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?

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 solution to “Keep Watching.”

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.

What is our assessment?

Our analysis finds that Deploy Ocean Biomass Sinking could have high potential environmental risks, including unknown impacts on marine ecosystems. It is also unclear how effective or durable carbon storage in the deep sea is from this approach. There are likely better alternative uses for terrestrial biomass, and cultivating seaweed at climate-relevant scales is probably not feasible. Even if it were, seaweed would probably provide greater value through other applications. Therefore, Deploy Ocean Biomass Sinking is currently “Not Recommended” as a climate solution.

What is it?

Ocean biomass sinking relies on sinking terrestrial plant material and/or seaweed grown in the ocean to the deep sea or seafloor where it can be stored long-term. Cultivating and sinking seaweed removes carbon from the surface ocean, whereas sinking terrestrial biomass material can help reduce emissions that might otherwise occur if the material instead decomposed on land. While not a current practice, terrestrial biomass grown explicitly for sinking would also constitute a form of carbon removal. When biomass sinks naturally, most of it is degraded into CO₂ or other forms of carbon before reaching the deep sea. Deliberate sinking of biomass might avoid some of this degradation by expediting its delivery to the deep sea, depending on the method used. Once sunk, the biomass and any CO₂ or other forms of carbon produced from its degradation can be isolated from the atmosphere for decades to centuries due to the ocean’s slow circulation times at depth. Biomass sinking can be accomplished using active methods, like submersibles, or passive methods, like letting weighted bundles sink on their own. There has been a recent focus on sinking material in low-oxygen ocean basins (e.g., the Black Sea), which might help further minimize degradation, while improving the durability of sequestered carbon due to the long circulation time-scales typical of these regions.

Does it work?

Global estimates suggest that ~11% of carbon produced in natural seaweed ecosystems might be sequestered at depth, generally defined as below the mixed layer at around 1,000 m. However, very few studies have documented the export efficiency, or the fraction of carbon in surface waters that makes its way to the deep sea, of purposefully sunk terrestrial and seaweed biomass, as this practice is currently in the early stages of development and research. If biomass is quickly sunk, most carbon might make its way to the deep sea, while passive sinking techniques, if slower, could result in higher degradation rates. Sequestration also depends on the storage efficiency and durability of carbon once at depth. Some initial research suggests that biomass degradation may be slowed in low-oxygen basins, but this also remains poorly characterized in field studies. It is similarly unclear how durable the carbon stored below the mixed layer will be over climate-relevant timescales, both in the deep sea in general and in low-oxygen basins specifically.

Why are we excited?

The advantages of ocean biomass sinking include its potential ability to use land-based biomass that might otherwise be degraded in landfills or incinerated, both of which lead to CO₂ emissions. In some regions, seaweed cultivation could help reduce nutrient pollution, provide habitat for marine organisms, and locally buffer against ocean acidification. Estimates of potential climate impacts suggest that ocean biomass sinking using biomass from seaweed farms could theoretically exceed 0.1 Gt CO₂‑eq/yr. Still, those estimates remain highly speculative and require more research. Costs are poorly quantified, but some estimates suggest they could be low to moderately expensive compared to other marine carbon dioxide removal approaches, close to US$100/t CO₂.

Why are we concerned?

Ocean biomass sinking has many environmental and social risks that, though not currently fully understood, could make it unfeasible to deploy the technology at scale. Deep-sea and seafloor ecosystems are highly understudied, and it's unclear how new biomass might alter these unique environments. Potential impacts include increased acidification, nutrient pollution, and oxygen depletion of the deep sea, which could affect diverse marine life. Large-scale seaweed cultivation could reduce phytoplankton abundance, disrupt food webs, and deplete nutrients needed by other ecosystems. Cultivation in open ocean areas might relieve demand for coastal space, but they are often nutrient-poor, and adding nutrients raises serious concerns (see Deploy Ocean Fertilization). Terrestrial biomass sources could introduce contaminants into the ocean due to inadvertent inclusion of plastics or other pollutants in sunken biomass. This practice also comes with social risks. Some countries might disproportionately bear negative impacts wherever biomass is cultivated and/or sunk, as it could alter marine food webs and livelihoods. There could also be issues with public perception due to historical injustices around ocean dumping, potentially impeding future projects without meaningful community engagement and transparency. 

Moreover, there are numerous technical challenges relating to the effectiveness and durability of carbon sequestration. Biomass sources differ in how easily they break down, affecting how much carbon is stored at depth. Sunk biomass could also potentially release other greenhouse gases, such as methane and nitrous oxide. The location where biomass is disposed of also matters, impacting how much carbon reaches and stays at depth. However, all of these factors remain poorly constrained. Operational and technical challenges are also significant. To remove at least 0.1 Gt CO₂‑eq/yr using marine biomass, nearly 7 million ha of ocean – over 60% of the global coastline – could be needed for seaweed cultivation, which is impractical. Measurement and verification pose additional hurdles. In the case of seaweed cultivation, tracking carbon removal requires monitoring both CO₂ uptake at the ocean’s surface and export as well as storage at depth across large spatial and temporal scales. In addition, the opportunity cost of sinking terrestrial biomass is high due to competing land-based uses, as waste biomass and crop residues are finite resources. Growing new biomass explicitly for ocean sinking would introduce new risks, given that land is also a finite resource. Similarly, seaweed probably has higher value and carbon benefits as food, fertilizer, and other products.

What is our assessment?

Based on our analysis, 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?

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.