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.

What is our assessment?

Based on our analysis, blue hydrogen is feasible and ready to deploy, but there is little real-world evidence for its effectiveness or ability to scale. The expansion of this technology to replace current gray hydrogen production or to support the transition to a global hydrogen economy will perpetuate and possibly expand the use of fossil fuels. Because of this risk, we conclude that producing blue hydrogen is “Not Recommended.”

What is it?

Blue hydrogen production is an industrial process that produces hydrogen (H₂) from fossil fuels – either natural gas or coal – combined with carbon capture and storage (CCS) technology to reduce CO₂ emissions produced during the process. Today, most hydrogen is gray hydrogen made from natural gas without any CCS. The addition of CCS prevents the release of some of the CO₂ generated during the hydrogen production process; capturing, concentrating, and then storing it permanently underground. 

Does it work?

The technologies for making hydrogen from natural gas, predominantly steam methane reformation (SMR), are well established and have been used to produce hydrogen for close to a century. CCS technology is also available and currently deployed in multiple industrial and power generation applications. The SMR hydrogen production process generates GHG emissions from two sources: methane leaks from the gas used as feedstock and fuel used to power the production process, and GHG emissions from both the SMR process and combustion of gas (or other fuels) for energy, including CO₂, methane, nitrous oxide, and black carbon. CCS can be applied to capture CO₂ produced during the SMR process, for post-combustion capture of CO₂ from the plant’s energy use, or for both. Incorporating CCS to capture emissions from the hydrogen production process adds costs and increases energy use, but it could theoretically reduce CO₂ emissions by more than 90%. However, current adoption of blue hydrogen is very low – less than 1% of global hydrogen production – and there is little real-world evidence to support its effectiveness and scalability. The few commercial facilities currently in operation capture only about 60% or less of the emitted CO₂. Because CCS is energy-intensive, it requires more fuel to power the blue hydrogen production plant. This can also increase fugitive methane leaks due to increased gas-powered energy consumption. If implemented adequately, carbon storage can be permanent. The captured CO₂ can also be used as a chemical precursor for the manufacture of other products or for enhanced oil recovery; however, these post-capture uses of CO₂ emit GHGs, thereby reducing or eliminating the emissions reduction efficacy of CCS. Currently, only ~8% of CO₂ captured from blue hydrogen production is injected into dedicated geological storage, with the rest used in industry, enhanced oil recovery, and other applications. 

Why are we excited?

Hydrogen can be combusted as a zero-emissions fuel, used to store energy to produce electricity, or deployed as a feedstock in industrial, transportation, and energy systems. The production of any hydrogen type – blue, gray, or green hydrogen – could facilitate the expansion of hydrogen infrastructure and the development of the global hydrogen economy, which is an important step in scaling hydrogen. Blue hydrogen is more technologically ready and cheaper than green hydrogen, which is made from water using electrolysis powered by renewable energy. Blue hydrogen is more expensive to produce than gray hydrogen, but the cost per metric ton of CO₂ removed could be relatively low. Estimates range from US$60–110/t CO₂, although these costs are uncertain and, with lower CCS effectiveness, they could increase to ~US$260/t CO₂. If implemented with low fugitive methane emissions and high CCS efficiencies, blue hydrogen could substantially reduce emissions compared to current gray hydrogen production. The climate impact of scaling blue hydrogen could be high. Estimates and targets for blue hydrogen production by 2050 range from ~30–85 Mt hydrogen. At that scale, even modest emissions savings relative to gray hydrogen would have a climate impact above 0.09 Gt CO₂‑eq/yr by 2050. However, achieving this depends on the quality of the infrastructure and rate of technology scaling, both of which are unproven. 

Why are we concerned?

Currently, 6% of the world’s natural gas and 2% of its coal are used to make hydrogen. As hydrogen production ramps up, blue hydrogen – even though it reduces production emissions compared to gray hydrogen – would perpetuate and could even increase the global market for fossil fuels. If the future implementation of green hydrogen is set back, blue hydrogen could create a long-term dependence on fossil fuels. Furthermore, any hydrogen produced from natural gas leads to methane leaks, regardless of whether CO₂ is captured. Methane is a potent short-lived GHG, meaning its impact on climate warming is stronger in the near-term. This is why reducing methane emissions is an urgent emergency brake climate action. Building and expanding a new industry that relies on natural gas as both a feedstock and fuel, and which inevitably leaks methane, is counterproductive to solving the climate crisis. 

If and when there is a transition to a global hydrogen economy, blue hydrogen is a less effective climate solution than green hydrogen. Although this technology could be a transitional solution between gray and green hydrogen, blue hydrogen risks diverting resources away from green hydrogen development or ready-to-deploy renewable energy technologies, such as onshore wind or distributed solar PV. Expert opinions are mixed regarding the realistic level of avoided emissions that blue hydrogen may reach. Additionally, there is uncertainty around whether CCS can meet its technical potential at a reasonable cost.