What is our assessment?

Based on our analysis, improving fuel-burning cooking equipment is an important strategy for reducing GHG emissions from stoves and unsustainable wood harvesting, especially in low-income countries where the additional social and health benefits are significant. However, the lack of quantitative data on the emissions impacts precludes a full analysis. We will “Keep Watching” this potential climate solution. 

What is it?

Improved fuel-burning cooking equipment reduces emissions from combustion and unsustainable biomass harvesting by switching to more efficient cooking equipment or cooking equipment that burn cleaner fuels. In many countries around the world, it is common to cook meals over open fires with solid fuels such as wood, charcoal, dung, and coal or over kerosene stoves Cooking using solid fuels occurs mainly in low-income countries and in rural areas, with one report estimating that 84% of people in sub-Saharan Africa lacked cleaner cooking solutions in 2020 (Stoner et al., 2021). Improved fuel-burning cooking equipment includes more efficient cookstoves and stoves that use cleaner fuels such as liquid petroleum gas, natural gas, biogas, and alcohol. Use of such equipment reduces emissions from combustion and unsustainable biomass harvesting (International Energy Agency [IEA], 2023). Improving fuel-burning cooking equipment is widely seen as a stepping stone toward cooking with electricity in regions lacking reliable and affordable access to electricity (Couture & Jacobs, 2019). Solar-powered cookstoves are not included here, and shifting to electric cooking is addressed in Deploy Electric Cooking Equipment. 

Does it work?

Improved fuel-burning cooking equipment is widely available, is proven to reduce GHG emissions, and can reduce unsustainable biomass harvesting. This is because cleaner fuels are less carbon intensive and make the cooking equipment more energy efficient, resulting in less emissions per unit of heat delivered (Johnson, 2009; Khavari et al., 2023). Meanwhile, more efficient solid-fuel cookstoves can reduce fuel usage by 25-40% (MacCarty et al., 2010). Using less biomass for cooking also means less unsustainable wood harvesting that can lead to deforestation (Bailis et al., 2015). The IEA (2023) estimates that 0.8 Gt CO₂‑eq/yr could be avoided by 2030 if all households cooked with cleaner cooking fuels or electricity, and a similar amount of emissions could be prevented by avoiding deforestation. 

Why are we excited?

Improving fuel-burning cooking equipment is a low-cost climate solution that has had high rates of adoption in some countries driven in part by the significant additional benefits. Most improved cooking equipment has up-front costs of less than $US100 (Modern Energy Cooking Services & Energy 4 Impact 2021), and there can be operational cost savings because the new equipment is more fuel efficient. China, India, and Indonesia saw 2–4% of their populations gain access to cleaner cooking technologies (including electric appliances) each year between 2010 and 2022 (IEA, 2023), highlighting how adoption can grow with supportive policies and funding. 

Improved cooking equipment provides numerous social benefits. The World Health Organization (2024) estimated that 2.9 million premature deaths occur per year from household air pollution, and inefficient cooking contributes to that. The World Bank (2020) estimated that cooking with solid fuels and kerosene causes US$1.4 trillion/yr in negative health impacts, mainly to women and girls. Women and girls also may be vulnerable to violence and assault when collecting biomass fuel. The IEA (2023) estimates that an average of 5 hours a day are spent on collecting biomass and cooking in some regions when inefficient cooking methods are used, time that might otherwise be spent on schooling or additional employment. Finally, biodiversity loss and deforestation are reduced when less biomass is used for cooking (Pearson et al., 2017). 

Why are we concerned?

Adoption of improved fuel-burning cooking equipment is held back by cost and cultural barriers, while lack of data hinders emissions impact analyses. Despite being low cost, improved fuel-burning cooking equipment and their fuels are financially inaccessible to many households in low-income countries, including half of households in sub-Saharan Africa (World Bank, 2020). Households using improved cooking equipment may revert to old equipment if fuel becomes unaffordable or cookstoves do not support traditional cooking practices (Jewitt et al., 2020). Finally, the full potential of this solution to reduce emissions cannot be assessed due to a lack of studies specific to improvements to fuel-burning equipment and the types of GHGs involved.

