By: Ella Gilbert
Picture yourself in the window seat of an aeroplane, cruising along at 30,000 feet, occasionally admiring the clouds below and watching that cheesy blockbuster you were too shy to go and see in the cinema. If you’re like me, you might also be trying not to think about the impact of this flight on the climate – after all, we are increasingly reminded that travelling by air is one of the most carbon-intensive things we can do.
But when you hear the phrase ‘climate impacts of aviation’, chances are you think about the emissions of greenhouse gases like carbon dioxide (CO2) from aircraft. Unfortunately, that’s only a third of the story. What you probably don’t think about are the non-CO2 impacts, which have a climate warming effect twice as large. Bad news if you’re already worried about that flight.
Flying from London to Inverness, for example, is equivalent to eating 13 beef steaks if we consider the CO2 emissions alone, while if we consider the non-CO2 effects it’s more like 24. And if you’re flying from London to San Francisco, those numbers rise to a whopper-ing 117 and 224 steaks. Now, how’s that for an in-flight meal?
It’s not just CO2
Many of the non-CO2 impacts of aviation act in opposing directions. Some cool the atmosphere overall, while others warm it. To make matters more complicated, some effects even have different impacts on the climate over different timescales. Because these non-CO2 impacts are so complex and difficult to observe, there is still a great deal of uncertainty around their magnitude.
Advancing the Science for Aviation and Climate (ACACIA) is a multi-institutional European project trying to dispel some of the ambiguity about the various effects of aviation on climate, many of which you can see on the schematic below. At the University of Reading, we’re working on one of the most uncertain impacts: the effect of aviation aerosol-cloud interactions.
Figure 1: – Schematic overview of how aviation impacts the climate. From Lee et al. (2021)
Aircraft emit lots of gases and particles at the high altitudes where they fly. Their exhaust plumes spew gases like CO2, nitrogen oxide (NOx) and water vapour, as well as soot and sulphur particles into the atmosphere.
Those soot and sulphur particles are also known as aerosols, and they act like tiny seeds on which ice crystals and liquid droplets can grow. In the right conditions, soot aerosols can trigger the formation of ice crystals, which make up cirrus clouds – the wispy, indistinct clouds you see high up in the sky.
A cloudy blanket
Cirrus clouds tend to warm the Earth overall. That’s because they are very thin and so let solar energy travel through them easily, but at the same time absorb lots of outgoing infrared radiation, preventing it from escaping to space and so warming the surface like a blanket (aka the Greenhouse effect). But aerosols change the properties of those cirrus clouds in ways we’re still learning about.
Think about your flight blazing its way through the sky, its engines releasing aerosols into the atmosphere. As long as the conditions are right for cloud formation, the more aerosols there are in the exhaust plume to act as seeds, the more ice crystals that will form in its wake.
Cloud properties like the number, size and mass of ice crystals influence a cloud’s ‘optical thickness’, which describes how easily radiation can travel through it and so the degree to which those clouds warm or cool the atmosphere.
It’s cirrus-ly complicated
Different characteristics of the cloud compete with each other to push the balance in favour of warming or cooling. For instance, clouds containing many small ice crystals will stick around for longer because it takes more time for crystals to get big enough to fall out of the cloud. Very small crystals (a few thousandths of a centimetre across) tend to reflect more solar energy back to space, which has a cooling effect, but most cirrus clouds contain ice crystals that are larger than this, and so have an overall warming effect.
Aircraft can change how much cirrus clouds warm the climate by injecting more aerosols into atmosphere and influencing how many ice crystals form, as well as their size, shape and lifetime.
Aviation-aerosol-cloud interactions are hugely complex and difficult to measure. And because cloud processes push and pull in different directions, we’re still finding out how aircraft aerosol emissions influence the overall characteristics of cirrus clouds. In fact, the question marks are so large that we don’t actually have a precise number to tell us whether their impact is to warm or cool the atmosphere.
Evidence suggests that it’s probably a warming effect, although a recent review study was unable to provide a best estimate of the effect of aerosol-cloud interactions, leaving a conspicuous gap, and an even newer study shows that the warming impact of aviation-aerosol-interactions may be negligible.
One thing at least is clear: it’s still very much a hot topic of research.
Filling in the blanks
Enter, stage left: ACACIA. Our main task at Reading as part of the ACACIA project is to use very fine-scale computer models (called large eddy simulation, or LES) to explore the processes acting on pre-existing cirrus clouds and to find out how they interact with emissions of aviation aerosols like soot.
Understanding these processes will help us quantify the exact effect of aviation aerosols on cirrus clouds: for instance, how do they impact the number of ice crystals that form? How fast do these crystals grow? How quickly do they disappear? How do the prevailing weather conditions impact these effects?
Reducing the non-CO2 impacts of aviation
Hopefully, the work of the ACACIA project will allow us to fill in some of the blanks when it comes to aviation’s effect on climate – the crucial first-step that will allow us to mitigate its effects. Understanding the science is key, and will allow us to develop solutions that reduce the non-CO2 impacts of aviation.
Using aviation fuels that have less soot, avoiding areas where contrails and cirrus clouds preferentially form or avoiding some airspaces entirely might all be helpful solutions – but more work is needed before these strategies can be implemented, especially because there is no clear winner and many proposed options come with trade-offs like increased CO2 emissions.
So – for now at least – your flight won’t be getting diverted away from those spectacular cirrus clouds. I’ll let you get back to watching Fast and Furious 82 now.