What is our assessment?

Based on our analysis, restoring large herbivores is a promising climate solution. Still, there is limited (and mixed) evidence on its effectiveness across several ecosystem types and its potential global impact. We will  “Keep Watching” this potential climate solution.

What is it?

Restoring large (>45 kg) wild herbivores – such as bison, moose, and elephants – means reintroducing or increasing their populations on natural and degraded lands where they are absent. Large herbivores play an important role in carbon, water, and nutrient cycling and shape habitats for plants and animals. Herbivores alter the diversity and abundance of plant species, the habitat structure (such as forest vs. grassland), and plant productivity. These changes help mitigate climate change in three main ways. First, carbon storage can increase if plant productivity increases. Second, herbivores generally create more open landscapes that reflect more sunlight, which has a local cooling effect. Third, herbivores can reduce the risk of intense wildfires by creating more open ecosystems and reducing fuel loads. 

Does it work?

There is limited and mixed data on the solution's effectiveness. It is difficult to measure responses to a single change within a complex ecosystem and to account for differences across ecosystem types, pre-implementation levels of degradation, herbivore species, and herbivore density. Several examples illustrate these context-dependent nuances. Forest elephants prefer palatable tree species, favoring those with denser wood (more carbon), and disperse large fruits from larger, carbon-rich trees. In contrast, elephants in the savanna reduce carbon stocks by browsing branches and knocking over trees, creating a more open, grassy habitat that stores less carbon in vegetation. However, this may be offset if soil carbon builds up over time. In the boreal forest and tundra, caribou reduce tree and shrub cover (lowering carbon stocks), creating more open habitat. The open habitat reflects more sunlight than the forest (creating local cooling). Still, the exposed ground may warm enough to increase the rate of decay of organic matter in the soil (increasing CO₂ emissions). 

Where restoring herbivores increases carbon storage in the ecosystem, it complements or contributes to Protect Forests, Restore Forests, Protect Grasslands and Savannas, and Restore Grasslands and Savannas. Restoring large herbivores, such as manatees, in coastal and marine ecosystems could boost carbon storage, though there is even less evidence in marine ecosystems than in terrestrial ones. 

Why are we excited?

Restoring large herbivores is a key component of active or passive ecological restoration approaches to increase species diversity, restore natural processes, and aid species dispersal on natural and degraded lands. Where restoring herbivores increases carbon storage in the ecosystem, it complements the climate solutions for protecting and restoring ecosystems. Even when it has limited mitigation potential, the solution still has many biodiversity benefits. It helps protect and restore large herbivore species, ~60% of which are threatened with extinction. In addition, herbivory is an important ecological process in many ecosystems. In boreal and tundra regions, reintroducing caribou reduces the risk of intense wildfires in the forest and can limit the northward expansion of forests, which accelerates warming. Reintroducing bison to the Great Plains on grasslands previously grazed by cattle has little impact on the carbon storage, but it increases plant species diversity, reduces methane emissions, and rebuilds cultural heritage for Indigenous people. This solution is becoming more common, particularly in Europe, but we are unable to estimate the potential global adoption and impact.

Why are we concerned?

The disadvantage of this solution for climate mitigation is that it can be expensive per metric ton of carbon, often has a limited impact, and can, in many cases, decrease carbon stocks. For example, reintroducing herbivores, such as elephants or deer, at high densities can reduce carbon stocks and limit further carbon sequestration. As a result, excluding or reducing herbivore abundance is a strategy in many habitat restorations. Changes in carbon storage, particularly in soils, are very difficult to measure and to attribute solely to the reintroduction of herbivores. 

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.

What is our assessment?

Because incineration of waste to produce electricity and/or heat does not reduce emissions in most circumstances, and it can displace or disincentivize other, more effective waste treatment practices, Deploying Waste to Energy is “Not Recommended” as a climate solution.

What is it?

