Carbon Dioxide Removal via Macroalgae Open-ocean Mariculture and Sinking: An Earth System Modeling Study

Abstract

Abstract. In this study we investigate open-ocean macroalgae mariculture and sinking (MOS) as ocean-based carbon dioxide removal (CDR) method. Embedding a macroalgae model into an Earth system model, we simulate macroalgae mariculture in the open-ocean surface layer followed by fast sinking of the carbon-rich macroalgal biomass to the deep seafloor (depth > 3,000 m). We also test the combination of MOS with artificial upwelling (AU), which fertilizes the macroalgae by pumping nutrient-rich deeper water to the surface. The simulations are done under RCP4.5 a moderate emission pathway. When deployed globally between years 2020 and 2100, the simulated CDR potential of MOS is 270 PgC, which is further boosted by AU to 447 PgC. More than half of MOS-sequestered carbon retains in the ocean after cessation at year 2100 until year 3000. The major side effect of MOS on pelagic ecosystems is the reduction of phytoplankton net primary production (PNPP) due to the nutrient competition and canopy shading by macroalgae. MOS shrinks the mid layer oxygen minimum zones (OMZs) by reducing the organic matter export to, and remineralization in, subsurface and intermediate waters, while it creates new OMZs on the seafloor by oxygen consumption from remineralization of sunken biomass. MOS also impacts the global carbon cycle, reduces the atmospheric and terrestrial carbon reservoir when enhancing the ocean carbon reservoir. MOS also enriches the dissolved inorganic carbon in the deep ocean. Effects are mostly reversible after cessation of MOS, though recovery is not complete by year 3000. In a sensitivity experiment without remineralization of sunk MOS biomass, the entire MOS-captured carbon is permanently stored in the ocean, but the lack of remineralized nutrients causes a long-term nutrient decline in the surface layers and thus reduces PNPP. Our results suggest that MOS has a considerable potential as an ocean-based CDR method. However, MOS has inherent side effects on marine ecosystems and biogeochemistry, which will require a careful evaluation beyond this first idealized modeling study.

Quantifying baseline costs and cataloging potential optimization strategies for kelp aquaculture carbon dioxide removal

“…Kelps may compete with local planktonic communities for limited nutrients, such as N and P, and light, leading to a decline in NPP and the efficiency of the biological carbon pump (Frieder et al, 2022). The increased oxygen demand at the sea floor of kelp deposition sites could also reduce sediment aerobic depth with trickle down effects on the understudied benthos (Wu et al, 2022). The space required for kelp CDR to effectively draw down atmospheric CO 2 could not only pose a major bottleneck to scaling, but also displace and compete with other ocean users.…”

Economic and biophysical limits to seaweed farming for climate change mitigation

“…This has driven surging interest in ocean-based carbon dioxide removal, including via cultivated macroalgae (seaweed), which would not require inputs of land or freshwater and might have environmental co-benefits (for example, see refs. [13][14][15][16][17][18][19][20][21]. Seaweed products might also help to lower greenhouse gas emissions, for example by reducing methane emissions from ruminants 22 , and replacing fossil fuels 23 and emissions-intensive agricultural products 24 .…”

“…To contribute to such climate goals, seaweed farming must therefore expand tremendously, and in turn contend with large uncertainties in the productivity of different types of seaweed in different places, the net costs of farming, the magnitude of emissions avoided or carbon sequestered, and the potential for undesirable ecological impacts. Recent studies of seaweed farming have examined localized opportunities and dynamics in particular regions 15,16,30 , made rough estimates of the global potential 13,14,31,32 and modelled the Earth system response to gigaton-scale production 19 . Yet the productivity, costs and potential climate benefits of such farming are spatially heterogeneous and scale-dependent, and the key sensitivities and trade-offs important to investors and decision-makers have not been comprehensively evaluated.…”

Kelps in Korea: from population structure to aquaculture to potential carbon sequestration

“…Seaweed, therefore, is not considered as a net carbon sink. However, recent studies have suggested potential contribution of seaweed to climate change mitigation strategies (Krause-Jensen and Duarte 2016, Duarte et al 2017, Wu et al 2022. For example, Oceans 2050 project led by Carlos M. Duarte and colleagues is evaluating sediment containing organic carbon originated from seaweed farms.…”

“…Offshore seaweed aquaculture can be another possibility for ocean-based CDR (see above for more details about offshore seaweed aquaculture). Wu et al (2022) estimated offshore seaweed aquaculture for carbon sink. They suggested that mature seaweed from the farms will rapidly sink to the seafloor and unload the biomass there.…”