Table 2.

Summary of probable main effects of a range of proposed geoengineering approaches on ocean acidification, assuming climatically significant deployment and in comparison to unabated CO2 emissions. Within approaches, there may be relatively large differences in effects depending on specific details of techniques and their deployment arrangements. Additional details in text. OA, ocean acidification; SRM, solar radiation management; CDR, carbon dioxide reduction.

SRM or CDRapproachprobable effects and comments
1. Techniques that either might slightly ameliorate, or slightly worsen, future OA or have no net effectSRMspace-based reflectionstabilized temperatures but increased CO2 expected to have adverse direct impact on OA due to effects on solubility of CO2 and CaCO3. However, the comparison is complicated by temperature effects on terrestrial carbon biomass in the non-SRM control [110,111]; direct SRM impacts on the hydrological cycle [112]; and the potential for second-order impacts (e.g. irradiance- and CO2-induced changes in terrestrial and marine primary production) [113]
neithercirrus cloud manipulation
SRMstratospheric aerosols (SO2)as above, plus effect of decreased pH of precipitation (although likely to be slight [114])
SRMmarine cloud brighteningtemperature/solubility effects, with increased likelihood of significant impact due to decreased marine primary production due to change in light quantity and quality [115]
SRMocean surface albedo
SRMland surface albedotemperature/solubility effects, that may vary inter-hemispherically due to asymmetric SRM cooling [116]
2. Techniques that displace OA from ocean surface to mid- or deepwaterCDRdirect ocean fertilizationadditional primary production and carbon export would reduce OA in upper ocean but decrease pH in ocean interior. On century-scale, potential for modest net benefit (due to enhanced CaCO3 dissolution at depth) [117119]
CDRup/downwelling modification
CDRdirect air capture with ocean storagepotential for severe local OA impacts at site of liquid CO2 injection (both midwater and seafloor have been proposed) [120,121]. Long-term fate of injected CO2 may be highly location-sensitive [122]
3. Techniques that might counteract OA globally, but with some risk of locally severe deepwater impactsCDRdirect air capture with sub seafloor storagesmall risk of potentially severe OA impacts due to reservoir failure [123125]; such risks might be reduced if CO2 injected into basaltic rocks [126]
CDRocean storage of terrestrial biomassvery slow decomposition (with low CO2 release and OA impacts) could be achieved if biomass (e.g. crop waste) is deposited in high-sedimentation sites; e.g. off major river-mouths [127]
4. Techniques that, in theory, could counteract OA, if achievable at necessary scaleCDRenhanced ocean alkalinitycould directly ameliorate OA at ocean surface, but with local risk of high pH/alkalinity impacts [128131]. Range of techniques proposed, with most likely to be slow acting [132]
CDRenhanced soil alkalinityriver run-off of minerals and enhanced alkalinity could have second order OA impacts (negative or positive) for coastal areas
CDRafforestation/ reforestationif successful in reducing atmospheric CO2, would also reduce future OA without significant unintended side effects on ocean chemistry
CDRbiochar and other techniques to enhance soil C
CDRdirect air capture with land-based geological storage