How to cut contrails - and their greenhouse effect

UK scientists have discovered a way to cut the airline industry’s contribution to global warming by as much as 35%. The answer lies in minimising the contrails that form when a plane flies through a humid patch of sky. These contrail clouds reflect heat heading for space back down towards the planet’s surface, forcing up the temperature. Now, a new technique is enabling scientists to spot which flights are the worst offenders, and how they can avoid the problem. Megan McGregor’s been hearing how from Imperial College’s Marc Stettler...

Marc - What we were able to do is use a high resolution data set of aircraft trajectories in the Japanese airspace, and what that showed us was that 2% of the flights contributed to 80% of the climate forcing. So it's a really skewed distribution where you have very small number of flights contributing to the vast majority of the warming effect.

Megan - And how does this contrail cloud cause warming?

Marc - There's a balance between two processes: the contrail if it persists and spreads, it will both reflect incoming sunlight back out to space - and that's a cooling effect; but it also trap outgoing heat that would otherwise go out to space - and that's a warming effect. Now during the day there's sunlight and so the cooling effect and the warming effect, they can balance each other out. But then over night if a contrail persists, there's no sunlight, so there's no cooling effect and so what we're left with is a warming effect. So it's really the ones that persist for a long period of time into the nighttime that have the warming effect.

Megan - If you wanted to take a targeted action towards this sort of small number of flights to reduce their contrail formation impact, what would it be that you could potentially could do?

Marc - Because contrails need this humidity in the atmosphere - actually the regions of the atmosphere that are humid enough are relatively thin layers of the atmosphere - and so small altitude changes either up or down by 2000 feet, which is what we simulated in our study would mean that an aircraft could fly outside of that contrail forming region. So an aircraft that originally had formed a contrail, if we just change the altitude by plus or minus 2000 feet may end up not forming a contrail.

Megan - And that sort of is a different approach to mitigating, for example, carbon dioxide emission?

Marc - The plane is still going to admit carbon dioxide. What we were careful to look at was whether the changes that we were making to the altitude of flight would have a significant effect on the fuel consumption of the aircraft because CO2 scales with fuel consumption, and that effect of CO2 that we put into the atmosphere will persist for a few hundred years. Compare that with a contrail which lasts for less than a day. If we only made changes to the altitude of flights, this effect of contrail is something that we could almost switch off, get rid of that effect on the climate system from aviation.

Megan - If the sort of high humidity areas are not that difficult to avoid, why is it that that's not part of the sort of route planning now?

Marc - The process of flight planning, you have to rely on forecasted weather data and so one of the pieces of work that we're looking at now is how accurate those weather forecasts are in terms of predicting where there is that high levels of humidity for contrails to form. What's encouraging is that pilots and air traffic controllers already have a discussion while a flight is ongoing, so en-route. Adding contrail set up discussion would be another dimension - complicating the calculation slightly - but it's something that we could incorporate. But we are working towards that and we're looking at engaging with air traffic management service providers.