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 grey 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 grey 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 H₂ 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, grey, 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 grey hydrogen, but the cost per 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 grey 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 H₂. At that scale, even modest emissions savings relative to grey 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 grey 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 dependency 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 grey 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. There are mixed expert opinions about 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.

What is our assessment?

Injecting huge amounts of reflective aerosols into the stratosphere to counteract or mask GHG-driven warming is not a serious or plausible climate solution. Its effectiveness is highly uncertain, and its potential for harmful unintended impacts to Earth and ecological systems, as well as on human well-being, is extremely high. Based on these significant problems and risks, we conclude that deploying Stratospheric Aerosol Injection is “Not Recommended.”

What is it? 

Stratospheric aerosol injection (SAI) is a geoengineering technology that uses airplanes or balloons to inject fine particles, usually sulfates, into the stratosphere, the layer of air that begins about 6 to 20 km (20,000 to 65,000 ft) above the Earth’s surface. These aerosols would scatter some of the sunlight striking the planet, reflecting it back into space. Reducing the amount of sunlight hitting the Earth is intended to cool the planet and counteract the warming effects of increasing GHG concentrations. Because SAI does not affect the atmospheric concentration of GHGs, the direct cause of global warming, this technology is not actually a solution to climate change. Instead, it is a temporary action to mask the ongoing warming effects of GHG emissions. 

Does it work?

The injection of large amounts of reflective aerosol particles into the stratosphere does have a cooling effect on the planet. Following the 1991 eruption of Mount Pinatubo, which injected 20 Mt of sulfur dioxide into the stratosphere, average global temperatures were about 0.5°C lower for more than a year. In another example, modeling studies suggest that recent reductions in East Asian air pollution have contributed to the acceleration of global warming. Therefore, in theory, deploying SAI could achieve a similar effect. However, other than modeling simulations, SAI has never been tested in the field, and researchers agree that there are substantial uncertainties and risks. For example, the ways that GHGs and stratospheric aerosols affect global temperatures differ. GHGs warm the planet more in winter than in summer, and more in the high latitudes, especially in the Northern Hemisphere, than in the equatorial regions. Because aerosols reflect solar radiation, they have a greater impact during the summer and in the equatorial zone. Finally, the solar radiation reflective effect of SAI is temporary. Depending on the location and altitude of injection, the aerosols remain in the stratosphere for only months to a few years.  

Why are we excited?

We’re not. The only argument in favor of deploying SAI is based on the concern that we cannot reduce GHG emissions fast enough to avoid the catastrophic environmental and societal impacts of climate change. SAI proponents argue that this geoengineering approach to reduce global temperatures could be a “bridge,” buying time to cut GHG emissions and remove atmospheric CO₂ over the longer term. 

Why are we concerned?

SAI is an untested technology designed to alter planetary energy balance and atmospheric dynamics. Numerous modeling studies indicate that its efficacy to reduce global or regional temperatures as intended is highly uncertain and that it has high risks for unintended impacts on Earth, ecological, and human systems. These studies show that SAI could have substantial effects on the physics, chemistry, and circulation of the upper atmosphere, including harm to the ozone layer. It could destabilize weather and rainfall patterns, reducing the amount of sunlight striking the Earth’s surface, and changing the balance of “direct” and “diffuse” sunlight, effectively making the sky look more hazy. These effects will, in turn, have profound impacts on ecosystems, including the rates of photosynthesis in forest carbon sinks, agriculture, and human well-being. Even if it works to lower temperatures as planned, SAI will have no impact on the non-climatic effects of increasing CO₂, such as ocean acidification. SAI is also inherently a temporary intervention; it will require sustained deployment for as long as 100 years, according to one study, to avoid “termination shock” and an abrupt temperature increase if GHG concentrations are still high. SAI also poses immense geopolitical, legal, and ethical challenges, including international responsibilities for implementation, financing, compensation for negative impacts, and procedural justice questions, such as those around informed consent. And finally, beyond these scientific, environmental, political, and socio-economic concerns, SAI poses a serious “moral hazard” that could distract or delay action on real solutions to climate change.

What is our assessment?

Based on the difficulty of capturing low concentrations of CO₂ from the air and the associated technological, energy consumption, and financial challenges facing DAC, it is unlikely that this climate technology can be scaled up to remove globally meaningful amounts of CO₂. Furthermore, based on the current financial and policy support for DAC from fossil-fuel interests, there is a clear risk that the technology will be used to enable and perpetuate the production and use of fossil fuels, which is antithetical to solving the climate crisis. Therefore, we conclude that deployment of DAC is “Not Recommended” as a climate solution.

