In the biggest experiment of its kind, an international team "fertilised" a swathe of the Southern Ocean with iron. This boosted the growth of algal plankton which suck up carbon dioxide.

The researchers argue that variations in the amount of iron blowing onto the oceans from land in the past would have also affected algal growth. The consequent changes in atmospheric CO2 could have been enough to both freeze and thaw the planet.

But the researchers warn that the technique is not a quick fix for global warming.

Danger zone
Next month, governments meet in The Hague to agree plans to soak up CO2 from the air by planting trees. Adding iron to the oceans on a large scale, however, might disrupt the ocean's ecosystem and is far too dangerous to be considered as a solution, the researchers say.

Scientists working on the Southern Ocean Iron Release Experiment (SOIREE) released 8 tonnes of iron across a patch of ocean 8 kilometres across, south of New Zealand. The iron produced a sixfold increase in algal growth.

Six weeks later, satellites could still see the plankton covering a thousand square kilometres of ocean, says Ed Abraham of the National Institute for Water and Atmospheric Research in New Zealand. The growing algae sucked up 10 per cent of the CO2 dissolved in the surface water, which was replaced by CO2 from the air.

The iron experiment was first tried out in the Pacific (New Scientist, 1 July 1995, p 5). But its successful repetition in the Southern Ocean has profound implications for understanding the beginning and end of ice ages, says Andrew Watson of the University of East Anglia in Norwich.

Dust storms
He believes that the amount of iron reaching the Southern Ocean in dust from the land could determine how much CO2 the oceans can absorb from the atmosphere.

"We think Patagonia may be crucial," says Watson. It was very dry during the last glaciation. Dust storms containing iron rained onto the ocean, stimulating the algal growth that absorbed atmospheric CO2 and helping to maintain the global freeze.

But at some point Patagonia became wetter. The iron rain ceased, ultimately causing a global rise in atmospheric CO2. Modelling studies suggest that the loss of iron caused a 20 per cent rise in atmospheric CO2, says Watson, half of what was needed to end the glaciation.

So could the trick be repeated in reverse? Could we seed the Southern Ocean with iron to reduce global temperature? The SOIREE results suggest this could yield "a modest increase" in the ocean's take-up of carbon, agrees Abraham.

But all the researchers urge great caution. "It would be extremely inadvisable to even consider such a radical and potentially dangerous step," warns Cliff Law of Britain's Plymouth Marine Laboratory. It could trigger changes in ocean ecosystems that "might increase production of other greenhouse gases and toxins," he says.

Source: Nature (vol 407, p 695, 727 and 730)

This article appeared in the October 14 issue of New Scientist New Scientist. Copyright 2000 - All rights reserved. The material on this page is provided by New Scientist and may not be published, broadcast, rewritten or redistributed without written authorization from New Scientist.

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