Flying a bit less may be enough for climate

When world leaders gathered in Glasgow for thejust recently ended COP26 summit in Glasgow, something that got some of the delegates into hot water was how many of them got there: most came by air. And, of course, flying releases significant amounts of greenhouse gases, which is why we’re repeatedly being reminded that even cheap flights actually cost the Earth in climate terms and we should try to minimise them. But rather than go cold turkey and cut flying dramatically, Milan Klöwer, from the University of Oxford, has found that with just small reductions in flights each year, we can actually neutralise aviation’s warming effects, as Verner Viisainen found out...

Plane instructor - Ladies and Gentlemen, may I have your attention, please as we go through the following safety instructions? Make sure all seats are in the upright position and trays are put away before we take off.

Milan - For many people, flying is the biggest contribution to global warming. COVID really forced us to rethink how important is it to fly. Suddenly 10 days quarantine for everyone, right? People really thought about whether it was worth it. And in our study, we find that an annual decrease of 2.5% in terms of flights would limit aviation's contribution to global warming with an immediate effect. It's a bit tricky to understand why this is because usually we think in terms of carbon emissions and carbon emissions mean that the more we emit, the more increases the warming. However because of the non-CO₂ two effects that aviation contributes aircraft engines at the altitude at which they fly, they also emit nitrogen oxides. Nitrogen oxides then start triggering chemical reactions. One thing they do is they decrease the amount of methane that actually has a tiny cooling effect. They also interfere with the ozone, that has a warming effect. And one of the major ones is the creation of these condensation tracks that we can see, they can basically form cirrus clouds. And these cirrus clouds then act as a net warming. If you take all of these effects that are not the carbon dioxide emissions together, we usually call them non-CO₂ effects, that increase the warming that is caused by CO₂ alone by a factor of two. These effects decrease over time. Meaning if you fly less than in the year before this effect slowly decreases and balance the increasing CO₂ emissions. At this rate that we've calculated, the -2.5% per year, we actually find that there's this balance of the CO₂ and the non-CO₂ effects such that there's no further warming introduced.

Verner - Does that mean we would have to stop flying completely?

Milan - No and this is exactly the point that we want to raise. The only thing that we need right now is a change of direction. If we get a change of direction. So we slowly decrease, then the additional warming that is caused by aviation is halted. And I think this is a very important message.

Verner - Could you describe your method?

Milan - Basically, we connected the whole dots between knowing what the fuel consumption is in a given year down all the line to what is the temperature increase that is caused by these emissions. And because we've connected all these dots, we also assume for the future, a given consumption of this jet fuel. And we can see what will happen if it increases or if it decreases,

Verner - Could we theoretically achieve the same reductions in emissions, through technological advances?

Milan - At the moment, it is not possible because we do not have alternative low carbon fuels available. The production of these sustainable aviation fuels, even if they were sustainable, is currently not available at scale that is needed. So before the pandemic, annually every day one billion litres of jet fuel was burned. Of course people also talk about electric aeroplanes, but at the moment the batteries are just too heavy and evening the next decades, if something like that will be available, it's only very short haul flights.

Verner - You mentioned this 2.5% reduction in flying for a year. What would this look like for the average person in one of the richer countries?

Milan - At the moment it wouldn't change that much because the global levels of air traffic during COVID decreased about by a factor of two. What I often hear is these messages 'in the future, I'll try to fly half as much as I did before.' What physically is needed is a reduction of 2.5%, which is obviously much less. Technically what we need is general awareness that we have to distinguish between necessary and unnecessary flights. So everyone has to ask themselves this question. 'Is it really important that I fly? Or can I replace this flight? Can I combine several journeys into a single one?' The only thing that will really have to change is this idea that the whole aviation sector should grow and grow and grow.