Aviation’s dirty secret: Airplane contrails are a surprisingly potent cause of global warming

The aviation industry has long been criticized for its large environmental footprint, particularly its climate-warming carbon emissions. But a new study suggests that another byproduct of airplanes—the white contrails they paint across the sky—has an even bigger warming effect, one that is set to triple by 2050.

Planes create their mesmerizing contrails as they soar high in the thin, cold air. Water vapor quickly condenses around soot from the plane’s exhaust and freezes to form cirrus clouds, which can last for minutes or hours. These high-flying clouds are too thin to reflect much sunlight, but ice crystals inside them can trap heat. Unlike low-level clouds that have a net cooling effect, these contrail-formed clouds warm the climate.

A 2011 study suggests that the net effect of these contrail clouds contributes more to atmospheric warming than all the carbon dioxide (CO 2 ) produced by planes since the dawn of aviation. And those effects are predicted to get worse as air traffic—and the resulting cloud coverage—increases: Some estimates suggest global air traffic will quadruple by the year 2050.

One of the researchers from the 2011 study wanted to explore how contrail clouds could affect the climate in the future. Along with colleagues, atmospheric physicist Ulrike Burkhardt from the German Aerospace Center’s (DLR’s) Institute for Atmospheric Physics in Wessling created a new atmospheric model that—for the first time—gave contrail clouds their own category, separate from natural clouds. That allowed them to model particular qualities of the humanmade clouds that affected everything from their formation to how they interacted with the rest of the atmosphere.

The researchers modeled the effect of global contrail cloud coverage in 2006, a year for which they had accurate aviation data. Then, taking into account predictions for future air traffic and emissions, they modeled the effect of contrail clouds for 2050. They found a threefold increase in their warming effect over that time, they report this week in Atmospheric Chemistry and Physics.

The study is one of the first to make a detailed prediction of how these special clouds affect the future climate, says DLR cloud physicist Bernd Kärcher, who was a co-author on the 2011 paper. He says the new cloud classification scheme was crucial to the model and its results.

The researchers looked at another scenario for 2050, one with a 50% reduction in airplane soot emissions. They found that such a reduction could lead to a 15% decrease in the contrail clouds’ atmospheric warming effect.

But little is known about the relationship between climate warming and cloud coverage, and how atmospheric warming affects temperatures on the surface. What researchers do know is that high levels of soot lead to more and longer-lived contrail cirrus clouds, which could alter weather and climate at the surface, Burkhardt says. However, she adds, even a 90% reduction in soot emissions with the help of cleaner aircraft fuels would fail to bring the cloud’s climate impact back to its 2006 levels.

A more likely scenario, Burkhardt says, is that levels of soot and contrail cirrus clouds will continue to rise. That’s because most aviation regulations and pollution-reduction plans fail to consider the climate impact from anything other than CO 2 emissions. A United Nations scheme, for example, requires all signatory nations to keep their CO 2 emissions under a certain level, and report them annually, but says nothing about the climate impact from contrails.

Burkhardt says that considering contrails in such schemes would be difficult, however, because climate impact varies based on weather, location, and time of day. One solution, Burkhardt says, is to reroute flights. However, such rerouting may force planes to burn more fuel and release more CO 2 . She says 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.