Waste to energy (WTE) uses high temperature incineration (above 800^oC) of waste, including municipal solid waste (MSW), medical or hazardous waste, and waste biomass from agriculture and forestry, to generate electricity and/or heat. This energy production technology can be used to displace fossil fuel energy sources. However, like fossil fuels, the incineration process produces GHGs during combustion, primarily CO₂ and nitrous oxides, as well as ash and other pollutants as byproducts. WTE also diverts waste from landfills and open dumps, avoiding high-impact methane emissions from decomposing organic materials and pollution and health risks from inorganic or toxic materials. Globally, there are more than 1,700 WTE plants. According to the International Energy Agency (IEA) World Energy Balances, WTE from industrial and municipal waste accounted for less than 0.5% of global energy production in 2022. Around 62% of WTE plants are in Asia, 33% are in Europe, and 4.5% are in North America. Other waste to energy technologies, such as pyrolysis and gasification, which use heat to convert organic materials into various forms of syngas, are evaluated in other Drawdown Explorer solutions and not included here. 

Does it work?

The effectiveness of WTE for reducing GHG emissions when substituting for fossil fuels for heat and energy, while also avoiding landfill emissions, is highly variable. Effectiveness varies with waste type and quality, combustion characteristics, air pollution controls, alternative disposal methods, and the type of electricity generation that it displaces. WTE incineration is less energy efficient than natural gas or coal for producing electricity (20–30% for WTE compared to 40–60%), and relatively more waste must be burned to produce comparable amounts of energy. WTE incineration is also less energy efficient than pyrolysis (40–75%), gasification (40–60%), and even methane digestion (30–40%). In regions that incinerate large proportions of plastic waste, such as South Korea and China, emissions are higher than for landfilling the waste. For some waste streams in some regions, other treatment strategies such as recycling, composting, pyrolysis, gasification, or the production of biochar, bio-oils, or bio-bricks yield greater emissions reductions.

Why are we excited? 

WTE can reduce waste volumes by up to 90%. In high-income regions without available land for sanitary landfills with landfill gas capture systems (see Improve Landfill Management), incineration is a viable alternative for post-recycling, hazardous, industrial, or medical waste. However, incineration of these waste streams requires strict standards, advanced air pollution control systems, and continuous monitoring to minimize harmful emissions of toxic pollutants. 

Why are we concerned?

In almost all circumstances, incinerating waste to produce reliable electricity or heat is not an effective method for reducing GHG emissions. WTE plants require a steady stream of waste feedstock to ensure ideal combustion conditions for electricity and heat production. This can incentivize waste production and disincentivize alternative waste treatments that reduce emissions more effectively. For example, in the European Union (EU), where incineration is widely used, some countries need to import waste to maintain energy production. This offsets some or all of the potential climate benefit due to emissions during transport and may divert attention from better waste management solutions, such as regulations on packaging, recycling, and composting. In other countries, high incineration rates of MSW (above 30%) are correlated with declines in recycling rates. For example, in 2018, Japan incinerated over 80% of MSW while only 4.9% was recycled. Recent policy initiatives there now focus on increasing recycling, which effectively and consistently reduces emissions, and decreasing incineration through source separation of waste. According to one study, chemically recycling plastic waste rather than incinerating it saves 0.82 kg CO₂‑eq /kg of feedstock. 

WTE is a substantial source of toxic air pollution. Poorly constructed, unregulated incinerators generate air pollution and large amounts of ash that will need further treatment. Advanced air pollution control systems are required to minimize emissions of GHGs and pollutants, including particulate matter, sulfur dioxide, nitrogen oxides, carbon monoxide, other acid gases, heavy metals, and persistent organic pollutants like dioxins. Even in high-income countries with strict air quality standards, polluting incinerators are disproportionately sited in under-resourced communities, which exacerbates environmental justice issues. In the United States, 79% of incinerators are located in low-income or minority communities. 

Finally, WTE is the most expensive waste management method in most regions, largely due to high capital costs. In 2018, all countries using industrial incinerators for incinerating more than 10% of MSW were high-income, except China. The addition of necessary emission control and monitoring systems further increases costs, making WTE more expensive than methane digesters or landfills with gas capture systems.

What is our assessment?

Using CCS on fossil-fueled power plants will reduce electricity production emissions, but it is more expensive, more energy-intensive, and more polluting than readily available, cheaper, and cleaner alternatives like wind, solar, and geothermal. Based on this, and the risk that large-scale deployment of CCS on fossil-fueled power plants could drive continued production and use of coal and gas, we conclude that using CCS on fossil fuel power plants is “Not Recommended” as a climate solution.

