Air‐sea carbon dioxide equilibrium: Will it be possible to use seaweeds for carbon removal offsets?

Abstract

To limit global warming below 2°C by 2100, we must drastically reduce greenhouse gas emissions and additionally remove ~100–900 Gt CO2 from the atmosphere (carbon dioxide removal, CDR) to compensate for unavoidable emissions. Seaweeds (marine macroalgae) naturally grow in coastal regions worldwide where they are crucial for primary production and carbon cycling. They are being considered as a biological method for CDR and for use in carbon trading schemes as offsets. To use seaweeds in carbon trading schemes requires verification that seaweed photosynthesis that fixes CO2 into organic carbon results in CDR, along with the safe and secure storage of the carbon removed from the atmosphere for more than 100 years (sequestration). There is much ongoing research into the magnitude of seaweed carbon storage pools (e.g., as living biomass and as particulate and dissolved organic carbon in sediments and the deep ocean), but these pools do not equate to CDR unless the amount of CO2 removed from the atmosphere as a result of seaweed primary production can be quantified and verified. The draw‐down of atmospheric CO2 into seawater is via air‐sea CO2 equilibrium, which operates on time scales of weeks to years depending upon the ecosystem considered. Here, we explain why quantifying air‐sea CO2 equilibrium and linking this process to seaweed carbon storage pools is the critical step needed to verify CDR by discrete seaweed beds and nearshore and open ocean aquaculture systems prior to their use in carbon trading.

Limited understanding of basic ocean processes is hindering progress in marine carbon dioxide removal

“…To assess the CDR potential of seaweeds, two coupled measurements are required: track the fate of DOC and POC and determine how much of that is stored in long-term pools (e.g. deep ocean and deep sediments) and mapping the seawater from which CO 2 was removed for weeks-months to determine, mostly via modelling, the degree of atmospheric CO 2 equilibration (Hurd et al 2023). These costly, difficult, and specialised measurements will be required for monitoring, reporting and verification (MRV) of any CDR farming proposals including sinking seaweeds to the deep ocean.…”

Algae in the Anthropocene: Managing, conserving, and utilizing algae in an era of rapid environmental change

“…In particular, in our first perspective of this special series, Hurd et al. (2024) discuss the knowledge gaps around quantifying air‐sea CO 2 equilibrium and linking this process to seaweed carbon storage pools. Overcoming these knowledge gaps is essential for verifying carbon removal by seaweeds prior to their use as carbon offsets or credits and in carbon trading.…”

Seaweeds for carbon dioxide removal (CDR)–Getting the science right

“…populations, when air-sea CO 2 equilibration is accounted for, CDR is just 6-33% of the maximal potential [10]. This is explained by the water body from which Sargassum removed CO2 being subducted under another water body before full equilibrium is reached [9,10].…”

“…All CDR methods will be required to undergo Monitoring, Reporting and Verification (MRV) to ensure that CO 2 removed from the atmosphere is securely stored and does not return to the atmosphere in the near future [6]. For marine systems, MRV is particularly difficult because of the complexity of measuring air-sea CO 2 equilibration [7][8][9]. Any seaweeds reaching the deep ocean may form a carbon storage pool but this is not directly linked to CDR.…”