An international team of scientists has completed the first comprehensive study of the ocean storage of carbon dioxide derived from human activity, called anthropogenic CO2, based on a decade-long survey of global ocean carbon distributions in the 1990s.
The findings, along with those detailed in a companion paper on the impacts of anthropogenic CO2 on the chemistry of the oceans and the potential response of marine animals and plants to changes in CO2 levels, will be published in the July 16 issue of the journal Science.
"About half of the anthropogenic CO2 taken up over the last 200 years can be found in the upper 10 percent of the ocean," said Christopher Sabine, an oceanographer at NOAA's Pacific Marine Environmental Laboratory (PMEL) in Seattle, Wash.
Sabine is the lead author of one of the papers. "The ocean has removed 48 percent of the CO2 we have released to the atmosphere from burning fossil fuels and cement manufacturing."
Over the long-term, the ocean has been the only reservoir on Earth to consistently take up anthropogenic CO2 from the atmosphere. This uptake changes seawater chemistry, and can have significant impacts on the biology of the upper oceans.
The global survey combined carbon and other ocean measurements (such as temperature, salinity, oxygen, nutrients and chlorofluorocarbon tracers) in the Atlantic, Pacific, and Indian Oceans. These oceans have taken up about 118 billion metric tons of anthropogenic CO2 between 1800 and 1994, about a third of their long-term potential.
The researchers, including scientists from the United States, South Korea, Australia, Canada, Japan, Spain, and Germany, reviewed data gathered during the 1990s as part of three major research programs: the National Science Foundation (NSF)-led World Ocean Circulation Experiment (WOCE) and Joint Global Ocean Flux Study (JGOFS); and the National Oceanic and Atmospheric Administration (NOAA)'s Ocean-Atmosphere Carbon Exchange Study (OACES).
The new global data set of ocean carbon system observations, co-sponsored in the United States by NSF, NOAA and the Department of Energy, is unprecedented, say scientists, with 10 times more observations and 10 times better accuracy than the previous global survey in the 1970s.
"This research presents the first complete synthesis of modern global ocean inorganic carbon measurements," said James Yoder, director of NSF's ocean sciences division.
"The results are among the most intriguing yet from the WOCE and JGOFS programs."
"These new measurements, when coupled with how much anthropogenic carbon dioxide is known to accumulate in the atmosphere, show that the ocean and atmosphere are the two primary 'sinks' [holding tanks] of this carbon dioxide since the beginning of the industrial revolution. The land is the 'source.'"
But that ocean "sink" may be changing, the studies conclude. "Feedbacks between the ocean and atmosphere involving air-sea exchange, ocean circulation and ocean biological processes need to be better understood," said Yoder.
Analysis of CO2 levels in ice cores have shown scientists that for the 400,000 years before the industrial revolution began in the 1800s, atmospheric CO2 concentrations remained between 200 and 280 parts per million.
Today CO2 levels are reaching 380 parts per million in the atmosphere.
"If the ocean had not removed 118 billion metric tons of anthropogenic carbon between 1800 and 1994, the CO2 level in the atmosphere would be about 55 parts per million greater than currently observed," said Sabine.
"Because CO2 is an acid gas, the surface ocean pH is dropping," said Richard Feely, a marine chemist at PMEL, and lead author of the companion paper.
If current scenarios are realized, surface ocean pH could drop lower than it has been for more than five million years, he said.
Feely and colleagues describe two major impacts of the oceanic uptake of anthropogenic CO2: They demonstrate that a substantial amount of the calcium carbonate, found in shells of marine animals living in surface waters, dissolves in the upper ocean. They then summarize the available evidence on the response of marine calcifying organisms to elevated CO2.
Feely noted that scientists have seen a reduced ability to produce protective calcium carbonate shells in many species of marine organisms at high CO2 levels, including corals and plankton, drifting plants and animals on which other marine life feeds.
"Based on our present knowledge, it appears that as seawater CO2 levels rise the skeletal growth rates of calcareous plankton will be reduced as a result of the effects of CO2 on calcification," said Victoria Fabry, a biologist at California State University at San Marcos and a paper co-author.
Recent studies have shown that calcification rates can drop by as much as 25 to 45 percent at CO2 levels equivalent to atmospheric concentrations of 700 to 800 parts per million. Those levels will be reached by the end of this century if fossil fuel consumption continues at projected levels.
The scientists note that the dissolving calcium carbonate shells also partially act to neutralize CO2, thus allowing the ocean to take up more carbon dioxide from the atmosphere.
However, the effects of decreased calcification in microscopic algae and animals could alter marine food webs and, combined with other changes in salinity, temperature and nutrients, could substantially alter the diversity and productivity of the oceans.
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