What is our assessment?

Based on the potential for harmful impacts, the risks of using AOE to destroy atmospheric methane outweigh its uncertain benefits. Because of this, as well as the fact that there are other effective solutions to reduce methane emissions already available, this climate solution is Not Recommended.

What is it?

AOE of methane is a technology designed to accelerate the natural decay of methane to CO₂  by increasing the concentration of hydroxyl and chlorine radicals in the air. In the presence of sunlight and oxygen, these molecules convert methane to CO₂. This solution aims to increase the atmospheric concentration of these molecules by injecting precursors, such as iron salts and hydrogen peroxide, into the air.

Does it work?

Methane is a potent greenhouse gas, more than 80 times stronger than CO₂ at trapping heat on a 20-year basis. Under natural conditions, it persists in the atmosphere for about 10 years before it converts into CO₂. Therefore, artificially accelerating methane conversion reduces its disproportionate warming impact. 

There is evidence that the concentration of hydroxyl radicals affects the rate at which methane is converted into CO₂ in the atmosphere. However, research into AOE for methane removal is still in its early stages and limited to a few modeling and laboratory studies. There are currently no real-world examples of atmospheric methane removal. The effectiveness of the solution is unknown and uncertain. Based on current research, no one knows if it is possible to remove atmospheric methane at a meaningful scale in a safe and cost-effective manner. 

Why are we excited?

Because methane is such a potent greenhouse gas, any actions to reduce its concentration in the atmosphere would be emergency brake solutions with immediate and disproportionate climate benefits. In addition, unlike direct air capture or carbon capture and storage, there is no need to capture or store the gas that the process produces. 

Why are we concerned?

AOE for methane removal is an untested technology designed to alter atmospheric chemistry that presents novel and potentially uncontrollable risks to Earth systems and ecosystem processes. Hydroxyl and chlorine radicals are highly reactive molecules, and they do not react solely with methane. When they react with other atmospheric constituents they can generate other, even stronger, climate pollutants such as nitrous oxide as well as other air pollutants such as PM2.5, carbon monoxide, nitrogen dioxide, and ground-level ozone, and they could deplete stratospheric ozone. Some proposed AOE methods, such as atmospheric injection of iron salt aerosols, create chlorine radicals. The chlorine- and iron-containing byproducts of these aerosols could adversely affect ocean chemistry and food webs when they are deposited on the ocean surface (see Deploy Ocean Fertilization). 

Other serious concerns include technical feasibility, scalability, cost, monitoring, reporting and verification, and governance. For example, in order for methane in the atmosphere to be reduced at climate-relevant scales, the production of chlorine or hydroxyl radicals would need to be magnitudes greater than the current global production. Costs have not been estimated, but they would likely be high. New tools for monitoring atmospheric methane would need to be developed to quantify the amounts of methane removed for accurate accounting and verification. Similar to stratospheric aerosol injection, deployment of atmospheric methane removal could pose geopolitical, legal, and ethical challenges. In addition, it could distract from or delay action on other methane reduction approaches, such as managing oil and gas methane, managing coal mine methane, improving landfill management, and increasing centralized composting.

What is our assessment?

Based on our analysis, expanding grazing increases methane emissions and often displaces land uses that would deliver greater carbon sequestration and biodiversity outcomes. It is therefore “Not Recommended” as a climate solution. 

What is it?

Expanding grazing includes converting cropland, degraded land, or native landscapes (e.g. forests, savannas) to pasture to increase beef production. As a climate solution, it assumes that soil organic carbon (SOC) can be increased sufficiently to offset methane emissions, resulting in net climate mitigation.

Does it work?

Expanding grazing does not generate net carbon removal because SOC gains are insufficient to offset associated enteric methane emissions. Claims that alternative grazing systems can generate net carbon removal or that grass-finished beef offers climate advantages are not supported by independent scientific evidence, with any gains limited to narrow, context-specific cases that exclude full-scope emissions. SOC gains, where they occur, are slow, saturating, and reversible, while ruminants emit methane continuously. Offsetting global current ruminant emissions would require sequestering 135 Gt of carbon (≈495 Gt CO₂ ) over 100 years, nearly twice the carbon stock in all the world’s managed grasslands. Because sequestration potential is uneven and many regions are degraded or constrained by soil and climate conditions, the required gains, to offset enteric methane emissions, would be concentrated on a subset of lands. This translates into required increases in soil carbon of roughly 25–2,000% locally over 61–225 years. This not only is unlikely, but also overlooks evidence that removing grazing livestock can increase plant growth, biomass, and SOC across grasslands, meaning that expanding grazing would compete with more effective forest or grassland carbon sequestration restoration pathways on degraded lands.

