Over the past several decades, airplane travel has been discussed in the scientific community and the media as a contributing factor to climate change, but there is a lot of disagreement about the claim’s validity and the contributing factors. Climate change is defined as the “periodic modification of Earth’s climate … [because of] changes in the atmosphere [and] interactions between the atmosphere and various other geologic, chemical, biological, and geographic factors within the Earth system (Jackson, 2018)”. Contrails are a byproduct of airplane travel; the term contrail is a portmanteau of condensation trail and it refers to the visible streaks stemming outward from the back of an airplane. In streak form they are known as linear contrails but can also form into cirrus clouds. Cirrus clouds are considered high clouds, since they are located at altitudes above 20,000-26,000 ft (Winograd, 2012). Like contrail cirrus clouds, natural cirrus clouds consist of ice crystals, which make them indistinguishable in imagery and difficult to study and resolve.
Flying is a vital method of transportation that sees an upward trend in passengers and cargo flights (Grewe et al., 2014). According to the Federal Aviation Administration (FAA), they handle over 15.8 million flights yearly, 43 thousand daily, over 24.1 million square miles of oceanic airspace, 5.3 million square miles of domestic airspace, and with 5,000 planes in the sky at peak times (FAA, 2018). That is a lot of flights, but steps can be taken to minimize the effect on climate change. According to a study funded by the European Union Fp7 Project REACT4C, the most impactful shifts were found to be avoidance of contrails and reduction of carbon dioxide and nitrogen oxide emissions. The study looked at all transatlantic flights on a single winter day using a climate-chemistry model (EMAC) and traffic simulator (SAAM). The study found that due to wind the biggest minimizing impact is on flights from east to west, rather than west to east, particularly when considering the financial impact on airline companies and passengers. Direction of travel impacts the ease of altering flight trajectories, with westward flight changes minimizing additional costs - 0.5% increase in cost will offer a 25% climate impact benefit. However, the study found that redirecting flights to avoid all contrails will only have a 1% cost increase. (Grewe et al., 2014). Such changes while considering costs may extend flight lengths (Climate Science). The results are not comprehensive for all locations, altitudes, and seasons, but it shows how small changes can have a significant impact on contrail formation and climate impact.
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The formation, duration, size, and type of contrail predominantly depends on temperature and humidity, which is the amount of water vapor in the air (Paoli et al., 2016). Higher levels of humidity increase the length of time that a contrail is visible, but they won’t form at temperatures higher than -38 degrees Celsius (Jansen et al., 2015). Since temperatures are lower at higher altitudes, flying higher results in more contrail formation. Contrails that don’t dissipate within an hour are known as persisting contrails. Longer persistence has a greater potential effect on climate change, since contrails can overlap. For persistence, the amount of ice cannot exceed the humidity in the air. If the ice exceeds the humidity the contrail disappears via evaporation. (Paoli et al., 2016)
There are other factors that play a role in determining the effect on climate change. Certain geographical locations and conditions are linked to the problem. In addition to the previously mentioned factors of temperature and humidity, studies found that nighttime flights particularly in busy air space lead to longer contrail persistence and overlapping (Carleton et al., 2016). During the day, contrails are less harmful because of “shortwave reflectance,” which is a cooling ability to reflect radiation away from Earth (Carleton et al., 2016). At night, contrails are more harmful because of the “greenhouse effect,” which is a heating ability attributed to climate change. According to the Environmental Protection Agency (EPA), “[greenhouse gases] are gases that trap heat in the atmosphere.” A 2013 global study by Schumann and Graf found that, “aviation induces additional cirrus cover and thickens existing cirrus,” which is a general statement from a global perspective some locations are affected more than others (Brasseur et al., 2016). Studies disagree whether flights over land or water are more harmful, still busy air space is noted to be over the United States, northern Atlantic Ocean, northern Europe, and China (Brasseur et al., 2016). These locations require contrail reduction more than places with fewer consecutive flights.
