This solution can still help mitigate climate change but does not meet the scale to be considered a major climate solution.
Improved manure management refers to the use of impermeable covers and physical or chemical treatments applied during the storage and processing of wet manure. These techniques can reduce methane emissions under anaerobic storage conditions and nitrous oxide emissions under aerobic conditions. They offer multiple environmental benefits, including reduced air pollution, reduced nutrient leaching and eutrophication of downstream aquatic systems, and reduced demand for energy-intensive synthetic fertilizers. Disadvantages include a relatively small climate impact and, except for covers, high costs. Even at an optimistic level of adoption, the climate impact is unlikely to be globally meaningful (<0.1 Gt CO₂‑eq/yr ). Despite this modest climate impact, we conclude that Improve Manure Management is a “Worthwhile” solution.
Based on our analysis, improved manure management using impermeable covers and physical or chemical treatments will reduce emissions, although not by a globally meaningful amount. However, because these manure management techniques are broadly available, we conclude this climate solution is “Worthwhile.”
| Plausible | Could it work? | Yes |
|---|---|---|
| Ready | Is it ready? | Yes |
| Evidence | Are there data to evaluate it? | Yes |
| Effective | Does it consistently work? | Yes |
| Impact | Is it big enough to matter? | No |
| Risk | Is it risky or harmful? | No |
| Cost | Is it cheap? | ? |
Manure generated from industrial livestock production contains significant quantities of organic carbon and nitrogen. Under low-oxygen conditions, bacteria convert organic material in manure to methane through anaerobic decomposition. Liquid manure, particularly from pigs and cows, produces significant quantities of methane. In oxygen-rich conditions, organic nitrogen in manure undergoes chemical reactions to produce nitrous oxide. Once produced, these GHGs diffuse towards the surface of the manure storage tank, where they are emitted into the atmosphere.
Improved manure management interrupts the production or release of methane and nitrous oxide through a structural barrier, or physical or chemical treatment processes. Manure storage covers made from impermeable synthetic materials effectively prevent the release of GHGs, and can be utilized in conjunction with biogas systems for energy generation. Chemical treatments, such as acidification and the addition of additives, suppress microbial activity, thereby inhibiting methane and nitrous oxide production. Physical processes, such as aeration and temperature reduction, similarly limit optimal conditions for microbial growth. Separating the solids and liquids from manure can also reduce the potential for methane production, enabling more effective solutions such as composting and anaerobic digestion.
Available technologies for manure management are mature and market-ready. However, empirical evidence of their effectiveness for reducing methane emissions is limited. Pilot studies indicate high effectiveness of manure acidification, moderate effectiveness of impermeable synthetic covers, and low effectiveness of manure additives. Except for the use of natural and synthetic impermeable covers, the overall adoption of these techniques is low.
Improved manure management can provide environmental benefits by reducing air pollution, preventing nutrient leaching from organic solids that settle into sludge, mitigating eutrophication in downstream aquatic ecosystems, and preventing soil acidification. In the food system, manure management allows for better alignment between crop needs and natural fertilizer characteristics. Since hauling liquid manure is expensive, manure storage and treatment methods promote efficient nutrient cycling and reduce the need for energy-intensive synthetic fertilizers. Abated methane in manure also limits ground-level ozone production upon application, thereby improving crop yields.
At the farm scale, the wide range of treatment options allows for a high level of customization in the manure management process to achieve joint goals of nutrient management, revenue generation, and emission reductions. Covers also directly mitigate risks to farmworker health and safety from manure handling, and manure treatment can further limit exposure to irritants and noxious gases, improving the health of surrounding communities.
Compared to no treatment and other manure-related solutions, such as composting and anaerobic digesters, evidence for the effectiveness of impermeable covers and manure treatment technologies is limited. At realistic levels of adoption, improving manure management is unlikely to have a globally meaningful climate impact (<0.1 Gt CO₂‑eq/yr ). High costs are also a key barrier to wider adoption, ranging from US$110–145/t CO₂‑eq for synthetic covers to US$500–3,000/t CO₂‑eq for other treatments.
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Protect Seafloors is the long-term protection of the seafloor from degradation, which helps preserve existing sediment carbon stocks and avoid CO₂ emissions. Advantages of seafloor protection include the conservation of biodiversity and marine ecosystems, potentially low costs, and the ability for immediate implementation. Disadvantages include uncertainties in the effectiveness of legal protection at preventing degradation and in the amount of CO₂ emissions avoided, as well as the risk of displacement of degradation to non-protected areas and/or an increase in other types of degradation. Given these considerations, we conclude that Seafloor Protection is a “Worthwhile” climate solution.