What is it?

Carbon capture and storage (CCS) is a technology that reduces GHG emissions from fossil fuel-powered electricity generation facilities by selectively capturing CO₂ from the power plant’s exhaust flue, preventing it from entering the atmosphere. The captured CO₂ is then concentrated, compressed, and permanently stored underground. There are other commercially available CCS technologies, such as pre-combustion capture and oxy-fuel combustion, but these are used almost exclusively for industrial processes like gas processing and cannot be readily retrofitted to existing power plants. CCS can also be applied to capture CO₂ from other industrial facilities that generate emissions from fuel combustion or production processes, like cement or ethanol production plants, or from biomass energy power plants. Instead of permanent storage, captured CO₂ can also be used as a chemical precursor for the manufacture of other products or for enhanced oil recovery, but, compared with geologic storage, these post-capture uses of CO₂ emit GHGs, thereby reducing or eliminating the emissions reduction efficacy of CCS. 

Does it work?

The technology and chemistry for the selective capture of CO₂ from the exhaust of a power plant are effective. There are numerous chemical, membrane, and cryogenic methods for capturing CO₂, but monoethanolamine (MEA) is the predominant commercially available chemical absorbent currently in use in power plants with CCS. CO₂ capture efficiency varies with the type of reactive absorbent material and plant operations. Most CCS installations target 90% CO₂ capture rates, although actual capture rates are usually lower. CCS infrastructure is large, and the process of capturing CO₂ from power plant exhaust is complex, expensive, and energy-intensive. CCS requires the flue gas to be pumped to different parts of the power plant, the CO₂ to be captured and then separated from the sorbent material, and the concentrated CO₂ to be compressed for transport and storage. Energy for all these processes comes from the power plant. Various studies estimate CCS consumes at least 15–25% of the plant’s total generation capacity, with most of the energy used to separate the CO₂ and regenerate the sorbent material. 

CCS has been used in pilot studies and commercial operations in a few dozen coal and natural gas power plants since the late 1990s. Despite the functional effectiveness of the technology, use of CCS to reduce power plant emissions has not been broadly adopted, and most CCS projects initiated in the past three decades have failed or been discontinued. Based on various assessments and projections, deployment of CCS on power plants has consistently lagged behind its expected contribution to emissions reduction. There are currently only four power plants with CCS in operation in the world, less than 0.05% of the global fossil-fueled power plant fleet. According to a 2021 study, only 10% of proposed CCS projects for power plants have actually been implemented. Based on another study, 78% of all power plant and industrial manufacturing CCS pilot and demonstration plants with a project size greater than 0.3 Mt CO₂ /yr have been cancelled or put on hold. 

Why are we excited?

Globally, emissions from coal- and gas-fired power plants are still increasing, primarily in China and India, where large numbers of new thermal power plants have been built in the last two decades. Given the typical 30- to 45-year operational lifespan for coal and gas power plants, retrofitting these newer plants with CCS could substantially reduce their operational emissions while also allowing plant owners and investors to recover their investments. Installation of CCS to reduce emissions can also be prioritized for power plants located near places with geologic storage and where alternative electricity generation options are limited. There is a large amount of research underway to develop and test alternative carbon capture technologies, most aimed at increasing carbon capture efficiencies and reducing energy demands and costs. Other research on the factors contributing to the failure of most CCS projects to date may lead to the development of regulations and policies that require or incentivize the use of CCS for power plants, which could increase the current low implementation and success rates for this emissions reduction technology. 

Why are we concerned? 

While CCS can reduce the operational CO₂ emissions from fossil-fueled power plants, large-scale deployment of this technology will likely drive continued production and use of coal and gas. Even before fossil fuels are burned, extraction, transport, and processing generate substantial GHG emissions, particularly for gas. Therefore, in addition to perpetuating the fossil fuel industry, even 90% efficient CCS reduces only a fraction of the life cycle emissions from coal and gas. 

