The world’s oceans have absorbed nearly half of the carbon dioxide emitted by humans during the last 200 years, creating potential long-term challenges for coral and free-swimming algae, say two new studies.
The international research, which included input from Australia’s CSIRO Marine Research and the Antarctic Climate and Ecosystems Co-operative Research Centre in Tasmania, was published in today’s issue of the journal Science.
The scientists found that oceans have taken in about 118 billion metric tonnes of carbon dioxide from human activities between 1800 and 1994, accounting for nearly a third of their long-term carrying capacity.
The researchers said this could pose a long-term risk for marine organisms, such as coral, which have greater difficulty in forming their outer shells as carbon dioxide levels increase.
“There is a price to pay in this process, and that is with living organisms,” said Associate Professor Richard Feely, a marine chemist with the National Oceanic and Atmospheric Administration (NOAA), and lead author of one of the studies.
Oceans, which cover about 75% of the Earth’s surface, have seen the amount of carbon dioxide they absorb fall to 30% as trees and plants soak up more of the gas before it reaches the water. A further 20% is taken in by foliage, with the remaining 50% staying in the atmosphere.
The 15-year study looked at nearly 72,000 samples taken in the Atlantic, Pacific and Indian oceans.
Analysis of carbon dioxide since the industrial age has shown that concentration levels in the atmosphere have increased to about 380 parts per million (ppm) from 280 ppm two centuries ago. Without ocean absorption, carbon dioxide levels in the atmosphere would be about 55 ppm higher.
The researchers found that even though the oceans continue to absorb more carbon dioxide, they are far from being saturated. Currents stir the ocean very slowly by pulling deep ocean water to the surface, where it is can absorb more carbon dioxide.
“The oceans have a capacity to continue to take in carbon dioxide for thousands of years with the slow mixing time,” said Professor Christopher Sabine, NOAA oceanographer and lead author of the other study.
The greatest threat to increasing levels of carbon dioxide is to species that live in the upper 10% of the ocean.
As ocean surfaces capture and store carbon, the slow circulation of water keeps the gas more highly concentrated where these creatures live.
The change in ocean chemistry reduces the level of carbonate ions needed by coral and other organisms to generate their shells. In areas where the ion level has fallen too low, calcium carbonate shells can begin to dissolve.
Researchers said that while the long-term impact on these creatures and other species that depend on them for food is uncertain, they will closely monitor how carbon dioxide absorption is affecting the food chain.
“We might see the structure of the food web change … and see shifts in species competition” in the ocean ecosystem, said Associate Professor Victoria Fabry, a biologist at California State University, San Marcos, who worked on the research.
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16 July 2010, Reuters