As seen in a previous article, ocean acidification has become a true cause of concern for ocean life, especially in the future. It is estimated that 22 million tons of carbon dioxide is being absorbed daily by our oceans. This has altered their natural pH, causing them to become 30% more acidic than they were in the early 1900s. In fact, Richard Feeley, an oceanographer with the National Oceanographic and Atmospheric Association (NOAA) has stated that the ocean will be more acidic in 2100 than it has been in the last 20 million years. While some are concerned that this increased acidity will dissolve the shells of the lucrative oysters and clams which depend on them, others have become cognizant of a new threat: reduced sound absorption. While this may seem trivial, whales, dolphins, seals, sea lions and a majority of fish use sound to locate food, communicate and hide from predators^1,2. If this is altered, so too could be their behavior and survival.

According to an October 3 issue of Science, ocean pH has dropped 0.12 units since the Industrial Revolution, which has led to an almost 15% drop in the ocean’s sound absorption. If carbon dioxide absorption continues, it is expected that the ocean pH will drop another 0.3 units by 2050. If calculations are correct, this would lead to a reduction in sound absorption of 40%³. This means the volume level for everyday sounds would not only increase, but the sound would also travel further, creating a louder environment over a greater area for all aquatic creatures³.

Man-Made Noises

One of the more problematic consequences of this is the increased volume of man-made noise, such as sonar or ships. Whales and dolphins would find this especially annoying since they are easily stressed by such foreign noises. It has been known for more than 30 years that a drop in ocean pH means an increase in acidity and, as a result, a decrease in sound absorption. It is believed this is due to the reduction in availability of pH-dependent ion pairs that occur in the ocean, such as carbonate, bicarbonate, borate and boric acid. These ions naturally absorb sound. Unfortunately, increased acidity tends to reduce the concentration of these ions³.

Dr. Keith Hester, an ocean chemist with the Monterey Bay Aquarium Research Institute in California, wanted to test if indeed the falling oceanic pH was affecting sound. By using the UN Intergovernmental Panel on Climate Change’s estimates for oceanic pH levels by 2050, Hester tested sound conductivity at varying pH levels. According to Hester, if the estimates from the UN are correct, sound will travel 70% further in 2050 than it does today³. This could lead to a problem when one realizes that whale behavior can be altered by sonar from over 500 kilometers away, and a reduction in the ocean’s ability to absorb sound waves will only increase this.

Effects Already Felt

It isn’t necessary to wait until 2050 to note the acoustical differences in the ocean. Changes have already been documented to have occurred off the Californian coast. Here, noise levels have increased by nine decibels over the last 50 years⁴. Considering an increase of 10 decibels roughly doubles how loud a sound is⁵, sounds in the ocean could be said to be almost twice as loud as they were in the 1960s. The full scope of how this could affect sea-life, fisheries and our oceans in general, however, is not yet known.

This is such a cause for concern that NOAA has become involved over the last several years. In 2004, they hosted a National Educational Lecture Series at points across the U.S. informing people of the consequences of increased sound volume on marine life⁶. They are also working to define Noise Exposure Criteria for sea life while funding research in this same area. In May 2007, there was even a symposium regarding technology to reduce the noise generated by commercial ships. Here, the problem of interference was discussed. It is thought that since the noise generated by ships is roughly the same frequency as the sound produced by marine life, ship noise will interfere, or mask, the marine sounds. The ability of aquatic life to find solution to the problems created by increased background noises is yet to be seen. It is hoped that with a concerted effort, these noises can be reduced in volume to become almost silent, disturbing ocean life as little as possible.

References:

1. Berardelli, Phil (30 Sept 2008), “Acidic Oceans Getting Noisy, Too,” Science, ScienceNOW Daily News.

2. NOAA Fisheries Office of Protected Resources. 2007 International Symposium.

3. Hester, K. C., E. T. Peltzer, W. J. Kirkwood, and P. G. Brewer (30 Aug 2008), “Unanticipated consequences of ocean acidification: A noisier ocean at lower pH,” Geophys. Res. Lett., 35, L19601, doi:10.1029/2008GL034913.

4. Ball, Philip. (3 Oct 2008), “Identifying the dripping taps of climate change,” Nature, doi:10.1038/news.2008.1148.

5. U. S. Department of Energy. Ask a Scientist; Environmental Earth Science Archive. 15 Sept. 2004.

6. NOAA Fisheries Office of Protected Resources. Ocean Acoustics Program.