Scientists have long suspected that the oceans serve as a major sink, or storehouse, of carbon dioxide. Past estimates of precisely how carbon dioxide was accumulating in the world's oceans were based on computer models or other indirect methods.
In the new study, however, researchers collected direct samples on dissolved carbon dioxide levels in oceans around the world throughout the 1990s. Data were collected at some 9,600 sites around the world on 95 separate research voyages.
The effort was led by two international collaborations: the World Ocean Circulation Experiment (WOCE) and the Joint Global Ocean Flux Study (JGOFS).
Using the data, Sabine and researchers from the United States, Europe, Australia, South Korea, Japan, and other nations have now completed the most comprehensive survey of ocean carbon chemistry.
Their results suggest that the oceans have taken up 48 percent of all carbon dioxide emitted from fossil fuel burning and cement manufacture (a major source of the gas) between 1800 and 1994.
The finding suggests an answer to a question that had previously puzzled scientists: Where did the missing half of the carbon dioxide scientists estimated humans released into the atmosphere go?
Taro Takahashi, a geochemist with Columbia University's Lamont-Doherty Earth Observatory in New York, says the answer is important for two reasons: It will help us understand Earth's natural carbon cycle and formulate a sound strategy for managing carbon dioxide emissions.
In a commentary that will also appear in tomorrow's edition of Science, Takahashi, who was not involved in the study, wrote: "This extensive data set [has] provided a solid estimate for the amount of carbon dioxide taken up by the global oceans."
Sabine, lead author of the first study, said that, next to the atmosphere, the world's oceans have served as the only other major warehouse for human-made carbon dioxide over the last two centuries.
He noted that most previous studies have looked at how much carbon dioxide land plants have taken up in the last few decades and found that plants store carbon dioxide.
But "over longer time scales however, we get a very different story," Sabine said. It's possible that plants are only taking up the gas now, as forests cleared a century ago regrow, but "we may not be able to count on this sink continuing into the future," he said.
According to the study by Sabine and his colleagues, the amount of carbon dioxide the oceans have currently taken up is about a third of what they can hold. After that, the researchers warn, the rate of global warming could accelerate.
In the second, related study, scientists found that while the oceans are helping to mitigate global warming, the dissolved carbon dioxide is already having a detrimental effect on marine life.
Richard Feely, a marine chemist with the National Oceanic and Atmospheric Administration's Pacific Marine Environmental Laboratory in Seattle, is the study's lead author.
Feely said, "Because carbon dioxide is an acid gas, the surface ocean pH is dropping" (pH is a measure of acidity in solutions).
If predictions made by Feely's team are right, the surface of oceanswhere most marine life is foundcould soon become more acidic than they have been in five million years.
This increase in acidity makes it difficult for shell-forming animals and some algae to amass carbonate ions from the seawater to form their calcium carbonate shells.
Corals, some types of mollusk, and tiny planktonic organisms called foraminifers and coccolithophorids could all be affected. Many of these species form key links in the marine food chain.
Past studies have shown that at atmospheric carbon dioxide concentrations of 700 to 800 parts per millionwhich some scientists say could be reached by the end of this centurythe rate at which these organisms can form shells could be reduced by as much as a 25 to 45 percent.
"Increasing [carbon dioxide] may reduce the fitness of [these shell-forming] organisms," Sabine said. "And this could alter food web structures in ways that we cannot predict at this time."
Jean Jaubert, a coral expert and director of the Monaco Oceanographic Museum, said it's possible that prior to the industrial age, the world's oceans might have absorbed so much atmospheric carbon dioxide that the process played a role in limiting terrestrial plant growth.
The process of calcification, which occurs when marine animals form shells, actually returns carbon dioxide to the atmosphere, Jaubert noted.
The two new studies support a theory that calcification might have worked to regulate atmospheric carbon dioxide levelsand the temperature of the planetin the past.
"In the absence of human activities, variations of calcification may have played a major role for the maintenance of optimum temperature conditions for the [continuing] development of life on Earth," Jaubert said.
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