Why are we excited?

Expanding grazing can involve converting intensive cropland, such as land used for biofuels, to perennial pastures, which may increase SOC under low ruminant stocking rates and favorable environmental conditions (see Reduce Grazing Intensity). However, these benefits are driven by perennial vegetation and can be achieved more effectively through restoration approaches that do not introduce ongoing methane emissions.

Why are we concerned?

The core climate concern with expanding grazing is that it introduces cattle, a continuous methane source, on land that could capture carbon below and above ground more rapidly in the absence of cattle grazing. SOC gains under grazing, if they occur, are slow, reversible, and limited by saturation, while emissions from ruminants are immediate and ongoing.

Grazing is already the largest human use of land, and many grazing lands are affected by degradation and overstocking. Approximately 42% of global pastureland could support forests. Restoring these areas could sequester 445 Gt CO₂ by 2100, equivalent to more than a decade of global fossil fuel emissions. In addition, 50% of all global natural nonforest ecosystem conversion between 2005 and 2020 was driven directly by pasture expansion.

Demand for food is rising, while climate change is already reducing agricultural productivity and increasing crop losses. Some projections show a 36 to 50% decline in climatically viable areas for grazing by 2100 due to rising heat and shifting water conditions, alongside risks of broader yield declines. Expanding grazing, which would increase cattle herds and their feed demands, is an inefficient way to increase food supply, converting large amounts of land and feed into relatively small amounts of food. The global food system loses 7.22 quadrillion calories annually through the conversion of crops into animal products and other nonfood uses rather than direct human consumption, enough to feed 7.2 billion people. Cattle are the most inefficient of these pathways, with a 91% caloric loss when crops are converted into beef. Beef production uses 40% of global cropland yet provides only 9% of animal-source calories.

Improving diets and other demand-side changes are critical to avoid expanding grazing. The impacts of beef production exist along a spectrum: more extensive pasture-based systems require more land and typically produce more methane per unit of output, while more intensive, feedlot-based systems use less land per unit of beef but rely more heavily on cropland for feed, antibiotics, and concentrated waste management. Whether expansion occurs through more extensive or intensive systems, beef remains among the most emissions- and land-intensive ways to produce food.

In addition, animal-sourced foods are a major driver of biodiversity and habitat loss globally, with grazing cattle bearing disproportionate responsibility. Beef is the largest single contributor to the loss of biodiversity in Key Biodiversity Areas (KBAs), at around 31% of total biodiversity loss; 60% of KBAs are used for livestock ranching. Expanding grazing therefore reinforces the leading driver of biodiversity loss.

What is our assessment?

Based on our analysis, restoring seagrass ecosystems can remove carbon with no major environmental risks. Despite a likely small climate impact due to the limited area available for restoration and uncertainty around costs, we consider it “Worthwhile.” 

What is it?

Restore Seagrass Ecosystems removes carbon from the air by reestablishing seagrasses – subtidal marine flowering plants with roots – in areas where they were previously destroyed, degraded, or otherwise lost. Seagrasses remove CO₂ from seawater for photosynthesis, accumulating carbon in their biomass that can later break down and settle into sediments on- or off-site. The removal of CO₂ allows seawater to absorb additional CO₂ from the atmosphere. Restoration typically involves seeding or transplanting seedlings and might also require infilling with sediment to compensate for previous sediment loss and accommodate sea-level rise. Seagrass restoration likely also avoids emissions by curtailing the continued loss of sediment carbon due to degradation (e.g., coastal development). 

Does it work?

Restoration of seagrass ecosystems is a relatively new practice in many regions of the world. While its global climate benefit is uncertain but expected to be small (<0.1 Gt CO₂‑eq/yr ), research suggests that restored seagrass ecosystems generally act as net carbon sinks. Nearly 7 Mha of seagrass have been lost worldwide since the 1970s due to a wide range of stressors, such as coastal development and water quality degradation, though it is unclear precisely how much of this loss is restorable. Areas with the greatest observed losses occur in the Tropical Atlantic, Temperate North Atlantic East, Temperate Southern Oceans, and Tropical Indo-Pacific regions.

Why are we excited?

Restoration of seagrass ecosystems provides numerous benefits for the environment and humans. Seagrass meadows can reduce coastal flooding risk while stabilizing seafloor sediment. Restored seagrass ecosystems also increase biodiversity and habitat available for other organisms, including fish and other animals that transiently use seagrass meadows for foraging or as nurseries.

Why are we concerned?

