London, September 24 : The big blue ocean is getting noisier, thanks to carbon emissions that have made the oceans more acidic.
Researchers have 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's call, for example, travels further in the north Pacific than in the north Atlantic, due to differences in pH.
Exactly how the process works is unclear, especially at frequencies below 1 kilohertz, which include whale calls, crashing waves and whirring ship engines.
"At these frequencies the exact molecular mechanism is still a bit fuzzy," said Peter Brewer of the Monterey Bay Aquarium Research Institute in California.
Some suggest that "ion pairs" of carbonate, bicarbonate, boric acid and borate are naturally "tuned" to absorb sound wave energy of 1 kHz and below.
The acidity of the water affects the balance between these chemicals.
Oceans are becoming more acidic because of rising levels of CO2 (carbon dioxide) in the atmosphere, which dissolves in seawater to form carbonic acid.
According to a report in New Scientist, Brewer and his colleagues wanted to find out if these changes could be enough to affect sound transport.
They analyzed 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 - the note "A" used to tune an orchestra.
They found that by the early 1990s, sound was being absorbed 15 percent less 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.
The team calculates that this would cause a 40 percent decrease in the absorption of sounds below 1 kHz.
"The ocean will have higher levels of ambient noise, marine mammals will communicate at greater range, and military or industrial sounds will travel further," said Brewer.
According to Tim Leighton of the University of Southampton, UK, the changes so far are relatively small, so the effect may be significant only in deep, quiet waters.
"Whether or not the differences will affect animal communication or military operations will require further study," said Brewer.