Carbon dioxide buildup creating crisis in oceans
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Recognition has spread that carbon dioxide and other "greenhouse gases" in the atmosphere could lead to warmer global temperatures and extremes in weather.
But now scientists are warning that the oceans, which have for two centuries absorbed huge quantities of industrial-age emissions, are turning acidic. That poses a threat to coral reefs and some types of plankton.
"Drastic emission cuts at some point in the future might be an option in terms of climate change, but it could be too late for coral reefs and other marine organisms," said Richard Zeebe, of the University of Hawaii, one of the authors in a research paper in Friday's issue of Science.
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Drastic cuts are needed immediately in human carbon emissions to prevent a potential crisis in the world's oceans, warns a group of chemical oceanographers led by Richard Zeebe of the University of Hawaii.
For more than 200 years, the oceans have absorbed 40 percent of human greenhouse gas emissions, which alleviates climate change but seriously affects coral reefs and other marine organisms, they said.
In a paper in the Friday issue of the journal Science, Zeebe and three mainland colleagues point out that scientific and public concern about human carbon dioxide emissions has largely focused on climate impacts.
"It's mostly overlooked that carbon dioxide has other consequences other than changing climate," Zeebe said in an interview.
Recent studies show ocean acidity is a second major impact, he said, pointing out the tremendous amount of carbon dioxide taken up by the oceans is changing their chemistry.
"If we continue with business as usual and don't cut carbon dioxide emissions, carbonate reefs will ultimately start to dissolve." he said. "This is basic chemistry. We need to reduce emissions and put funding into research and development of alternative energy sources. If we can do this and reduce carbon dioxide in the next decade or so, prospects are better."
But the United States is lagging behind Europe, which began taking measures 10 to 15 years ago to reduce emissions, he said.
Zeebe co-authored the Science paper with James Zachos of the University of California, Santa Cruz; Ken Caldeira of the Carnegie Institution, Stanford University; and Toby Tyrrell, Southampton University, England.
They said in a news release that the amount of carbon dioxide absorbed by the oceans over the past two centuries equals roughly 500 billion metric tons, "equivalent in weight to about 28 inches of water across the whole state of Texas."
Although this has slowed the rise in atmospheric carbon dioxide and global warming, it has caused the ocean's average surface pH -- a measure of acidity -- to drop by 0.1 unit from pre-industrial levels, they said.
This is a 25 percent increase in hydrogen-ion concentration, they said, pointing out that "small changes in the pH value can make a big difference" because it is measured on a logarithmic scale similar to the Richter seismic scale.
"For example, a drop by one pH unit means a tenfold increase in acidity."
The ocean's pH could drop by as much as 0.35 units -- more than twice as many hydrogen ions -- by the mid-21st century, depending on the rate of future emissions, they said.
Once standards for future ocean pH changes have been implemented, the results will help to establish carbon dioxide emissions targets, they said.
"Drastic emission cuts at some point in the future might be an option in terms of climate change, but it could be too late for coral reefs and other marine organisms," Zeebe said.
Changes as small as 0.2 to 0.3 units of surface pH can harm corals and some plankton and other marine organisms that build their skeletons from pH-sensitive carbonate minerals, the researchers said.
The Hawaiian Islands Coral Reef Reserve and Australia's Great Barrier Reef are among ocean areas in danger of exceeding those levels of pH change by midcentury, they said.
Reduced calcification will hurt shellfish such as oysters and mussels and affect worldwide commercial aquaculture production, the scientists said.
"To prevent the pH of surface waters from declining more than 0.2 units, the limit set by the U.S. Environmental Protection Agency in 1976, carbon dioxide emissions would have to be reduced immediately," they said.
It is more difficult to predict the ocean's biological response to higher carbon dioxide levels than the chemical response, Zeebe said.
"We've just started to look at the effects of higher acidity in the ocean on different organisms," he said. "But the majority of studies at the moment clearly show most calcified organisms are struggling under these conditions."
He said it is difficult to predict their long-term reaction, let alone responses of entire marine ecosystems.