Pacific's CO2 levels: cause for concern?

Pacific's CO2 levels: Cause for concern?

One of the greatest concerns associated with the world's burning of some 5 billion tons of fossil fuels annually is the large amount of carbon dioxide (CO2) it generates. Right now, only about half of that combustion-generated CO2 stays in the atmosphere. Much of the rest, it is generally believed, is taken up by the oceans. But new research indicates that the ocean might not remain as robust a sink for CO2 as it has been. That would leave even more of the gas to accumulate in the atmosphere, potentially triggering a more rapid and devastating global warming from the so-called "greenhouse effect.'

Researchers from the University of South Florida in St. Petersburg have been studying the calcium carbonate shells of pteropods (planktonic mollusks) in the north Pacific (SN: 12/15/84, p. 376). Not only have their shells incorporated some of the carbon that entered the water as CO2, but the creatures are also a mechanism by which a portion of that carbon is eventually removed from upper ocean waters; as the creatures die, their shells fall toward the ocean bottom, carrying the carbon along. If a shell falls a long way before dissolving, it carries the carbon far from the surface, potentially making it easier for more CO2 to enter, helping to reduce atmospheric CO2 levels. This is usually the case, since high acid levels, which help dissolve the pteropod shells, are normally present only at great depths.

But Robert Byrne and his colleagues have identified regions in the north Pacific where the pteropod shells begin to dissolve at depths of only 170 meters-- well within the top 10 percent of the ocean depth--and in far shallower water than generally expected. Since CO2 is one source of water acidity, Byrne notes, the regions of shallow acidity they've identified may be an indication of higher CO2 levels beginning to accumulate in the surface waters.

If true, this suggests a couple of causes for concern, he says. First, the more acidic water is, the less CO2 it will absorb. So a trend toward more acidic surface waters could spell a long-term decline in the amount of CO2 the ocean will accept from the atmosphere. Moreover, if shell dissolving begins too high in the water column, there is a risk that the shells will be less effective at removing carbon from surface waters. That could exacerbate the acidity problem and the potential inability of the ocean surface to accept as much atmospheric CO2.