Scientists are planning this week to start a highly controversial experiment in changing the composition of the oceans, in apparent contravention of international law.

The experiment – to be conducted in the Southern Ocean – aims to create a bloom of plankton so big that it will be visible from outer space. But, at the last minute, the scheme has sailed into an international storm as environmentalists have called for it to be abandoned. The researchers – mainly from Germany and India, but including two Britons – plan to add some 20 tons of iron sulphate to a 186-square-mile patch of ocean about half way between Cape Horn and the Cape of Good Hope, to demonstrate a way both of combating global warming and of saving the whale.

As the waters are short of iron, this is expected to lead to an explosive growth of plankton, which will take up carbon dioxide from the air. The scientists hope that, when the plankton die and their bodies sink deep into the ocean, they will take the carbon with them, keeping it out of the atmosphere for centuries. Applied on a large enough scale, they believe this could help stave off climate change, while increasing food for whales. Commercial firms have already announced plans to make money from such schemes.

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But other scientists are deeply concerned that the practice could have devastating unintended effects on the oceans, including killing off large areas of sea, and releasing methane and nitrous oxide, which are even more potent causes of global warming. They also fear that the plankton could absorb sunlight, heating up surface waters and hastening climate change.

Last May the UN's Convention on Biological Diversity banned the practice, allowing exceptions only for "small-scale scientific research studies within coastal waters". Nevertheless, the expedition – jointly organised by the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven and the National Institute of Oceanography in Goa, India – set off this month.

Alarmed environmentalists, led by the Canada-based ETC Group, urged Germany's Environment Minister, Sigmar Gabriel, to stop the experiment. The German government suspended it while legal and environmental reviews were carried out, and the scientists expect to hear the result early this week.

Dr Richard Lampitt of the University of Southampton's National Oceanography Centre, which has two scientists on board, says: "We desperately need to make this sort of experiment if we are going to make rational decisions in the future."

The Alfred Wegener Institute accuses objectors of "indulging in disruptive activities merely to draw attention to themselves".