Widespread deployment of CCS in the electricity sector could also delay or crowd out deployment of wind, solar, and geothermal energy, slowing the clean energy transition that is already underway. Beyond these risks, the three-decade-long failure of power plant CCS to make the transition from pilot-scale science and technology to large-scale commercial deployment reflects its systemic problems and limitations. Unlike wind and solar energy, which have seen costs decline rapidly with development and deployment, CCS on power plants shows little evidence of a learning curve. It remains very expensive and very energy-intensive. A large-scale CCS demonstration project can cost more than US$1 billion to build and, in addition to its operational costs, CCS consumes at least 15–25% of the energy that the plant could otherwise sell to customers. CCS-related energy requirements could mean that a power company would need to build an additional power plant to compensate for reduced electricity deliveries from every four of its power plants equipped with CCS. 

Due to these high project risks and costs, as well as the lack of regulations and policies to require or support CCS on power plants, public and private investments in the technology have been falling. Despite all this, recent research shows that the vast majority of lobbying spending for government support of CCS comes from fossil fuel interests, which have publicly stated that they view the technology as a strategy to extend society’s use of fossil fuels. Finally, in contrast to most other climate solutions that provide other benefits to natural systems or human well-being, CCS on power plants does nothing to address or alleviate the current harm from toxic air pollution produced by fossil-fueled power plants.

What is our assessment?

Overall, ocean electrochemical approaches appear plausible, but their costs, environmental impacts, and effectiveness require further research. Furthermore, improving energy efficiency and increasing the use of low-carbon electricity supply will be critical to its viability as a large-scale global climate solution. Therefore, we will “Keep Watching” this solution.

What is it?

Ocean electrochemical systems pass an electrical current through seawater to drive chemical reactions that redistribute major ions, creating separate acidic and basic solutions that can be used for CO₂ removal in different ways. One common method uses an acidic solution to lower the pH of seawater. This shifts the chemical balance in the carbonate system toward dissolved CO₂, which then degasses from the water and can be captured for long-term storage in underground geologic reservoirs. Another variation uses the basic solution to increase the alkalinity of seawater, which shifts the carbonate system so that the ocean can absorb and remove more atmospheric CO₂, similar to Deploy Ocean Alkalinity Enhancement. Several other variations exist, including designs that use the acidic stream to weather alkaline rocks and the basic stream to precipitate carbon-containing minerals, such as carbonates. These systems can operate as closed systems, in which carbon removal is achieved with on-site infrastructure, as open systems, which rely on effluent discharge into the ocean to complete carbon removal, or as combination hybrid systems that combine both approaches.

Does it work?

The fundamental chemistry and physics underlying electrochemical methods for shifting chemical reactions and ionic balances in solutions are well understood and sound. These approaches are already used on a large scale in industrial applications, such as the chlor-alkali process for producing sodium hydroxide and chlorine. However, using electrochemistry to remove CO₂ from seawater remains in early development. Studies show that CO₂ can be removed using these methods, and some pilot projects are currently underway; however, the effectiveness and costs of approaches, particularly when energy requirements are factored in, are still uncertain. In addition, the effectiveness of ocean electrochemistry for CO₂ removal also depends on how the removed CO₂ is stored. For example, to have a beneficial climate impact, CO₂ degassed from acidified seawater must be permanently stored deep underground rather than used as a chemical precursor for the manufacture of other products or for enhanced oil recovery, both of which create additional GHG emissions.

Why are we excited?

Ocean electrochemical systems for CO₂ removal offer several potential advantages, including useful co-products, opportunities to leverage existing infrastructure, and possible environmental benefits. Some electrochemical variations generate beneficial co-products, such as hydrogen gas, which could help offset costs and displace external fossil fuel energy demand. Some designs can also be coupled with existing brine streams in desalination or industry to leverage both infrastructure and waste feedstocks. In other cases, seawater itself serves as an abundant natural feedstock for these electrochemical processes. In some variants, environmental benefits may include local reductions in ocean acidification, which may benefit some marine organisms. Additionally, monitoring and verification of electrochemical variations using closed-system approaches may be more straightforward compared to other marine carbon dioxide removal (mCDR) approaches that require tracking carbon across large regions and depths of the ocean. 

Why are we concerned?

