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The world's oceans have absorbed nearly half of the carbon dioxide that humans have released into the atmosphere by burning fossil fuels and manufacturing cement, according to research published in Science this week. As carbon dioxide contributes to global warming, this may sound like good news, but the flipside is that the gas is acidifying the seas and posing a threat to marine life.

By absorbing large quantities of carbon from the atmosphere, the oceans provide a considerable service to humanity. A large part of this is done by microscopic plants, called phytoplankton, which draw carbon dioxide out of the atmosphere much like terrestrial plants do. Tiny creatures also use atmospheric carbon to build their shells. When these organisms die, they sink down to the seabed, and take the carbon with them.

To estimate how much carbon the ocean stores, two international ocean research programmes, the World Ocean Circulation Experiment and the Joint Global Ocean Flux Study, spent ten years and some 100 research trips collecting data from nearly 10,000 different sampling points across the globe's oceans.

The findings, say the researchers, constitute the most accurate and comprehensive description of carbon distribution in the oceans.

They show that since humans began contributing large amounts of carbon dioxide to the atmosphere during the 19th century industrial revolution, the oceans have absorbed 118 billion metric tonnes of carbon from human sources.

Christopher Sabine, principal investigator and a scientist from the US National Oceanic and Atmospheric Administration (NOAA), says this corresponds to nearly half the carbon dioxide humans have released to the atmosphere from burning fossil fuels and cement manufacturing.

In the long-term, the authors estimate that about 90 per cent of human carbon emission will end up in the oceans. Because of the slow mixing of the waters, however, the oceans are currently only storing about one third of their capacity. In the coming decades, the amount of carbon the oceans are able to draw out of the atmosphere may in fact decrease as saturated areas slowly combine with less saturated areas.

A related study led by Richard Feeley, also from NOAA, has shown that this extra carbon in the seas is acidifying the water and having adverse affects on the marine life. When carbon dioxide dissolves in ocean water, it forms a compound that can dissolve the shells and skeletons of sea creatures, including planktons and corals.

Link to full paper by Sabine et al in Science

Link to full paper by Feeley et al in Science

Reference: Science 305, 362 (2004) and Science 305, 367 (2004)