Editorial: "We can't afford to neglect ways to halt global warming"
IF YOU want to help stop climate change, try tipping some iron into the sea. For years, this geoengineering idea has been considered a busted flush, but new results suggest it really can work.
Tiny floating algae called phytoplankton pull carbon dioxide out of the atmosphere. When they die, the plankton sink to the seabed, taking the carbon with them. Over thousands of years, this strips CO2 from the air, lowering temperatures.
But many ocean regions are short of iron, which plankton need to grow, so the process does not occur. Adding iron should stimulate plankton growth in these areas.
That was the theory, at least. In practice, it is charitable to say the results have been mixed. For many people, the idea died in 2009, when a field trial called Lohafex failed in the South Atlantic. The iron triggered a bloom, but it was eaten by crustaceans before it could sink.
However, another trial, called Eifex, was carried out in the Southern Ocean in 2004. The results have finally been published - and they are promising. The Eifex ship found an ideal testing ground: a slowly rotating eddy 60 kilometres across and 4 kilometres deep, which was more or less isolated from the surrounding waters.
Victor Smetacek of the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven, Germany, and colleagues dumped iron sulphate into the eddy's core and studied the resulting bloom.
The water was rich in silicic acid, so the bloom was dominated by phytoplankton called diatoms. These algae build silica cell walls, which makes them harder to eat and more likely to sink than plankton with calcium carbonate shells. "They are not the pastures of the ocean, they are the thistles," Smetacek says.
The diatom bloom grew for three weeks, then died and sank. At least half of it sank far below 1 kilometre, and probably reached the sea floor (Nature, DOI: 10.1038/nature11229).
No other study has tracked a sinking bloom. "This confirms what we expected to happen," says Richard Lampitt of the UK's National Oceanography Centre in Southampton, who was not involved in the study. Lampitt says Lohafex failed because the trial site was low in silicon, so the bloom contained few diatoms.
Eifex's success is far from a green light for iron fertilisation, though. At most, a global programme could mop up about 1 gigatonne of carbon per year, about a tenth of our current emissions, according to a modelling study by Ken Caldeira at the Carnegie Institution of Washington in Stanford, California (Climatic Change, DOI: 10.1007/s10584-010-9799-4). "It's too little to be the solution," agrees Smetacek, "but it's too much to ignore."
Fertilised patches create algal food sources but burn through ocean nutrients. This could be a boon to some threads of the food web (see "Iron fertilisation and the whales"), but it could suppress diatom formation elsewhere to the detriment of other marine species.
All those contacted by New Scientist agreed that any tests should be run as a public good, not for profit. Some firms had planned to use iron fertilisation to accrue carbon credits which they could sell on, but in 2008 the London Convention and Protocol - an international treaty - ruled that the practice should not be allowed.
Iron fertilisation and the whales
Iron fertilisation is mainly seen as a way to engineer the climate (main story), but it could also help boost whale populations by restoring their natural ecosystem, says Victor Smetacek of the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven, Germany.
Many Southern-Ocean whales feed on Antarctic krill (Euphausia superba). This krill is one of the few species that eat algae called diatoms in large quantities, but krill numbers have been plummeting for decades.
Increasingly, ecologists suspect that declining krill numbers are linked to humanity's over-hunting of whales. Whale faeces are rich in the iron that helps fuel diatom growth. This in turn benefits the krill - and ultimately the whales. "Whales might be effectively fertilising their own foods, and a reduction in whale populations would impact on that food resource," says David Raubenheimer of Massey University in Auckland, New Zealand.
There is no guarantee that the boosted diatoms would end up being eaten by krill, though, or that the resulting increased krill would be eaten by whales.
"There are many other competitors in the ecosystem," notes Ian Boyd at the University of St Andrews in the UK.
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