Ocean electrochemical approaches for carbon removal present several challenges with respect to cost, effectiveness, and potential environmental impacts. Many designs are energy-intensive, require substantial infrastructure, and need large volumes of seawater. Their effectiveness and overall climate impact will depend on access to low-carbon electricity and accurate accounting of full life cycle energy use, since use of fossil-based electricity could offset much of the CO₂ removal benefits. Costs are unclear but are largely estimated to be well above US$500/t CO₂ for many variations. Additionally, some approaches, such as those that aim for carbonate mineral precipitation, do not inherently yield net CO₂ removal unless the alkalinity used for mineral formation is replenished, which can be costly. 

If the electrochemical approach relies on capturing CO₂ from seawater, geologic storage will be required to dispose of the degassed CO₂. Furthermore, some co-products of electrochemical approaches, such as acidic brines or chlorine gas, could exceed viable market demand at large scales and may require waste disposal or the development of technologies to reduce or eliminate their production. Finally, for open-system electrochemistry variants that rely on effluent interactions in the ocean to complete carbon removal, carbon accounting will likely need to extend far beyond the facility itself, making monitoring and verification of carbon removal similarly complex to other marine carbon dioxide removal approaches. 

Environmental impacts are not well understood but will depend on the specific electrochemical approach. Approaches that discharge chemically altered effluent into the ocean could impact marine organisms near discharge points, with ecosystem responses that remain highly uncertain. Large-scale pumping of seawater could affect marine organisms through entrainment and other intake impacts, due to the large volume of influent needed for these approaches at scale. When these approaches require alkaline rocks, they raise concerns about the potential effects of mining, similar to Deploy Ocean Alkalinity Enhancement and Enhanced Rock Weathering.

What is our assessment?

Based on our analysis, OAE could be a promising carbon removal technique, but it is not ready for large-scale deployment until the risks, costs, and effectiveness become clearer. We will “Keep Watching” this potential climate solution.

What is it?

OAE is the practice of adding alkalinity to seawater to increase the ocean’s ability to remove atmospheric CO₂. The addition of alkalinity through OAE mimics the natural process of weathering, or the physical and chemical breakdown of rocks. Rock weathering on land produces alkaline substances that eventually flow into the ocean through rivers and groundwater. This natural supply of alkalinity reduces ocean acidity, which affects the distribution of various carbon forms in the ocean. As alkalinity increases, CO₂ dissolved in seawater shifts toward more stable carbon forms, like bicarbonate and carbonate ions, that cannot exchange with air. This allows the ocean to remove more gaseous CO₂ from the atmosphere because the ocean and the atmosphere maintain a balance of CO₂ through gas movement at the sea surface. Most of the dissolved carbon in the ocean is bicarbonate and carbonate ions, which can persist in seawater for thousands of years. Under natural conditions, the ocean removes nearly 0.5 Gt of CO₂ annually. OAE generally relies on dissolving large amounts of ground-up rocks, either directly in the ocean or indirectly in water that is added to the ocean, to increase alkalinity and remove CO₂. This practice typically requires mining for alkaline rocks, though alkaline materials can also be sourced from waste by-products of other industries (e.g., steel slag, mine tailings) or commercially through human-made substances.

Does it work?

The science behind OAE is theoretically sound, and OAE is expected to result in durable storage over long time periods (>100 years). At scale, OAE could potentially remove over 1 Gt CO₂ /yr, but additional lab and field-based studies are needed to understand whether this approach is effective and safe. The majority of our understanding of OAE comes from models and laboratory experiments. However, when crushed minerals have been dispersed in field studies, the dissolution has not always occurred as expected. Several large-scale experimental trials are currently underway or planned, which will produce real-world data and inform monitoring and verification tactics needed to help refine and guide future implementation. These tests will also provide critical information on any ecological or community impacts. Various ways of implementing OAE are being developed, including ship-based dispersal, shoreline-based systems, and other approaches that leverage existing industrial waste streams or combine with other marine carbon dioxide removal (mCDR) techniques, such as electrochemical alkalinity generation.

Why are we excited?

