Besides controlling emissions, a wide range of more active ways to combat global warming has been proposed. One particular proposal is the addition of iron to the oceans.

The objective is to promote phytoplankton growth in order to reduce atmospheric carbon dioxide though photosynthetic uptake.

Old idea, new approach
Scientists became interested in the concept in the late 1980s when oceanographer John Martin famously told colleagues: “Give me half a tanker of iron and I’ll give you the next ice age.” Iron fertilization has since been tested in at least a dozen experiments around the world.

Though common on land, dissolved iron is in short supply in the ocean which limits growth of plankton. Therefore, researchers and commercial enterprises have recently proposed that adding this missing nutrient on a large scale could create artificial blooms.

Once absorbed by the algae, some of this carbon will be deposited on the ocean floor in the form of dead cells and fecal matter, and sequestered in deep ocean sediments for millions of years. The rest will enter the food chain when larger organisms consume the algae, or will be transported by ocean currents to middle-depth waters where the carbon will remain for a few decades at most.

Putting it to the test
Laboratory experiments suggested that every ton of iron added to the ocean could remove 30,000 to 110,000 tons of carbon from the air. In a series of small scale ocean experiments conducted since 1993, Woods Hole Oceanographic Institution has shown that iron additions do indeed draw carbon into the ocean, though perhaps less efficiently or permanently than first thought.

One recent large scale experiment done by a team of scientists from the German National Institute of Oceaonography and the Alfred Wegener Institute showed less than promising results. The so-called LOHAFEX experiment showed that iron fertilization of nutrient-rich waters did not necessarily lead to algal blooms, carbon export, and thus, CO2 uptake. The accumulation rates of phytoplankton increased for a very short time only (if at all) because of heavy grazing pressure by zooplankton.

Could iron fertilization of oceans combat global warming?
Although iron fertilizers have been shown to promote plant growth in surface waters, many scientists still remain very skeptical to the long term effects of iron fetilization. In addition, there is no certain knowledge of the side effects from these kinds of experiments, especially done on a large scale. Making major changes to one of the largest ecological systems on the planet, the ocean, is not anything to taken lightly.

And the claim that artificial ocean iron fertilisation will indeed cause any significant reduction of atmospheric carbon dioxide still remains to be substantiated. In theory, fertilisation should boost phytoplankton biomass, which is the base of the marine food chain. In turn, krill and other consumers of phytoplankton, should benefit from this increased food-supply.

Down the line, this would increase fisherman’s catches from the over-fished seas, and even help restore whale populations, which are not yet recovering from over-hunting. However, ocean ecosystems are poorly understood relative to their terrestrial counterparts, and tampering with the base of the food chain could yield undesirable consequences. ■

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