What is it? 

DAC is a suite of engineered technologies that remove CO₂ directly from the atmosphere, concentrate it, and then inject it underground for permanent storage. CO₂ is captured from the atmosphere by moving large volumes of air, usually with large fans, past a reactive material that selectively binds CO₂, either a solid sorbent (referred to as solid-DAC or S-DAC) or a liquid solvent (referred to as liquid-DAC or L-DAC). The captured CO₂ is recovered from the reactive material by applying heat, pressure, or chemical reactions, and collected and compressed for transportation and storage. The concentrated CO₂ is then injected deep underground into geological formations, such as saline aquifers or basalt formations, where it can be permanently stored. 

Does it work?

The technology and chemistry for the selective capture of CO₂ from air are effective, although the CO₂ capture efficiency varies with the reactive material and other factors. A variety of solid and liquid reactive materials have been developed, along with material-specific processes for recovering captured CO₂ and regenerating the sorbents. This process is very energy-intensive and, for liquid-DAC, water-intensive. To capture and recover 1 t CO₂, solid-DAC uses about 1,100 kWh, while liquid-DAC uses about 2,500 kWh and consumes as much as 7 t of water. Most of the energy for DAC (70–90%) is used to generate heat for recovery of the captured CO₂ and regeneration of the sorbent material. Liquid-DAC requires temperatures up to about 900 °C (1,652 °F), while solid-DAC requires temperatures of only about 100 °C (212 °F). Because the process is so energy intensive, DAC achieves net carbon removal – capturing and sequestering more CO₂ than it emits – only if it is powered by zero or low-carbon energy sources and/or uses waste heat. For example, recent reporting showed that the amount of CO₂ captured and stored by Climeworks, the largest commercial DAC company currently in operation, was insufficient to offset the facility’s operational GHG emissions. CO₂ captured by a DAC facility can also be used for other purposes, such as enhanced oil recovery or production of algae biofuels. However, life cycle analyses conducted by the National Energy Technology Laboratory show that these pathways do not result in net carbon removal due to the emissions from production and/or use of these other products. Therefore, in addition to its requirements for zero or low-carbon energy, DAC can only be an effective method for net carbon removal if the CO₂ it captures is permanently stored deep underground. With appropriate pre-injection site selection, geologic testing, and post-injection monitoring, underground storage of CO₂ is safe and effectively permanent.

Why are we excited about it?

Unlike some other carbon removal technologies and practices, a DAC facility has a relatively small footprint and can be located anywhere there is sufficient low-carbon energy and infrastructure and capacity to transport or store captured CO₂. In addition, the amount of CO₂ removed from the atmosphere can be directly measured by monitoring the flow and concentration of captured CO₂ at the point of storage. Compared to many other carbon removal approaches, this method provides a higher level of confidence in the amount of CO₂ being removed for investors and carbon credit purchasers. The geological sequestration of captured CO₂ has high permanence, effectively removing CO₂ from the atmosphere for thousands of years with a low risk of reversal. There are numerous research and pilot projects underway to improve CO₂ capture efficiency, reduce energy use, and reduce costs, which may improve the effectiveness and cost of this technology. 

Why are we concerned?

The concentration of CO₂ in the atmosphere is small, currently about 420 parts per million, or about 0.04%. This means that a DAC facility must process huge amounts of air – more than 1,600 t by one estimate – and consume more energy than a typical U.S. household uses in a month to capture 1 t CO₂. Scaled up to remove a globally meaningful amount of CO₂ (>0.1 Gt CO₂ /yr), DAC would consume more energy than the annual energy consumption of 10 million U.S. households. In addition, removing and storing CO₂ using DAC is very expensive, costing up to US$1,000/t CO₂ stored. This is more than twice the cost per t for all other commercially available carbon removal technologies and practices. 

For these reasons, the technical and financial feasibility of scaling DAC to remove globally meaningful amounts of CO₂ from the atmosphere is low. Despite these challenges, as of September 2025, more than 30 companies have sold more than 2.4 million t of future carbon removal credits. However, less than 1,300 t CO₂ has actually been removed so far – or only 0.05% of these promised credits. To put this in perspective, despite spending billions of dollars, DAC has removed about as much CO₂ as would be saved by keeping 250-300 cars off the road for a single year.

