Russian and South Korean scientists made a disturbing discovery recently in the Sea of Japan. They found that the amount of carbon dioxide, the main global warming gas, being absorbed in the water dropped by half between 1992 and 2007.

French scientists last month concluded that there had been an even sharper drop in the capacity of the Southern Ocean, surrounding Antarctica, to soak up excess carbon dioxide (CO2) being released into the atmosphere, largely as a result of burning fossil fuels and deforestation.

In its latest report in 2007, the U.N. Intergovernmental Panel on Climate Change estimated that human activity produced 32 billion tons of CO2 worldwide each year but that only about 15 billion tons actually stayed in the atmosphere and affected climate change. The oceans, forests, vegetation and soils store the rest.

In this natural system of “carbon sinks,” the oceans — which cover about three quarters of the Earth — play a key role. They are thought to absorb some 8 billion tons of CO2, one quarter of the annual total.

The Southern Ocean is rated as the biggest ocean sink, but the scientists affiliated with France’s National Center for Scientific Research reckon that it may now take in 10 times less CO2 than previously estimated, around 50 million tons annually instead of 500 million tons.

The weakening of this carbon absorbing cycle would leave much more CO2 in the atmosphere, increasing the pressure on governments to adopt stricter controls on emissions to prevent rises in temperature.

The Russian and Korean scientists also reported that not as much of the CO2 being absorbed in the Sea of Japan was being held at depths of over 300 meters, where it was more likely to stay permanently. The French scientists studying the Southern Ocean concluded that climate change was affecting atmospheric pressure in the region, causing higher wind speeds in the often stormy ocean. This caused increased mixing of deep waters with surface waters.

Water near the surface contains less CO2 than deep water. The gas is absorbed by vast numbers of minute algae known as phytoplankton. They not only provide the basic food sustaining oceanic life but also help to regulate concentrations of CO2 in the atmosphere. As the organisms die, they sink and get broken down by bacteria, thus enriching the deep water with CO2 and trapping the greenhouse gas.

Some scientists believe it may be possible to absorb in the sea much larger amounts of excess CO2 from the atmosphere by seeding the oceans with iron or other nutrients that make phytoplankton multiply. A study published in January by researchers at the University of Southampton in Britain looked at a natural source of iron released into the sea near the Crozet Islands in the Southern Ocean, 2,250 km southeast of South Africa. It showed that iron — which is added by volcanic rocks to the north, but not to the south, of the island — tripled the growth of phytoplankton and also the amount that sank into the depths of the sea.

Meanwhile, last week a team of scientists were in the Southern Ocean conducting an experiment to fertilize about 300 square km of sea with up to 20 tons of iron sulfate, then observe the development and impact of the phytoplankton bloom on the environment and the progress of the carbon sinking to the deep ocean as the algae died.

The experiment was the most comprehensive of six carried out since 2000 by Germany’s Alfred Wegener Institute for Polar and Marine Research and India’s National Institute of Oceanography. But scientists involved say that based on current knowledge, they oppose large-scale iron fertilization to regulate the climate. Fertilizing oceans with iron is advocated by those who believe that geo-engineering is a promising way of preventing extensive climate change. Some say it could remove as much as 1 billion tons of CO2 from the air every year if applied widely.

Private companies in the United States and Australia have come up with schemes to seed the oceans with iron and then sell carbon credits to energy-intensive firms that need to offset their emissions by buying the credits. They estimate that ocean iron fertilization could be worth $100 billion in the carbon-trading market.

However, no one knows exactly how much CO2 can be captured and stored in this way, for how long, or the risks to ocean ecosystems. Until these questions can be answered with reasonable certainty, it would be irresponsible to allow geo-engineering ventures to proceed.

Michael Richardson is a visiting senior research fellow at the Institute of Southeast Asian Studies in Singapore.

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