Despite its widespread environmental benefits, seagrass ecosystem restoration can be expensive and is not always successful. In addition, an estimated 33% of its carbon removal benefits can be offset by emissions of methane, a greenhouse gas that microbes can produce using compounds released by seagrass plants. While costs are uncertain, studies suggest they can be high, with a median cost of US$537,140/ha and an average cost of US$979,335/ha (2023 US$), though other regional projects suggest costs can be closer to US$1,200/ha. Also, restoration of seagrasses is not always successful. On average, 55% of seagrass meadows restored succeed (≥50% survival), and future success may be affected by impacts of climate change such as sea-level rise, which is already driving losses of native seagrass meadows. 

What is our assessment?

Based on our analysis, restoring mangrove forests is a highly effective and relatively inexpensive tool for carbon removal, but it has a small climate impact due to the limited global area available for restoration. While the climate impact is probably low, we consider it a “Worthwhile” climate solution because it poses no major risks and provides widespread co-benefits for humans and the environment.

What is it?

Restore Mangrove Forests is a climate solution that removes carbon from the air by re-establishing mangrove ecosystems in areas where they were previously drained, filled, or otherwise degraded and lost. Nearly 2 Mha of mangroves have been destroyed since 1970. As mangrove plants are re-established and grow, they take up CO₂ through photosynthesis and store carbon in above- and below-ground biomass. They also trap and bury carbon-containing sediments, allowing additional carbon to accumulate in waterlogged soils where decomposition is slow. Restoration actions typically involve re-planting and restoring hydrologic conditions to allow tidal exchange with the ocean. Restoration also often replaces land uses that can be sources of large CO₂ (and other GHG) emissions.

Does it work?

Mangrove restoration is a well-established carbon removal approach that has been practiced for at least 40 years in many regions of the world. Research shows that restored mangrove ecosystems can act as large, durable carbon sinks, with sediment carbon likely able to persist for centuries or longer, similar to natural systems. However, because the estimated global area available for restoration is ~1 Mha, its climate impact is expected to be under 0.1 Gt CO₂‑eq/yr. Despite this limitation, restoration can still be a regionally important intervention in certain regions and countries that hold a disproportionate share of restorable mangrove area, due to its high effectiveness. Indonesia (~186,600 ha) and Mexico (~145,500 ha) contain the two largest national areas of restorable mangroves globally. In a relative sense, countries such as Belize, Honduras, Mexico, Nicaragua, Sri Lanka, the United States, and Vietnam are estimated to have at least 10% of their original mangrove area restorable. 

Why are we excited?

Restoration of mangrove ecosystems is an established practice that can be low cost with widespread environmental benefits. Recent global assessments suggest that restoration and natural expansion together added about 393,000 ha of mangrove area from 2000–2020. Restoration can recover ecosystem function, support biodiversity, and reduce exposure to coastal hazards, such as coastal flooding and erosion. Costs can vary from US$3,000–9,800/ha, with the removal of an estimated 0.78 Gt CO₂ over the next 40 years estimated to cost under $20/t CO₂. Low-cost restoration potential is greatest in countries such as Indonesia, Brazil, Mexico, Myanmar, and India. 

Why are we concerned?

This practice, while highly effective at removing carbon, is unlikely to scale to a globally relevant climate impact level given the limited area available for restoration. Although a large area of mangroves has been lost, not all of these areas remain feasible for restoration. For example, nearly 20% of all lost mangrove areas have been converted to open water habitats that are no longer suitable for restoration. Additionally, methane emissions can occur in restored mangroves, which might offset 20% of the carbon removed. Mangrove restoration is also not always successful, and reported outcomes vary widely across projects, with an estimated average success rate of ~62%.

What is our assessment?

Nitrous oxide removal technology is at a very early stage of development. Other available technologies and practices can effectively reduce nitrous oxide emissions. However, because this solution aims to remove such a potent GHG from the atmosphere, we will Keep Watching it. 

What is it?

Nitrous oxide removal is a technology that uses photocatalytic chemicals to convert nitrous oxide, a GHG that has 270 times more warming potential than CO₂ and persists in the atmosphere for more than 100 years, into gaseous nitrogen and oxygen. 

Nitrous oxide is found naturally in the atmosphere, but 40% of emissions come from human activities, and human-caused emissions have increased more than 30% during the past four decades. Most anthropogenic contributions are from fertilizers applied to croplands and other farming activities, while the rest are from fossil fuel use, industrial activities, and waste and wastewater. 

