It has been known for quite some time that excessive anthropogenic carbon dioxide is modifying ocean chemistry, increasing acidity, and compromising shell growth in calciferous sea life. The effects of this have been confirmed in sea snails, corals, and oysters, but also in marine phytoplankton – the organisms that provide a significant share of the oxygen we breathe.
In these alarming times I don’t want to increase our collective stress levels any more, except to say that turning our backs on this additional cost of a fossil-fueled civilization is not a wise survival strategy. But there is an acoustical component of a warming planet that I’d like to explore.
In 2008 researchers determined that changes in ocean chemistry also had an effect on sound propagation – with the concern that noise would not be so readily absorbed by a more acidic ocean. This spawned a concern that increasing ocean noise introduced by human opertions would be even noisier.
To some extent this is true, but in the already complex array of anthropogenic impacts on marine habitat, this sound propagation component was not really at the top of the concern list. Although there is another acoustical component of global warming that will have unanticipated and potentially profound acoustical impacts in the polar regions of our planet.
Last week Acoustic Ecologist Jim Cummings sent us an article which details the acoustical impacts of increased freshwater melt in the Beaufort Sea. Due to the high rate of ice melt there are now “sound channels” in the Arctic created by interfaces between water with differing densities.
There are two factors affecting water density in the ocean; salinity and temperature, with freshwater being less dense than saltwater (because doesn’t contain salt) and warm water being less dense than cold (due to thermal expansion). But the lower warm water is also saline – so it is denser by composition, and the warmth creates a thermal boundary that reflects sound back into the cold layer.
The differences in density between the salt water and the freshwater, as well as warmer and colder water is creating a sound channel in the Beaufort Sea that readily conducts sounds (and noise) over much greater distances. The initial research indicates that sound caught in this channel can transmit four times farther than it did in the 1970’s – when the impacts of global warming were not as pronounced.
How this impacts marine life remains to be determined. The Navy is exploring how this will effect sonar transmission. But this sound channel will also transmit shipping and other industrial noise over longer distances. This will particularly impact arctic marine life as more ice-free Arctic ocean becomes available for human activities it will also increase the sources of that noise.
This all could be a serious problem for Arctic marine animals, who have evolved an extensive repertoire of acoustical adaptations to a world that is dark for many months of the year, and until recently has also been thick with ice. Having to compete with ever more noises coming at them from greater distances does not bode well for these highly adapted acoustic communicators.