The oceans have long buffered the effects of climate change by absorbing a substantial portion of the greenhouse gas carbon dioxide. But this benefit has a catch: as the gas dissolves, it makes seawater more acidic. Now an international panel of marine scientists says this acidity is accelerating so fast it threatens the survival of coral reefs, shellfish and the marine food web generally.

A coral reef in the depth of Ras Mohammed protection area near Sharm el-Sheikh in Egypt, July 2005. Carbon dioxide (CO2) emissions must fall sharply to avoid inflicting acid damage to the world's marine ecosystems, more than 150 scientists warned Friday. (AFP/File/Tarik Tinazay) The panel, comprising 155 scientists from 26 countries and other international groups, is not the first to point to growing ocean acidity as an environmental threat. For example, a group of eminent scientists convened by The Nature Conservancy issued a similar assessment in August. But the new report's blunt language and international backing give its assessment unusual force. It called for "urgent action" to sharply reduce emissions of carbon dioxide.

"Severe damages are imminent," the group said Friday in a statement summing up its deliberations at a symposium in Monaco last October. The statement, called the Monaco Declaration, said increasing acidity was interfering with the growth and health of shellfish and eating away at coral reefs, processes that would eventually affect marine food webs generally.

Already, the group said, there have been detectable decreases in shellfish and shell weights, and interference with the growth of coral skeletons.

Jeremy B. C. Jackson, a coral expert at the Scripps Institution of Oceanography in San Diego who has no connection to the Monaco report, said "there is just no doubt" that the acidification of the oceans is a major problem. "Nobody really focused on it because we were all so worried about warming," he said, "but it is very clear that acid is a major threat."

Carbon dioxide, principally from the burning of fossil fuels, is the major component of greenhouse gas emissions, which have risen steadily since the beginning of the Industrial Revolution in the 18th century.

Oceans absorb about a quarter of carbon dioxide emissions, the group said, but as the gas dissolves in the oceans it produces carbonic acid.

The group says acidity of ocean surface waters has increased by 30 percent since the 17th century.

"The chemistry is so fundamental and changes so rapid and severe that impacts on organisms appear unavoidable," according to James Orr, who headed the symposium's scientific committee. Dr. Orr is a chemical oceanographer at the Marine Environmental Laboratory in Monaco, an affiliate of the International Atomic Energy Agency, a United Nations body.

According to the declaration, "ocean acidification may render most regions chemically inhospitable to coral reefs by 2050." The group said that acidification could be controlled only by limiting future atmospheric levels of the gas. Other strategies, including "fertilizing" the oceans to encourage the growth of tiny marine plants that take up carbon dioxide, may actually make the problem worse in some regions, it said.