WASHINGTON – The oceans have soaked up nearly half of the carbon dioxide released by industrial activity during the past two centuries, researchers say, and changes in ocean chemistry could pose a long-term risk to corals and shell-forming marine life.

The findings suggest that burning of fossil fuels such as coal and oil could have ecological consequences by the end of the century that go beyond the current concern with carbon dioxide as a greenhouse gas that can contribute to global warming.

The research is reported in a pair of papers to be published Friday in the journal Science. Oceanographer Christopher Sabine of the National Oceanic and Atmospheric Administration and his colleagues found that oceans absorbed 48 percent of the carbon dioxide emissions from fossil fuel burning and cement manufacturing between 1800 and 1994, or about 118 billion metric tons of carbon.

At a news briefing Thursday, Sabine said the oceans still have the capacity to take up carbon dioxide “for thousands of years.”

But the impact on marine life could be felt much quicker.

A companion study, led by marine chemist Richard Feely, also of NOAA’s Pacific Marine Environmental Laboratory in Seattle, reports that the dissolution of acidic carbon dioxide in ocean surface waters can decrease their alkalinity.

If, as some computer models suggest, the concentration of carbon dioxide in the atmosphere increases to as much as 800 parts per million by century’s end – up from about 380 parts per million now – the authors say corresponding changes in seawater chemistry “can potentially have significant impacts on the biological systems in the oceans in ways we are only beginning to understand.”

Of primary concern are the many mollusks, corals and single-celled creatures that use ingredients in seawater to build their shells and other hard parts. They pull carbonate ions out of the water and make a hard material called calcium carbonate or limestone. As the concentration of carbon dioxide increases in seawater, the carbonate ion concentrations decrease. At depths where the ion levels fall too low, the calcium carbonate shells will likely start to dissolve and the creatures die.

Recent laboratory studies suggest that calcification rates in marine organisms can drop as much as 25 to 45 percent at carbon dioxide levels equivalent to atmospheric concentrations of 700 to 800 parts per million. The shell of one small snail species started to dissolve after being exposed to 1,000 parts per million for just 48 hours in one lab study, said Victoria Fabry, a biologist at California State University, San Marcos, and a member of Feely’s team.

Feely and his colleagues note that as the limestone shells of dead animals settle through the water and are dissolved over a wider range of depths, they will partially neutralize the effects of the industrial carbon dioxide being absorbed.

Still, if present trends in carbon dioxide emission continue for several hundred years, they say, the shell dissolution will occur over wider areas. The trend likely would start with colder waters in the Northern Hemisphere and proceed toward the equator, they say.

Sabine’s team based its uptake estimates on an analysis of carbon dioxide concentrations and other measurements taken by three global oceanographic programs during the 1990s. The information was drawn from more than 72,000 water samples taken during 95 research cruises.

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