An international team of scientists is setting off for the Antarctic to test whether iron could help to slow down climate change.

The team includes nine Britons, from Plymouth Marine Laboratory and the university of East Anglia.

During their four-week cruise, they will use several tonnes of iron sulphate to fertilise the southern ocean.

This will help them to understand the role of iron in enhancing marine productivity.

They expect the addition of the iron sulphate powder to stimulate the growth of substantially larger quantities of plankton in the water.

Checking for unforeseen effects

They will take measurements to see if this leads to fluctuations in the levels of carbon dioxide (CO2), the main greenhouse gas produced by human activities.

And they will also check to see whether the plankton produces other greenhouse gases.

The scientists will use an inert tracer, sulphur hexafluoride, to add to the water to show where the iron sulphate goes.

A harsh environment for study

The theory that iron can affect marine productivity was first put forward in the 1920s.

If it is right, it could have several significant consequences.

Using iron to stimulate plankton growth would mean an increase in the creatures at the bottom of the food chain.

That might mean increased levels of life further up the chain, including food fish.

And there is particular interest in the role of plankton in climate change.

Professor Peter Liss, of the school of environmental sciences at the University of East Anglia, says two experiments conducted several years ago provide useful pointers.

Startling results

Both took place in the equatorial Pacific, several thousand kilometres off the coast of south America.

The first provided modestly encouraging results. But the second was quite dramatic.

"We used iron sulphate to fertilise squares of ocean about 10km by 10", says Professor Liss.

"There was a huge plankton bloom in that second experiment, vastly more diatoms in the water.

"And chlorophyll levels were 30 times higher than normal - the blue Pacific turned green.

"The level of CO2 dissolved in the water dropped by 90 parts per million, because the plankton had used it up.

"And they produced about four times more dimethyl sulphide (DMS) than normal."

It would need vast amounts of iron to have much effect

DMS helps to cool the atmosphere, in addition to the reduction in CO2 levels.

The experiment seems to bear out the evidence from Antarctic ice cores.

These show that during the last ice age there was less CO2 in the atmosphere, but more iron.

"The point of repeating the Pacific experiment in the southern ocean, which is a much harder place to work in, is simple", says Professor Liss.

"Other nutrients, like nitrate and phosphate, are present. The Antarctic is the biggest ocean area where we think a lack of iron may be limiting productivity.

"And it is a big CO2 sink - it soaks up the carbon - unlike the Pacific, which is a source of CO2."

The scientists hope to have some preliminary results before they end their cruise.