OAE removes CO₂ from the atmosphere and stores it in the ocean as bicarbonate and carbonate ions, which are stable over long time periods. This means the CO₂ would be durably stored from the atmosphere for thousands of years. OAE could be scaled globally and can also mitigate local ocean acidification, a growing issue that threatens a range of marine ecosystems. Indeed, adding alkalinity to seawater has already been shown to mitigate ocean acidification in some coral reefs. Mitigating ocean acidification could also benefit fisheries and aquaculture, highlighting the potential for OAE to provide additional local benefits beyond carbon removal.

Why are we concerned?

Several technical, environmental, and social concerns surround OAE. The effectiveness could be limited by real-world conditions that either transport the alkaline materials away from the ocean’s surface before CO₂ can be absorbed or result in unexpected chemical reactions or biological uptake of the added alkalinity. Measuring and verifying the amount of CO₂ permanently stored using OAE is also challenging and will rely on a combination of field data and complex numerical models, which will require significant effort to collect and develop. Beyond these technical challenges, OAE poses potential environmental risks on land and in the ocean. On land, OAE could require an expansion of mining that rivals the cement industry, which could have negative environmental impacts on human and ecosystem health. In the ocean, increased alkalinity and the potential release of metals from the source rocks could negatively affect some marine life, but our understanding of the effects on individual species and food webs is limited. OAE could also interfere with existing ocean uses (e.g., fisheries, recreation) in some places and could have other unintended consequences as well. For instance, research suggests that OAE reduces natural alkalinity production in some ocean areas. In addition, OAE faces several social challenges. To be successful, mCDR approaches, like OAE, will require rapid, meaningful, and just community engagement. Public concerns about OAE have already led to a pilot project cancellation, highlighting the importance of public perception for OAE feasibility. It is also unclear if OAE can be scaled globally at reasonable costs, with current estimates highly variable but generally over US$100/t CO₂. Lastly, acquiring and dispersing sufficient alkaline materials could be challenging at scale, particularly because some materials are currently energy-intensive to source, transport, and/or produce.

What is our assessment?

Based on our analysis, enhanced rock weathering is a promising carbon removal technique, but it is not ready for large-scale deployment. We will “Keep Watching” this potential climate solution.

What is it?

Enhanced rock weathering is a practice that removes CO₂ from the atmosphere by accelerating the natural chemical and physical breakdown, or weathering, of rocks such as basalt, olivine, or limestone. This is typically achieved by crushing the rocks into dust or sand-sized particles to increase their surface area before applying them to croplands, beaches, or directly into the ocean, the latter of which is also a form of carbon removal known as ocean alkalinity enhancement. During weathering, the rock surface chemically reacts with atmospheric CO₂ that is dissolved in rain or ocean water. This reaction produces bicarbonate ions containing the carbon from the captured CO₂ and positively charged cations, such as magnesium or calcium, depending on the type of rock. For land-based enhanced rock weathering, the bicarbonate needs to be flushed out to the ocean, where it is stable and can be securely stored for thousands of years.  

Does it work?

The basic idea of enhanced rock weathering is scientifically and geologically sound. Its effectiveness in converting atmospheric CO₂ into bicarbonate has been demonstrated in laboratory and field trials for several rock types and application sites. There are currently numerous research and demonstration projects underway. More than a dozen companies are selling enhanced rock weathering-based carbon removal credits, with nearly 10,000 t CO₂ reported to have been removed as of early 2025.

Why are we excited?

Enhanced rock weathering has several features that improve the likelihood that it can be scaled up to remove and store globally meaningful amounts of atmospheric CO₂ (>0.1 Gt CO₂/yr). Since enhanced rock weathering utilizes a natural process – mineralization – it does not need to be combined with other technologies to capture CO₂ from the air or durably store it. Moreover, it does not require external energy for the carbon capture and storage process, although it does use energy and generate emissions from the mining, crushing, transport, and deployment of the crushed rock. Suitable rock types, such as basalt, which is widely used in construction, paving, and concrete, are common and often locally available. Globally, there are large areas of land and ocean surface on which enhanced rock weathering could be deployed, including on croplands where current agricultural practices often already include regular application of soil amendments. A recent study suggested that extensive deployment of enhanced rock weathering on U.S. agricultural lands could sequester 0.16–0.30 Gt CO₂/yr by 2050. Other studies have shown that the application of crushed rock to croplands for enhanced rock weathering can improve soil pH, provide essential soil nutrients, and improve crop yields.

