Climate change, ocean processes and ocean iron fertilization

Abstract

Observations indicate that the rate of increase in concentration of atmospheric CO2 is increasing faster than projected in any of the Intergovernmental Panel on Climate Change (IPCC) emission scenarios. Several mitigation measures, referred to as 'geoengineering options', have been proposed to remove CO2 from the atmosphere. To be successful, such a mitigation operation must remove 'significant' CO2 from the atmosphere for many decades, be verifiable, and not cause deleteri- ous side effects. One option, purposeful addition of iron to fertilize photosynthetic uptake of CO2 by phytoplankton in regions of the ocean where iron is a limiting nutrient, has received considerable sci- entific attention. In the last 15 yr, a dozen small-scale open ocean iron fertilization experiments have been performed and a succession of models of large-scale fertilization have been developed. As succes- sive models have become more realistic, the amounts of CO2 forecast to be sequestered have dropped, and in all cases are small relative to the amounts of CO2 projected to be released through fossil fuel burning over the next century for any of the IPCC emission scenarios. Possible side effects include a long term reduction in ocean productivity, alteration of the structure of marine food webs, and a more rapid increase in ocean acidity. Most importantly, increased remineralization associated with the increased downward export of organic carbon particles would result in increased production of the third most important long-lived greenhouse gas, N2O. The magnitude of this effect is poorly known.