There is also an opportunity cost for DAC. Even if a DAC facility is powered by solar, wind, geothermal, or nuclear energy, that carbon-free energy could have been used to displace coal- and gas-powered electricity instead, reducing emissions by far more than a DAC facility can capture and store. Similarly, the large amounts of public and private sector funding going to DAC could be more cost-effective and carbon-effective if used for other, more effective actions to cut emissions or remove CO₂. There is also the risk that DAC will be used to delay or avoid emissions reduction actions or for greenwashing by fossil fuel companies and other emitters. Substantial amounts of the funding supporting the development of DAC are coming from fossil fuel companies, which have publicly stated that they view carbon capture as a strategy to extend society’s use of fossil fuels. Finally, unlike most other emissions reduction or carbon removal actions, DAC provides no obvious other benefits to nature or human well-being.

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 or is transported 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 ~1,000 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 GHGs, 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 such as 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 reduce 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 CO₂ 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 because 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 need for fertilization to be done continuously to minimize carbon returning 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 promoting harmful algal blooms that 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.
 

What is our assessment?

The use of corn ethanol as a transportation biofuel, which has led to the expansion and intensification of corn production, does not reduce GHG emissions compared to gasoline. Based on this finding, using corn ethanol is not a plausible approach for reducing emissions and is “Not Recommended” as a climate solution.

What is it?

Corn ethanol is a liquid biofuel that is blended with gasoline to displace a fraction of the petroleum-based fuel with a renewable fuel derived from plants. Proponents claim that blending corn ethanol with gasoline reduces emissions because the CO₂ produced from combusting the ethanol is offset, or balanced out, by the atmospheric CO₂ absorbed by the corn plant during growth. Corn ethanol is made from corn grain by breaking down the starch in the kernels into sugar and then fermenting it into a liquid. In the United States, the world leader in biofuel production, almost 90% of biofuel is corn ethanol. Most gasoline now sold in the U.S. contains about 10% corn ethanol, and, in 2025, the Renewable Fuel Standard (RFS) program requires production of more than 15 billion gallons of this biofuel. Currently, it is primarily made from corn kernels; the technology for producing biomass-derived ethanol from other, non-edible parts of the corn plant is not yet commercially viable. Brazil is the second-largest producer of ethanol, but uses sugarcane as a feedstock. 

Does it work?

The Renewable Fuel Standard requires that the life cycle emissions from corn ethanol be at least 20% lower than those of conventional gasoline. However, based on comprehensive life cycle emissions analyses, using corn ethanol does not reduce emissions compared to gasoline. The main reasons for this are that the production of corn and processing it into ethanol generate large amounts of emissions, including from land conversion, fertilizer-related nitrous oxide emissions, and the industrial process of fermenting the corn into ethanol. The most prominent recent study reported that corn ethanol life cycle emissions were, at best, no less than gasoline and, more likely, were 24% higher. Corn ethanol is also more emissions-intensive than ethanol made from other plants, like sugar cane. 

Why are we excited?

Ethanol has been used as a transportation fuel, including as a blend with gasoline, for more than a century. It boosts the octane number of fuel, improves engine performance and fuel economy, and reduces emissions of harmful pollutants like unburned hydrocarbons, nitrogen oxides, and particulates. Ethanol has also been used to replace other harmful and polluting gasoline additives, including lead and methyl tert-butyl ether (MTBE). Ethanol produced from non-edible biological feedstocks with lower production emissions, such as switchgrass or cellulose from crop residues, has the potential to reduce emissions. 

Why are we concerned?

The Renewable Fuel Standard (RFS) program requires that biofuels be blended into the transportation fuel supply at annually increasing increments. The United States now uses one-third of its corn to generate more than 15 billion gallons of ethanol per year. Not only does this mandated program not reduce emissions (it more likely increases emissions), but it also consumes corn that could otherwise be used for food or animal feed. The increased demand for corn for ethanol has increased corn prices, which in turn have contributed to the conversion of grasslands and semi-natural ecosystems to grow more corn. When grasslands, woodlands, or other natural ecosystems are plowed and converted to cropland, the carbon stored in the vegetation and soil is emitted to the atmosphere. Between 2008 and 2016, the conversion of 1.8 Mha of natural and semi-natural land in the U.S. released about 400 million metric tons of CO₂ from vegetation and soil. The increased corn production also increased the application of synthetic fertilizers, which has increased nitrate leaching, phosphorus runoff, and emissions of nitrous oxide, a powerful GHG (see Improve Nutrient Management). These problems are particularly severe in the U.S. Midwest and the Mississippi River drainage.