This solution involves spraying a chemical photocatalyst onto agricultural fields. When the photocatalyst is exposed to sunlight and nitrous oxide, it drives a chemical reaction that decomposes nitrous oxide into gaseous nitrogen and oxygen. 

Does it work?

Research into atmospheric nitrous oxide removal is still in its early stages. The concentration of nitrous oxide in the atmosphere is very low, so nitrous oxide removal would likely be implemented in agricultural areas where fertilizer use locally elevates atmospheric concentrations. Laboratory testing has shown that nitrous oxide can be converted into nitrogen and oxygen using light energy and photocatalysts. However, the effectiveness of the solution in practice is uncertain because few experiments have been conducted in real-world settings. The single field study that applied titanium dioxide to a field crop did report a measurable reduction in nitrous oxide emissions. However, there is no evidence that this technology can remove atmospheric nitrous oxide at a meaningful scale. 

Why are we excited?

Because nitrous oxide is such a potent GHG, reducing its concentration in the atmosphere could have a disproportionately beneficial climate impact. In addition, unlike direct air capture or carbon capture and storage, there is no need to capture or store any gases because the nitrous oxide breaks down into gases that have no climate impact. Also, titanium dioxide application to crops is being researched as a method for improving crop resilience.

Why are we concerned?

Serious concerns include technical feasibility, environmental risk (including environmental and food safety), scalability, cost, and monitoring, reporting, and verification. While there is currently very little research on the real-world use of photocatalysts to destroy atmospheric nitrous oxide, ongoing research on the application of nanoparticles of titanium dioxide to crops to enhance productivity and resilience to stress suggests that high concentrations of titanium dioxide can have adverse effects. Furthermore, these nanoparticles are not approved for direct food consumption, and their fate and environmental impacts are poorly understood. 

Tools, methods, and standards need to be developed to quantify nitrous oxide removal for accurate accounting and verification. Costs are unknown. Finally, numerous other approaches for reducing human-caused nitrous oxide emissions exist, including improving nutrient management, rice production, manure management, and industrial processes, as well as reducing fossil-fuel use for power generation and transportation and increasing use of centralized composting. 

What is our assessment?

Reduced grazing intensity can temporarily reduce soil degradation and erosion. However, SOC outcomes depend on a number of factors, such as climate zone, land use history, soil properties, and grass type. Therefore, until stronger, long-term evidence is available to guide more effective implementation, we will Keep Watching this solution.

What is it?

Reducing grazing intensity refers to lowering ruminant livestock stocking rates or shortening grazing duration to reduce pressure on grazing lands. As a climate solution, it is intended to remove carbon from the atmosphere by increasing SOC through enhanced plant productivity, root inputs, and soil stability. Grazing intensity is typically classified as heavy, moderate, or light, based on the proportion of forage removed per unit time.

Does it work?

In general, while heavy grazing reduces SOC, the effects of grazing intensity on SOC recovery varies with climate zones, grass types, soil properties, and prior land use. Increases in SOC under reduced grazing intensity are largely limited to wetter regions, often with high annual rainfall. In arid and semi-arid regions, which represent a major share of global grazing land, reduced grazing intensity often results in neutral or negative SOC responses. A global review and meta-analysis that normalized SOC to 30 cm depth found that even grazing below carrying capacity was associated with an overall decline in SOC, with gains limited to lower-intensity grazing conditions in specific climate zones.

Why are we excited?

Reducing grazing intensity provides ecological benefits. This usually involves reducing the number of ruminant livestock on a farm, which in turn reduces the farm’s methane emissions, land-use pressure, and threats to biodiversity–at least in isolation. It can reduce soil degradation, erosion, and vegetation loss. It is already practiced in many contexts and requires no new technology or infrastructure, making it easy and relatively low cost as a climate intervention, though not necessarily cost-neutral for ruminant livestock producers.

Why are we concerned?

Several limitations, risks, and trade-offs are associated with reducing grazing intensity as a carbon removal strategy.

First, even low-intensity grazing can prevent ecosystem recovery when pastures are seeded with, or invaded by, aggressive grasses that suppress native plants, prevent tree regrowth where ecologically appropriate, and lock landscapes into lower-biodiversity, grass-dominated states.

Second, SOC gains are limited, slow, and reversible. Soil organic carbon is a finite sink that approaches saturation within decades and can be lost through drought, warming, fire, or management changes. SOC accumulation through reduced grazing intensity has been shown to be a temporary and fragile form of carbon storage. 

Third, SOC gains are difficult to measure and verify. Many studies lack baseline SOC measurements, adequate controls, sufficient duration, and/or adequate soil-depth sampling, making it difficult to attribute carbon gains to grazing intensity. To show an increase in SOC from reduced grazing intensity, an ideal experiment would adopt a before-and-after control intervention at a commercial scale and follow SOC changes for 5–10 years.