What is our assessment?

Based on our analysis, deploying electric cooking equipment can reduce emissions while providing significant additional benefits for health. However, the lack of quantitative data on emissions impacts precludes a full analysis. We will “Keep Watching” this potential climate solution. 

What is it?

Using electricity for cooking instead of burning fuels such as liquid petroleum gas (LPG), natural gas, biogas, biomass, charcoal, coal, or kerosene reduces the emissions from burning fuels and can reduce emissions overall when powered by low-emissions electricity. The World Health Organization (WHO, n.d.) estimated that 60% of the global population cooked with gas and 26% cooked with biomass in 2023, while only 8.4% used electricity. Natural gas is the most common fuel for gas cooking in countries where gas distribution infrastructure exists, while liquid petroleum gas is common in low- and middle-income countries (Kojima, 2021). Cooking with electricity typically involves single or multi-element electric resistance or induction cookstoves and electric ovens. The Improve Fuel-Burning Cooking Equipment solution focuses on the adoption of more efficient fueled cookstoves and the switch to cleaner fuels such as LPG as a stepping stone toward eventual electric cooking in low-income communities where reliable and affordable access to electricity is currently lacking (Couture & Jacobs, 2019). 

Does it work?

Cooking with electricity rather than fuels reduces emissions from cooking in areas with low grid emissions (Gould et al., 2023), and relies on well-established technologies. Burning fuels for cooking generates greenhouse gas emissions, and methane leaks from natural gas stoves also contribute to emissions (Lebel et al., 2022). Cooking with electricity can be two or more times more energy efficient than alternatives (Rao & Sagar, 2024). Furthermore, electric cooking equipment is widely available. However, the total emissions impact of shifting to electric cooking equipment will be limited because cooking is a minor contributor to total building energy use worldwide (IEA, 2025). 

Why are we excited?

Electric cooking can have significant benefits for health, safety, and energy use, with modern electric appliances growing in popularity among chefs. Electric cooking equipment also supports all-electric new developments and provides benefits in communities with mini-grids. Burning fuels for cooking contributes to household air pollution (Lebel et al., 2022), which is responsible for an estimated 2.9 million premature deaths per year (WHO, n.d.), and also generates poisonous carbon monoxide. Explosions can also result when fuel leaks from fuel-burning cooking equipment (Lebel et al., 2022). Electric cooking reduces or eliminates these health and safety risks because fuels are not used. Furthermore, electric cooking is more energy efficient (Gould et al., 2023), and therefore releases less unwanted heat to the kitchen area which can improve comfort and the need for air conditioning (Li et al., 2024; Luo et al., 2023). Modern induction cookstoves are gaining in popularity among chefs because they can heat food faster with better temperature control than alternatives (Hawking, 2025) and have the added advantage of being easy to clean. In new developments, electric cooking enables all-electric developments that avoid locking in new fossil-fuel infrastructure (Tan et al., 2022). In some regions in Africa, electric pressure cookers are also a promising solution for some cooking tasks because their lower energy needs make them easier to integrate into mini-grids (Efficiency for Access et al., 2020; Kweka et al., 2021). 

Why are we concerned?

Electric cooking adoption can be hampered by costs, unreliable access to electricity, and cultural preferences. In addition, net emissions can increase when used in areas with high grid emissions, and data for analysis are limited. In high-income countries, switching to electric cooking can trigger additional electrical upgrade costs (Pergantis et al., 2025). In low-income countries, up-front costs are a major barrier, as is reliable, safe, and affordable access to electricity (Das et al, 2025; IEA, 2024). An estimated 750 million people lack access to electricity worldwide, with more than 80% living in Africa (IEA, 2024). Households may also continue to use traditional cooking methods alongside electric appliances due to cultural preferences (Crentsil et al., 2025). The effectiveness of this solution varies regionally, with grid emissions factors, electrical equipment efficiency, and the fuel being substituted all playing a role in whether electric cooking provides net emissions benefits and the magnitude of those benefits. The biggest factor is the emissions due to the grid mix providing electricity, which varies regionally. Finally, a lack of data on the relative proportions of fuels in use today for cooking is a barrier to deeper analysis. 

