What is our assessment?

Based on our analysis, Deploy Artificial Upwelling is not ready for large-scale deployment, as it has not been tested or proven effective for carbon removal. Even if demonstrated to be effective, it comes with considerable feasibility and cost concerns, as well as potentially insurmountable and widespread environmental risks at scale, and is therefore “Not Recommended” as a potential climate solution.

What is it?

Artificial upwelling generally involves using pumps or other devices to bring deep, nutrient-rich ocean water to shallower depths, where it can increase phytoplankton growth via photosynthesis by reducing nutrient limitations. This can increase biological uptake of dissolved CO₂ from the ocean, allowing it to absorb additional CO₂ from the atmosphere. Carbon removed by phytoplankton can then be transported to the deep ocean, where it may be stored long-term. A variety of pumps and devices have been described (e.g., air-lift, gravity wave, air bubble, electrical, wave-based) for bringing deep ocean water to the surface. Additionally, some efforts are considering artificial downwelling, or the deliberate transport of surface water to depth, both with and without artificial upwelling, as a means of moving surface carbon to deep waters before it is degraded and potentially returned to the atmosphere as CO₂.  

Does it work?

The fundamental biology underlying artificial upwelling relies on well-understood principles for the natural upwelling of deep, nutrient-rich seawater to the surface ocean, which supports biological production and carbon removal. Some efforts have demonstrated the ability to bring deep ocean water to the surface. However, no proof-of-concept field trials have demonstrated net carbon removal, which depends not only on carbon uptake by phytoplankton and subsequent CO₂ exchange with the atmosphere in the surface ocean, but also on its long-term storage in the deep ocean; both aspects remain largely unexplored in the context of artificial upwelling. Model simulations have been conducted, most of which indicate that artificial upwelling is ineffective for large-scale carbon removal.     

Why are we excited?

Artificial upwelling may provide some environmental advantages. For example, deliberate upwelling of deep ocean water could lower the temperature of surface ocean water, benefiting some marine organisms. Artificial upwelling could also be coupled with aquaculture operations to improve nutrient availability in nutrient-poor regions.

Why are we concerned?

Artificial upwelling presents significant challenges in terms of effectiveness, feasibility, cost, and environmental risk. Large-scale deployment is estimated to potentially require millions to hundreds of millions of pumps. Even short-duration deployments have operationally failed. Moreover, for pumps that require external power, energy requirements could be substantial but remain unclear at this stage. Another current limitation of this technology is the engineering challenge of physically moving large volumes of seawater from depth. Costs remain highly uncertain but are expected to be high given these major operational needs. 

The effectiveness of artificial upwelling is also unclear. Upwelled deep water often contains high concentrations of dissolved inorganic carbon (and low oxygen concentrations), meaning that upwelling might actually result in more CO₂ being emitted into the atmosphere if upwelled waters release more carbon than is removed. Existing studies suggest that additional carbon removal in the surface ocean from artificial upwelling is unable to compensate for this release of deep water dissolved CO₂. Circulation and ocean mixing could further limit the durability of carbon removed, as some research suggests that more than 70% of carbon is returned to the surface ocean within 50 years. 

Finally, environmental effects are poorly constrained but potentially significant. This solution shares similar ecological risks as Deploy Ocean Fertilization, wherein manipulating nutrient availability to increase biological productivity can alter the function of marine ecosystems across large areas and in unclear ways. Artificial upwelling could also alter oxygen availability and exacerbate ocean acidification in some regions. By redistributing cold, dense water and altering the layering of ocean water, some models suggest that large-scale deployment might also increase ocean heat uptake and alter ocean circulation dynamics in ways that impact processes in the lower atmosphere, such as precipitation and temperature modulation. 

What is our assessment?

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

What is it?

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

Does it work?

As a carbon removal technique, ocean fertilization requires that the nutrient addition enhances phytoplankton uptake of seawater CO₂ and subsequent absorption of additional CO₂ from the atmosphere, and that the carbon is transported and durably stored in the deep sea. Research since the 1990s has shown that ocean iron fertilization does lead to increased seawater CO₂ uptake due to enhanced photosynthesis. However, the ultimate fate and durability of that carbon are less well understood. To be sequestered, carbon must be transported below water depths where annual mixing occurs, often considered to be ~1,000 m, but research suggests that, on average, 66% of carbon at these depths can be re-exposed to the atmosphere in less than 40 years. Ocean fertilization might also increase production of GHGs, such as nitrous oxide and methane, which could impact the effectiveness of this practice, although these effects remain understudied. In places like the Southern Ocean, sunlight and changes in the availability of other nutrients, such as silicate, can also limit the effects of iron addition. Additionally, nutrients such as iron can have high loss rates, up to 75%, after dispersal into seawater due to conversion into forms inaccessible to phytoplankton, potentially further reducing the effectiveness of nutrient addition.

