The team envisages 100 vast nets full of quick-growing seaweed, each measuring six miles by six miles, floating off the northeast coast of Japan.

The seaweed in each net, growing to a weight of 270,000 tonnes a year, will absorb prodigious quantities of greenhouse gases and convert them to oxygen before being harvested 12 months later as a rich source of biomass energy.

If a pilot version of the project indicates that the idea is viable, and sufficient funding can be found, the concept of fighting global warming through giant seaweed farms across the world’s oceans could be included in the upcoming revision to the Kyoto Protocol.

The Japanese Ministry of Economy, Trade and Industry mentioned carbon dioxide absorption by seaweed in its Technology Roadmap for 2005. The project is led by Masahiro Notoya, a world expert on seaweed from the Tokyo University of Marine Science and Technology. Dr Notoya believes that Sostera marina and sargassum, herded to the right parts of the ocean, will grow up to 40ft every year, absorbing about 36 tonnes of carbon dioxide in the process. Those seaweeds are also popular fare for a variety of fish whose stocks have dwindled.

Working with Dr Notoya are scientists at the Mitsubishi Research Institute and Tokyo University. Mitsubishi Heavy Industries, Mitsubishi Electronics, Toshiba and NEC are among a large group of companies involved. The Japanese Government has provided a small grant and is expected to give more when a pilot version of the giant seaweed farm opens next year.

Another obstacle is finding enough empty sea to accommodate the vast farms. “It’s the main problem we face — particularly because of the two strong currents that run off the Japanese archipelago, but also because there are only certain parts of the ocean where seaweed grows well,” Dr Notoya said.

The nets will be equipped with a technology allowing them be tracked by global positioning satellites, so that they could be dragged back into position should they stray into a shipping lane.

Dr Notoya’s team will present its proposal this month to the Japan Research Industries Association and will attempt to convince industry leaders that the idea is worth the 570 billion yen (£2.8 billion) needed to implement it on the grand scale necessary.

“It’s actually thanks to seaweed that we’re here at all,” Dr Notoya said. “When the world was young, it was the little blue-green algae and other seaweeds that, over the years, converted so much of the carbon dioxide in the air into oxygen and eventually pushed it up to the levels it is at today. Now that the balance is being thrown off, it’s time for the seaweed to come and help again.”

The most critical part of the plan is to then convert the seaweed into useful energy — a process that draws on technology produced by the Mitsubishi Research Institute. When blasted with superheated steam, seaweed discharges hydrogen and carbon monoxide gases that can be used to create a biofuel, which, in turn, discharges no extra carbon dioxide when burnt.

“I think the project is entirely viable on the technology side,” Katori Yoshishige, of the Mitsubishi Research Institute, said. “We need to improve methods to generate methanol from seaweed, for example, but I don’t envisage too much difficulty. The biggest challenges at the moment are financial.”

This is not the first time that seaweed has been identified as mankind’s potential saviour. In the 1970s the United States-led Giant Kelp Project failed because it was unclear what to do with all that seaweed once it was hauled back to shore. Now that seaweed can be converted to energy without expensive fermentation, the idea is back on course.

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