Contrails, short for condensation trails, are the white streaks often observed in the sky trailing behind aircraft. The international cloud atlas, which classifies clouds, has a dedicated category for them: cirrus homogenitus, an example of man-made clouds.
Climate Impact of Contrails
Contrails contribute to climate change, adding to the warming caused by carbon dioxide emissions from aviation. Although the exact amount of warming from these wispy clouds remains uncertain, current understanding suggests that reducing contrail numbers could lessen the climate impact of flights.
Contrails consist of ice crystals. They reflect sunlight, reducing the energy reaching Earth's surface, but also trap some outgoing infrared radiation. Depending on the balance between these opposing effects, individual contrails can either warm or cool the planet over their lifetime. However, when averaged globally and annually, warming dominates.
How Contrails Form
Contrails form behind aircraft at altitudes around 10–11 km. They only develop in sufficiently cold and humid atmospheric regions, where water vapor condenses onto soot particles emitted by aircraft engines, forming liquid droplets that freeze into ice crystals. The regions with the most contrails are over Europe, the North Atlantic, and eastern North America, while they are rarer in Asia.
Soot particles are necessary for contrail formation, yet even engines emitting very few soot particles still produce contrails. Other particles, often formed in the engine plume, take over and lead to contrail formation. However, some combinations of fuel and engine technology may offer ways to generate fewer contrails, or at least contrails with a smaller climate impact.
Factors Influencing Contrail Characteristics
The characteristics of a contrail depend initially on the size, shape, and engine position of the aircraft that created it, but atmospheric conditions are ultimately more important. In a dry atmosphere, contrails last only a few minutes and cover a tiny surface area, making their climate impact negligible. However, if the atmosphere remains cold and moist enough, many contrails form, grow, and merge into fields of ice clouds called contrail cirrus.
Contrail cirrus impact the climate because they persist for several hours and can cover vast areas, sometimes spanning entire countries, as observed over the UK and France. Some contrail cirrus clouds can exert a climate impact equivalent to tens or even hundreds of tons of carbon dioxide.
Why Contrails Are Potent
Two effects make contrails particularly potent. First, although they initially form from a few hundred kilograms of water vapor and dozens of grams of soot released per minute of flight, contrails gain mass from atmospheric humidity. Second, ice crystals absorb infrared radiation at virtually all wavelengths, whereas carbon dioxide absorbs only in narrow wavelength ranges.
Contrail cirrus strongly affect the flow of energy in and out of the Earth for a few hours. In contrast, carbon dioxide causes comparatively weaker changes that last for centuries. Therefore, the warming caused by a flight is initially dominated by contrails, but carbon dioxide dominates a few years after the flight.
Mitigation Strategies
Routing aircraft to avoid flying in regions where contrails form could slow the climate warming caused by a growing aviation sector. However, scientists still need to understand how to predict which flights would see the greatest reduction in climate impact through such planning.
Weather forecasts of humidity at flight altitude need improvement. One way to achieve this is through more accurate and frequent humidity measurements. This is the aim of the Mist research project, in collaboration with Honeywell Aerospace UK and Boeing UK, to develop a humidity sensor to detect contrail formation, integrate it on commercial aircraft, and evaluate how better humidity measurements affect the predicted climate impact of contrails.
Many research projects are seeking to better quantify the climate impact of contrails and find ways to form fewer warming contrails. Changing fuel or engine technology is slow, but optimising flight trajectories with weather forecasts to avoid cold, moist regions where contrails form might be achievable more quickly.
About the author: Nicolas Bellouin is a Professor of Climate Processes at the University of Reading. This article is republished from The Conversation under a Creative Commons license. Read the original article.
