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, deploying electric irrigation pumps will reduce emissions but will not provide a globally significant climate impact (>0.1 Gt CO₂‑eq/yr ), even under high adoption scenarios, until electrical grid emissions decline further. Therefore, this potential climate solution is “Worthwhile.”

What is it? 

This solution reduces emissions from irrigation by replacing pumps powered by natural gas, diesel, propane, or gasoline with electric pumps. Irrigation is the practice of adding water to croplands or pastures to reduce crop water stress and increase productivity. Pumps are used on some irrigated croplands to extract groundwater, transport surface water, and pressurize water for application through sprinklers or drip irrigation systems. Electric pumps have much higher motor efficiencies (~88%) than fossil fuel pumps (~21–31%), so pump switching reduces the energy required to pump the same amount of water. The extent to which emissions are reduced depends on the emissions intensity of the electrical grid mix. Electric pumps reduce emissions when the emissions intensity of the grid is below ~0.75 kg CO₂‑eq /kWh, or when they are powered by on-site solar or wind energy. In some places, additional emissions reductions can be achieved through Improving Irrigation Water Use Efficiency.

Does it work?

The efficiency and emissions benefits of electric pumps over fossil fuel pumps are well established. On-farm pumping emissions, currently estimated at approximately 0.2 Gt CO₂‑eq/yr, could feasibly be eliminated if all fossil fuel pumps are replaced with electric pumps and electrical grid emissions reach net-zero, or if they are powered by on-farm solar or wind energy. However, the climate impact of electric pump adoption today would be much lower, as electricity generation still produces substantial emissions. Under current conditions, replacing a diesel pump with an electric pump will reduce emissions in most, but not all, places around the world.

Why are we excited?

Electric pumps can reliably reduce emissions, are already cost-competitive and widely used, and adoption is increasing. Irrigation is a major energy user, and its energy use is increasing as irrigated areas expand. These trends are expected to continue in the coming decades as climate change exacerbates heat and water stress and agricultural production intensifies in low- and middle-income countries. Coupled with ongoing reductions in electrical grid emissions intensity, the potential climate benefits of this solution are growing.

Electric pump adoption can also be geographically targeted, as just five countries (China, India, the United States, Pakistan, and Iran) account for almost 70% of irrigation energy use. Areas with high groundwater reliance can also be targeted, as groundwater pumping accounts for 89% of irrigation energy use.

Pump switching also provides additional benefits, such as lowering long-term energy costs for farmers and reducing air pollution from on-farm fossil fuel use. Access to the electrical grid is the primary technical barrier to electric pump adoption, but small-scale solar installations can be used where grid connectivity is limited. Powering pumps with on-site solar also eliminates operational emissions, reduces the load on the electrical grid, and insulates farmers from variability in energy costs. 

Why are we concerned?

The climate impacts of pump switching are highly dependent on the emissions factor of the electrical grid. A large share of the potential reduction in fossil fuel pumping is located in India and China, which currently have relatively high electrical grid emissions intensities. Under the current grid mix, we estimate that pump switching in these countries will result in only modest benefits or a small increase in emissions.

What is our assessment?

There is strong evidence that green roofs sequester carbon and may reduce building energy consumption, although emissions reduction data are limited and vary with geography, roof design, and other factors. The potential climate impact of increasing green roofs is likely too small to be globally significant (>0.1 Gt CO₂‑eq/yr ). The solution, however, is considered “Worthwhile” because it can reduce energy use in buildings and sequester carbon while helping communities adapt to climate change and benefiting human health, the environment, and building owners.

What is it?

Vegetation planted on specially engineered rooftops sequesters CO₂ through photosynthesis and provides indirect cooling for buildings through evapotranspiration, reflecting heat back to the atmosphere, and shading. This cooling plus the added insulation inherent in the design can reduce the air conditioning loads of the building, particularly compared to dark rooftop surfaces, and therefore reduce emissions from the electricity used to power cooling systems. Green roofs can also reduce heating energy use and corresponding GHG emissions due to the insulation that soils and plant matter provide. Green roofs are in use in all regions of the globe, but concentrated in high-income countries. 

Does it work?

There is strong evidence that green roofs sequester carbon and can reduce the energy consumption and therefore emissions from cooling and heating buildings. Carbon sequestration by vegetation on green roofs has been documented in several studies. A study in Germany found that plants absorbed 141 g carbon/m²/yr (517 g CO₂ /m²/yr) over a 5-year period. However, carbon sequestration rates are difficult to generalize due to variations in design, plant types, and climates. 

Reported building energy savings from green roofs can range from negligible to 60% or more for cooling. For heating the savings can reach 45% or more, but some studies also show a roughly 10% increase in heating energy use with a green roof. The large variability in energy savings outcomes is due to differences in climate; existing insulation and other properties of buildings; green roof design, vegetation and maintenance practices; and measurement and modeling approaches. The highest energy savings potential has been calculated in dry-winter subtropical highlands for cooling and in humid subtropical climates for heating. Areas with short and mild winters are most likely to see heating energy use increase with green roofs, but these areas often have net energy savings when heating and cooling are combined, and most studies of green roofs show a reduction in heating energy use. 

When combined with the carbon sequestration effect of vegetation, green roofs appear to consistently reduce GHG emissions. 

Why are we excited?

Green roofs and other urban green spaces (see Increase Urban Vegetation) provide valuable climate adaptation, human health, environmental, and economic benefits. Green roofs can help cities adapt to climate change because the vegetation reduces heat exposure during extreme heat, while the soil and root systems absorb stormwater – thereby reducing runoff and flooding risks during extreme rainfall. Green roofs improve human health because vegetation filters the air and reduces noise transmission, and interactions with green spaces, including green roofs, have been shown to improve mental well-being. Green roofs can increase biodiversity and habitat and remove water pollution. They also can increase the property value of a building and prolong the longevity of the roof.

Why are we concerned?

Increasing green roofs can be challenging due to high up-front cost, lack of supportive policies, structural and climate limitations, maintenance requirements, and lack of awareness. A green roof can cost three to six times more than a conventional roof, and although it can save energy for cooling and heating, the returns on investment can be lengthy and savings may not be enough to fully offset the higher costs. In addition, not all roofs can support vegetation, rooftop plants can struggle to survive in hot and dry climates, and green roofs may increase heating energy use in buildings in climates with short and mild winters. A green roof also requires maintenance such as watering, plant care, weed control, pruning, and regular inspections. Finally, a lack of awareness is a major barrier to greater adoption. We also noted a lack of measured, rather than modeled emissions reduction data and on current and potential green roof adoption globally.