What is our assessment?

Based on our analysis, using low-flow fixtures is a cost-effective strategy for reducing water consumption, but has only a modest impact on GHG emissions. Therefore, this climate solution is “Worthwhile.”

What is it?

Low-flow fixtures lessen the total consumption of water by reducing flow rates through a household faucet or shower. Less hot water use means fewer emissions from the energy source used to heat the water, and it also means fewer emissions from pumping and treating tap water. Heating water for showers, sinks, and other domestic appliances is often the second largest source of emissions from buildings after space heating. Modern low-flow showerheads can produce comparable pressure and coverage to traditional showerheads through aeration and/or laminar flow. Aerators for faucets and low-flow showerheads are relatively low-cost investments that users can install themselves.

Does it work?

Low-flow fixtures reduce emissions from heating, delivering, and treating water by reducing hot water consumption. There is ample evidence for water savings with low-flow fixtures, as well as for the linkage between quantity and source of energy used for water heating and GHG emissions. Additionally, there is substantial research on the emissions from treating and pumping water, which can be reduced through water conservation. Low-flow fixtures are readily available, and performance labels are available to help consumers select quality products.

Why are we excited?

Low-flow fixtures conserve water, which reduces emissions, reduces energy demand, saves consumers money, and helps with sustainable water resource management. Households that adopt low-flow fixtures can enjoy significant utility bill savings because these fixtures reduce both water consumption and the energy used to heat water in the home. Faucet aerators also produce a smoother water stream with less splashing, and along with low-flow showerheads, are low-cost and simple to install. Household water conservation practices, such as low-flow fixtures, can help with regional sustainable water resource management and defer infrastructure expansion projects. This is particularly important in areas where water resources are increasingly strained due to climate change, growing populations, and other factors. In some regions, community water conservation efforts have had measurable impacts on water treatment costs, resulting in lower water rates for consumers.  

Why are we concerned?

Even with widespread adoption, low-flow fixtures would have a relatively small impact on GHG emissions. Moreover, the low cost and ease of replacement mean that low-flow fixtures can be easily reverted to less efficient fixtures, eliminating the emissions impact and other benefits. Lastly, although modern quality low-flow showerheads are comparable to traditional fixtures, the poor quality of early low-flow showerheads may have contributed to decreasing levels of adoption in some areas.

What is our assessment?

Based on our analysis, restoring salt marsh ecosystems is a “Worthwhile” carbon removal technique that is ready for large-scale deployment. While the capacity for adoption is limited, limiting climate impact, this solution has no major risks and provides widespread added benefits for people and the environment.

What is it?

Restore Salt Marsh Ecosystems removes carbon from the air by reestablishing salt marshes in areas where they were previously drained, filled, or otherwise degraded and lost. As plants take up CO₂ through photosynthesis and vegetation traps sediments, some of this carbon is stored long term in waterlogged soils with slow decomposition rates. Restoration typically reconnects land to tidal exchange and rebuilds marsh elevation and vegetation, which promotes plant growth and sediment accumulation. Active restoration can include breaching levees or removing barriers to restore tidal flow, regrading or adding sediment to raise elevations, planting native marsh vegetation, and controlling invasive species. In many cases, restoration can also reduce GHG emissions by replacing land uses, such as drained agriculture, that emit CO₂.

Does it work?

The fundamental idea of restoring salt marsh ecosystems is scientifically sound, and, on average globally, restored salt marshes have been shown to remove carbon over long timescales through vegetation recovery and sustained carbon burial in waterlogged soils, even after accounting for methane and nitrous oxide emissions. This solution has been in practice worldwide for many decades, and global assessments suggest it could expand to roughly 2 million hectares because ~67% of salt marshes have been destroyed since the early 1900s. Restoration success rates are high relative to those of many other marine habitats. However, its potential adoption ceiling is still low relative to other nature-based solutions (e.g., Restore Forests) because restoration is limited to suitable coastal areas, which are constrained by coastal development and other human stressors. As a result, its climate impact is likely well below 0.1 Gt CO₂‑eq /year. 

Why are we excited?

Restoration of salt marsh ecosystems is a well-established, scalable practice with many benefits for the environment. Restored salt marshes can reduce shoreline erosion and costal flooding, improve water quality by retaining nutrients and sediments, and provide habitat for fish and birds. While global impact is limited, this intervention can be an important multi-benefit tool for building climate resilience and removing carbon in some countries and coastal regions. Restoration is already widely implemented. In some restorations, such as those that reestablish tidal exchange in previously impounded ecosystems, increases in salinity can reduce methane and nitrous oxide production relative to pre-restoration conditions.

