Aviation warming slashed with gradual changes

Non-CO2 effects can be exploited to balance out CO2 emissions from flights...
25 November 2021


A plane flying across the sky, leaving a streaky contrail cloud behind it


Flying is very emission-intensive and we should probably cut down the number of flights we take. For many of us, the pandemic forced us to do just that. So, what effect could a small reduction in your flying habits actually have on the climate? Turns out, quite a lot, as described in a new study published in Environmental Research Letters.

Milan Klöwer, the lead author of the study, summarises their surprising finding, “we find that an annual decrease of 2.5% in terms of flights [globally] would limit aviation's contribution to global warming with an immediate effect”. With only gradual reductions in flying each year, we could make the impact of flying on climate change negligible.

Typically we think of global warming as caused mainly by carbon dioxide emissions, which are also emitted by jet turbines during flight. However, there are other gases that also contribute to the greenhouse effect and it is the so-called “non-CO2 effects” associated with aviation that hold the key to this finding.

Klöwer explains, “aircraft engines also emit nitrogen oxides. Nitrogen oxides (NOx) trigger chemical reactions... [These reactions] decrease the amount of methane in the atmosphere, which has a cooling effect.” NOx also interferes with the ozone causing a warming effect as well as creating condensation trails, the long white wispy features that trail behind aircraft.  Klöwer points out that these condensation trails “can form cirrus clouds, which have a net warming effect”. Overall, these non-CO2 effects can increase the warming effect of aviation by a factor of 2, meaning that aviation has contributed 4% of all human-induced warming, instead of 2% based on just CO2 emissions.

However, the overall contribution of these non-CO2 effects reduces with time, in contrast to CO2 which stays in the atmosphere for hundreds of years and cumulatively contributes to warming. As a result, with reductions in flying, you can even reverse their contribution to such an extent that it could cause a net cooling effect, through the mechanism by which NO breaks down methane, counteracting the CO2 released due to aviation  Klöwer and his co-authors calculated that this negative growth rate of 2.5% per year corresponds to a scenario where all of these factors balance each other out, and the impact of aviation on global warming becomes negligible.

Klöwer clarifies that this finding does not mean we need to stop flying entirely, “the only thing we need right now is a change of direction”. So we need the number of flights taken a year to gradually decrease rather than continue to increase  Klöwer makes the point that this change of direction is the only pragmatic way to reduce the emissions from flying at the moment, because technological solutions such as low-carbon fuels and electric planes aren’t feasible or available on a commercial scale and probably won’t be for a long time.

So what would this 2.5% annual reduction mean for the average person? Not much, Klower thinks we all need to stand back, take a minute, and consider our flying habits more carefully before adding our next holiday to the basket, “everyone has to ask themselves [these questions]: ‘Is it really important that I fly?’, ‘Can I replace this flight?’ and ‘Can I combine several journeys into a single one?’”


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