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Concentrations of atmospheric CO2 (carbon dioxide), one of the longest lasting and most potent greenhouse gases, have nearly doubled since the end of the last ice age and it's almost all because of fossil fuel combustion. The result is an increase in global temperatures and climate instability—while governments quibble over policies to reduce carbon emissions, 'geoengineers' are exploring ways to pull CO2 back out of the atmosphere.

One approach is carbon storage, where captured CO2 is pumped deep into the Earth. A catch 22 in this approach is that the easiest. Phytoplankton contain the same green pigment as land plants (chlorophyll) and their blooms are visible from space.place to store CO2 is in pore spaces left vacant by fossil fuel extraction. In fact, the 'stored' CO2 is sometimes used to flush oil and natural gas up for future combustion! It's better than nothing, but there's still a net release of CO2 into the atmosphere.

Another more 'natural' approach is to get nature's best carbon capturers involved — plants that photosynthesize, converting solar energy, water and CO2 into sugar. Roughly 70% of the Earth is covered by ocean so the floating plants, or phytoplankton, are a major extractor of atmospheric CO2. Plus, when phytoplankton die, they can sink, carrying carbon away from the atmosphere.

So, is there a way to get the phytoplankton to pull more carbon into the deep ocean? Scientists hoped that by fertilizing ocean water with dissolved iron they could kick-start large CO2-capturing phytoplankton blooms.

Did you know? The first large-scale iron fertilization experiment was the IronEx-II in 1995 which produced a 30x increase in phytoplankton growth.

But, phytoplankton don't just have to bloom, they have to sink in order for CO2 to be sequestered in the deep ocean — and that may be trickier to control. This past March the Lohafex expedition showered 300 square kilometers (km2) of the Southern Atlantic with six tones of dissolved iron. Phytoplankton bloomed, but then swarms of copepods (tiny shrimp-like crustaceans) moved in to eat them — while CO2 cycled in and out of the ocean ecosystem, there was no evidence forlarge-scale carbon sequestration.

Iron fertilization may only work if certain phytoplankton are involved - most importantly 'diatoms', algae whose silica skeletons make tough eating for predators. But diatoms will only thrive if there is enough silica present. And to complicate things more, phytoplankton growth also depends on how acidic the seawater is, which depends on how much CO2 the ocean is storing.

Ocean chemistry is complex, and dissolved iron is expensive, both of which make iron fertilization a risky approach for geoengineers trying to combat climate change. But that's not dissuading companies eager to make money peddling the approach for carbon 'offsets.'

Did you know? Governor Schwarzenegger hired Planktos Inc., an iron-fertilizing and reforestation company, to help California meet their carbon emission goals.

There's no shortage of companies selling carbon offsets for tree planting, either. And that picture is no simpler. While planting trees in the tropics may have a climate cooling effect, northern reforestation may actually accelerate global warming by making the earth less reflective to solar energy.

CO2 can linger in the atmosphere for hundreds of years, and in the ocean for thousands - can we keep fertilizing the oceans and planting trees on those kinds of geologic timescales? No way! We'd run out of iron and land space long before CO2 was brought down to historical levels. But, in combination with efforts to reduce emissions and develop alternative energy sources, geoengineering is poised to play an interesting and most likely important role in fighting climate change.

References

Alfred-Wegener-Institut Press Release, March 23,2009: Lohafex provides new insights on plankton ecology - Only small amounts of atmospheric carbon dioxide fixed

Bala, G. 2009. Problems with geoengineering schemes to combat climate change. Current Science, volume 96(1).

PlanktosInc. Offers California Affordable Green Tech to Fulfill New GlobalWarming Law - Ecosystem Restoration: The Life-Giving Key to CO2Reduction and Economic Progress. Business Wire , August 31, 2006.

Maritnez-Garcia,A. Rosell-Mele, A. Geibert, W., Gersonde, R., Masque, P., Gaspari, V.,Barbante, C. 2009. Links between iron supply, marine productivity, seasurface temperature, and CO2 over the last 1.1 Ma. PALEOCEANOGRAPHY,volume 24, PA1207, doi:10.1029/2008PA001657

Learn More:

Geoengineering: The Top 10 Most Controversial Ways to Save the Planet

Lohafex Iron Fertilization Experiment

Hungry shrimp eat climate change experiment

The Politics of Iron Fertilization

Reforesting the Tropics — TREE Centre Hawai'i

Sandy Marie Bonny

I am a geobiologist with a doctorate in Earth and Atmospheric Sciences from the University of Alberta. I work as a sessional lecturer, science writer, and creative writer - and yes, the science is almost always stranger than my fiction!

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