Up, Up and Away – Aviation

Looking up at the sky you will sometimes see thin ribbons of cloud. Often these form behind high flying aircraft. They can be regular or irregular in shape, perhaps looking like a zip at times. They may soon disappear, but on occasions they will persist for several hours or merge into other cloud. These are contrails, condensation trails, and they have only been around for just over a hundred years. We are seeing a new type of cloud that most of our ancestors wouldn’t have observed. They form as aircraft fuel (kerosene) is burnt producing carbon dioxide (CO2) and water vapour. The water vapour forms ice crystals in the high, cold atmosphere and these form the contrails.

Contrails are just one product of flying. Greenhouse gases from burning fuel are another and these are changing our climate. Aviation currently emits approximately three per cent (in tonnes) of the global total of CO2 and this is increasing. Action is being taken to find alternative fuels for aviation to reach the industry’s net zero target by 2050.

Battery powered aircraft
Using battery power produces negligible amounts of greenhouse gases, plus they require less maintenance and are quieter. There are hurdles to overcome. Currently, the power produced by batteries compared to standard aviation fuel is less for a similar weight. Improvements are taking place in battery technology and will narrow the gap but presently the development of battery powered aircraft is concentrating on smaller aircraft with relatively short ranges.

Hydrogen Fuel Cells
These combine hydrogen with oxygen to produce electricity. The only waste product is water vapour and they will have lower maintenance costs. Weight is again an issue compared with current aviation fuel and there are unknowns such as the impact of both low air temperature and atmospheric pressure. Development is again concentrating on smaller aircraft. In some cases the fuel cells are combined with batteries to meet the high-power demands required at take off. Other considerations are the water vapour produced, a greenhouse gas, and if the hydrogen used is produced using renewable energy or fossil fuels.

Hydrogen combustion
Here hydrogen is burnt in gas turbines similar to current aircraft engines. However, development appears to have stalled, with the use of hydrogen in aviation concentrating on fuel cells.

Sustainable Aviation Fuel (SAF)
These fuels are presented as the way aviation will significantly reduce greenhouse gases. SAF is made from natural materials containing carbon and hydrogen to make a synthetic substitute. At the moment it is largely produced using processed fats and vegetable oils but there isn’t sufficient available to come anywhere near to meeting aviation’s requirements. Other future sources may be household waste but this is more expensive and the results less energy efficient. Also, there is disagreement as to how much SAF reduces the carbon footprint but it is claimed to have a much lower one than current fuels.

Synthetic fuels
A synthetic fuel can be made from hydrogen using electricity and CO2 directly captured from the air and other sources. These have a very low carbon footprint if the electricity used is produced from renewable sources. Cost is an issue and very large amounts of electricity will be required. In addition, direct CO2 capture from the air is still in its infancy. However, there are now places making small amounts of synthetic fuels.

UK Government policy
Current policy is set out in the government’s Jet Zero Strategy (2022). Here the use of SAF, aviation system efficiencies, zero-emission flight technology, carbon markets, and offsets form the strategy to reduce UK’s aviation carbon footprint. The UK’s Climate Change Committee (CCC) view this as a high-risk strategy and they predict aviation’s contribution to total UK greenhouse gas emissions will increase from 7 to 16 per cent by 2035. Currently only domestic aviation emissions count towards the UK’s net zero target but from 2033 the UK’s international emissions will also be included.

The government’s growth policies include airport expansion and they believe this is still compatible with their legally binding net zero target. However recent and planned airport expansions will generate many more flights and millions more passengers. Department of Transport modelling (2023) showed a 52 per cent increase in passenger demand by 2050.

In summary, various technologies may reduce the use of the current aviation fuels. However, aviation’s contribution will increase both in quantity and as a percentage of the UK’s carbon budget. The UK government has a strategy, but only time will tell whether this will be successful or a change in direction will be required.

And back to contrails, whilst they have a beauty they contribute to warming temperatures as they trap more heat in our atmosphere than they block. We can forecast their formation, methods for this were originally developed in the 1950s using data from Spitfires flying at high levels. Now computer models forecast where they will occur and can be used in flight plans to reduce their occurrence and mitigate against climate change.