"The deep waters are always more acidic than surface waters," Feely explained.
In addition to carbon dioxide absorbed from the atmosphere, deep waters contain carbon dioxide that is a by-product of the breathing of marine organisms and the decay of organic matter.
"What's more, there is a 50-year lag between the time when ocean surface waters [are] exposed to the atmosphere, its sinking, and ultimate upwelling on the continental shelf," noted co-author Burke Hales from Oregon State University.
"This means that even if we were to stop instantaneously the current rate of rise of CO2 in the atmosphere ... the corrosivity of these upwelled waters would increase for the next 50 years," he said in a telephone briefing with reporters.
These corrosive waters already reach the surface at one point in northern California. Elsewhere, they reach depths between 130 and 390 feet (40 and 120 meters).
Feely, Hales, and colleagues report their findings in tomorrow's issue of the journal Science.
"Amazing and Frightening"
Ken Caldeira is a geoscientist who studies ocean acidification at the Carnegie Institute of Washington's office in Stanford, California. He said the new finding is dramatic.
"The idea that [calcium carbonate] shells might already be dissolving in coastal waters is pretty amazing, and it's frightening," he said.
Global models, he noted, had predicted corrosion in coastal waters at the Poles around the middle of this century and the California coast sometime next century.
However, the impact of these acidic waters remains uncertain, Caldeira added.
Experiments have shown that ocean acidification makes coral reefs vulnerable to erosion, potentially impacting thousands of species that depend on reef habitat.
Shellfish such as crabs, mussels, and oysters also weaken with increasing carbon dioxide levels, and experiments suggest squid and sea urchins are susceptible to acidic oceans.
But scientists have not conducted any experiments on how entire coastal ecosystems respond to increasing ocean acidification, Caldeira said. Most research has been in the lab and focused on single organisms.
In addition, the chemistry of coastal waters is generally more variable than the open ocean, suggesting coastal environments may be more resilient to the changing conditions.
"It could be either something serious that could completely restructure the coastal ecosystem, or it could be something that within a few years these things basically adapt to," he said.
He added, however, that no experiments have shown an organism becoming more efficient at making shells in response to ocean acidification.
"It looks like the biological capacity for adaptation there in calcification is pretty low."
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