Melting Ice May Slow Global Warming

Collapsing antarctic ice sheets, which have become potent symbols of global warming, may actually turn out to help in the battle against climate change and soaring carbon emissions.

Professor Rob Raiswell, a geologist at the University of Leeds, says that as the sheets break off the ice covering the continent, floating icebergs are produced that gouge minerals from the bedrock as they make their way to the sea. Raiswell believes that the accumulated frozen mud could breathe life into the icy waters around Antarctica, triggering a large, natural removal of carbon dioxide from the atmosphere.

And as rising temperatures cause the ice sheets to break up faster, creating more icebergs, the amount of carbon dioxide removed will also rise. Raiswell says: ' It won't solve the problem, but it might buy us some time.'

As the icebergs drift northwards, they sprinkle the minerals through the ocean. Among these minerals, Raiswell's research shows, are iron compounds that can fertilise large-scale growth of photosynthetic plankton, which take in carbon dioxide from the air as they flourish.

According to his calculations, melting Antarctic icebergs already deposit up to 120,000 tonnes of this 'bioavailable' iron into the Southern Ocean each year, enough to grow sufficient plankton to remove some 2.6 billion tonnes of carbon dioxide, equivalent to the annual carbon pollution of India and Japan. A 1 per cent increase in the number of icebergs in the Southern Ocean could remove an extra 26 million tonnes of CO2, equivalent to the annual emissions of Croatia.

Raiswell, a Leverhulme Emeritus Fellow, said: 'We see the rapid ice loss in Antarctica as one obvious sign of climate warming, but could it be the Earth's attempt to save us from global warming?' He added that the effect had not been discovered before because scientists assumed that the iron in the iceberg sediment was inert and could not be used by plankton.

In a paper published in the journal Geochemical Transactions, Raiswell and colleagues at the University of Bristol and the University of California describe how they chipped samples off four Antarctic icebergs blown ashore on Seymour island by a storm in the Weddell Sea.

They found that they contained grains of ferrihydrite and schwertmannite, two iron minerals that could boost plankton growth. 'These are the first measurements of potentially bioavailable iron on Antarctic ice-hosted sediments,' they write. 'Identifying icebergs as a significant source of bioavailable iron may shed new light on how the oceans respond to atmospheric warming.'

No rivers flow into the Southern Ocean and the only previously identified major source of iron for its anaemic waters is dust blown from South America. The team says that icebergs could deliver at least as much iron as the dust.

A key question is how much of the carbon soaked up by the growing plankton is returned to the atmosphere. 'We simply don't know the answer to that,' Raiswell said. Seeding the oceans with iron will only benefit the climate if the plankton sink to the bottom when they die, taking the carbon with them.

David Vaughan, a glaciologist with the British Antarctic Survey, said: 'It's a very interesting new line of research and one that should be looked at in more detail.'

He said the number of icebergs in the Antarctic was expected to rise by about 20 per cent by the end of the century, which could remove an extra 500 million tonnes of carbon dioxide each year, if they all seeded plankton growth.