16-20 October 2017






Concept of the Week: Seawater Salinity and Carbon Dioxide

The contemporary concern regarding global climate change has caused scientists to study the various factors that govern the ocean's ability to absorb atmospheric carbon dioxide. Concentrations of atmospheric carbon dioxide, a greenhouse gas, are on the rise primarily because of increased burning of fossil fuels. Higher levels of atmospheric carbon dioxide may be contributing to increased global temperatures, a condition often identified as 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.

Studies show that the ocean's ability to absorb carbon dioxide is primarily temperature dependent. As noted in Chapter 8 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. We also found in Chapter 1, photosynthetic organisms assimilate carbon dioxide and release oxygen. Through cellular respiration, all organisms release carbon dioxide. Therefore, biological activity affects the ocean's ability to absorption of atmospheric 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. For nearly 20 years, scientists have been collecting physical, chemical and biological data through a large column of ocean water at Station ALOHA, a sampling site about 100 km (62 mi) north of Oahu that appears representative of oceanic conditions in the central North Pacific. 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 2001, the rate of carbon dioxide uptake was only about 15% of the rate in 1989. Why the change in carbon dioxide 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 associated with drought 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 carbon dioxide uptake to the saltier waters. The balance of the decline may be due to changes in biological productivity or ocean mixing.

Projected changes in global climate indicate significant changes in precipitation around the globe including reduced precipitation over various large areas of the oceans, resulting in potential "drought" conditions. Since changes in oceanic salinity result from changes in precipitation, the contribution that salinity plays on future assimilation of atmospheric carbon dioxide by the ocean also becomes an important consideration.

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Prepared by Edward J. Hopkins, Ph.D., email hopkins@aos.wisc.edu
© Copyright, 2017, The American Meteorological Society.