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 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 ready to deploy and feasible, but there is mixed consensus and limited data on its effectiveness in reducing emissions. Its climate impact has the potential to be high, but only if adopted effectively and at a large scale, both of which are currently unproven. If deployed correctly, this technology could serve as an interim solution to reduce gray hydrogen emissions while progress is made on scaling and reducing the costs of green hydrogen. Based on our assessment, we will “Keep Watching” this potential solution.

What is it?

Hydrogen is a fuel that can be used in place of fossil fuels in industrial, transportation, and energy systems. To deploy hydrogen as an energy source or feedstock, it first needs to be extracted from other compounds. Each “color” is an informal term specifying hydrogen produced through a unique hydrogen production path, each with different associated supply chains, processes, and energy GHG emissions. Gray hydrogen, which uses natural gas as the source of hydrogen atoms and electricity, is the most produced and has high production emissions, estimated at 10–12 t CO₂-eq/t hydrogen on a 100-yr basis. One way to reduce emissions is by switching to blue hydrogen, which still uses fossil fuels but also uses CCS technologies to prevent the release of some of the CO₂ generated during production. Blue hydrogen has the potential to be a lower-emission source of energy relative to gray hydrogen or direct fossil fuel combustion.

Does it work?

Blue hydrogen is a plausible way to reduce emissions from gray hydrogen production. However, expert opinions are mixed on the magnitude of emissions that can be abated by producing blue hydrogen in place of gray hydrogen. The effectiveness of emissions reduction hinges on two main factors: the rate at which upstream methane leaks and carbon capture rates, both of which are challenging to predict on a global scale. There is uncertainty around these performance metrics and the ability to effectively store and transport captured CO₂ at scale. Due to low current adoption, there is little real-world data to answer these questions. As of 2023, blue hydrogen comprised <1% of worldwide hydrogen production. While adding carbon capture to gray hydrogen production should help prevent emissions, there is limited evidence for both effectiveness and the ability to scale of this technology.

Why are we excited?

Compared to other types of low-carbon hydrogen, including green hydrogen produced from electrolysis powered by renewable energy, blue hydrogen is a technologically developed and lower-cost option. Without a currently viable and cost-effective alternative to gray hydrogen that does not use fossil fuels, blue hydrogen can be a near-term option to facilitate the transition to a global hydrogen economy. Expert estimates of cost per emissions avoided range widely, but the International Energy Agency (IEA) estimates US$60–85/t CO₂ for lower carbon capture rates (55–70%) and US$85–110/t CO₂ for higher carbon capture rates (>90%). However, these costs are uncertain: with lower estimates of effectiveness, the cost could increase to ~US$260/t CO₂, including the cost to transport and store CO₂. If implemented with low GHG fugitive emissions and high CCS efficiencies, blue hydrogen can reduce emissions by more than 60% relative to current gray hydrogen production on a 100-yr CO₂‑eq basis. In this case, 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 (3 t CO₂‑eq/t hydrogen, 20-yr basis) 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?

While it has some advantages, blue hydrogen is still a less effective solution than green hydrogen, while costing more than gray hydrogen. Though it could be useful for near-term energy decarbonization, this risks taking resources away from renewable energy and green hydrogen development while perpetuating and increasing fossil fuel use. The infrastructure required to scale hydrogen-based energy is expensive and will require technical advances and policy incentives to be competitive with fossil fuels. There are mixed expert opinions about the realistic level of avoided emissions that blue hydrogen may reach. The theoretical worst-performing blue hydrogen plants (low capture rates, high methane leaks, high-emission electricity sources) have been predicted to lead to more emissions on a near-term basis than direct natural gas combustion. Additionally, there is uncertainty around whether CCS can meet its technical potential at a reasonable cost. While experts predict >95% carbon capture rates are possible, facilities currently in operation capture less than this target, some less than 60% of all emitted carbon. For blue hydrogen to be feasible and scalable, CO₂ transport and storage need to be low-emitting, stable, and available. Only ~8% of CO₂ currently captured from blue hydrogen production is injected in dedicated storage, with the rest used in industry, enhanced oil recovery, and other applications. Finally, an understudied risk is hydrogen leaks. Hydrogen transport and storage require larger volumes than fossil fuels, increasing the risk of leaks. Hydrogen has an indirect planet-warming effect by increasing the levels of other atmospheric GHGs. At scale, the IEA estimates that high hydrogen leak rates could contribute 0.1 Gt CO₂-eq/yr in emissions, potentially canceling out any positive climate impacts.

What is our assessment?

Based on the scientific uncertainties regarding its effectiveness and the potential serious environmental and social risks, we conclude that Ocean Fertilization is “Not Recommended” as a climate solution.

What is it?

Ocean fertilization involves adding nutrients, such as iron, to seawater to promote photosynthesis in the surface ocean. As phytoplankton draw in seawater CO₂ and convert it into biomass, the ocean can absorb more CO₂ from the atmosphere. Some of the carbon eventually sinks to the deep sea or seafloor, where it can be stored for decades or centuries. Most ocean fertilization efforts are focused on iron because it is a micronutrient already required in small amounts for photosynthesis and because iron limitation is common in many global ocean regions. The Southern Ocean, in particular, has been highlighted as a potential target due to its widespread iron limitation.

Does it work?

