11-15 September 2017

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Ocean in the News

Concept of the Week: Sea Water Salinity and Carbon Dioxide

In view of the contemporary concern regarding global climate change, scientists are studying the various factors that govern the ocean's ability to absorb atmospheric carbon dioxide. Concentrations of atmospheric carbon dioxide are on the rise primarily because of the burning of fossil fuels (i.e., coal, oil, natural gas). Carbon dioxide is a greenhouse gas (an atmospheric gas that absorbs and radiates infrared radiation) so that higher levels of atmospheric carbon dioxide may be contributing to global warming. The ocean's role in regulating the concentration of atmospheric carbon dioxide depends on the temperature, salinity, and biological components of surface waters.

As noted in Chapter 3 of your textbook, gases are more soluble in cold seawater than warm seawater. Hence, changes in sea surface temperature affect the ability of the ocean to absorb carbon dioxide. As noted in Chapter 1 of your textbook, photosynthetic organisms take up carbon dioxide and release oxygen. And through cellular respiration, all organisms release carbon dioxide. What about the effects of changes in salinity on the ocean's uptake of atmospheric carbon dioxide? Research from the Pacific Ocean near Hawaii provides some insight on this question.

Since the late 1980s, scientists have been recording ocean conditions at a site (dubbed ALOHA) about 100 km (62 mi) north of Oahu. In 2003, David M. Karl, a biogeochemist at the University of Hawaii in Honolulu, reported a decline in the rate at which surface ocean waters were absorbing carbon dioxide from the atmosphere. In fact, in 2001, the rate of CO2 uptake was only about 15% of what it was in 1989. Why the change in CO2 uptake? In this region of the Pacific north of Hawaii, sea surface temperatures showed no significant change during the period of observation but precipitation decreased and evaporation increased. Less precipitation coupled with higher rates of evaporation caused the surface water salinity at ALOHA to increase by about 1%. Increasing salinity inhibits water's ability to absorb gases including carbon dioxide. Karl and his colleagues attribute 40% of the decline in the ocean's CO2 uptake to the saltier waters. The balance of the decline may be due to changes in biological productivity or ocean mixing.

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Prepared by Ocean Studies Central Staff and Edward J. Hopkins, Ph.D.,
© Copyright, 2017, The American Meteorological Society.