Abstract
Proposals to increase ocean alkalinity may make an important contribution to meeting climate change net emission targets, while also helping to ameliorate the effects of ocean acidification. However, the practical feasibility of spreading large amounts of alkaline materials in the seawater is poorly understood. In this study, the potential of discharging calcium hydroxide (slaked lime, SL) using existing maritime transport is evaluated, at the global scale and for the Mediterranean Sea. The potential discharge of SL from existing vessels depends on many factors, mainly their number and load capacity, the distance traveled along the route, the frequency of reloading, and the discharge rate. The latter may be constrained by the localized pH increase in the wake of the ship, which could be detrimental for marine ecosystems. Based on maritime traffic data from the International Maritime Organization for bulk carriers and container ships, and assuming low discharge rates and 15% of the deadweight capacity dedicated for SL transport, the maximum SL potential discharge from all active vessels worldwide is estimated to be between 1.7 and 4.0 Gt/year. For the Mediterranean Sea, based on detailed maritime traffic data, a potential discharge of about 186 Mt/year is estimated. The discharge using a fleet of 1,000 new dedicated ships has also been discussed, with a potential distribution of 1.3 Gt/year. Using average literature values of CO2 removal per unit of SL added to the sea, the global potential of CO2 removal from SL discharge by existing or new ships is estimated at several Gt/year, depending on the discharge rate. Since the potential impacts of SL discharge on the marine environment in the ships' wake limits the rate at which SL can be applied, an overview of methodologies for the assessment of SL concentration in the wake of the ships is presented. A first assessment performed with a three-dimensional non-reactive and a one-dimensional reactive fluid dynamic model simulating the shrinking of particle radii, shows that low discharge rates of a SL slurry lead to pH variations of about 1 unit for a duration of just a few minutes.
Highlights
Increasing the alkalinity of the ocean is gaining mounting attention for the potential to mitigate ocean acidification, a major threat for some marine ecosystems, while simultaneously removing atmospheric CO2
A very high potential discharge of slaked lime in the sea can be achieved by using the existing global commercial fleet of bulk carriers and container ships and low discharge rates. This could reach several Gt/year of SL discharged if a significant share of the existing maritime traffic of dry bulk carriers and containers is used
In case between 20 and 40% of the existing fleet is involved in such operation of ocean liming, this would imply a potential of 1 Gt/y of CO2 removal from the atmosphere
Summary
Increasing the alkalinity of the ocean is gaining mounting attention for the potential to mitigate ocean acidification, a major threat for some marine ecosystems, while simultaneously removing atmospheric CO2. After Kheshgi (1995) first proposed to add alkaline materials to the ocean surface for removing atmospheric carbon and storing it in the seawater, some authors (Keller et al, 2014; Renforth and Henderson, 2017; Lenton et al, 2018; Rau et al, 2018) indicated a large potential for carbon removal by “ocean alkalinity enhancement,” via spreading Ca(OH) (commonly named slaked lime, SL) or NaOH in seawater, as well as for mitigating ocean acidification and enhancing the net calcification of reef (Albright et al, 2016). While chemical reactions of ocean alkalinity enhancement are theoretically known, and some processes to produce SL without CO2 emissions into the atmosphere have been proposed (Renforth and Henderson, 2017; Caserini et al, 2019), less is known regarding the practical aspects of spreading large amounts of alkaline materials in the seawater for ocean alkalinity enhancement. The magnitude of the task and the rates required to achieve net carbon removal suggests that a portfolio of options should be implemented, and in the race to remove this tremendous amount of CO2 research should be devoted to current frontrunners, i.e., bioenergy and carbon capture and storage, and to approaches less formally evaluated in terms of cost, effectiveness, resource availability, and acceptability (Rau, 2019)
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