Saltier Water: Climate change can slow ocean’s absorption of carbon dioxide gas

A decrease in precipitation over the Pacific Ocean just north of Hawaii in recent years has left the ocean there saltier and has diminished its capacity to soak up planet-warming carbon dioxide, a new analysis shows.

Each month since the late 1980s, researchers have recorded ocean conditions about 100 kilometers north of Oahu, far enough out that waters aren’t affected by nutrients washed from any island. At 5 km deep, the water at this site–which scientists have dubbed station ALOHA–has ocean layers that mix just as much as they do in more remote waters, says David M. Karl, a biogeochemist at the University of Hawaii in Honolulu.

The scientists’ measurements indicate that, averaged over a year, surface waters at the site soak up carbon dioxide from the atmosphere. However, the rate of absorption of that greenhouse gas has been slowing in recent years. In 2001, the ocean at station ALOHA absorbed only about 15 percent of the carbon dioxide that it did in 1989, says Karl.

Other changes in the water during that same period–together with simple thermodynamics–help explain why. From 1989 to 2001, the ocean-surface salinity at station ALOHA went up about 1 percent. That’s because in recent years there’s been less rainfall and more evaporation in the area, both of which concentrate salt in the surface water. When the concentration of dissolved substances goes up, it becomes more difficult for a liquid to absorb a gas, says Karl.

Increased salinity accounts for about 40 percent of the decrease in carbon dioxide absorption over the 13-year period, says Karl. He and his colleagues haven’t identified the cause of the rest of the absorption slowdown, but some candidates are changes in biological productivity and fluctuations in ocean-surface mixing. The dip in carbon sequestration doesn’t seem to be related to sea-surface temperature, however, because annual averages at the site haven’t changed over the period. The researchers report their findings in the Aug. 14 Nature.

Station ALOHA is located in the North Pacific subtropical gyre, a swirl nearly the width of the Pacific that typically has little biological activity near the sea’s surface. Such gyres account for 40 percent of Earth’s ocean area, says Karl.

The team’s study is the first to look at the effect that a change in precipitation over an ocean would have on the rate at which the water absorbs carbon dioxide, says Rik Wanninkhof, an oceanographer at the Atlantic Oceanographic and Meteorological Laboratory in Miami. The salinity changes that Karl and his colleagues have measured aren’t huge, he notes, but their effect “is much larger than I’d have thought.”


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