Containing the contrails

As the airline industry battles to reduce the environmental impact of going about its business, the fluffy white streaks that form behind planes have become a complex issue.

Contrails materialise at cruising altitude when aircraft emit black carbon particles from incomplete fuel combustion, providing a surface on which moisture condenses to form ice particles. 

Most last only a few minutes, but if the atmosphere is supersaturated with ice, these tracks can spread and mix with other contrails and cirrus clouds, forming “contrail cirrus” that last up to 18 hours. This can create a kind of thermal blanket, which in turn has an impact on atmospheric temperatures.

Nine years ago, a study by the Institute of Atmospheric Physics (IAP) in Oberpfaffenhofen, Germany, found that the net effect of contrail clouds contributes more to atmospheric warming than CO2 emissions from planes ever have.

Last year, things got more complicated still. Another IAP team predicted that the impact of contrails could triple by 2050, in part because the airline industry is trying to reduce its CO2 emissions. Making engines more fuel-efficient would create more contrails that last longer, they said.

Now there is some positive news. A study in the journal Environmental Science & Technology suggests the impact of contrails could be reduced by having a few planes make small flight path adjustments. 

Researchers from Imperial College London (ICL) and the IAP, led by ICL’s Marc Stettler, wanted to refine previous models to try to more accurately predict the characteristics and impact of contrails, and to evaluate mitigation strategies

They say they combined their recently developed model to estimate black carbon emissions for specific aircraft engine types and power with a model to estimate the characteristics and climate impact of contrails from individual flights and detailed weather information. 

Studying flight activity in Japanese airspace, they found that 80% of warming caused by contrails could be traced to 2.2% of flights.

Further analysis suggested that having just a few aircraft fly 600 metres higher or lower than planned could limit formation of contrails and thus reduce their impact – even taking into account the fact that diversions could make flight paths less efficient, leading to increased CO2 emissions, which would offset the gains. 

“A small-scale strategy of selectively diverting 1.7% of the fleet could reduce the contrail EF [energy forcing] by up to 59.3%, with only a 0.014% increase in total fuel consumption and CO2 emissions,” Stettler and colleagues write in their paper. 

“A low-risk strategy of diverting flights only if there is no fuel penalty, thereby avoiding additional long-lived CO2 emissions, would reduce contrail EF by 20.0%.

“In the longer term, widespread use of new engine combustor technology, which reduces BC [black carbon] particle emissions, could achieve a 68.8% reduction in the contrail EF. A combination of both interventions could reduce the contrail EF by 91.8%.”

While the research studied a specific time and airspace, the researchers say the findings “are likely valid for other mid-latitude regions”.