To fly or not to fly? That is the question environmentally conscious people are increasingly grappling with. Sometimes taking the plane seems the only viable option, perhaps when time is tight or when a loved one lives far away.
I think we can have some flying as part of a sustainable future – but only if we put a few misleading myths to rest and instead lay out some realistic opportunities to cut its impact on global warming.
The first myth is that so-called sustainable aviation fuel (SAF) can solve the problem. It is a complete misnomer, as SAF turns out not to be sustainable at all.
Here’s why. There are three main types of SAF. The first is fuel from waste products, especially cooking oil. The problem is, the whole world’s waste cooking oil supply would be enough to power only 2 to 3 per cent of flights. The second is synthetic SAF, produced from raw materials such as captured carbon dioxide using energy from renewable sources. The process involved is so inefficient (at least 2 kilowatt-hours of electricity input per 1 kWh of fuel generated) that it is a self-defeating use of our limited renewable power. The third type of SAF is created from crops, so needs a lot of farmland. The detrimental pressure that this puts on both our food system and on nature is a big problem. The hard reality is that sustainable aviation fuel really isn’t a “game changer”.
The next piece of wishful thinking I often come across is that we can decarbonise aviation through electrification or by using hydrogen as a fuel. Electrification will be practical only for short-haul flights because the weight of batteries renders it unworkable for long-haul operations. Hydrogen is tricky because it takes up so much space, even when compressed to 700 times atmospheric pressure. This means it needs to be stored in large, pressurised cylinders, making it unfeasibly bulky compared with liquid fuels.
The good news is that I also see some clear opportunities that aren’t getting enough attention.
The closest thing we have to an elusive silver bullet to make aviation greener has flown under the radar until very recently. Contrails – those high, cloud-like streaks left by aircraft engine exhaust fumes – account for over 60 per cent of the climate change impact of all our flights, and much more if you look at the short-term impacts over, say, 20 years.
They do this by reflecting heat released at Earth’s surface back down, acting like a blanket. But the overall impact of a contrail is complex. Not only do they trap Earth’s radiant heat, but on a sunny day they can have a cooling effect by reflecting incoming sun rays. This happens only in the daytime, and they cool most when contrails are above a darker surface, such as an ocean. The more frequent, warming effect happens most at night when they occur over warm, dark surfaces.
It is possible to manage contrails by making small tweaks to flight paths: taking a plane above, below or around a specific pocket of weather that will cause them to form. When flying over a sunny ocean, it may even be an advantage to deliberately create them. Small tweaks to just 1.7 per cent of all flight paths could have the effect of cutting the contrail warming impact by nearly 60 per cent. All that is required is some real-time modelling built into the flight path calculation, and no more flexibility than is already routinely applied to avoid storms and other aircraft.
It is a relatively cheap solution, one that just needs the aviation industry to get behind it. Once contrail management is in place, there may even be a role for SAF, as it burns more cleanly and could be used to mitigate contrails on the worst-offending flights.
Does this mean we can all relax about the climate impact of flying? Sadly not. But it might help us continue to fly when we have a good enough reason to do so.
Mike Berners-Lee is the author of A Climate of Truth: Why we need it and how to get it
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