Carbon-Neutral Flying

According to the aviation industry, it was responsible for 7.3% of UK CO₂ emissions in 2017 (37 MtCO₂ out of a total of 503 MtCO₂*). Note: this is a massive underestimate of aviation's true climate impact - see Aviation's True Climate Impact below.

In 1990 UK aviation emissions were 17 MtCO₂; ie they've been rising at a rate of 2.9% per year.

There are several approaches which could allow us to continue flying without further increasing CO₂ in the atmosphere: 1) biofuels, 2) sustainable synthetic fuels, 3) carbon capture and storage.

Biofuels

Existing planes can run on a 50:50 mix of traditional and sustainable jet fuel. A handful of airports already allow airlines to add biofuel when they refuel. For example, at Oslo Avinor airport in 2016 0.2% of total fuel loaded was biofuel.

It would be possible to design planes that run off 100% biofuel. And it would be possible for biofuel production and its shipping to be carbon-neutral.

We could halve aviation per-mile emissions in the short-term and eliminate them entirely in the long-term, if flyers were willing to pay the extra to switch to biofuels produced carbon-neutrally. This would increase the cost of flights several times over. Note: this would represent the true cost of flying. If we all had to pay the true cost, it's likely demand would fall significantly.

Warning! biofuel production uses a lot of land and agriculturally viable land is a globally limited resource.

Synthetic Aviation Fuel

After Rolls Royce's Director of Electrical explained how electric planes will never power long-haul flights, the company's chief exec, Warren East, popped up on the BBC Radio 4 Today programme on 14th October 2021 (from about 1h17m) to share the company's vision of how long-haul flights might continue:

To create the quantities of sustainable aviation fuel required, you do require huge amounts of zero-carbon electricity, and we believe ... small, modular nuclear reactors [are] an ideal solution ... We'll have those ready by around the end of this decade.

This might work. But nuclear is expensive and carbon capture and storage is hard work, so it's going to mean long-haul flights become significantly more expensive. On the flip side, if (a big if) Rolls Royce can pioneer this technology, it will assist in efforts to decarbonise the atmosphere more generally.

Carbon Capture and Storage

Carbon capture and storage (CCS) is a technology that would remove CO₂ from the atmosphere and lock it away in a stable solid such as calcium carbonate, aka limestone. This technology does not exist yet! at least there's no large-scale, financially viable solution.

Microsoft and Jeff Bezos have independently announced billion-dollar programmes "to accelerate the global development of carbon reduction, capture, and removal technologies".

It might be possible to burn wood in power stations and capture most of the CO₂ released. This would significantly reduce the electricity output but would be an overall carbon-negative system, which would remove from the atmosphere the CO₂ released by our fossil-fuel-powered flights. Warning! growing trees to burn in power stations uses a lot of land and land is a globally limited resource.

Rolls Royce plans to use nuclear power to capture carbon from the atmosphere - see section Synthetic Aviation Fuel.

If we were able to invent a viable method of carbon capture and storage, it would also be urgently required to remove the CO₂ generated by other industrial processes. For example, concrete production causes 4-8% of worldwide man-made CO₂ emissions, and half of those emissions come from the chemical process, even if the process is powered by sustainable energy.

Bidding against other users for a finite supply of carbon capture and storage to offset CO₂ released by aviation is likely to significantly increase the cost of flying.

Offsetting Your Flight?

Many airlines offer (or allow you to pay a surcharge) to offset the CO₂ emitted by your flight by emissions-reduction measures. Maybe, someone in the developing world will be given a more fuel-efficient stove or some trees will be planted or another business will be paid to reduces its emissions.

The big problem with offsetting new emissions is that there is limited total capacity: there's only so many people who need stoves, only so many trees that can be planted (see Geo-engineering By Reforesting) and only so far that other businesses can reduce their emissions. The available offsetting capacity isn't sufficient to remove from the atmosphere all the CO₂ emissions we've already released.