What is our assessment?

Advanced geothermal systems (AGS) are emerging as one of the most promising technologies for reliable, utility-scale, zero-carbon energy that can complement wind and solar, strengthening grid resilience, and providing heat for district heating and industrial uses. The technology, which is built on an existing base of technical and industrial expertise and capacity, is advancing rapidly through major R&D efforts, pilot projects and, just recently, small scale commercial operations. While large-scale deployment is still in its early stages and challenges remain around cost, execution, and social acceptance, we expect meaningful progress by the 2030s. For now, we will “Keep Watching” this solution.

What is it?

Advanced geothermal systems (AGS) are a suite of renewable energy technologies that extract heat from deep within the Earth’s crust to generate electricity, provide high-temperature heat for industrial processes or district heating, and enable geothermal energy storage by storing heat underground. Unlike traditional geothermal systems that tap naturally occurring hot water or steam reservoirs, such as geysers or volcanic areas, AGS access geothermal heat by drilling into the earth, injecting and circulating water (or other fluids) through hot, dry rock formations underground, and then recovering the heated fluid or steam to generate electricity before reinjecting it back underground. Circulation of the water between the surface and the geothermal reservoir can be a in closed loop system, where the water or other fluid is contained within pipes throughout the heat exchange circulation cycle, or in an open loop, enhanced geothermal system (EGS) where the subsurface rocks are hydraulically fractured, or “fracked,” to increase permeability and allow water to flow between an injection well and a production well.

Does it work?

Electricity and heat production by an advanced geothermal power plant emits virtually no greenhouse gases. Analysis by the National Renewable Energy Laboratory showed that the median life cycle emissions from enhanced geothermal power plants were 32 g CO₂‑eq/kWh, just 6% of the median life cycle emissions from a natural gas power plant, with most of the emissions generated during construction rather than operation. Geothermal energy has been used for more than a century, but AGS that use the directional and horizontal drilling and hydraulic fracturing techniques developed by the oil and gas industry to access previously inaccessible underground heat resources are relatively new. To date, several small-scale and experimental AGS projects have successfully produced electricity, and in December 2025, the first commercial plant for electricity and heat production delivered electricity to the grid in Germany. 

Why are we excited?

Advanced geothermal energy systems are a potentially transformative climate solution for several reasons. First, they could massively expand clean energy availability. AGS can be deployed in almost any region with hot subsurface rocks. Experts estimate the Earth’s accessible geothermal resources are staggeringly large, and that tapping just 0.1% of the heat under our feet could meet global energy needs for millennia. If technology improvements continue, advanced geothermal could supply around 15% of the world’s electricity by 2050. Second, unlike solar and wind energy, advanced geothermal power plants produce steady baseload power, dispatchable power, and even energy storage. Currently, coal and gas power plants are commonly used to provide stability and backup power to electricity grids around the world. AGS can provide the same energy benefits, complementing wind and solar energy by providing firm capacity and grid stability services to a renewable-heavy electricity grid, without the harmful climate impacts. Third, AGS plants have a relatively small land footprint and can potentially be sited near demand centers (including repurposing old fossil plant sites), improving energy security for regions with limited solar or wind resources.

Recent technological breakthroughs have improved the prospects for AGS. The application of directional drilling and hydraulic fracturing techniques has produced higher fluid flow rates and extended reservoir life. This has dramatically increased the heat extraction per well, overcoming previous limitations and boosting the energy output and economics of AGS. Industry reports show drilling rates in hot rock have increased by 300–500% in the last few years, slashing upfront costs. A recent U.S. Department of Energy report projects that the cost of next-generation geothermal projects, including AGS, will fall below that of other baseload power sources such as nuclear and natural gas with carbon capture and storage (CCS) by 2035. Other projections suggest that geothermal electricity could drop to around US$50/MWh by the 2030s, competitive with other renewables and nuclear. Finally, AGS leverage a skilled workforce and supply chain from the oil and gas sector. The necessary drilling rigs, subsurface imaging, and engineering expertise already exist, which could help scale up AGS faster than entirely new industries.

