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Part I: Impacts of Climate-related Geoengineering on Biological Diversity geoengineering technique. Whilst enhanced dimethyl sulphide (DMS) emissions from plankton might be considered a “beneficial”, unintended outcome of ocean fertilization, due to albedo effects,297 the scale (and even sign) of this response is uncertain, and the overall linkage between DMS and climate is now considered relatively weak.298 5.3 MODIFICATION OF UPWELLING AND DOWNWELLING Artificial upwelling is an ocean fertilization technique that has been proposed to bring deep water (from 200–1000 m) naturally rich in a range of nutrients to the surface, through some type of pipe, to fertilize the phytoplankton299. Limited field experiments have been carried out in the Pacific.300, 301 The intended effects are essentially the same as for externally-adding nutrients, as above and will therefore not be repeated here. However, there is a major problem with the concept, as the nutrient-rich water brought up to the surface also contains high concentrations of dissolved CO2 derived from the decomposition of organic material. The release of this CO2 to the atmosphere302, 303 would counteract most (if not all) of the potential climatic benefits from the fertilization of the plankton. Upwelling in one area necessarily also involves downwelling elsewhere. Modifying downwelling currents to carry increased carbon into the deep ocean by either increasing the carbon content of existing downwelling or by increasing the volume of downwelling water has also been proposed as a possible geoengineering approach, without necessarily involving enhanced biological production. While the view of some authors304 is that “modifying downwelling currents is highly unlikely to ever be a costeffective method of sequestering carbon in the deep ocean”, lower-cost structural approaches have recently been proposed, with the claim that the downwelling would stimulate adjacent upwelling, increasing primary production and carbon drawdown, and benefitting fisheries.305 In order to estimate the number of such structures necessary to achieve global climate impact, the hydrodynamics and biogeochemistry of such systems would need further attention. However, such an approach is likely to require coverage of a significant proportion of the ocean surface, since— as for other CDR techniques—the geoengineering requirement is for long-term sequestration of anthropogenic carbon, not stimulating carbon cycling per se. In particular: i) if the increased phytoplankton growth is stimulated by nutrients from deeper water, such water will also contain higher CO2, thus net drawdown of atmospheric CO2 is unlikely to be achieved; ii) there is considerable variability in the timescale of carbon re-cycling in the ocean interior, determining the rate of return of additional, biologically-fixed CO2 to the atmosphere and iii) enhancement of biological production at the scale required for climatic benefits is likely to significantly deplete mid-water oxygen, resulting in increased CH4 and N2O release. Because of these complications, the overall effectiveness of any upwelling/downwelling modification is questionable. Furthermore, high costs are likely to be involved in achieving reliable data on any long term carbon removal (needed for international recognition of the effectiveness of the intervention), and in quantifying potentially counter-active negative impacts, over large areas (ocean basin scale) and long time periods (10–100 years). 297 Wingenter et al. (2007). 298 Quinn & Bates (2011). 299 Lovelock & Rapley (2007). 300 Maruyama et al. (2011). 301 White et al. (2010). 302 Oschlies et al. (2010). 303 Yool et al. (2009). 304 Zhou & Flynn (2005). 305 Salter (2009). 60
5.4 GEOCHEMICAL SEQUESTRATION OF CARBON DIOXIDE Part I: Impacts of Climate-related Geoengineering on Biological Diversity CO2 is naturally removed from the atmosphere by the weathering (dissolution) of carbonate and silicate rocks, forming bicarbonates and other compounds. However, the process of natural weathering is very slow: CO2 is consumed at less than one hundredth of the rate at which it is currently being emitted.306 It has therefore been proposed that, in order to combat climate change, the natural process of weathering could be artificially accelerated. There is a range of proposed techniques that include releasing calcium carbonate or dissolution products of alkaline minerals into the ocean, or spreading abundant silicate minerals such as olivine307 over agricultural soils—as discussed below. 5.4.1 Enhanced ocean alkalinity This proposed approach is based on adding alkaline minerals (e.g., carbonate or silicate308 rock) or their dissolution products to the ocean in order to chemically enhance ocean storage of CO2; it also expected to buffer the ocean to decreasing pH, and thereby help to counter ocean acidification.309 It has been proposed that dissolution products of alkaline minerals could be released into the ocean through a range of techniques that include: i) CO2-rich gases dissolved in sea water to produce a carbonic acid solution that is then reacted with a carbonate mineral to form calcium and bicarbonate ions;310, 311, 312 ii) addition to the ocean of bicarbonate ions produced from the electrochemical splitting of calcium carbonate (limestone);313 and iii) addition of magnesium and calcium chloride salts from hydrogen and chlorine ions produced from the electrolysis of sea water to form hydrochloric acid which is then reacted with silicate rocks.314 For deployment for geoengineering purposes, all of these techniques would require very large volumes of feedstock minerals, abundant (non-carbon) energy, water, and extensive associated operational infrastructure. Most proposals envisage the addition of material through a pipeline into the sea or indirectly through discharge into a river: hence constraining their application to coastal zones, and limiting the potential for rapid dilution (thereby increasing the risk of local negative impacts on ecosystems). Other proposals involve the direct addition of limestone powder315 or calcium hydroxide316 to the ocean from ships, thereby increasing flexibility with the sites of application and also potentially achieving much higher dilution rates (therby minimizing short-term pH spikes). Note that the manufacture of calcium hydroxide requires energy and releases CO2 (that would need to be captured and safely stored), although the overall process is theoretically capable of net uptake. Such processes undoubtedly could have high long term effectiveness, i.e. on geological timescales. However, for geoengineering purposes, the maximum potential effectiveness of generic enhanced alkalinity techniques (in terms of their radiative forcing) has been estimated317 as very low, at ≤ 0.03 W m-2.. This value is less than 1% of the 306 Uchikawa & Zeebe (2008). 307 Schuiling & Krijgsman (2006). 308 The weathering of calcium carbonate (CaCO3) by CO2 mostly produces bicarbonate ions and calcium ions; the weathering of magnesium silicate (olivine; Mg2SiO4) mostly produces bicarbonate ions, magnesium ions, and silicic acid (H4SiO4). In theory, the latter is more efficient, absorbing four molecules of CO2 for each molecule of magnesium silicate, with potential sequestration of 0.34 tonnes of carbon for each tonne of olivine. 309 Kheshgi (1995). 310 Rau & Caldeira (1999). 311 Caldeira & Rau (2000). 312 Rau (2011). 313 Rau (2008). 314 House et al. (2007). 315 Harvey (2008). 316 Cquestrate (http://www.cquestrate.com/). 317 Vaughan & Lenton (2011). 61
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PART I IMPACTS OF CLIMATE-RELATED G
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Part I: Impacts of Climate-related
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