Why are we concerned?

The climate impact of salt marsh restoration is constrained by its limited adoption ceiling, variable but potentially high costs, vulnerability to future loss, and potentially low effectiveness. Adoption is limited by where marshes can actually be restored, such as on low-elevation coastal lands that are not heavily developed, and where they can be maintained into the future with climate change stressors, such as sea-level rise. If salt marshes are not restored with consideration of projected sea level rise, loss or conversion to mud flats or open water habitats in the future is possible, which would result in the loss of carbon benefits. Restored salt marshes can also emit potent GHGs such as methane and nitrous oxide as low oxygen conditions and ecosystem function are reestablished, which can offset some of the climate benefits of restoration. As a result, costs vary widely by site, and can exceed US$500/t CO₂‑eq (~US$1,000–7,000/ha), depending on site-specific effectiveness rates. Additionally, few data are available for understanding long-term, multi-decadal changes in carbon accumulation rates in restored sites, and some regions remain underrepresented globally.

What is our assessment?

Based on our analysis, grassland and savanna restoration is a promising climate solution, but there is insufficient evidence to ascertain how much carbon it could remove at the global scale. Restoring Grasslands and Savannas is therefore “Worthwhile.” 

What is it?

Restoring grasslands and savannas removes carbon from the atmosphere via photosynthesis and stores it in soils and vegetation. Grassland and savanna restoration includes a spectrum of practices, such as returning ecologically appropriate grazing and fire regimes, reseeding with native species, and controlling invasive and woody plants. 

Because grasslands and savannas are diverse, widespread ecosystems spanning a large climatic range, appropriate restoration and management strategies vary depending on the type of degradation and the natural history of the area. For this solution, we considered only degraded areas that were historically grassland and savanna and are not currently used as croplands or grazing lands. Other Project Drawdown solutions, including Deploy Silvopasture, Reduce Grazing Intensity, and Deploy Alternative Grazing, address increasing carbon removal in grasslands managed for grazing. Protect Grasslands & Savannas addresses protecting existing carbon stocks by reducing ongoing ecosystem degradation.

Does it work?

Grassland and savanna restoration will generally remove carbon when implemented with ecologically appropriate strategies on grasslands and savannas with depleted carbon stocks. Restoration efforts covering millions of hectares have already been initiated in some regions, though data tracking restoration progress are sparse. Although grassland and savanna restoration will remove carbon in principle, very little information is available to quantitatively assess the amount of carbon removed by restoration of degraded, ungrazed grasslands and savannas. One study in the United States found that planting diverse species on degraded grasslands increased total carbon uptake by up to 178% of that associated with natural succession over 22 years; however, the generalizability of this finding is unclear. Other studies that focused on activities outside of the scope of this solution, such as changing grazing practices, restoring croplands to grasslands, planting legumes, and adding fertilizers, found an average increase in carbon uptake rates of ~1.7 t CO₂‑eq /ha/yr with a range of 0.1–3.2 t CO₂‑eq /ha/yr. These estimates may serve as a rough benchmark of the maximum per-hectare carbon removal that grassland restoration could achieve.

Why are we excited?

Grassland and savanna restoration may be an effective, low-risk strategy for sequestering carbon on hundreds of millions of hectares while also providing substantial benefits for biodiversity and other ecosystem services. Grasslands and savannas are the largest ecosystem on Earth, covering more than 2.8 billion hectares (see Protect Grasslands & Savannas) from the tropics to the tundra. Some studies estimate that roughly half of grasslands are degraded, suggesting that the opportunity for grassland and savanna restoration is in the range of hundreds of millions of hectares even after excluding grazed areas. Grasslands and savannas also play a critical role in the global carbon cycle, containing roughly 30% of the world’s soil carbon stock. Therefore, even small relative increases in grassland and savanna carbon stocks could translate into large absolute climate benefits. Because most grassland and savanna carbon is stored in below-ground biomass and soils, these carbon stocks can be more resilient to disturbance, such as fire, than carbon stored in above-ground biomass. 