Why are we concerned?

There are numerous challenges for enhanced rock weathering, as well as potential risks and adverse impacts from its large-scale deployment. Numerous studies on both land- and ocean-based enhanced rock weathering have shown that the amounts of atmospheric CO₂ converted into bicarbonate are highly variable, dependent on rock type, soil type, application rates, and other variables, and are therefore difficult to accurately predict and model. This makes measurement, reporting, and verification of the amount of CO₂ captured and stored, which is essential for the carbon market, reliant on extensive and expensive field measurements and customized models. There are also concerns about the harmful impacts of heavy metals, like nickel or chromium, that can be released during weathering, as well as other ecological impacts and environmental justice concerns, particularly for crushed rock deployed on beaches or in the ocean. Finally, costs for deployment and the purchase of enhanced rock weathering-based carbon credits are relatively high (>US$200–US$500/t CO₂ removed) and will likely remain high if verification continues to depend on large numbers of field measurements and carbon removal cannot be easily modeled. There is a general consensus in the scientific community that the current knowledge base is not sufficient to reliably or accurately quantify the CO₂ captured and stored by most land- or ocean-based enhanced rock weathering deployments.

What is our assessment?

Based on our analysis, cultivated meat is promising in its ability to reduce emissions from meat production, but the impact on a large scale remains unclear. Based on our assessment, we will “Keep Watching” this potential solution.

What is it?

Cultivated meat (also called lab-grown or cultured meat) is a cellular agriculture product that, when used to replace meat from livestock, can reduce emissions. Cultivated meat is developed through bioengineering. Its production uses sample cells from an animal, in addition to a medium that supports cell growth in a bioreactor. Energy is required to produce the ingredients for the growth medium and to run the bioreactor (e.g., for temperature control, the mixing processes, aeration).

Does it work? 

Since the development of cultivated meat is still in its infancy, there is limited evidence on its emissions savings potential from large-scale production. Preliminary estimates differ by an order of magnitude, depending on the energy source used in the lab environment. Using fossil energy sources, emissions generated from the production of 1 kg of cultivated meat could reach 25 kg CO₂‑eq. If renewable energy is used, emissions could be about 2 kg CO₂‑eq/kg of cultivated meat. By comparison, producing a kilogram of beef from livestock generates 80–100 kilograms CO₂‑eq, on average. Almost half of those emissions from livestock beef are in the form of methane. Producing pig meat and poultry meat generates about 12 kg and 10 kg CO₂‑eq, respectively. Based on these estimates, cultivated meat could substantially reduce the emissions of beef. Compared to pork and chicken, however, its emissions depend on the source of energy used during production.

Why are we excited?

The cultivated meat industry is fairly new but growing rapidly. The first cell-cultivated meat product was developed in 2013. In 2024, there were 155 companies involved in the industry, located across six continents, mostly based in the United States, Israel, the United Kingdom, and Singapore. Agriculture is responsible for about 22% of global GHG emissions, and raising livestock, especially beef, is particularly emissions-intensive. Therefore, cultivated meat has great potential to reduce related emissions as demand for meat continues to grow across the world. Cultivated meat enables the production of a large amount of meat from a single stem cell. This means that far fewer animals will be needed for meat production. Cultivated meat is also more efficient at converting feed into meat than chickens, which reduces emissions associated with feed production and demand for land.

Why are we concerned?

Concerns about cultivated meat include scalability, cost, and consumer acceptance. Because cultivated meat is still an emerging area of food science, the cost of production may be prohibitive at a large scale. Although cell culture is routinely performed in industrial and academic labs, creating the culture medium for mass-market production at competitive prices will require innovations and significant cost reductions. There are still many unknowns about the commercial potential of cultivated meat and whether consumers will accept the products. In 2024, companies began to move from research labs to larger facilities to start producing meat for consumers. Several countries now allow the sale of cultivated meat. In the United States, about one-third of adults find the concept of cultivated meat appealing, and only about 17% would be likely to purchase it, according to a poll conducted on behalf of the Good Food Institute. However, even substituting a fraction of the beef consumed in the United States with cultivated meat could have an important impact on reducing emissions. Cultivated meat is a novel food and may require consumer education and producer transparency on production methods and safeguards in order to become more widely accepted.

