Planetary Watch: An Unexpected Impact of Global Warming

Global warming is going to make the ocean noisier. So says Peter Hester and colleagues from the Monterey Bay Aquarium Research Institute.  Is that possible? And should we care? Yes to the first, and yes to the second – at least if you are a fish or marine mammal.

Global warming equals climate disruption

Most people think of it as global warming, but climate disruption is probably more appropriate. Scientists have long appreciated that the effects of rising global temperatures would ripple through the entire climate system leading to profound and widespread impacts. However, I did not anticipate that those ripples would include sound waves traveling though the ocean.

Sound wave basics

First let’s review some facts about sound. Sound travels as a wave – that’s why we use the term sound waves.  The number of times the peak or the trough of the wave passes a point in space is referred to as its frequency. The term Hertz (or Hz) is used to quantify this frequency. A sound wave at one Hz has a trough that passes by one time in a second; one with a frequency of 1000 Hz does so 1000 times in a second.

We humans hear sounds with different frequencies as different pitches. The extreme low end of the sounds we are able to hear have frequencies of about 1 Hz, the high notes clock in at about 20,000 Hz.

Sound waves in the ocean

Sound traveling though the ocean is quite different from sound traveling through the atmosphere. Water and the dissolved salts along with bubbles and particles make it harder for the sound waves to travel. The higher frequency of the sound, the faster it is absorbed and the shorter the distance it can travel; the lower the frequency, the farther it can travel.

In 1943, scientists Ewing and Worzel from the United States and Brekhovskikh from the Soviet Union independently discovered a horizontal layer of water that channels low frequency sound waves 1,000’s of miles across the ocean at a depth of about ½ – 1 mile (1 -2 km).  This channel or waveguide is referred to as the sound fixing and ranging (SOFAR) channel and carries sound with frequencies less than 1 Hz.

Importance of sound in oceans

Perhaps for this reason, marine mammals such as whales and some other fish (i.e., sharks) use low frequency sound to listen and even communicate. It is also the reason why scientists use low frequency sounds to monitor ocean temperatures in experiments such as the Acoustic Thermometry of Ocean Climate (ATOC) experiment. And it is also the reason why some marine biologists have been concerned that the addition of artificial low frequency sound to the ocean, either inadvertently by ocean ships or intentionally by ATOC may disrupt ocean life.

How global warming makes more noise

To this mix, we now add the work of Peter Hester and colleagues published in Geophysical Research Letters.  It has been known that changes in the acidity (or pH) or seawater affects the propagation of low frequency sound. As acidity increases, the farther low frequency sound travels. They find that global warming will increase ocean acidity in two ways:

1.    A portion of the carbon dioxide we emit each year from burning fossil fuels and forests ends up dissolving in the ocean. When carbon dioxide dissolves in the ocean it acidifies the ocean.
2.    As the globe warms, ocean mixing decreases and this leads to more efficient breakdown of dead organic matter in the deep ocean and this in turn leads to the generation of additional acidity.

Hester and colleagues estimate that the propagation of low frequency sound has already been enhanced by as much as 10% or more and by mid-century the enhancement will exceed 20% on average.

Implications

So the ocean is getting to be a noisier place at the low frequency end of the sound spectrum. For some that’s a good thing – it’s going to be easier to communicate across the ocean, and it’s going to probably be harder for boats such as submarines to hide. For scientists, this change might afford new and improved ways to study the ocean. But for fish and marine mammals, it could prove to be just a whole lot of static or worse. We don’t know how much worse.

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