Fertilising the oceans to boost the growth of tiny plants that soak up greenhouse gases is unlikely to work as a way to slow climate change, a UN-backed study showed on February 01.
Such "geo-engineering" schemes would be hard to monitor and were likely to store away only small amounts of carbon dioxide, the main greenhouse gas, according to a report by the Intergovernmental Oceanographic Commission.
"Geo-engineering schemes involving ocean fertilisation to affect climate have a low chance of success," according to the 20-page study by the Commission, part of the UN Education, Scientific and Cultural Organisation (UNESCO).
The review, by scientists in seven countries, said 13 experiments in recent years showed faded optimism that iron dust or other nutrients could spur growth of microscopic marine plants and permanently suck carbon dioxide from the atmosphere.
Twenty years ago, scientists had optimistically reckoned that one tonne of iron could make the seas absorb 100,000 tonnes of carbon dioxide, it said. But one recent study suggested that one tonne of iron would only soak up 5,000 tonnes of the gas.
Firms including Climos, Atmocean Inc and the Ocean Nourishment Corp have been looking into ocean fertilisation technology as a way of offsetting emissions of greenhouse gases from factories, power plants and cars.
But the study said that some absorbed carbon simply returns to the atmosphere. Just one to 15 percent falls as what scientists call "marine snow" - dead plants, animals or their faeces - to be stored on the deep ocean floor.
"At best it can make a small contribution" to storing industrial emissions of carbon dioxide, Doug Wallace, lead author of the report at the Leibniz-Institut fur Meereswissenschaften, told Reuters.
In 2008, the UN's Convention on Biological Diversity imposed a moratorium on ocean fertilisation experiments outside coastal waters, awaiting wider scientific evidence.
The study said that some experiments had successfully triggered a bloom of plankton and bacteria - such one in 2002 that produced a patch visible to satellites in the north-east Pacific Ocean.
But it was impossible to say how much of that carbon stayed in the oceans. And the extra plants might help fish grow in some areas by providing more food, but might also rob nutrients from other parts of the ocean.