the difference arises is unclear, especially at frequencies below 1 kilohertz, which includes whale calls, crashing waves and whirring ship engines. 鈥淎t these frequencies the exact molecular mechanism is still a bit fuzzy,鈥� says Peter Brewer of the Monterey Bay Aquarium Research Institute in California. Some suggest that 鈥渋on pairs鈥� of carbonate, bicarbonate, boric acid and borate are 鈥渢uned鈥� to absorb energy from sound waves of 1 kHz and below. The acidity of the water affects the balance between these chemicals.
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Oceans are becoming more acidic because of rising levels of CO2 in the atmosphere, which dissolves as carbonic acid in seawater. Brewer and his colleagues wanted to find out if these changes could be enough to affect sound transport. They analysed a database of ocean acidity during the 20th century, which showed that, on average, ocean pH levels dropped by 0.12. Using previous experimental data and field observations of how pH affects sound, they calculated how much this drop would affect the absorption of sound waves at 0.44 kHz 鈥� the note A used to tune an orchestra. They found that by the early 1990s, 15 per cent less of the waves鈥� energy was absorbed than in the late 19th century.
Some studies predict ocean pH could drop by an average of 0.3 before the end of this century (Nature, vol 425, p 365). The team calculates that this would cause a 40 per cent decrease in the absorption of sounds below 1 kHz (Geophysical Research Letters, ). 鈥淭he ocean will have higher levels of ambient noise, marine mammals will communicate at greater range, and military or industrial sounds will travel further,鈥� Brewer says.
鈥淭hey have done well to raise this issue,鈥� says Tim Leighton of the University of Southampton, UK. He says these relatively small changes may be significant only in deep, quiet waters.
Whether or not the differences will affect animal communication or military operations will require further investigation, Brewer says. However, he points to a study by Mark McDonald of Whale Acoustics, Colorado, and John Hildebrand at Scripps Institution of Oceanography, California, suggesting that blue whales are calling at lower frequencies than they used to, possibly because it is easier to communicate this way.
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