Fourth, while reducing grazing intensity compares favorably with alternative grazing when it reduces total stocking numbers, it is still less durable and certain as a carbon removal strategy than protecting intact ecosystems, restoring degraded grasslands, or restoring forests where ecologically appropriate. 

Fifth, reducing grazing intensity often lowers herd sizes, but under rising global beef demand this can simply shift production elsewhere. This underscores the value of improving diets and shifting food system infrastructure away from ruminant consumption rather than simply altering ruminant production practices.

Overall, reducing grazing intensity can reduce some local damage from heavier grazing, but in climate-favorable regions especially, the stronger opportunity is often restoring ecosystems or producing higher-yielding plant-based foods.

What is our assessment?

Based on our analysis, the climate impact of restoring seaweed ecosystems is unclear but likely to be low. While restoration offers important ecological benefits, its effectiveness in removing carbon is understudied, and the implementation costs may be prohibitively high, but require further research. Therefore, we conclude that Restore Seaweed Ecosystems is a “Worthwhile” solution.

What is it?

Seaweed ecosystem restoration is the deliberate action of reestablishing seaweed in degraded, damaged, or destroyed ecosystems. Seaweed removes CO₂ from seawater through photosynthesis, which allows the ocean to absorb additional CO₂ from the atmosphere. Some of the fixed carbon can be sequestered through export to the deep sea or burial at the seafloor, while a portion may also persist as carbon forms that resist degradation even in the surface ocean. Restoration of seaweed ecosystems helps restore biomass and therefore the productivity of these ecosystems, which can enhance their sequestration capacity. Restoration can occur in a number of ways, but commonly includes transplanting adults, controlling grazers, building artificial reefs, seeding with propagules or spores, remediating pollution, removing competitive species, and culturing. Most restoration efforts have focused on canopy-forming species, such as giant kelp (Macrocystis pyrifera). 

Does it work?

Seaweed ecosystem restoration can be somewhat effective, with nearly 60% of restoration efforts achieving survival rates of over 50%. The first large-scale restoration is thought to have occurred in Japan in the late 1800s. Still, few projects have been implemented at scale, with most restoration efforts below 0.1 ha in size. Moreover, little data exist to evaluate the effectiveness of restored seaweed ecosystems at removing carbon. While theoretically, they should regain functional equivalence to intact systems, this requires further research. The extent of lost and degraded seaweed ecosystems is also poorly understood, making it unclear how restoration efforts might be scaled globally. Additionally, the air-to-sea transfer of CO₂ to replace the CO₂ taken up by photosynthesis in the ocean is not always efficient, meaning removal in the water column may not always translate to equivalent atmospheric CO₂ removal. However, this aspect of effectiveness also remains understudied. Consequently, the climate impact of restoration is uncertain.

Why are we excited?

Healthy seaweed ecosystems provide a range of ecological benefits. Seaweed can help buffer against ocean acidification in some places as functional systems better regulate pH. These systems also provide complex habitats that support a wide range of marine life, such as fish and invertebrates, so restoring seaweed ecosystems can help recover biodiversity. Seaweed ecosystem restoration can also improve nutrient cycling and overall ecosystem resilience to climate stressors.

Why are we concerned?

Restoration of seaweed ecosystems is currently expensive, with costs varying widely depending on the method used. In kelp forests, chemical or manual urchin removal, which reduces grazing pressure, may cost between US$1,700/ha and US$76,000/ha in 2023 dollars, while most other approaches exceed US$590,000/ha.

It’s also unclear whether seaweed restoration efforts could scale enough to have a globally meaningful impact on GHG emissions. Using estimates from intact subtidal brown seaweed ecosystems, which are among the most productive and represent a likely upper limit on the effectiveness of seaweed restoration as a whole, restoration might remove 2.3 tCO₂‑eq /ha/yr. At this rate, over 40 Mha would need to be restored to exceed 0.1 GtCO₂‑eq/yr. However, most restoration projects are under 0.1 ha. For kelp forests, only roughly 2% (19,000 ha) have been restored out of the Kelp Forest Challenge’s target of 1 million ha by 2040, suggesting that this practice may not be scalable currently.

The effectiveness of restoration can also be offset by the life-cycle emissions of products required to re-establish some seaweed ecosystems. For example, emissions from the production of cement blocks needed to afforest some seaweed habitats have been estimated to potentially delay carbon removal benefits for 5–13 years in some systems.