The world’s oceans absorb a third of CO2 emissions produced by humans, according to an international study led by federal technology institute ETH Zurich. Determining what share of man-made carbon dioxide the oceans absorb has long been a priority for climate researchers.
The research, whose findings are published in the latest edition of Scienceexternal link, covered the period 1994-2007.
Over this period, the world’s oceans absorbed 34 billion tonnes of CO2 from the atmosphere, ETH Zurich said in a press releaseexternal link. That represents about 31% of all the CO2 produced by human activities.
This is important, because if oceans and other ecosystems did not absorb CO2, global warming would be even more rapid. Oceans absorb CO2 in two stages, the press release explains. First CO2 in the atmosphere dissolves in surface water. It is then carried by currents into the depths of the sea, where it accumulates.
Researchers from seven countries measured concentrations of CO2 gathered from oceans all over the world. A statistical model was developed to calculate the proportion generated by human activities.
ETH Zurich environmental physics professor Nicolas Gruber, who led the research, was already involved at the turn of the millennium in a similar study on the absorption of man-made CO2 from the beginning of industrialization around 1800 up to 1994. Comparing these two surveys, the results show that the proportional volume of CO2 stored by the oceans has remained stable in the 200 years since the industrial revolution. But it has increased in absolute terms as CO2 levels in the atmosphere have risen, leading to fears that the oceans might one day become saturated and no longer able to serve as a "sink" for CO2.
This has not yet happened, but researchers found variations in different oceans across the world. The North Atlantic stored about 20% less CO2 than expected, which was compensated by a significantly higher uptake in the South Atlantic.
The oceans’ capacity to store CO2 is a valuable service for mankind, writes ETH Zurich, but it also has a price. The CO2 dissolved in the sea makes the water more acidic. “Our data shows that acidification sometimes reaches over 3,000 metres deep in the world’s oceans,” says Gruber.
This acidity attacks corals and shellfish and changes the chemistry of the sea, which can also make it harder for fish to breathe.