Can air travel be made sustainable?

There has been a lot of coverage recently about sustainable aviation and the need to cut down on air travel.

A paper in Nature by a group of European academic technologists called "How to make climate-neutral aviation fly" described an in depth study of the problem and came up with 3 findings-

1. The role of non-CO2 effects ( such as NOx emissions and cirrus cloud formation) is key and nontrivial to consider.

2. We need to go beyond fuel replacement. Simply switching from fossil kerosene to sustainable synthetic jet fuel is not enough for aviation to be climate neutral. Carbon dioxide removal (CDR), such as direct air capture with CO2 storage, is also needed to mitigate climate impacts.

3.  If air traffic continues to soar, we must carefully consider the environmental and economic implications of climate-neutral aviation. Unless we actively reduce aviation demand, effectively offsetting its climate impact will require significant resource utilization (land, water, energy, etc.), regardless of the use of synthetic jet fuel

And their main conclusion was-

"Here, we demonstrate that a European climate-neutral aviation will fly if air traffic is reduced to limit the scale of the climate impacts to mitigate."

But is this finding realistic and necessary?

All the indicators for air traffic show a relentless rise in air travel - 2023 will see the last pre-covid peak in 2019 being passed. And order books for new aircraft are bulging as never before. Airbus has raised its own 20 year delivery forecast to 40,850 planes and Boeing forecasts 42,600 over the same period. The current global commercial fleet is about 27,000 strong. Of course a lot of planes will be retired in the next 20 years, so the outcome is difficult to tie down, but it is a pretty safe bet to say that the size of the global fleet will increase strongly.

So restricting air travel looks unlikely. But is it necessary in any case?

Item 2 of their findings provides the answer. Carbon neutral synthetic fuels made from hydrogen and carbon dioxide can do some of the work, but they are energy intensive. Direct Air Capture and Storage of carbon dioxide already in the air (DACS) uses less than a tenth of the energy for the same climate effect.

There are 3 reasons why DACS is advantageous-

1. Removing CO2 can reliably compensate for aviation emissions

2. It can also compensate for the non CO2 effects mentioned at item 1 above

3. Finally, emissions from legacy aviation have accumulated in the atmosphere and we need to remove them to attain a liveable climate

The size of the CDR in 1) is straightforward - it is the same as the emissions that are made as planes fly. Current annual aviation emissions are about 1 billion tons of CO2 annually. 2) is more tricky as the effect of things like contrails is difficult to model, but it is safe to say that they account for more than an additional 50% of the direct emission effect. 3) Legacy emissions from aviation are about 30 billion tons of CO2 in total, so over 50 years that represents another 600 million tons of CDR. In total we are talking over 2 billion tons of CDR annually.

We can expect some relief before 2050 from new technology, rerouting and some alternative options such as high speed train travel. But at best these will hold the projected total emissions to around current levels.

So we urgently need to grow CDR using DACS from the modest level that it will be in 2025, at around 1 million tons of CO2 annually, to billion ton scale by 2050.

That looks like a monumental task, but it is actually less difficult than it appears.

Thanks to more than as decade of development by a few DAC startups in North America and Europe, we already have workable and efficient DAC technology. Growing installed DACS by a factor of 10 in each of the 3 decades before 2050 is really a finance issue. And the finance model is already in place.

The US firm 1PointFive is building a DACS plant in the US Permian Basin that will capture and bury 500,000 tons of CO2 a year. It has done a deal with Airbus to create 400,000 tons of carbon credits which have been sold in advance. That forward sale will secure some of the capital needed to build the plant. This is similar to an upfront Power Purchase Agreement for a renewable energy plant.

We are heading for DAC costs at $100/tonCO2 but reaching that milestone quickly will depend on the rate at which we deploy it. Fuel represents about 25% of airline costs so at that price the CDR adder to ticket prices will be under 25%.

(one ton of Jet fuel costs more than $500 and emits about 3 tons of CO2)

To give some idea of the carbon reduction for aviation versus energy input, for example relative to the electrification of cars, it can easily be shown that removing the emissions from aviation uses a similar amount of energy per unit of carbon dioxide eliminated. So aviation CDR using DACS is a really worthwhile candidate for global carbon reduction to net zero.

Low cost production oil reserves such as from Saudi are well able to absorb the additional cost of DACS and it could be that initiatives such as the EU carbon border tax will force Aramco to develop DACS at scale at least for aviation in order to avoid that new tax. To give some idea of the scale involved, Saudi could supply the current global demand for net zero aviation fuel for at least 50 years, which would give a cushion with which to develop and scale alternative low carbon technologies.

As a report from the Royal Society found earlier this year, it is completely unrealistic to think that Sustainable Aviation Fuel will solve the aviation emissions crisis. We dont need to restrict flying if we get on with CDR using DACS now.

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Sacchi, R., Becattini, V., Gabrielli, P. et al. How to make climate-neutral aviation fly. Nat Commun 14, 3989 (2023). https://doi.org/10.1038/s41467-023-39749-y

https://royalsociety.org/topics-policy/projects/low-carbon-energy-programme/net-zero-aviation-fuels/

https://www.linkedin.com/pulse/we-need-start-growing-direct-air-capture-now-leon-di-marco