Abstract
The oceans sequester carbon from the atmosphere partly as a result of biological productivity. Over much of the ocean surface, this productivity is limited by essential nutrients and we discuss whether it is likely that sequestration can be enhanced by supplying limiting nutrients. Various methods of supply have been suggested and we discuss the efficacy of each and the potential side effects that may develop as a result. Our conclusion is that these methods have the potential to enhance sequestration but that the current level of knowledge from the observations and modelling carried out to date does not provide a sound foundation on which to make clear predictions or recommendations. For ocean fertilization to become a viable option to sequester CO2, we need more extensive and targeted fieldwork and better mathematical models of ocean biogeochemical processes. Models are needed both to interpret field observations and to make reliable predictions about the side effects of large-scale fertilization. They would also be an essential tool with which to verify that sequestration has effectively taken place. There is considerable urgency to address climate change mitigation and this demands that new fieldwork plans are developed rapidly. In contrast to previous experiments, these must focus on the specific objective which is to assess the possibilities of CO2 sequestration through fertilization.
Highlights
It is generally accepted (IPCC 2007) that emissions of CO2 to the atmosphere are the dominant cause of global warming, and that cuts in these emissions are needed in the few years
The solubility pump has increased since pre-industrial times as a result of increasing atmospheric CO2 concentration and this historically recent ocean sink for anthropogenic CO2 is currently estimated to be approximately 2 GtC yrK1 (IPCC 2007)
If ocean fertilization is to be useful as a geoengineering option, any carbon removed from the atmosphere must remain separated from the sea surface and out of contact with the atmosphere for periods in excess of a century (IPCC 2007)
Summary
It is generally accepted (IPCC 2007) that emissions of CO2 to the atmosphere are the dominant cause of global warming, and that cuts in these emissions (currently approx. 8 GtC yrK1 and rising) are needed in the few years. If ocean fertilization is to be useful as a geoengineering option, any carbon removed from the atmosphere must remain separated from the sea surface and out of contact with the atmosphere for periods in excess of a century (IPCC 2007) This requirement demands that any enhanced production must lead to ‘sequestration’ of the material by settling into the deeper water masses (200–1000 m) below the depth of winter mixing. The vast majority is DIC but the particulate matter in the surface and subsurface has a ratio of C : N : P of approximately 106 : 16 : 1, while dissolved organic matter (DOM) has a ratio of 199 : 20 : 1 (Hopkinson & Vallino 2005) The implication from this is that if nutrients were provided solely from deep water, and if the settling biogenic particles (or DOM) had the same composition as the upwelled water, sequestration could not be enhanced in any sustained way. The only sources available to fuel primary production are the stocks in deep water or those supplied from rivers or on airborne dust and unless such sources exist, productivity will cease once local production exhausts the upper ocean pool
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