Why are we concerned?

Despite its promise, AGS face several challenges that temper its near-term prospects. To bridge the gap from pilot stage to commercialization, the industry needs more demonstration projects, case studies of success, and greater public trust. This is challenging because advanced geothermal projects today have high upfront capital costs, primarily due to deep drilling and, for EGS, hydraulic stimulation expenses, as well as high operational costs. Current AGS electricity is also far more expensive than conventional renewables, often hundreds of dollars per MWh. Until these costs decline, the industry may struggle to attract the investment financing needed to scale up. Moreover, the geological uncertainty in any given project is high because limited geophysical data in many regions makes it hard to pinpoint the best spots to drill. Developers must invest in exploration with no guarantee of finding an adequate resource, so early projects carry a significant risk of cost overruns.

Safety and environmental concerns also pose challenges. In some types of geologies, enhanced geothermal systems, which use hydraulic fracturing to create the heat exchange reservoirs and circulate fluid underground, can trigger small earthquakes. Some EGS have been halted after local earthquakes caused alarm and minor damage. Because they use water and circulate hot brines, AGS could pose risks for groundwater contamination or water consumption in arid regions, although geothermal system designs that use closed-loop systems or non-potable water can avoid these problems. Finally, geothermal projects often face regulatory and logistical hurdles and lengthy permitting processes. In many countries, regulatory regimes and incentives have focused on solar, wind, and even nuclear, while geothermal energy (and especially AGS) has received comparatively little support. This means AGS developers may struggle with financing and grid access due to policy gaps or obstacles. 

What is our assessment?

Based on our analysis, Improve Steel Production using H₂-DRI-EAF powered by clean electricity has the potential to significantly reduce emissions. However, while the individual technologies for H₂-DRI-EAF are mature and their combined use has been piloted, the process has not yet been adopted in a meaningful way. We will “Keep Watching” this solution, but it is not ready for widespread adoption.

What is it?

Currently, making steel from iron ore relies heavily on coal and other fossil fuels to provide heat and reducing agents (chemicals that remove oxygen from iron ore). Improve Steel Production refers to using electric heat and hydrogen produced by electrolysis to reduce the iron ore (H₂-DRI) and electric arc furnaces (EAF) to melt the resulting iron and alloy it with carbon to make steel. The solution also requires the electricity used in these processes to include significant renewable energy or other low-carbon generation. The output is varying grades of steel with different degrees of hardness and brittleness determined by slight variations in carbon content. This solution does not include processes that rely on bioenergy or CCS, since the emissions from burning bioenergy contribute to climate change and CCS is not an effective climate solution.  

Does it work?

Replacing fossil fuels in steelmaking with H₂-DRI-EAF that uses electrolytic hydrogen and where all electricity comes from relatively clean sources results in significantly reduced emissions. Steel made today using fossil fuels for heat and as a reducing agent results in an estimated 1.8 t CO₂‑eq /t of steel. By contrast, steel made using H₂-DRI-EAF and low-carbon electricity would generate an estimated 0.12 t CO₂‑eq /t of steel and is a more energy-efficient process. EAF furnaces are already very common in steelmaking and for recycling existing steel, but are rarely combined with H₂-DRI. Although H₂-DRI was first used on an industrial scale in 2001, that plant was shut down for economic and political reasons, and economics remain a barrier. Finally, technologies to make industrial hydrogen from electricity are mature, but most hydrogen produced today is made from fossil fuels and is carbon-intensive. Active research is exploring other technologies that could become important for improving steel production in the future, most notably aqueous or molten oxide electrolysis, both of which use electricity to directly remove oxygen from iron ore, and can be combined with EAF to make steel.  

Why are we excited?