Why are we excited?

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

Why are we concerned?

Ocean fertilization poses several technical challenges, along with significant environmental and social risks. Tracking the amount of carbon sequestered from ocean fertilization is difficult because carbon export efficiencies – the amount of carbon produced in surface waters that makes its way to the deep sea – can be low and highly variable in time and space. Addressing this will require both field studies and models capable of capturing global and multi-decadal changes in carbon cycling due to fertilization, given the long time scales and large spatial areas involved. Implementing ocean fertilization at globally meaningful carbon removal levels could raise additional feasibility concerns, given the potential difficulty of dispersing sufficiently large quantities of nutrients across vast areas and the need for fertilization to be done continuously to minimize carbon returning to the atmosphere. 

Beyond these technical challenges, ocean fertilization also poses several potentially severe environmental risks. Enhancing primary production could disrupt existing nutrient pools in the ocean, reducing the nutrients available for ecosystems far from dispersal sites. Another consequence of ocean fertilization is that increased organic carbon supply can enhance microbial processes that consume dissolved oxygen, potentially impairing respiration in marine organisms and leading to mortality. Other unintended consequences of nutrient fertilization include promoting harmful algal blooms that can release toxins that negatively impact a wide array of life, from shellfish to marine mammals to humans. Ocean fertilization also carries significant social risks because global-scale modification of marine ecosystems is likely to create inequities in environmental and economic impacts.

What is our assessment?

Restoring whales will have ecological benefits, but there is limited and uncertain evidence on its effectiveness. The limited evidence available suggests it would be a small contribution to climate change mitigation.  Therefore, we will “Keep Watching” this as a climate solution.

What is it?

Large whales can remove and store carbon via two pathways. They remove carbon when, after they die, some of their biomass reaches the deep ocean, where it can remain for long periods. In addition, while alive, defecation adds nutrients, such as iron, to surface ocean waters, stimulating the growth of phytoplankton that absorb CO₂ via photosynthesis. As phytoplankton draw in seawater CO₂ and convert it into biomass, the ocean can absorb more CO₂ from the atmosphere. Some of this biomass carbon eventually sinks or is transported to the deep sea or seafloor, where it can be stored for decades or centuries. Current whale populations are low due to commercial whaling, climate change, and habitat disturbance.  An estimated 3 million whales were killed for commercial purposes during the 20th century, leaving populations at only a fraction of what they once were (Stewart et al., 2025). Increasing the number of large whales through various restoration activities could increase carbon removal.

Does it work?

Restrictions on whaling have allowed for the humpback whale and other species to make a dramatic comeback over the past 40 years (Zerbini et al., 2019). However, climate benefits from whale restoration are still not clear (Ritchie, 2025). The amount of carbon that could be stored directly in whale tissue is better understood than the extent to which whales stimulate phytoplankton growth, although it’s difficult to directly measure either mechanism. Carcasses that sink to the deep seafloor can sequester carbon for hundreds to thousands of years (NAS, 2022). However, only a small fraction of whales die each year, and only a small fraction of their biomass sinks to the deep ocean. An analysis of the life cycle of great whales estimates that whale carcasses were responsible for storing about 0.0001 Gt CO₂ in the deep ocean in 2001 (Pershing et al., 2010). However, when whales were more numerous and larger during the pre-whaling era, it’s estimated that they could have stored 0.0007 Gt CO₂. (This assumes that none of the CO₂ removed returns to the atmosphere, which is not always the case [Hurd et al., 2024]). Whales’ role in enhancing phytoplankton growth is even trickier to estimate. Whales forage at different ocean depths, depending on the species, but much of the defecation occurs at the ocean surface (Lavery et al., 2010). If whale populations were restored to 10 times their current numbers, estimates suggest that about 0.01 Gt CO₂ could be sequestered (Pershing et al. 2010), which is less than .002% of annual global GHG emissions (59 Gt) in 2019. 

Why are we excited?

Restoring whale populations to their pre-whaling numbers could have multiple ecological benefits, including enhanced growth of phytoplankton and other marine life. Whales play multiple roles in the biological carbon pump of the ocean, including by linking trophic levels (Boyd et al., 2024). 

Great whales are vulnerable to entanglement in fishing gear, whaling, ship strikes, climate change, and ocean pollution (Pearson et al., 2023). Although commercial whaling became illegal in most countries in 1986, a handful of countries still allow it (WWF, 2026). Restricting whaling, establishing marine protected areas, and regulating fishing and shipping in known migratory routes could counter threats to whales and allow their populations to thrive (NAS, 2022). 

Why are we concerned?

The limited published research on whales’ contribution to carbon uptake from the atmosphere estimates that its potential carbon removal is likely quite small (<0.02 Gt CO₂). Overstating the role of great whales in carbon capture may hinder, rather than facilitate, effective climate mitigation efforts (Duvall et al., 2024).

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?

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.