Controversial proposals to slow global warming by sowing the oceans with iron have been dealt a blow by one of the most complex oceanography research projects ever undertaken, writes Steve Farrar.
Preliminary results from the Southern Ocean Iron Fertilization Experiment - dubbed SOFeX - revealed a more muted effect than many had expected.
The "iron hypothesis" was proposed by the late American oceanographer John Martin as a possible trigger for the onset of ice ages.
He suggested that a surge in the amount of iron-rich dust going into the oceans would cause great blooms of phytoplankton to grow.
These single-celled organisms need two key nutrients - iron and nitrate. When both are present they can photosynthesise, using up carbon dioxide in the water, reducing the levels of the greenhouse gas in the atmosphere and ultimately cooling the planet.
Some people suggested this process could be used to artificially counter the build-up of carbon dioxide and hence reduce the impact of global warming.
The SOFeX collaboration involved 76 scientists from 17 institutions backed by the National Science Foundation and the US department of energy. Over six weeks at the start of the year, they used three ships to carry out two iron-enrichment experiments in the Southern Ocean around Antarctica, where the waters have high levels of nitrate but little iron.
A team of scientists led by Ken Johnson, chairman of science at the Monterey Bay Aquarium Research Institute in the US, spread 1.3 tonnes of iron sulphate and an inert chemical tracer across two swaths of ocean. A second team led by Kenneth Coale, director of the Moss Landing Marine Laboratory, then studied the response of the phytoplankton.
Finally, a third team led by Ken Buesseler, a senior scientist at the Woods Hole Oceanographic Institution, assessed the amount of carbon that had ultimately been removed from the iron-fertilised patches.
The blooms of phytoplankton grew within weeks to cover two areas thousands of square kilometres across.
But Dr Buesseler found that the amount of carbon sinking below 100m was low - just 10 per cent of the total. Most of the carbon was retained within the food chain above this depth, leaving atmospheric levels virtually unaltered.
He also found evidence that the communities of microscopic creatures living in the ocean below the phytoplankton bloom changed, an unexpected demonstration of iron fertilisation's ecological impact.
The findings were announced at the American Geophysical Union autumn meeting in San Francisco last week.