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.