Steelmaking is classified as a hard-to-abate industry, and H₂-DRI-EAF powered by clean electricity is considered one of the best strategies for cutting emissions in this sector. The Net Zero Industry project forecasts that under an emissions-neutral steel scenario by 2050, roughly 40% of global steel production could depend on H₂-DRI-EAF, with the remainder consisting of recycled steel (47%), steelmaking with CCS (11%), or technologies not yet defined (2%). The impact is potentially significant, given that steelmaking accounted for an estimated 3.7 Gt of CO₂‑eq in 2019. Improved steelmaking has the additional benefit of reducing air and land pollution, as burning coal releases fine particulate matter, heavy metals, and other pollutants. In China, steel production is the largest industrial source of air pollution. As demand for steel is expected to increase up to 30% by 2050 due to demand from India and other low- and middle-income countries, it is critical that new and existing production shift to cleaner, lower-emission technologies, and that policies supporting this shift be implemented.  

Why are we concerned?

While proposed low-emission steel projects have attracted significant attention from the press, many have since been canceled or put on hold. As of 2025, we could find references to only a few pilot facilities producing improved steel as we have defined it here. The entire H₂-DRI-EAF process is considered to be at the large-scale prototype demonstration stage. However, contributing technologies such as electrolytic hydrogen production and EAF are more mature, and H₂-DRI was first used on an industrial scale in 2001. The higher cost of making low-emission steel is a significant barrier to industrial adoption and consumer demand. Electricity accounts for nearly half the cost of producing low-emission steel from iron ore. To increase adoption, improved steel facilities need to be located in areas that can readily supply both iron ore and abundant low-carbon, low-cost electricity. In areas such as China, where the electricity grid still relies heavily on fossil fuels, transitioning to H₂-DRI-EAF risks increasing emissions unless dedicated renewables are integrated into the project. To move this solution forward, new policies are needed to create an international market for low-emission steel. Meanwhile, existing steelmaking facilities typically have lifetimes of 25–40 years, which increases the likelihood of stranded assets or continued reliance on fossil fuels by 2050. Under its Sustainable Development Scenario, the International Energy Agency (IEA) projects that, by 2050, only 12% of cumulative direct emissions reductions in steelmaking will be due to electrification and the use of hydrogen (the IEA considered emissions from electricity to be indirect). Reducing demand for steel, incremental efficiency gains, and CCS are expected to make up the bulk of cumulative direct emissions reductions, according to the IEA projections.

What is our assessment?

Nitrous oxide removal technology is at a very early stage of development. Other available technologies and practices can effectively reduce nitrous oxide emissions. However, because this solution aims to remove such a potent GHG from the atmosphere, we will “Keep Watching” it. 

What is it?

Nitrous oxide removal is a technology that uses photocatalytic chemicals to convert nitrous oxide, a GHG that has 270 times more warming potential than CO₂ and persists in the atmosphere for more than 100 years, into gaseous nitrogen and oxygen. 

Nitrous oxide is found naturally in the atmosphere, but 40% of emissions come from human activities, and human-caused emissions have increased more than 30% during the past four decades. Most anthropogenic contributions are from fertilizers applied to croplands and other farming activities, while the rest are from fossil fuel use, industrial activities, and waste and wastewater. 

This solution involves spraying a chemical photocatalyst onto agricultural fields. When the photocatalyst is exposed to sunlight and nitrous oxide, it drives a chemical reaction that decomposes nitrous oxide into gaseous nitrogen and oxygen. 

Does it work?

Research into atmospheric nitrous oxide removal is still in its early stages. The concentration of nitrous oxide in the atmosphere is very low, so nitrous oxide removal would likely be implemented in agricultural areas where fertilizer use locally elevates atmospheric concentrations. Laboratory testing has shown that nitrous oxide can be converted into nitrogen and oxygen using light energy and photocatalysts. However, the effectiveness of the solution in practice is uncertain because few experiments have been conducted in real-world settings. The single field study that applied titanium dioxide to a field crop did report a measurable reduction in nitrous oxide emissions. However, there is no evidence that this technology can remove atmospheric nitrous oxide at a meaningful scale. 

Why are we excited?

