The experiment involved "fertilising" a 300-square-kilometre (115-sqare-mile) area of ocean inside the core of an eddy -- an immense rotating column of water -- with six tonnes of dissolved iron.

As expected, this stimulated growth of tiny planktonic algae or phytoplankton, which it was hoped would take out of the atmosphere carbon dioxide, the principal greenhouse gas blamed for climate change, and absorb it.

However, the scientists from India's National Institute of Oceanography (NIO) and Germany's Alfred Wegener Institute (AWI) did not count on these phytoplankton being eaten by tiny crustacean zooplankton.

"The cooperative project Lohafex has yielded new insights on how ocean ecosystems function," an AWI statement published on Monday said.

"But it has dampened hopes on the potential of the Southern Ocean to sequester significant amounts of carbon dioxide (CO2) and thus mitigate global warming."

Earlier projects with iron fertilisation were more successful because they used algae protected by hard shells that do not thrive in the Southern Ocean, the AWI said.

The team set sail from Cape Town on January 7 and spent an "ardous" two and half months conducting the experiments, buffetted by the treacherous waves of the notorious "Roaring Forties" and twice having to escape approaching storms.

Spicy Indian curries at each meal "contributed to the good atmosphere" however in an "exciting experience laced with the spirit of adventure and haunted by uncertainty quite unlike other scientific cruises," the AWI said.

The experiment is one of several schemes collectively known as geo-engineering which have been getting a closer hearing in recent years in the absence of political progress to roll back the greenhouse gas problem.

But these projects have been heavily criticised by environmentalists for failing to tackle the human behaviour that causes global warming and for having unforeseen and potentially catastrophic consequences.

Other geo-engineering ideas include sowing sulphur particles in the stratosphere to reflect solar radiation and erecting mirrors in orbit that would deflect sunrays and thus slightly cool the planet.

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