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Age of iron and ice
14 October 2000
From New Scientist Print Edition.
Fred Pearce

Ocean scientists believe they have recreated one of the key processes that trigger the beginning and end of ice ages.

In one of the largest experiments 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 in dust blowing onto the oceans from land in the past would have affected algal growth, changing atmospheric CO₂ enough to either freeze or thaw the planet.

But the researchers warn that the technique is not a quick fix for global warming. 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 over 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 in the Pacific (New Scientist, 1 July 1995, p 5). But the Southern Ocean success has profound implications for understanding the beginning and end of ice ages, says Andrew Watson of the University of East Anglia in Norwich. He believes that the amount of iron reaching the Southern Ocean in dust from 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 algal growth that absorbed atmospheric CO₂ and helped 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 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.

From issue 2260 of New Scientist magazine, 14 October 2000, page 11

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