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Contrail Clouds and Global Warming

Paul Kando

The aviation industry is justly criticized for its large climate-warming carbon emissions. But studies suggest that another byproduct of air travel — the white contrails airplanes create as they fly at high altitudes in the thin, cold air — cause even more warming than the planes’ CO2 emissions. Water vapor quickly condenses around soot from the plane’s exhaust and freezes to form cirrus clouds, which can last for just minutes to several hours. These high-altitude clouds are too thin to reflect much sunlight, but ice crystals inside them can trap heat. So, unlike low-level clouds that have a net cooling effect, these contrail-formed clouds warm the climate. And this is predicted to get worse as air traffic — and the resulting cirrus cloud coverage — increases. Some estimates suggest global air traffic will quadruple by the year 2050 and the contrails’ warming effect is likely to triple by the same year.

Criss-crossing contrails
photo credit: engineeringafuture.com

To explore how contrail clouds could affect the climate in the future, atmospheric physicist Ulrike Burkhardt and colleagues from the German Aerospace Center created a new atmospheric model that, for the first time, puts contrail clouds into their own category, distinct from natural clouds. This makes it possible to model the manmade clouds’ particular qualities that affect everything from their formation to how they interact with the rest of the atmosphere.

The researchers modeled the effect of global contrail cloud coverage in 2006, a year for which accurate aviation data were available. Then, accounting for predictions of future air traffic and emissions, they modeled the effect of contrail clouds in 2050. They found a threefold increase in the contrail-clouds’ warming effect from 2006 to 2050, according to their recent report in the journal Atmospheric Chemistry and Physics.

The researchers also modeled an alternative scenario for 2050, one with a 50% reduction in airplane soot emissions, finding that such a reduction would lead to only a 15% decrease in the contrail clouds’ atmospheric warming effect.

There is still too little known about the relationship between cloud coverage and climate warming, and about the precise impact of atmospheric warming on surface temperatures. Researchers do know, however, that high levels of exhaust-soot lead to more and longer-lived contrail cirrus clouds, and that these can alter weather and climate on the ground. They also know that even a 90% reduction in soot emissions — with the help of cleaner aircraft fuels — would fail to bring the clouds’ climate impact back to its 2006 level.

The more likely scenario is that soot levels and contrail clouds will continue to increase, since most aviation regulations and pollution-reduction plans fail to consider the climate impact from anything other than CO2  emissions. The United Nations Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA), for example, requires all signatory nations to keep their CO2 emissions under a certain level and report them annually, but says nothing about the climate impact from contrails.

Considering contrails in such schemes would, of course, be difficult because the contrails’ climate impact varies with the weather, the location, and the time of day. Rerouting flights could be one solution. However, that might force planes to burn more fuel and release more CO2 . It would be better to find more efficient fuels that release less soot but, with the likely increase in air traffic, even that might not be enough. Can we hope for a timely replacement of the jet engine? Or for replacing short-to-intermediate distance flights with electric high-speed rail?

This latest study is the first to make a detailed prediction of how contrail clouds affect the future climate, and to establish the new cloud classification scheme so crucial to the modeling and its results. Contrail cirrus clouds are a complex problem, but their warming effect is small compared with the overall amount of CO2  society belches into the atmosphere. Contrails alone don’t cause global warming. However, it is still important for the aviation industry to understand the science and to manage their overall environmental impact accordingly.

It is equally important for human society to consider transportation as an integrated system, perhaps even as a public utility, each component of which is optimized to provide the best service at the least cost — both monetary and environmental.