As a carbon removal technique, ocean fertilization requires that the nutrient addition enhances phytoplankton uptake of seawater CO₂ and subsequent absorption of additional CO₂ from the atmosphere, and that the carbon is transported and durably stored in the deep sea. Research since the 1990s has shown that ocean iron fertilization does lead to increased seawater CO₂ uptake due to enhanced photosynthesis. However, the ultimate fate and durability of that carbon are less well understood. To be sequestered, carbon must be transported below water depths where annual mixing occurs, often considered to be ~1000 m, but research suggests that, on average, 66% of carbon at these depths can be re-exposed to the atmosphere in less than 40 years. Ocean fertilization might also increase production of greenhouse gases, such as nitrous oxide and methane, which could impact the effectiveness of this practice, although these effects remain understudied. In places like the Southern Ocean, sunlight and changes in the availability of other nutrients, such as silicate, can also limit the effects of iron addition. Additionally, nutrients like iron can have high loss rates, up to 75%, after dispersal into seawater due to conversion into forms inaccessible to phytoplankton, potentially further reducing the effectiveness of nutrient addition.

Why are we excited?

If ocean fertilization were broadly deployed and functioned as intended, its global climate impact could reach 0.1–1.0 Gt CO₂ /yr. Ocean fertilization is expected to increase surface water pH, which could help temporarily mitigate ocean acidification locally. However, some studies suggest this benefit will come at the cost of increased acidification of deeper ocean regions. While costs remain highly uncertain, estimates of ocean fertilization range between US$80/t CO₂ and US$457/t CO₂, suggesting this practice might also be relatively inexpensive compared to other marine carbon dioxide removal practices.

Why are we concerned?

Ocean fertilization poses several technical challenges, along with significant environmental and social risks. Tracking the amount of carbon sequestered from ocean fertilization is difficult, as carbon export efficiencies – the amount of carbon produced in surface waters that makes its way to the deep sea – can be low and highly variable in time and space. Addressing this will require both field studies and models capable of capturing global and multi-decadal changes in carbon cycling due to fertilization, given the long time scales and large spatial areas involved. Implementing ocean fertilization at globally meaningful carbon removal levels could raise additional feasibility concerns, given the potential difficulty of dispersing sufficiently large quantities of nutrients across vast areas and the requirement that fertilization be done continuously to prevent the rapid return of sequestered carbon to the atmosphere. Beyond these technical challenges, ocean fertilization also poses several, potentially severe, environmental risks. Enhancing primary production could disrupt existing nutrient pools in the ocean, reducing the nutrients available for ecosystems far from dispersal sites. Another consequence of ocean fertilization is that increased organic carbon supply can enhance microbial processes that consume dissolved oxygen, potentially impairing respiration in marine organisms and leading to mortality. Other unintended consequences of nutrient fertilization include the promotion of harmful algal blooms, which can release toxins that negatively impact a wide array of life, from shellfish to marine mammals to humans. Ocean fertilization also carries significant social risks, as global-scale modification of marine ecosystems is likely to create inequities in environmental and economic impacts.

What is our assessment?

Based on our analysis, vertical farms are not an effective climate solution. The tremendous energy use and embodied emissions of vertical farm operations outweigh any potential savings of reducing food miles or land expansion. Moreover, the ability of vertical farms to truly scale to be a meaningful part of the global food system is extremely limited. We therefore classify this as “Not Recommended” as an effective climate solution.

What is it?

Vertical farms are facilities that grow crops indoors, with multiple layers of plants stacked on top of each other, using artificial lights, large heating and cooling systems, humidity controls, water pumps, and complex building automation systems. In principle, vertical farms can dramatically shrink the land “footprint” of agriculture, and this could help reduce the need for agricultural land. Moreover, by growing crops closer to urban centers, vertical farms could potentially reduce “food miles” and the emissions related to food transport.

Does it work? 

The technology of growing some kinds of crops – especially greens and herbs – in indoor facilities is well developed, but there is no evidence to show that doing so can reduce GHG emissions compared to growing the same food on traditional farms. Theoretically, vertical farms could reduce emissions associated with agricultural land expansion and food transportation. However, the operation and construction of vertical farms require enormous amounts of energy and materials, all of which cause significant emissions. Vertical farms require artificial lighting (even with efficient LEDs, this is a considerable energy cost), heating, cooling, humidity control, air circulation, and water pumping – all of which require energy. Vertical farms could be powered by renewable sources; however, this is an inefficient method for reducing GHG emissions compared to using that renewable energy to replace fossil-fuel-powered electricity generation. Growing food closer to urban centers also does not meaningfully reduce emissions because emissions from “food miles” are only a small fraction of the life cycle emissions for most farmed foods. Recent research has found that the carbon footprint of lettuce grown in vertical farms can be 5.6 to 16.7 times greater than that of lettuce grown with traditional methods.

Why are we excited?

While vertical farms are not an effective strategy for reducing emissions, they may have some value for climate resilience and adaptation. Vertical farms offer a protected environment for crop growth and well-managed water use, and they can potentially shield plants from pests, diseases, and natural disasters. Moreover, the controlled environment can be adjusted to adapt to changing climate conditions, helping ensure continuous production and lowering the risks of crop loss.

Why are we concerned?

Vertical farms use enormous amounts of energy and material to grow a limited array of food, all at significant cost. That energy and material have a significant carbon emissions cost, no matter how efficient the technology becomes. On the whole, vertical farms appear to emit far more GHGs than traditional farms do. Moreover, vertical farms are expensive to build and operate, and are unlikely to play a major role in the world’s food system. At present, they are mainly used to grow high-priced greens, vegetables, herbs, and cannabis, which do not address the tremendous pressure points in the global food system to feed the world sustainably. There are also concerns about the future of the vertical farming business. While early efforts were funded by venture capital, vertical farming has struggled to become profitable, putting its future in doubt.