Proposals to combat global warming by sowing the sea with iron to promote carbon-gobbling plankton may be badly overblown, according to a study published on Wednesday.
Ocean “fertilization” has ignited fierce scientific controversy, with supporters saying these schemes could stave off damaging climate change and critics warning that swathes of ocean may turn stagnant or acidic.
The idea behind fertilization is to scatter iron powder in swathes of the ocean, providing nutrients for algae in the warm upper layers of the sea called phytoplankton. These tiny marine plants suck in carbon dioxide by photosynthesis.
When they die, some of them would sink to the depths and their carbon remains would be stored, or sequestered, there. In other words, greenhouse gas would be transferred from the atmosphere to the depths of the ocean, and so would not be around to trap solar radiation.
But new research, published in the science journal Nature, casts doubt on some claims of the effectiveness of the process.
Researchers led by Raymond Pollard of the National Oceanography Centre in Southampton, southern England, looked at seas around the Crozet Islands, a small archipelago on the northern rim of the Southern Ocean.
The seas just north of the islands are rich in natural iron and causes blooms of plankton in the southern summer that last for months. South of the islands, though, is nutrient-poor, and plankton blooms there are far smaller and short-lived.
Comparing the two zones, the so-called CROZEX mission found that iron-rich seas doubled or even tripled plankton growth and the absorption of carbon dioxide.
But the amount of carbon dioxide that was actually stored was just 5 percent or 6 percent, senior researcher Richard Sanders said.
“If we think of a hundred units of carbon being fixed by phytoplankton in the upper ocean, around 90 percent of that will be recycled in the upper ocean and around 10 percent will sink out of this sunlit upper layer,” he said.
“Of those 10 units sinking, one will get to the sediment at the bottom,” where it will be effectively stored forever.
The rest, though, will be recycled in the midwater region or, in the lower depths, eventually get pushed to the surface by deep ocean currents, which would prompt them to surrender their carbon, he explained.
This overturning takes place on a time scale of decades to a couple of hundred years, meaning the carbon would be stored out of harm’s way for a long while but not permanently.
The study said carbon sequestration around the Crozet islands fell massively short of some estimates, although it was 20 times more than that calculated for a fertilization experiment called SERIES.
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