Because nitrous oxide is such a potent GHG, reducing its concentration in the atmosphere could have a disproportionately beneficial climate impact. In addition, unlike direct air capture or carbon capture and storage, there is no need to capture or store any gases because the nitrous oxide breaks down into gases that have no climate impact. Also, titanium dioxide application to crops is being researched as a method for improving crop resilience.

Why are we concerned?

Serious concerns include technical feasibility, environmental risk (including environmental and food safety), scalability, cost, and monitoring, reporting, and verification. While there is currently very little research on the real-world use of photocatalysts to destroy atmospheric nitrous oxide, ongoing research on the application of nanoparticles of titanium dioxide to crops to enhance productivity and resilience to stress suggests that high concentrations of titanium dioxide can have adverse effects. Furthermore, these nanoparticles are not approved for direct food consumption, and their fate and environmental impacts are poorly understood. 

Tools, methods, and standards need to be developed to quantify nitrous oxide removal for accurate accounting and verification. Costs are unknown. Finally, numerous other approaches for reducing human-caused nitrous oxide emissions exist, including improving nutrient management, rice production, manure management, and industrial processes, as well as reducing fossil-fuel use for power generation and transportation and increasing use of centralized composting. 

What is our assessment?

Reduced grazing intensity can temporarily reduce soil degradation and erosion. However, SOC outcomes depend on a number of factors, such as climate zone, land use history, soil properties, and grass type. Therefore, until stronger, long-term evidence is available to guide more effective implementation, we will Keep Watching this solution.

What is it?

Reducing grazing intensity refers to lowering ruminant livestock stocking rates or shortening grazing duration to reduce pressure on grazing lands. As a climate solution, it is intended to remove carbon from the atmosphere by increasing SOC through enhanced plant productivity, root inputs, and soil stability. Grazing intensity is typically classified as heavy, moderate, or light, based on the proportion of forage removed per unit time.

Does it work?

In general, while heavy grazing reduces SOC, the effects of grazing intensity on SOC recovery varies with climate zones, grass types, soil properties, and prior land use. Increases in SOC under reduced grazing intensity are largely limited to wetter regions, often with high annual rainfall. In arid and semi-arid regions, which represent a major share of global grazing land, reduced grazing intensity often results in neutral or negative SOC responses. A global review and meta-analysis that normalized SOC to 30 cm depth found that even grazing below carrying capacity was associated with an overall decline in SOC, with gains limited to lower-intensity grazing conditions in specific climate zones.

Why are we excited?

Reducing grazing intensity provides ecological benefits. This usually involves reducing the number of ruminant livestock on a farm, which in turn reduces the farm’s methane emissions, land-use pressure, and threats to biodiversity–at least in isolation. It can reduce soil degradation, erosion, and vegetation loss. It is already practiced in many contexts and requires no new technology or infrastructure, making it easy and relatively low cost as a climate intervention, though not necessarily cost-neutral for ruminant livestock producers.

Why are we concerned?

Several limitations, risks, and trade-offs are associated with reducing grazing intensity as a carbon removal strategy.

First, even low-intensity grazing can prevent ecosystem recovery when pastures are seeded with, or invaded by, aggressive grasses that suppress native plants, prevent tree regrowth where ecologically appropriate, and lock landscapes into lower-biodiversity, grass-dominated states.

Second, SOC gains are limited, slow, and reversible. Soil organic carbon is a finite sink that approaches saturation within decades and can be lost through drought, warming, fire, or management changes. SOC accumulation through reduced grazing intensity has been shown to be a temporary and fragile form of carbon storage. 

Third, SOC gains are difficult to measure and verify. Many studies lack baseline SOC measurements, adequate controls, sufficient duration, and/or adequate soil-depth sampling, making it difficult to attribute carbon gains to grazing intensity. To show an increase in SOC from reduced grazing intensity, an ideal experiment would adopt a before-and-after control intervention at a commercial scale and follow SOC changes for 5–10 years.

Fourth, while reducing grazing intensity compares favorably with alternative grazing when it reduces total stocking numbers, it is still less durable and certain as a carbon removal strategy than protecting intact ecosystems, restoring degraded grasslands, or restoring forests where ecologically appropriate. 

