The world is safe from crackpot scientists - for now.

A United Nations moratorium on all geo-engineering projects has been hailed as a victory by environmentalists who have been fighting to stop scientific experiments such as "fertilising" the sea and creating artificial volcanoes to block sunlight.

The 193-member UN Convention on Biological Diversity ended its 10th biennial meeting yesterday in Japan with a temporary ban on all geo-engineering projects pending proper risk assessments.

South Africa was caught up in a geo-engineering row last year when a German-flagged ship set sail from Cape Town loaded with iron sulphate to "fertilise" a large stretch of southern ocean.

The 20-ton chemical cargo - normally used to treat lawns and sewage - was intended to provoke a massive algal bloom big enough to be seen from outer space.

Scientists were hoping the algae would provide a quick fix to climate change by absorbing carbon into the sea, rather than letting it escape as gas into the Earth's atmosphere.

An international outcry led to the cancellation of the mission, which contravened a UN moratorium on sea-fertilisation experiments.

Now the UN has extended the moratorium to all geo-engineering experiments, including any in the Earth's atmosphere. Greenpeace on Friday welcomed the moratorium.