In addition to the potential climate benefits, healthy grasslands and savannas support diverse biological communities, regulate hydrology, improve water quality, reduce erosion, and provide pollination, cultural, and provisioning services to local communities. 

Why are we concerned?

While grassland and savanna restoration can consistently remove carbon, large uncertainties remain in the magnitude of the effectiveness and adoption potential of this solution. 

First, most research on the carbon removal potential of grasslands and savannas focuses on improving grazing management or conversion of croplands back to grasslands, which are outside the scope of this solution. Effectiveness at removing carbon also depends on post-restoration management because many grasslands and savannas depend on establishment of ongoing, ecologically appropriate fire and grazing regimes. Additionally, climate change is reducing grassland and savanna productivity in many regions and may prohibit successful restoration in some places. 

Second, the area of degraded, ungrazed grasslands and savannas that are restorable remains largely unknown. The definition of land degradation varies across studies, and maps of degraded lands are inconsistent with one another. While maps of grazing extent have improved, they are still uncertain. Thus, it is difficult to assess the adoption potential of this solution. Without sufficient data on effectiveness and adoption potential, we are ultimately unable to assess whether the climate impact of this solution falls above or below our threshold of 0.1 Gt CO₂‑eq/yr. We encourage additional research to alleviate data limitations related to grassland and savanna restoration.

What is our assessment?

Our analysis concludes that, despite its limited global impact for reducing emissions, Reduce Overfishing is a “Worthwhile” climate solution that has other important benefits for ecosystem health and long-term food security.

What is it?

Reducing overfishing lowers fuel use and CO₂ emissions from wild capture fishing vessels by reducing fishing effort on overfished stocks. This is typically achieved through management actions, such as seasonal closures, gear restrictions, and catch limits. Fishing effort, whether measured as the hours spent fishing or distance traveled, is generally proportional to fuel use. In addition to immediate reductions in emissions, reducing overfishing can allow overfished stocks to recover, which can lead to reduced future emissions since fuel use is lowered when fish are easier to catch and harvested sustainably.

Does it work?

Reducing fishing effort in locations with depleted and overfished wild fish stocks is expected to reduce emissions from fishing vessels. When stocks are overfished, fishers must exert additional effort, traveling further and/or searching longer to make the same catch, which increases fuel use and CO₂ emissions. Reducing overfishing through management actions, such as harvest control rules, gear restrictions, seasonal closures, stronger enforcement of existing regulations, and establishment of marine protected areas, can help fish stocks recover. Other policy tools, such as reducing harmful fuel subsidies that currently enable many otherwise unprofitable fishing fleets, are also likely to result in lower fuel use and CO₂ emissions. Healthy fish stocks can be caught with lower fishing effort, translating to future fuel savings and reduced CO₂ emissions. Global estimates suggest that reductions in overfishing could avoid up to 0.08 Gt CO₂‑eq/yr, representing almost half of the entire capture fisheries sector's annual emissions (0.18 Gt CO₂‑eq/yr ).

Why are we excited?

Currently, overfishing affects more than 35% of global wild marine fish stocks, increasing by 1%, on average, every year. Reducing overfishing not only lowers fuel use and emissions but also allows overfished stocks to recover. Healthy fish stocks strengthen marine food webs and contribute to ecosystem resilience and biodiversity. Overfishing has widespread consequences for diverse marine ecosystems, such as kelp forests, where declines in fish have led to overgrazing of the kelp by sea urchins. Over time, management interventions will also likely improve the sustainability and long-term reliability of coastal livelihoods and food security by supporting sustainable fisheries.

Why are we concerned?

Policy and management tools for reducing overfishing and, by extension, fishing-related emissions come with some challenges. For instance, management measures or legal protections may not be fully effective if implementation or enforcement is weak. Management and enforcement can be particularly challenging on the high seas, where jurisdiction is limited or shared across many nations, and where illegal, unreported, and unregulated fishing can be widespread. Even when effective, fish stock recovery can take years to decades, and the costs and trade-offs are unlikely to be evenly distributed across fishing fleets. In the short term, efforts to reduce overfishing could create economic challenges for small-scale fishers who may have fewer resources and less capacity to adapt to management restrictions.

What is our assessment? 

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.”

What is it? 

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.

Does it work? 

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. 

Why are we excited? 

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.

Why are we concerned?

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. 

What is our assessment?

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.

What is it?

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.

Does it work?

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.

Why are we excited?

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. 

Why are we concerned?

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.