Scientists say sprinkling the ocean surface with trace amounts of iron or releasing other nutrients over many thousands of square kilometers promotes blooms of tiny phytoplankton, which soak up carbon dioxide in the marine plants.

When the phytoplankton die, they drift to the ocean depths, along with the carbon locked inside their cells where it is potentially stored for decades or centuries in sediments on the ocean floor.

Firms eyeing this natural carbon sink hope to commercialize it to yield carbon credits to help industries offset their emissions.

The problem is no one knows exactly how much carbon can be captured and stored in this way, for how long, or the risks to ocean ecosystems from such large-scale geo-engineering.

Some scientists fear such schemes could change species composition in the oceans, increase acidity or cause oxygen depletion in some areas, even promote the release of another powerful greenhouse gas, nitrous oxide.

“It is very important to recognize that if deleterious effects increase with scale and duration of fertilization, detection of these cumulative effects may not be possible until the damage is already done,” said John Cullen, professor of oceanography at Dalhousie University at Nova Scotia in Canada.