Fifth, reducing grazing intensity often lowers herd sizes, but under rising global beef demand this can simply shift production elsewhere. This underscores the value of improving diets and shifting food system infrastructure away from ruminant consumption rather than simply altering ruminant production practices.

Overall, reducing grazing intensity can reduce some local damage from heavier grazing, but in climate-favorable regions especially, the stronger opportunity is often restoring ecosystems or producing higher-yielding plant-based foods.

What is our assessment?

Based on our assessment, Reduce Airplane Contrails has the potential to rapidly reduce the direct climate warming impact of the aviation industry. However, because the solution is not already being adopted at scale and there is a lack of data on its effectiveness, we will “Keep Watching” this solution.

What is it?

This solution reduces the warming impact of contrails by rerouting airplanes to avoid areas where contrails are likely to form. Contrails (also known as condensation trails) are long, thin clouds that form behind aircraft when the exhaust combines with cold, humid air to produce ice crystals at high altitudes. Contrails can trap heat radiating from the Earth, producing a strong but short-lived warming effect similar to that of greenhouse gases in the atmosphere. Most contrails dissipate quickly (<10 minutes), but under some meteorological conditions, they can persist for many hours. In regions with high air traffic density, contrails can cover a large fraction of the sky area, and even though they may last for only hours, the heat trapped in the atmosphere and oceans by contrails is multiplied by the tens of millions of flights per year. It’s important to note that not all contrails have a warming impact. The degree to which contrails warm or cool the atmosphere varies with time of day, season, atmospheric conditions at cruising altitudes, and whether the clouds form over land or ocean. Contrails that form during the day can have a net cooling effect by reflecting solar radiation back into space. However, the scientific consensus is that contrails overall have a net warming effect.

Does it work?

Modeling studies and field testing suggest that strategically rerouting flights to avoid areas where warming contrails are likely to form can substantially reduce contrail formation and their warming impacts. It is estimated that less than 20% of flights produce persistent contrails with a net warming effect, and rerouting the most impactful of these flights could reduce contrail-induced warming by as much as 80%, providing an immediate climate benefit. Rerouting aircraft to avoid turbulence is already a standard industry practice. These same protocols could be used for contrail avoidance with the addition of model forecasts for contrail formation into pre-flight planning and in-flight sensors and satellite measurements for in-flight responses.  

Why are we excited?

Research suggests that the warming impact of contrails is roughly comparable to and additional to the warming from the direct GHG emissions from the aviation industry’s use of fossil fuels. Strategically rerouting air traffic to reduce the formation of warming contrails could have an immediate and globally meaningful climate impact, making this an “emergency brake” solution with the potential to deliver a beneficial impact more rapidly than many other climate solutions. In addition, this solution could be implemented at scale relatively quickly, even as supportive predictive models, meteorological monitoring, and instrument integration technologies improve. Progress is already being made. Industry trials are already underway, and on-board humidity sensors that can identify when an airplane is moving through a contrail-forming region are being developed. The European Union now requires major aircraft operators to report modeled data on their contrail formation as part of their emissions reporting. This sets the stage for policies that require warming contrail avoidance. Finally, this high-impact climate solution is relatively low-cost. The costs for additional sensors and fuel are estimated to be US$10–15 per flight, or the equivalent of US$1–6/t CO₂‑eq avoided.  

Why are we concerned?

Policy and regulatory changes will be needed to support the adoption of rerouting protocols to avoid warming contrails, and implementation could be restricted by uncertainties in the models and by safety concerns. Multilateral industry and government cooperation will be necessary to draft new regulations to support rerouting to avoid warming contrails, and timelines must be established for mandatory implementation. While models that forecast where warming contrails are likely to form exist, they are limited by a lack of data on humidity levels at cruising altitudes and require more validation to assess how accurately they project contrail formation. In addition, better tools to monitor and model the effectiveness of rerouting in preventing the formation of warming contrails are needed, especially when the added emissions from fuel use could exceed the climate benefits of the contrails avoided. Rerouting opportunities may also be limited by safety concerns in congested airspaces.