Based on our analysis, seafloor protection could avoid some CO₂ emissions while preserving critical marine ecosystems from degradation. However, the effectiveness of protection and the magnitude of avoided CO₂ emissions associated with protection are understudied and currently unclear. All told, we consider this a “Worthwhile” climate solution.
| Plausible | Could it work? | Yes |
|---|---|---|
| Ready | Is it ready? | No |
| Evidence | Are there data to evaluate it? | Limited |
| Effective | Does it consistently work? | No |
| Impact | Is it big enough to matter? | Yes |
| Risk | Is it risky or harmful? | No |
| Cost | Is it cheap? | Yes |
Protect Seafloors aims to reduce human impacts that can degrade sediment carbon stocks and increase CO₂ emissions. Protection is conferred through legal mechanisms, such as Marine Protected Areas (MPAs), which are managed with the primary goal of conserving nature. The seafloor stores over 2,300 Gt of carbon (roughly 8,400 Gt CO₂‑eq) in the top one meter of sediment. This marine carbon can be stable and remain sequestered for millennia. However, degradation of the seafloor from a range of human activities can disturb bottom sediments, resuspending the carbon and increasing its microbial conversion into CO₂. Currently, degradation of the seafloor primarily results from fishing practices, such as trawling and dredging, which are estimated to occur across 1.3% of the global ocean. Additional sources of degradation include undersea mining, infrastructure development (for offshore wind farms, oil, and gas), and laying telecommunications cables. Estimates of seafloor degradation are highly uncertain due to data limitations and the unpredictable nature of how these activities may expand in the future.
More evidence is needed to confirm whether legal seafloor protection is effective at reducing degradation and the extent to which degradation results in increased CO₂ emissions. While ~8% of the seafloor is currently protected through MPAs, there is mixed evidence that legal protection reduces degradation and CO₂ emissions. For instance, in a review of 49 studies examining the impacts of bottom trawling and dredging on sediment organic carbon stocks, most (61%) showed no change, while nearly a third (29%) showed carbon loss. More recent work suggests that trawling intensity might drive these mixed results, with more heavily trawled areas showing clear reductions in sediment organic carbon. Additionally, the few existing global estimates of CO₂ emissions from trawling and dredging range from 0.03 to 0.58 Gt CO₂/yr, highlighting the need for further research. The effectiveness of MPAs at preventing seafloor degradation is also mixed. In strictly protected areas with enforcement of no-take policies that prevent bottom fishing, MPAs could help minimize degradation and retain seafloor carbon. However, implementation can be challenging, as over half of existing MPAs generally allow high-impact activities. For instance, trawling and dredging occur more frequently in MPAs than in non-protected areas in the territorial waters of Europe.
Advantages of seafloor protection include its potential low cost and its ability to conserve often understudied biodiversity and ecosystems. Human activities, such as trawling and dredging, impact marine organisms on the seafloor, and ecosystem recovery can take years to occur. In the case of undersea mining, ecosystems may never fully recover. Increases in CO₂ emissions along the seafloor from degradation can also enhance local acidification and reduce the ocean's buffering capacity, both of which can affect marine organisms and the carbon sequestration capacity of seawater. Protection can also increase fisheries yields in neighboring waters and reduce other negative impacts of seafloor disturbances. While costs are somewhat uncertain, MPA expenses have been estimated to be an order of magnitude less than the often unseen ecosystem service benefits gained with protection, suggesting MPA expansion could provide cost savings.
Disadvantages of seafloor protection include uncertainties surrounding the effectiveness of preventing degradation and avoiding CO₂ emissions, as well as the potential increased risk of disturbance to other ocean areas. The amount and fate of CO₂ generated due to the degradation of seafloor carbon is complex and understudied. It can take months or even centuries for CO₂ produced at depth to reach the sea surface and atmosphere. Current estimates of CO₂ emissions due to dredging and trawling are widely debated and highly variable due to differing methods and assumptions. Large amounts of organic carbon will inevitably re-settle after seafloor disturbances, with no impact on CO₂, but estimates of just how much remain uncertain. The risk of protection-induced leakage, where a reduction in disturbances, such as trawling and dredging in MPAs, leads to increased fishing effort in other ocean areas, is also potentially high.
