Aviation’s non-CO2 climate impacts

Introduction

Over fifteen years ago, I started working on aviation and climate change, and was bemused by reports of aviation’s non-CO₂ effects on the climate. According to one study, in the year 2000 the total effect of aviation on the climate was many times greater than that generated by its CO₂ emissions alone [1]. Surely this enormous additional effect was really important? And yet the same article argued that it was not appropriate, at that time, to include such impacts in the EU Emission Trading Scheme. Indeed, there was widespread reluctance to formally include non-CO₂ effects in any policy decisions about the aviation sector.

Fast forward to today. There has been a substantial amount of research in the intervening years and yet, in many cases, there is still a disinclination to do more than mention that such effects exist. Because of the complexities of quantifying non-CO₂ effects, they are often left out of estimates of the climate contribution of different sectors, and Government ‘action’ is often limited to ‘more research’. I think this lack of direct policy focus on aviation non-CO₂ effects is wrong for several reasons.

The precautionary need to act on best available evidence

First, although there is undoubtedly scientific uncertainty about many of the non-CO₂ impacts, the major studies on the topic, spanning over 20 years, have concluded that the overall known, quantified non-CO₂ impacts from aviation are detrimental (i.e. they increase global warming) [2]. Surely the urgency of the climate crisis means we have to act according to the most sensible assumptions of the time, rather than waiting for certainty?

The general importance of short-lived forcing agents

Second, a big part of the debate is about the timescales of impacts. CO₂ has, rightly, had centre-stage in climate change discussions, because, once up in the sky, CO₂ will stay there for a very long time (unless we manage to remove it). However, this focus has possibly distracted from measures to address pollutants that cause shorter-term impacts. Of course, if we can choose to ‘switch off’ these impacts relatively quickly, they are less of a worry. However, if we don’t do so, they continue to contribute to the problem. Contrails from aviation have this characteristic – their effects are short-term but can be intense. Surely, given the urgency of the climate issue, it is time for more of a twin-track approach? This is particularly the case since short-lived non-CO₂ forcing can affect CO₂ levels via the complex feedbacks between atmosphere, land and ocean [3].

The potential to make a difference through targeted demand management

Third, one argument against focusing on non-CO₂ impacts is the danger that some measures to tackle them could increase CO₂ emissions. Whilst this is undoubtedly important for policies that would result in such trade-offs, it is much less relevant for demand management solutions where flights are discouraged or restricted. And one of the characteristics of non-CO₂ effects is that they are not uniform – typically, plane journeys made in certain locations or at certain times are likely to have much greater impacts than others. For example, for transatlantic routes, night-time winter flights are more likely to cause warming from contrails than those made at other times [4]. So maybe avoiding the issue is missing a trick? If there was more of a focus on the issue, perhaps we could clarify whether it is possible to have a fairly large effect by restricting particular types of flights.

Conclusion

A more detailed briefing note, arguing about why aviation’s non-CO₂ climate impacts strengthen the case for aviation demand management is given, including a simple mathematical illustration of why a range of different numbers can be quoted as to ‘how bad’ non-CO₂ is, compared to CO₂.

To try to get an idea of the scale of potential benefits that might occur if we tackled aviation non-CO₂, in 2020, I did some fairly simple calculations [5]. These suggested that, in that year, as a result of changes in activities due to Covid-19, the beneficial atmospheric effect from the global reduction in plane contrails could be seven to eight times greater than the beneficial atmospheric effect from the entire world’s reduced production of carbon dioxide emissions from all sectors (excluding land-use change). Latest IPCC data (undoubtedly based on more robust methods) suggest it was more like four times greater [6] – which is still huge. If it is possible to achieve such an enormous effect from only limiting aviation, can’t we use this to help buy time for sorting out longer-term pollutants? And if we allow aviation to resume previous growth trajectories, won’t this seriously add to the climate problem?

Meanwhile – as a somewhat different approach to the topic – I recently had the chance to attend a ‘fairytale writing workshop’, in which the idea was to use metaphors to explain scientific concepts. Read my attempt to explain short- and long-lived climate forcers using a clothing metaphor!

Related reading

• Briefing: The non-CO2 impacts of planes are a key reason to reduce aviation demand  
• Fairy tale: The princess and the puffer jacket

References

1. It is premature to include non-CO₂ effects of aviation in emission trading schemesOpens in a new tab
2. In particular: Aviation and the global atmosphereOpens in a new tab; Transport impacts on atmosphere and climate: AviationOpens in a new tab; The contribution of global aviation to anthropogenic climate forcing for 2000 to 2018Opens in a new tab
3. Short-lived climate forcers have long-term climate impacts via the carbon–climate feedbackOpens in a new tab
4. Aviation contrail climate effects in the North Atlantic from 2016 to 2021Opens in a new tab
5. The Potential Climatic Significance of the Global Reduction in Aviation During the PandemicOpens in a new tab
6. Chapter 6 – Short-Lived Climate ForcersOpens in a new tab – specifically, Cross-chapter box 6.1: Implications of COVID-19 restrictions for emissions, air quality and climate.

Banner photo credit: © Sally Cairns, 2023