Last year, the London Convention said that "knowledge about the effectiveness and potential environmental impacts of ocean iron fertilization currently was insufficient to justify large-scale operations."

Those doubts were "still valid", the Convention's Coenen said.

Firms such as Australia's Ocean Nourishment, Atmocean in New Mexico and Climos are working on varying sea-based projects. Another start-up, Planktos, indefinitely suspended operations in February after failing to raise cash.

Some like Climos hope that sucking carbon into the ocean, if it works, could qualify for credits as carbon trading.

"It is possible to design experiments to avoid harm to the oceans," said Leinin. Climos wants to test iron fertilization in the southern ocean, at the earliest in January 2010 in a test that could $15-20 million, she said. If it works, Leinin said it could be one of the cheapest ways to combat global warming.

Lesser risk

Among objections are that carbon makes water more acidic and could undermine the ability of shellfish, crabs or lobsters to build shells. That in turn could disrupt the marine food chain.

Backers of geo-engineering say the risks are slight compared to far bigger disruptions from climate change, stoked by human emissions of greenhouse gases, which could lead to heatwaves, floods, droughts, more disease or rising seas.

"We are already bludgeoning nature," said Victor Smetacek, a professor at the Alfred Wegener Institute in Germany, who is planning an iron sulphate fertilization experiment off Antarctica in early 2009.

( China Daily via Agencies October 28, 2008)