What is our assessment?

Based on our analysis, improving district heating for industry by integrating low-carbon heat sources has the potential to significantly reduce the use of fossil fuels and the emissions they generate. However, the lack of data, combined with the complexity of such projects and the growing interest in alternative decarbonization pathways, makes this a potential solution to “Keep Watching.”

What is it?

District heating systems consist of a network of underground pipes that distribute heat to a large number of buildings, including industrial buildings. In the industrial sector, district heating is used by light industries and for processes such as drying, paper making, food processing, as well as space heating and even heat-driven chillers for refrigeration. Industry is well-suited to district heating because it typically has steady and predictable heat demand throughout the year. Current district heating systems rely heavily on coal and natural gas for heat generation, often as part of combined heat and power generation. Low-carbon alternatives for district heating can include electric heat pumps, solar thermal, deep geothermal, and even waste heat from other industries. 

Does it work?

Shifting district heating for industry from conventional heat sources to low-carbon heat sources will significantly reduce emissions. Our analysis for district heating use by commercial and residential buildings shows that significant emissions can be avoided by shifting to electric boilers, heat pumps, and the use of waste heat (see Improve District Heating: Buildings). Similar outcomes are likely possible for industrial district heating use, and emissions reductions will increase as more renewables are integrated into the electricity systems used to power electric boilers and heat pumps. 

Why are we excited?

District heating for industry currently produces significant emissions. According to the International Energy Agency (IEA), district heating for all applications accounted for 4% of global emissions in 2022, and roughly 40% of the heat energy from district heating was delivered to industry. China is a major adopter of district heating for industries, with the combustion of coal supplying much of that heat. The shift to renewable heat sources is likely to increase because both China and the EU have policies targeting the adoption of renewables in district heating. Because district heating systems serve multiple buildings, a single project to replace fossil fuels with renewables can have a large impact. Such projects also have the benefit of reducing local air pollution. 

Why are we concerned?

Although simple on paper, replacing fossil fuel systems with lower-carbon alternatives in district heating systems can be an extended undertaking involving many stakeholders and years of planning. Some low-carbon options may not be suitable for industrial processes that require higher temperatures than those needed for space heating. There is also a significant lack of publicly available data about how industry currently uses district heating and the opportunities and challenges involved in shifting to renewables. In the meantime, industrial heat pumps with higher temperature outputs (100–200°C) are increasingly available and could become a low-carbon competitor to the use of a conventional district heating system.

What is our assessment?

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

What is it?

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

Does it work?

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

Why are we excited?

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

Why are we concerned?

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

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

What is our assessment?

Based on our analysis, restoring large herbivores can provide climate benefits, but there is limited (and mixed) evidence on its carbon removal effectiveness across different ecosystem types. 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. However, 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, in many cases, can 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?

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.

What is our assessment?

Based on our analysis, sustainable aviation fuel (SAF) has the potential to reduce emissions in the aviation sector, particularly for long-haul flights where few alternatives exist. However, pathways with the lowest emissions are not yet cost-effective and face significant challenges to scaling production due to feedstock constraints, land conversion pressure, and the need to meet robust sustainability standards. Based on our assessment, SAF is a climate solution to “Keep Watching.”

What is it?

Sustainable aviation fuel (SAF) is a low-carbon alternative to conventional jet fuel that uses non-petroleum feedstocks such as waste oils, agricultural residues, and municipal solid waste. SAF is produced through chemical processes that convert these feedstocks into fuels that meet the same technical standards as fossil-based jet fuel, allowing them to be blended and used in existing aircraft engines and fueling infrastructure without modification. As of 2025, existing SAFs are only approved for use in blends; no SAF is yet certified for 100% use in commercial aircraft (also known as “neat SAF”) for passenger flights.

Does it work?

Life-cycle emissions vary widely depending on the feedstock and production pathway. Multiple SAF production pathways – such as hydroprocessed esters and fatty acids (HEFA), Fischer-Tropsch synthesis (FT), and alcohol-to-jet (ATJ) – have been approved by international aviation standards bodies. However, as of 2025, the HEFA pathway (which uses vegetable oils, waste oils, or fats) is the only commercially deployed method to produce significant amounts of SAF. 

While some SAFs could achieve low emissions, others, especially those using food crops or poorly regulated waste streams, deliver uncertain climate benefits or can even increase emissions compared to conventional fuels. (We include emissions from burning biomass and biofuels, using default Intergovernmental Panel on Climate Change [IPCC] stationary combustion emission factors for each feedstock. See the Drawdown Explorer primer on “Effectiveness” of solutions.) Others, such as FT pathways, which use municipal solid waste or agricultural waste and residues, have the greatest potential for emissions reduction.

Real-world use of SAF is already underway: over 450,000 commercial flights have flown using SAF blends as of early 2025. SAF is currently being supplied at major airports in Europe, the United States, and Asia, with dozens of airlines integrating SAF into operations or entering offtake agreements. However, SAF supplies less than 0.5% of global jet fuel use. 

Why are we excited?

Sustainable aviation fuels may reduce contrails, potentially significantly reducing aviation’s climate impact. SAF can be used in existing aircraft and fueling systems without requiring new infrastructure or major redesigns. This makes it one of the few ready-to-deploy solutions for long-haul and international flights, which are difficult to electrify or replace. SAF production from waste oils and residues can deliver additional benefits, such as reduced methane emissions from organic waste streams and improved waste management. Growing policy support, rising carbon prices, and airline demand are accelerating development. 

Why are we concerned?

Despite its promise, SAF faces significant limitations and challenges that could constrain its impact and scalability. In the United States, soybean oil is one of the most commonly used feedstocks for HEFA SAF, and its production faces similar land use and ecological risks and constraints as corn ethanol. Whereas in Europe, waste oils and fats are more commonly used. Measurement, reporting, and verification of actual emissions reductions can be complex, especially when land-use change, indirect emissions, or supply chain impacts are involved. 

Due to limited feedstock availability, SAF is highly unlikely to meet the ambitious 2050 goals set by industry organizations and government institutions. Effective SAFs must be combined with other strategies, like demand reduction and new aircraft technologies, to achieve substantial emissions reductions. 

Another major concern is cost. Current SAF prices are substantially higher than fossil jet fuel, ranging from US$300 to over US$1,500 per ton of CO₂ avoided, depending on the pathway. Without strong policy support, this cost premium poses a barrier to widespread adoption.