What is our assessment?

Based on our analysis, bio-blocks offer a promising pathway for long-lasting storage of already-captured biogenic carbon, and could meaningfully scale using existing technologies and low-cost plant biomass. However, little is known about their real-world performance over long time scales, effectiveness depends on storage conditions, and land use impacts remain uncertain. As a result, we will “Keep Watching” Bury Bio-Blocks. 

What is it?

Bio-blocks are dried and compressed plant matter, typically forestry or agricultural residues, enclosed in a non-polluting, impermeable barrier for long-term carbon storage. To make carbon-storing bio-blocks, plant residues are dried to moisture levels low enough to prevent rotting driven by microorganisms, mechanically compressed into dense blocks, and encased inside multi-layer barriers that keep out water and oxygen (Crotty et al., 2026; Yablonovitch & Deckman, 2023; Zeng et al., 2023). The barriers often combine nontoxic geomembranes made of high-density or linear low-density polyethylene liners, compacted clay layers, and polymeric sealants (Yablonovitch & Deckman, 2023). Once packaged, the bio-blocks are buried in designated sites designed to keep the material dry and sealed so the stored carbon cannot return to the atmosphere (Amelse, 2025). The block-making process does not make carbon inert, but “locks” it into the organic matter. Bio-blocks can be made from many types of biomass, making them a versatile means of storing biomass carbon (Allen, 2025).

Does it work?

Bio-blocks are a physically plausible means to durably store carbon captured by plants. Microbes cannot sustain metabolism when water activity is below 0.60, preventing biomass decomposition in dry, oxygen-deprived storage (Crotty et al., 2026; Yablonovitch & Deckman, 2023). Bio-blocks’ net climate benefit depends on processing emissions, biomass type and source, and whether they remain dry and intact. Effectiveness is supported by peer-reviewed studies, concept papers, life-cycle analyses, and reactor studies (Crotty et al., 2026; Johnson et al., 2025; Wang et al., 2011; Ximenes et al., 2019), with the strongest support from wood vaulting, engineered dry biomass storage, and landfill-style wood preservation (Crotty et al., 2026; Yablonovitch & Deckman, 2023; Zeng & Hausmann, 2022). A 3,775-year-old wood sample found 2 meters below ground had near-perfect preservation, with less than 5% of its carbon lost due to decay over millennia (Zeng et al., 2024). Other assessments by the Intergovernmental Panel on Climate Change (IPCC) suggest wood biomass burial could retain 99.9% of stored CO₂‑eq over a century (Gooding, 2023). Despite this, bio-blocks are not yet a mature carbon storage strategy. They have little real-world validation beyond concept, operating guidance, and early implementation (Crotty et al., 2026; Zeng et al., 2023). Currently, only a handful of companies use proprietary “bio-block” or “carbon-stiffened brick” methods to sequester atmospheric carbon (Scafidi & Denvir, 2026). 

Why are we excited?

Bio-blocks, and related biomass burial, are an inexpensive form of durable carbon storage that could be scaled up toward globally meaningful sequestration of atmospheric CO₂ (>0.1 Gt CO₂ /yr). According to one study, terrestrial photosynthesis removes six times more CO₂ each year than fossil-fuel burning emits (Zeng et al., 2024). Since most of that carbon would return to the atmosphere through biomass decomposition, storing some long term in designated sites is a major opportunity. A study in the United States estimated that about 415 Mt CO₂‑eq/yr is available for wood harvesting and storage from coarse woody debris (Hausmann et al., 2024). Broader wood-vaulting studies project gigaton-scale climate impacts, with wood burial having a theoretical ceiling of 37 ± 18 Gt CO₂‑eq/yr, a level far above any sustainable deployment, supported by an estimated ~238 Gt CO₂‑eq contained in coarse woody debris currently accumulated on forest floors worldwide (Zeng, 2008; Zeng & Hausmann, 2022; Zeng et al., 2024). Furthermore, drying and compacting biomass into bio-blocks is mechanically and practically straightforward with existing technologies. Cost is favorable for wood burial including bio-blocks, ranging between US$10–50/tCO₂ stored in most studies to as high as US$100/tCO₂ (Allen, 2025; Johnson et al., 2025; Zeng & Hausmann, 2022), but this is based on estimates rather than commercialized outcomes. Bio-block burial also provides simple, high verifiability.

Why are we concerned?

There are several limitations to bio-block burial as a viable climate solution, including possible concerns from large-scale deployment. Dry biomass storage systems fail if the barrier is breached and internal humidity rises enough to restart microbial activity and anoxic digestion of biomass, forming and leaking methane (Crotty et al., 2026; Johnson et al., 2025). This reversibility concern is highest for feedstocks with larger degradable fractions, such as agricultural residues, and lower for coarse woody debris (Crotty et al., 2026; Ximenes et al., 2019). Therefore, the durability of this type of carbon storage depends on sealing requirements, site selection, and gas monitoring (Zeng et al., 2023). 

Burying plant matter from forest floors can lead to forest nutrient depletion, habitat removal, and biomass diversion from better climate uses (Burns, 2025). Permanence concerns include uncertain long-term barrier integrity (Crotty et al., 2026), the need for decades’ worth of monitoring to ensure stored carbon doesn’t leak (Crotty et al., 2026; Zeng et al., 2023), and potential soil-carbon release from excavating deep burial pits (Johnson et al., 2025; Zeng et al., 2023). For bio-blocks to sustainably store carbon, the feedstocks are ideally sourced from low-conflict waste streams in limited amounts, protect soil health, and are evaluated for land use changes, according to the World Resources Institute (Denvir & Leslie-Bole, 2025). 

