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. 

What is our assessment?

Injecting huge amounts of reflective aerosols into the stratosphere to counteract or mask GHG-driven warming is not a serious or plausible climate solution. Its effectiveness is highly uncertain, and its potential for harmful unintended impacts to Earth and ecological systems, as well as on human well-being, is extremely high. Based on these significant problems and risks, we conclude that deploying stratospheric aerosol injection (SAI) is “Not Recommended.”

What is it? 

SAI is a geoengineering technology that uses airplanes or balloons to inject fine particles, usually sulfates, into the stratosphere, the layer of air that begins about 6–20 km above Earth’s surface. These aerosols would scatter some of the sunlight striking the planet, reflecting it back into space. Reducing the amount of sunlight hitting Earth is intended to cool the planet and counteract the warming effects of increasing GHG concentrations. Because SAI does not affect the atmospheric concentration of GHGs, the direct cause of global warming, this technology is not actually a solution to climate change. Instead, it is a temporary action to mask the ongoing warming effects of GHG emissions. 

Does it work?

The injection of large amounts of reflective aerosol particles into the stratosphere does have a cooling effect on the planet. Following the 1991 eruption of Mount Pinatubo, which injected 20 Mt of sulfur dioxide into the stratosphere, average global temperatures were about 0.5°C lower for more than a year. In another example, modeling studies suggest that recent reductions in East Asian air pollution have contributed to the acceleration of global warming. Therefore, in theory, deploying SAI could achieve a similar effect. However, other than modeling simulations, SAI has never been tested in the field, and researchers agree that there are substantial uncertainties and risks. For example, the ways that GHGs and stratospheric aerosols affect global temperatures differ. GHGs warm the planet more in winter than in summer, and more in the high latitudes, especially in the Northern Hemisphere, than in the equatorial regions. Because aerosols reflect solar radiation, they have a greater impact during the summer and in the equatorial zone. Finally, the solar radiation reflective effect of SAI is temporary. Depending on the location and altitude of injection, the aerosols remain in the stratosphere for only months to a few years.  

Why are we excited?

We’re not. The only argument in favor of deploying SAI is based on the concern that we cannot reduce GHG emissions fast enough to avoid the catastrophic environmental and societal impacts of climate change. SAI proponents argue that this geoengineering approach to reduce global temperatures could be a “bridge,” buying time to cut GHG emissions and remove atmospheric CO₂ over the longer term. 

Why are we concerned?

SAI is an untested technology designed to alter planetary energy balance and atmospheric dynamics. Numerous modeling studies indicate that its efficacy to reduce global or regional temperatures as intended is highly uncertain and that it has high risks for unintended impacts on Earth, ecological, and human systems. These studies show that SAI could have substantial effects on the physics, chemistry, and circulation of the upper atmosphere, including harm to the ozone layer. It could destabilize weather and rainfall patterns, reducing the amount of sunlight striking the Earth’s surface, and changing the balance of “direct” and “diffuse” sunlight, effectively making the sky look more hazy. These effects will, in turn, have profound impacts on ecosystems, including the rates of photosynthesis in forest carbon sinks, agriculture, and human well-being. Even if it works to lower temperatures as planned, SAI will have no impact on the non-climatic effects of increasing CO₂, such as ocean acidification. SAI is also inherently a temporary intervention; it will require sustained deployment for as long as 100 years, according to one study, to avoid “termination shock” and an abrupt temperature increase if GHG concentrations are still high. SAI also poses immense geopolitical, legal, and ethical challenges, including international responsibilities for implementation, financing, compensation for negative impacts, and procedural justice questions, such as those around informed consent. And finally, beyond these scientific, environmental, political, and socioeconomic concerns, SAI poses a serious “moral hazard” that could distract or delay action on real solutions to climate change.