Changing flight trajectories in real time is one way to avoid creating persisting contrails by avoiding existing cirrus clouds and factors that facilitate the creation of new contrails. However, contrails can also be minimized by reducing airplane emissions from jet fuel. Airplane exhaust releases numerous substances, which mix with other substances in the atmosphere. The combination of hot (from combustion) airplane emissions such as carbon dioxide, carbon monoxide, water vapor, hydrocarbons, nitrogen oxides, sulfur oxides, and black carbon are known to have a net warming effect. Some substances like carbon dioxide are well documented as harmful toward climate change, meanwhile others receive less media coverage, but are harmful on their own or in combination with other substances. In regard to contrails, some substances and mixtures cause more harm than others. For instance, a study was conducted in 2013 by Wong et al. in the NASA Particle Aerosol Laboratory (PAL) for Phase II of the Federal Aviation Authority’s (FAA) Aviation Climate Change Research Initiative (ACCRI); the study recreated airplane emissions and contrail formation in a lab and found that eliminating sulfur affected the production of ice particles that are part of contrail formation. Meanwhile, soot affects the “reflectivity of contrails,” which means that more heat can bounce off the cloud and increase temperature in those places. The two substances interact by sulfur coating soot, with the combination resulting in a warming affect. (Brasseur et al., 2016)
Soot and sulfur cannot be eliminated while burning jet fuel and would require switching to some alternative fuel sources. (Brasseur et al., 2016) Eliminating jet fuel could potentially curb other airline climate change emissions, such as levels of carbon dioxide. Alternative jet fuel, which is referred to as biofuel, can be manufactured from a variety of sources. Some popular options include ethanol, algae, vegetable oil, and waste. After a decade of research and development, in 2016, the FAA approved the usage of alternative jet fuel (FAA). It is already becoming more mainstream with United Airlines being a major proponent of using bio fuel and pledging to reduce carbon emissions by 50% by 2050 (United Airlines). United Airlines has been testing algae fuel since 2009, investing in biofuel since 2015, since 2016 they run a daily flight between Los Angeles and San Francisco incorporating biofuel, and since 2018 their transatlantic flight from San Francisco to Zurich flight runs on 30% (16,000 gallons) biofuel. (United Airlines; Harvey, 2016)
According to Jessica Kowal of Boeing, alternative jet fuel must be able to meet demand and be comparable to jet fuel in price to become the primary fuel source. As of 2014, alternative jet fuel is striving to meet 1% of the annual demand for jet fuel, which is $60 billion gallons (Cha, 2014). There are economic benefits to expanding the use of alternative fuel sources. One reason is the volatility of fuel prices; currently, jet fuel prices are on the rise. In October 2018 jet fuel costs $2.25 per gallon, which is a 27.78% increase from November 2017 when jet fuel cost $1.76 per gallon; 10 years ago the price per gallon was $1.88, 20 years ago $0.37, and the 30 year high was in July 2008 when it was $3.89 (US Energy Information Administration, 2018). While jet fuel prices fluctuate, the cost of alternative fuel has been on a downward trend, but still more expensive than jet fuel (International Air Transportation Association, 2017). As a result, the higher jet fuel prices go the more attractive alternative fuel sources become, since the price gap starts to close. Alternative jet fuel can be manufactured from a broad number of sources, so the price is less volatile than jet fuel. As such, economic instability - like the 2008 recession that spiked fuel costs to $3.89 per gallon -will have a smaller impact and price hike.
There is a lot of public and governmental support for alternative fuel sources, as well as, cons and controversies. Among the controversies include the politics and intent behind such strong support in biofuels. For instance, the United States dictates ethanol prices by being the largest ethanol producer, which encourages use of ethanol as a fuel source over imported jet fuel (Oliveira et al., 2017). With widespread use, not only would it eventually decrease costs, but also increase fuel security. Currently, biofuel still requires the use of jet fuel. Biofuels vary, so the introduction of other harmful emissions and reduction of existing emissions is uncertain. With various fuel sources and mixtures, future studies may be harder to conduct – too many variables. Lastly, as the demand for biofuel increases, the production will cause various environmental damage. For instance, ethanol is made from crops such as corn, which requires land to grow and can lead to deforestation, which would remove animal habitats and decrease general food supply (Oliveira et al., 2017). The problem emerges as reliance on biofuel increases. Companies want to appear environmentally friendly but resolving one problem could invite a series of others.
As a component of a mainstream issue, there is a lot of misinformation, unsubstantiated evidence, and fake news spread about contrails. Numerous conspiracy theories exist, including purposefully creating contrails to warm the earth for oil drilling in cold climates, using clouds to spread poison for food supply control, using weather as a weapon, and general fear mongering tactics for monetary and political benefits (Cairns, 2016). Actions are being taken to curb contrail formation, but there are a lot of sceptics who don’t see contrails as problematic. Others see it as a major problem and demand action, while knowledge about contrails is perceived as if the link between contrails and climate change is definite. According to the FAA, after numerous studies knowledge on the topic went from “very low to low,” at best. (Brasseur et al., 2016). Even with years of observational studies, satellite imagery, modeling experiments, and other research, knowledge is still limited. which means that a lot of action is taken without much information.
One factor that undermines the credibility of studies is the metric known as radiative forcing (RF). RF refers to the amount of radiation that reaches Earth, rather than being reflected away from Earth within a certain time period - positive RF is measured as warming and negative RF as cooling (Mann, 2016). The RF measurement is in watts per square meter (W/m2). The metric is insufficient for use in contrail studies due to “nonlinear interactions (Brasseur et al., 2016),” meaning the studies are vastly different in terms of variables, so comparison loses accuracy. Variables include airplane type, fuel type, season, geographic location, altitude of airplane for temperature, humidity level at altitude, and many more; these variables increase the need for more studies and make definitive information harder to acquire.
Despite the controversy surrounding climate change and contrails, the current trend is toward the innovation, improvement, and implementation of alternative fuel sources. Based on current evidence, the use of alternative fuel would be most beneficial in geographic locations with heavy air traffic, as well as locations with high humidity and higher altitude requirements. Concurrently or alternatively, for a low-cost increase of 0.5-1%, flight trajectories can be altered mid-flight to avoid contrail-prone heights and locations. Lastly, heavier planes use more fuel, so redesigning planes to be lighter and more aerodynamic would reduce fuel consumption. United Technologies and NASA are currently developing a plane due in 2035 that has a “blended wing-body” that fits this criteria by reducing weight by 15% and fuel by 27% (National Geographic, 2017).
Over decades of research, information is limited, but based on the information available steps should be taken to reduce contrails and their effect on climate change. Particularly, since the cost exceeds the benefit and provides economic and political benefits to the United States. It would take a long time, if ever, to transition to biofuels entirely. In the meantime, deforestation and food supply rates can be monitored.
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