What is our assessment?

Our analysis finds that Deploy Ocean Biomass Sinking could have high potential environmental risks, including unknown impacts on marine ecosystems. It is also unclear how effective or durable carbon storage in the deep sea is from this approach. There are likely better alternative uses for terrestrial biomass, and cultivating seaweed at climate-relevant scales is probably not feasible. Even if it were, seaweed would probably provide greater value through other applications. Therefore, Deploy Ocean Biomass Sinking is currently “Not Recommended” as a climate solution.

What is it?

Ocean biomass sinking relies on sinking terrestrial plant material and/or seaweed grown in the ocean to the deep sea or seafloor where it can be stored long-term. Cultivating and sinking seaweed removes carbon from the surface ocean, whereas sinking terrestrial biomass material can help reduce emissions that might otherwise occur if the material instead decomposed on land. While not a current practice, terrestrial biomass grown explicitly for sinking would also constitute a form of carbon removal. When biomass sinks naturally, most of it is degraded into CO₂ or other forms of carbon before reaching the deep sea. Deliberate sinking of biomass might avoid some of this degradation by expediting its delivery to the deep sea, depending on the method used. Once sunk, the biomass and any CO₂ or other forms of carbon produced from its degradation can be isolated from the atmosphere for decades to centuries due to the ocean’s slow circulation times at depth. Biomass sinking can be accomplished using active methods, like submersibles, or passive methods, like letting weighted bundles sink on their own. There has been a recent focus on sinking material in low-oxygen ocean basins (e.g., the Black Sea), which might help further minimize degradation, while improving the durability of sequestered carbon due to the long circulation time-scales typical of these regions.

Does it work?

Global estimates suggest that ~11% of carbon produced in natural seaweed ecosystems might be sequestered at depth, generally defined as below the mixed layer at around 1,000 m. However, very few studies have documented the export efficiency, or the fraction of carbon in surface waters that makes its way to the deep sea, of purposefully sunk terrestrial and seaweed biomass, as this practice is currently in the early stages of development and research. If biomass is quickly sunk, most carbon might make its way to the deep sea, while passive sinking techniques, if slower, could result in higher degradation rates. Sequestration also depends on the storage efficiency and durability of carbon once at depth. Some initial research suggests that biomass degradation may be slowed in low-oxygen basins, but this also remains poorly characterized in field studies. It is similarly unclear how durable the carbon stored below the mixed layer will be over climate-relevant timescales, both in the deep sea in general and in low-oxygen basins specifically.

Why are we excited?

The advantages of ocean biomass sinking include its potential ability to use land-based biomass that might otherwise be degraded in landfills or incinerated, both of which lead to CO₂ emissions. In some regions, seaweed cultivation could help reduce nutrient pollution, provide habitat for marine organisms, and locally buffer against ocean acidification. Estimates of potential climate impacts suggest that ocean biomass sinking using biomass from seaweed farms could theoretically exceed 0.1 Gt CO₂‑eq/yr. Still, those estimates remain highly speculative and require more research. Costs are poorly quantified, but some estimates suggest they could be low to moderately expensive compared to other marine carbon dioxide removal approaches, close to US$100/t CO₂.

Why are we concerned?

Ocean biomass sinking has many environmental and social risks that, though not currently fully understood, could make it unfeasible to deploy the technology at scale. Deep-sea and seafloor ecosystems are highly understudied, and it's unclear how new biomass might alter these unique environments. Potential impacts include increased acidification, nutrient pollution, and oxygen depletion of the deep sea, which could affect diverse marine life. Large-scale seaweed cultivation could reduce phytoplankton abundance, disrupt food webs, and deplete nutrients needed by other ecosystems. Cultivation in open ocean areas might relieve demand for coastal space, but they are often nutrient-poor, and adding nutrients raises serious concerns (see Deploy Ocean Fertilization). Terrestrial biomass sources could introduce contaminants into the ocean due to inadvertent inclusion of plastics or other pollutants in sunken biomass. This practice also comes with social risks. Some countries might disproportionately bear negative impacts wherever biomass is cultivated and/or sunk, as it could alter marine food webs and livelihoods. There could also be issues with public perception due to historical injustices around ocean dumping, potentially impeding future projects without meaningful community engagement and transparency. 

Moreover, there are numerous technical challenges relating to the effectiveness and durability of carbon sequestration. Biomass sources differ in how easily they break down, affecting how much carbon is stored at depth. Sunk biomass could also potentially release other greenhouse gases, such as methane and nitrous oxide. The location where biomass is disposed of also matters, impacting how much carbon reaches and stays at depth. However, all of these factors remain poorly constrained. Operational and technical challenges are also significant. To remove at least 0.1 Gt CO₂‑eq/yr using marine biomass, nearly 7 million ha of ocean – over 60% of the global coastline – could be needed for seaweed cultivation, which is impractical. Measurement and verification pose additional hurdles. In the case of seaweed cultivation, tracking carbon removal requires monitoring both CO₂ uptake at the ocean’s surface and export as well as storage at depth across large spatial and temporal scales. In addition, the opportunity cost of sinking terrestrial biomass is high due to competing land-based uses, as waste biomass and crop residues are finite resources. Growing new biomass explicitly for ocean sinking would introduce new risks, given that land is also a finite resource. Similarly, seaweed probably has higher value and carbon benefits as food, fertilizer, and other products.