User:Clayoquot/OAA

Basic principle of operation The ocean alkalinity approach to GGR seeks to increase carbon uptake into the ocean by increasing the seawater concentration of stable, positively-charged ions, such as calcium (Ca2+). The ocean contains about 65 times more carbon than the atmosphere, some as carbonate (CO3 2-) but mostly as bicarbonate (HCO3 - ) ions. The concentration of these negatively charged forms of carbon is controlled by concentration of positively charged ions like Ca2+, described as alkalinityf. Intentionally increasing the ocean’s alkalinity leads to transfer of dissolved CO2 to the ionic forms of carbon, and hence to additional uptake of CO2 from the atmosphere184. This might be achieved directly by addition of lime (CaO or Ca(OH)2) to seawater. It might also provide an alternative to traditional CCS by reacting a CO2-rich gas with water and limestone and transferring the carbon-rich products to the oceans. Increased alkalinity would also result if the dissolved products of enhanced terrestrial weathering, including positively charged ions and carbon, are transported into the ocean. Technology readiness Increasing the carbon content of the ocean by increasing the alkalinity of seawater relies on well-understood chemistry and can readily be demonstrated in the laboratory. No field-scale trial of the approach has been undertaken, however. At its simplest, enhancing alkalinity would use existing technology; there is plentiful limestone, lime is already produced from it for the cement and other industries, and distribution from ships would not require technological advance. CCS would, however, be required where lime is produced to ensure that CO2 emissions generated during the calcination process do not outweigh the emissions removed by ocean alkalinisation. Reaction of CO2 in flue gases with limestone to produce high-alkalinity solutions has also been demonstrated in the lab, but not scaled up to industrial level. Application at scale also requires research into the environmental response to consider possible negative consequences on ecosystems, or reversal of the alkalinity addition.

Increased ocean alkalinity could theoretically remove many GtCO2 pa. The uptake of additional carbon would only lead to a small fractional change to the large natural carbon content of the oceans. Modelling suggests that increased alkalinity could lead to additional ocean storage of as much as 3500 GtCO2 by 2100185 though this assessment does not consider how this large alkalinity addition would be realised. Carbon stored in the ocean in this fashion is stable as long as alkalinity remains high. However, if the alkalinity increase were sufficient locally to cause mineral precipitation (either spontaneously or through increased shell production), this would reduce the carboncarrying capacity of the water and reverse the CO2 uptake. If alkalinity was initially added from weathering or reaction of limestone then this would completely reverse the benefit, while if it were from dissolution of silicates or from lime produced with CCS, only about half of the CO2 uptake would be reversed186. The likely extent of carbonate mineral formation in response to increased alkalinity is essentially unknown and represents a limitation in present assessment of the effectiveness and longevity of this GGR technique. Natural resources required The primary resource requirement for ocean alkalinity is a source of calcium or magnesium minerals to provide that alkalinity. A natural source is limestone, mostly composed of calcium carbonate, which covers around 10% of the earth’s surface, but it would need to be extracted and converted to lime for direct addition. Limestone extraction at levels similar to that of the global cement industry would be required to achieve uptake of 3.7 GtCO2 pa187. Around 5 GJ of energy is required per tonne of CO2 removed by calcium carbonate188, with the main energy requirements for the mining and grinding of limestone and, for direct lime addition, production of lime by calcination. This calcination process also produces about 60% of the mass of CO2 that is consumed on subsequent addition to the ocean, so efficient pursuit of alkalinity addition for GGR would require storage of the CO2 from calcination, or result in significantly reduced overall benefit. For approaches using reaction of CO2 in flue gas with limestone to produce alkaline solutions, water is an additional resource requirement, and may require location on the coast and the use of seawater. Land use requirements would arise from the additional (new) mining required. A full assessment of the UK’s potential for ocean alkalinity addition has not yet been conducted. The UK has plentiful limestone deposits, an active cement industry, coastal power-generation, and a strong shipping industry, so could pursue this approach. Environmental benefits and challenges An increase of alkalinity would partially offset the effects of ocean acidification caused by high atmospheric CO2 concentrations, so controlled addition of alkalinity could have beneficial consequences for ecosystems in some regions189, though these have not been assessed in the field. At locations of alkalinity addition, there is the potential for adverse effects on local ecosystems due to the resulting high pH. Lime(stone) also contains impurities, some of which may either be toxic or act as nutrients to perturb ocean ecosystems. The response of the ocean ecosystem to both high pH and to impurity addition has not yet been assessed.

Other environmental challenges are the environmental impacts commonly associated with mining (e.g. biodiversity loss, acidification of drainage, perturbation of nutrient cycles) and associated with the requirements for energy and CCS. Depending on the mechanism for distribution of the alkalinity, there would be additional impacts from shipping, for example due to the high-sulphur fuel currently used in shipping. Scalability and engineering challenges Addition of alkalinity to the ocean could rely on existing technology. Small scale application might use waste fines from existing limestone production190, but removal of 4 GtCO2 pa would require doubling the global production of lime, and around 100 ships for ocean distribution globally191. Extending this to reach the full potential capacity would require significantly more resources. New infrastructure would be required for lime production, ideally to produce pure CO2 for CCS, or for reaction of power-station flue gas with limestone. Coastal operation would minimise land-transport costs, and would likely be essential for limestone neutralisation of flue gas, given water requirements. The full cost of GGR by production and oceanic distribution of lime have been estimated as $72 to $159 per tonne of CO2 192,193. Risks to implementation The major risks are environmental, and reflect insufficient knowledge of the response of the ocean ecosystem to enhanced alkalinity and associated addition of mineral impurities. There is also risk associated with possible partial reversal of CO2 uptake if the precipitation of carbonate minerals were to occur. Monitoring and evaluation Direct measurement of CO2 uptake by the ocean in response to alkalinity addition would be challenging, but monitoring the addition of lime or the products of flue gas neutralisation would be relatively straightforward. The possibility of partial reversal by mineral precipitation would require evaluation and may need to be monitored, particularly close to sites of addition. Social factors The oceans hold a special place in the environmental awareness of many societies and there is often a dislike of processes that intentionally interfere with them. This may be an impediment to the large-scale application of alkalinity addition. Smaller scale deployment for mitigation of the impact of ocean acidification (for example on a coral reef) may not face the same opposition. Policy factors The London Convention and Protocol194 controls addition of material to the ocean and prohibits “deliberate disposal at sea of wastes or other matter from vessels… or other structures at sea”. Interpretation within the Convention of the intentional addition of lime for GGR rather than as a waste is unclear, but consideration in this regulatory framework would be required before pursuit of ocean alkalinity addition from ships.