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Cool roofs cut GHG emissions from electricity generation by lowering the amount of cooling required to condition indoor spaces, thereby decreasing the use of air conditioners. Using cool roofs in building design lowers electricity use, improves thermal comfort for building occupants, and is relatively cheap to deploy. However, its potential climate impact is relatively small, and its relevance is largely limited to hot climates where buildings need more cooling than heating to be thermally comfortable. Its application has mostly been in pilot projects, but we conclude that this solution is “Worthwhile” with potential for large-scale deployment.
Our analysis concludes that the projected climate impact of using cool roofs on buildings is not large enough to be globally significant (>0.1 Gt CO₂‑eq/yr ). However, we consider it “Worthwhile” because it helps reduce electricity consumption in buildings, makes indoor spaces more thermally comfortable, and lessens the urban heat island effect.
| Plausible | Could it work? | Yes |
|---|---|---|
| Ready | Is it ready? | Yes |
| Evidence | Are there data to evaluate it? | Limited |
| Effective | Does it consistently work? | Yes |
| Impact | Is it big enough to matter? | No |
| Risk | Is it risky or harmful? | No |
| Cost | Is it cheap? | Yes |
Using cool roofs reduces the amount of electricity needed to cool indoor spaces, thereby cutting GHG emissions from electricity generation. Cool roofs are generally defined as light-colored roofs designed to reflect more sunlight and transfer less solar energy into the interior compared to traditional roofs, thereby reducing cooling loads. Cool roofs can be achieved by applying coatings or using roofing materials with a high solar reflectance index (SRI), which results from high solar reflectance and thermal emittance. These properties ensure that surface temperatures on cool roofs remain substantially cooler than conventional roofs.
Using cool roofs can effectively reduce the amount of air conditioning needed to cool indoor spaces, though their potential to cut annual electricity use in buildings and resulting GHG emissions is minimal. Nonetheless, evidence from real-world applications demonstrates that the surface temperatures of cool roofs can be as much as 28–30°C cooler than conventional roofs on extremely hot afternoons. Other studies have shown that cool roofs can decrease indoor air temperatures by 2–3°C while simultaneously reducing surrounding outdoor air temperatures by about 10°C, thereby minimizing the urban heat island effect.
Several organizations are deploying initiatives to drive cool roof adoption as a passive cooling strategy in the building sector. For example, C40 Cities previously launched a cool roofs program across New York City. Over a six-year period (2009–2015), the initiative resulted in nearly 530,000 m2 of building roof tops being retrofitted as cool roofs. As of 2023, the United States is estimated to have over 232 million m2 of installed cool roofs. Recently, the Million Cool Roofs Challenge organized by the Global Cool Cities Alliance resulted in 1.1 million m2 of additional cool roofs in 2022 across 10 countries, including Indonesia, Mexico, and Rwanda.
Some studies estimate that about 229 billion m2 of roof space existed as of 2022. Given the existing building stock – and the fact that the bulk of projected new construction by 2050 is expected in regions with hot climates – the impact of this potential solution could grow.
There are several advantages to using cool roofs in buildings. First, it is cheap to implement, and the incremental cost of applying new coatings or selecting light-colored roofing materials during construction is often minimal compared to conventional roofs. Second, it is expedient as a cooling strategy when buildings are not mechanically air-conditioned or designed to be naturally ventilated. This is important because many countries in hot climates (where cooling is generally required for indoor thermal comfort more than heating) also lack access to reliable electricity, thereby necessitating the use of passive measures in building design.
In addition, a recent analysis of 77 low- and middle-income countries determined that cooling systems are not readily available, sustainable, or affordable, especially for building applications, placing nearly 4 billion people at risk. Deploying scalable strategies such as cool roofs in buildings helps reduce exposure to these risks, which could lead to greater adoption and climate impact. Several studies have also shown that using cool roofs can help reduce indoor heat stress, especially in hot and humid environments. Others are exploring the concept of cool-colored roofs, where non-white roof materials can provide similar cooling effects while preserving aesthetic choice for building owners and developers.
Despite the advantages of using cool roofs as a potential climate solution, a few challenges exist. Some studies have shown that cool roofs can slightly increase heating loads during winter, especially in cold climates. However, other studies conclude that the increase is marginal and often inconsequential. Another concern is that cool roofs can produce glare as the incident sunlight is reflected. This could adversely impact building users if the buildings with cool roofs are surrounded by taller structures with daytime occupancy, such as offices, which is an increasing reality in urban spaces. Lastly, we found examples of pilot projects and resources for cool roofs, but could not find reliable datasets for a comprehensive assessment of their current impact. Addressing such data gaps could help drive cool roofs research, integration into industry practices and building codes, and, ultimately, greater adoption.
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Henry Igugu, Ph.D.
Christina Swanson, Ph.D.
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