What is our assessment?

Based on our analysis, methane digesters can reduce emissions only under some conditions. However, because of high methane leakage rates, relatively high costs, and the risk that they may incentivize high-emission activities, methane digesters are not broadly applicable for large-scale deployment. We will “Keep Watching” this potential climate solution.

What is it?

Methane digesters are specialized devices that utilize anaerobic digestion to convert agricultural, industrial, or municipal organic waste into useful products, such as biogas and digestate, in a controlled environment. Using methane digesters to treat organic waste reduces emissions by capturing methane that would have otherwise been released during uncontrolled anaerobic decomposition in landfills, manure lagoons, or other waste facilities. Methane digester-produced biogas, which is roughly 50% methane and 50% CO₂, along with some trace gases, can be used directly as a fuel or refined further into biomethane to replace natural gas use for heat, electricity, transportation, or industrial processes. Biogas can potentially reduce emissions from electricity or heat generation if it is used to replace a more emissions-intensive fuel. Biomethane, also called renewable natural gas, can be used as a drop-in replacement anywhere natural gas is used, such as in industrial processes.

Does it work?

The effectiveness of methane digesters for reducing emissions varies depending on the type of feedstock used and the amount of methane that leaks from the digester and any associated tanks, valves, and pipes. For manure and municipal organic waste, which often degrade in anaerobic environments and can emit methane directly to the atmosphere, capturing that methane in a methane digester and using the resulting biogas to replace a fossil fuel energy source can result in a net emissions reduction if methane leakage rates are low. In contrast, using agricultural crop residues, which typically degrade aerobically and produce CO₂, as feedstock for a methane digester may yield little or no reduction in net GHG emissions, largely due to methane leaks. Use of biogas for energy can also reduce emissions if it replaces a more carbon-intensive fuel. 

Why are we excited?

Methane digesters have been deployed in many parts of the world at various scales, from household to centralized industrial-scale digesters. They can use a wide variety of organic feedstocks from the agricultural, municipal, and industrial sectors. For municipal organic waste and concentrated livestock manure, methane digesters are more effective at reducing emissions than landfilling or most other manure management strategies. The biogas produced can be stored and used for on-site energy needs or as a fuel source for dispatchable electricity that supports intermittent clean energy options such as solar and wind generation. It can also be sold to provide additional sources of revenue for farmers or waste disposal facilities. The use of methane digesters can reduce noxious odors from waste, and the digestate can be used as material for animal bedding or fertilizer, reducing demand for synthetic fertilizers. 

Why are we concerned?

The most serious problem with methane digesters is that they are a significant source of methane leaks. Few data are available on leakage rates; however, the International Energy Agency (IEA) reports that methane leakage from biogas production can range from 0% to 12%, which is significantly higher than the average global methane leakage rate for oil and gas production of 1.2% in 2024. Over a 20-year time frame, methane is more than 80 times more potent than CO₂ at trapping heat in the atmosphere. Therefore, using methane digesters to process waste material that would otherwise degrade aerobically in order to produce methane-rich biogas could, due to high methane leakage rates, have an even greater warming impact than if the waste were left alone. The collection, transportation, and processing of feedstocks and the operation of the methane digesters also produce GHG emissions from fuel use. 

Methane digesters have a high capital cost and are more expensive than other forms of manure methane abatement, such as covers, physical treatments, or chemical treatments. Financial support from governments can help with the upfront installation costs, while low-cost feedstocks and expensive conventional fuel prices could help create an environment where biogas production is economically viable. However, these economic incentives may encourage poor farming practices or increased waste production. For example, dairy farmers may consider increasing their herd size to capitalize on revenues from the outputs of methane digesters, an outcome that would increase total methane emissions from agriculture. There are also health and safety concerns, as there have been instances of digester explosions and leaks that have injured people or harmed the environment.

What is our assessment?

Based on our analysis, deploying district cooling is a potentially impactful option for reducing emissions from buildings as demand for cooling continues to grow. However, upfront cost and project complexity are major barriers to deployment, and a lack of data is a barrier to deeper analysis. This potential solution is therefore classified as “Keep Watching.”

What is it?

District cooling consists of a centralized cooling system that distributes chilled water to multiple buildings through a network of insulated underground pipes. The cooled water absorbs heat from the buildings, replacing the need for air conditioners or chillers in each building. District cooling can produce cooled water from a variety of renewable sources, such as renewable electricity, solar cooling, and natural cooling sources, including seawater, lakewater, rivers, and groundwater. It can even use waste heat from industry to generate cooling. Many systems include thermal energy storage facilities where frozen water, cold water, or phase change materials are cooled when electricity prices are low for use during peak hours to save costs and reduce strain on the electricity grid. District cooling is best applied to high-density areas and can be combined with district heating to provide year-round conditioning. 

Does it work?

When district cooling replaces conventional standalone systems in residential and commercial buildings, it can reduce emissions through two main mechanisms. First, many district cooling systems exchange heat with natural sources of cooling such as oceans, deep lakes, and rivers, a process that can be many times more energy efficient than conventional cooling systems. This results in reduced energy use and reduced emissions from the electricity used to operate the system. Second, district cooling systems can reduce the use of refrigerants with high global warming potentials, which can leak at all stages of a cooling system’s lifespan. When replacing standalone systems, district cooling can significantly reduce the total volume of refrigerants used. In addition, some district cooling systems do not use any refrigerants at all (e.g., exchanging heat with ocean or deep lake water), and many are able to use refrigerants with low global warming potentials. For instance, the Zuidas International Business Hub in the Netherlands adopted a district cooling system that uses lake cooling combined with chillers, reducing emissions by 75% compared to conventional cooling systems. 

Why are we excited?

According to the International Energy Agency (IEA), global carbon emissions from cooling buildings reached 1.02 Gt CO₂‑eq in 2022. The majority of emissions associated with cooling are from standalone systems such as window air conditioners and chillers that serve a single building. District cooling systems are relatively rare at this time, with most capacity found in the United States and the Gulf Arab States. While existing district cooling systems can be made less emitting, there may be greater potential for new systems because demand for cooling is increasing by ~4%/yr as global temperatures rise and as standards of living improve in regions that experience high temperatures. This is raising concerns about the new electricity generating capacity needed when demand peaks on very hot days. District cooling systems can reduce overall energy use for cooling relative to standalone systems, and when paired with cool thermal storage, can significantly reduce demand during peak hours and on hot days. Building owners can enjoy less maintenance costs, more reliable cooling, and increased floor space when district cooling systems replace bulkier standalone cooling systems. In dense areas with good access to natural or low-cost cooling sources, district cooling systems can cost less to operate and offer lifetime savings despite the higher upfront costs. 

Why are we concerned?

Deploying district cooling systems has high upfront costs and requires extensive planning and coordination among a wide range of stakeholders. These projects can face challenges in getting financing due to a lack of confidence for both investors and customers, uncertainty about future loads, and regulatory barriers. These can be especially challenging in low- and middle-income countries where demand for cooling is growing rapidly. Many buildings are likely to invest in standalone systems in the near term, locking them into alternatives and weakening the business case for district systems in the area. Meanwhile, the full potential is difficult to assess due to a lack of data on district cooling systems globally.