What is our assessment?

Based on our analysis, green hydrogen holds long-term potential in sectors that are difficult to decarbonize, particularly long-haul aviation and freight trucking. It is technologically feasible, but currently hampered by high costs, severe infrastructure gaps, and limited commercial readiness. While it is unlikely to be deployed at scale this decade, we will “Keep Watching” green hydrogen as innovation and policy evolve.

What is it?

Green hydrogen is a clean, emissions-free liquid fuel produced through electrolysis powered by renewable energy that can replace fossil fuels in some transportation sectors. Unlike hydrogen from fossil fuels (gray or blue hydrogen), green hydrogen generates no CO₂ emissions during production. For transportation, green hydrogen can be used in two main ways: (1) in fuel cell electric vehicles (FCEVs) to generate electricity onboard and power electric motors, or (2) combusted in specially designed hydrogen combustion engines or turbines. For aviation, liquid hydrogen may fuel aircraft engines directly, be used to produce synthetic jet fuels, or power fuel cell airplanes. For long-haul trucking, hydrogen can replace diesel by powering fuel cell trucks, which offer long range and fast refueling.

Does it work?

Green hydrogen is being produced and used in pilot projects and select transportation initiatives globally. Hydrogen combustion engines and fuel cells are currently in use and have been shown to reduce emissions compared to fossil fuels. For aviation, aircraft manufacturers, such as Airbus, have hydrogen-powered planes in development, with test flights expected by 2030, but it could be several decades before they are put into commercial use. In heavy-duty trucking, several major automakers, including Toyota and Hyundai, have already commercialized hydrogen trucks in limited markets, such as China and Japan.

Why are we excited?

Green hydrogen is one of the few near-zero-emission fuels with the potential to decarbonize aviation and long-haul trucking, where battery-electric solutions currently face range and weight constraints. If produced using abundant, low-cost renewables, green hydrogen could significantly cut emissions in sectors responsible for nearly 15% of global transport emissions. In aviation, hydrogen-based fuels like e-kerosene could save around five million tons of CO₂ per year in Europe by 2030. In trucking, hydrogen fuel cell vehicles are beginning to roll out but remain a niche market. Looking ahead, hydrogen has strong potential: by 2050, it could meet up to 30% of energy demand in long-haul trucking and significantly reduce aviation emissions, particularly for short- and medium-haul flights, but it will have to compete with advances in battery-electric options. Hydrogen enables fast refueling and long range, making it a strong candidate for freight and intercity applications. Additionally, investment in green hydrogen infrastructure could unlock cross-sectoral benefits, supporting decarbonization of industry, power, and potentially heating. As electrolyzer costs fall and renewable power expands, the economics and emissions profile of green hydrogen are likely to improve.

Why are we concerned?

Despite its promise, green hydrogen for transport faces significant technical, economic, and logistical hurdles. Electrolysis is energy-intensive, and green hydrogen production is still expensive (US$300–600/t CO₂ avoided for trucking and US$500–1500/t CO₂ for aviation), making it much more costly than diesel or jet fuel but comparable to sustainable aviation fuel today. It is also less energy-dense by volume than other fuels, requiring complex transportation and storage (especially for aviation, where cryogenic tanks are needed) and limiting payload capacity. In addition to producing contrails, hydrogen leakage, though not a GHG, can contribute to indirect global warming effects. There are also safety concerns related to flammability and explosiveness, and a complete overhaul of transportation and refueling infrastructure is needed for both aviation and trucking. Green hydrogen requires entirely new infrastructure for production, storage, and distribution, including refueling stations for trucks and specialized handling systems for liquid or compressed hydrogen at each airport the airplane uses. The absence of this infrastructure creates a major barrier to adoption in aviation and long-haul trucking, where fuel logistics, safety standards, and scale are critical for commercial viability. Hydrogen remains a niche fuel due to its low energy density per volume, the need for cryogenic storage in aviation, limited refueling infrastructure, and high cost. While technically viable, major deployment for aviation and trucking is still nascent. Without a clear business case or strong policy incentives, uptake will remain limited in the near term.