We should be prioritising removing existing CO₂ not legitimising additional emissions.

Other problems with offsetting:

  • Can you be absolutely certain that the stove reaches the recipient and is used correctly long term?
  • In some cases, landowners are paid simply to retain existing trees. But, unless they truly were going to clear that land, they're onto a nice little earner and no carbon has been removed from the atmosphere.
  • It seems weird to be planting a few trees while the Amazon rainforest is being burned with such alacrity.
  • The tree plantations need to remain in position or stored as wood without rotting in perpetuity, otherwise the CO₂ absorbed will eventually be released.
  • The tree-planting schemes take full account of the carbon captured by the new trees while remaining vague about the carbon that was already stored in that land, which can be lost.
  • At 5m40s into BBC's The Climate Question "Can putting a price on nature help us care about it more?" we are told about a rainforest community, which received a payment to preserve the trees in their area and used the money to deforest and farm land further away.
  • Paying people to plant trees can cause deforestation! by creating a perverse incentive for the cynical or the desperate to clear the land so they can be paid to replant it.
  • We should be incentivising businesses to cut emissions without all the CO₂ saved being emitted by aviation instead, which achieves net-zero overall improvement.
  • The CO₂ emissions from aviation account for only one third of aviation's climate impact - see Aviation's True Climate Impact.

Aviation's True Climate Impact

The even worse news for fans of powered flight is that CO₂ emissions represent only a fraction (31%) of the true climate impact of aviation. The Contribution of Global Aviation to Anthropogenic Cimate Forcing estimates that contrails (53%) and NOx emissions (16%) together lead to a total climate impact from aviation three times higher than the CO₂ emissions alone.

For more explanation see How Airplane Contrails Are Helping Make the Planet Warmer, which points out the more hopeful news:

A second approach to minimizing contrails is to change fuels - from kerosene-based fuels to biofuels, hydrogen, liquid natural gas... If burned "neat" the other alternative fuels could cut the soot emissions around which water vapour condense in contrails by 90% or more, according to Fabio Caiazzo of the Massachusetts Institute of Technology. Even a 50:50 mix of biofuel and kerosene-based fuel could halve soot.

And:

While CO₂ accumulates in the atmosphere and has a long-lasting effect, contrails last a matter of hours at most, and their warming impact is temporary.

Hence, as we reduce the amount we fly we will immediately eliminate over half of the climate-warming this is causing. However, until we switch to biofuels, each flight will still add to CO₂ in atmosphere causing extra warming for centuries to come.

Furthermore, we could immediately reduce the warming effect of contrails by 60% by changing the altitude of 2% of flights according to this computer-modelling: Mitigating the Climate Forcing of Aircraft Contrails by Small-Scale Diversions and Technology Adoption (https://www.imperial.ac.uk/news/195294/small-altitude-changes-could-contrail-impact/).

Electric Planes

Electric planes can't fly very far because batteries have much lower energy density than kerosene ie each kilo of battery holds much less energy to power the plane's engines than a kilo of kerosene.

Rob Watson, Director of Electrical at Rolls Royce, told the BBC Radio 4 Today programme on 24th September 2021 (from about 1h20m) the following:

At 200 seats, aircraft are too large [to be powered electrically because] you are limited by the power and energy density of batteries. By the time you're trying to produce the levels of power required to move that volume of people through the air, the weight of the batteries just overwhelms the platform

This proponent of the technology has pretty low ambitions: planes of up to 20 seats might be able to achieve a range of 150 miles if battery technology was significantly improved.

It's the laws of physics that drive ambitions low. The energy density of kerosene is 12.8kWh/kg. The best energy density yet achieved for a (lithium ion) battery is 265Wh/kg. Wow! kerosene carries approximately 50 times more energy per kilo. We might be able to improve this energy density of batteries significantly but, given that this problem has been worked on for decades, it's unclear whether we'll ever develop a battery with sufficient energy density to power current airframes.

Hydrogen Planes

The energy density of (liquified) hydrogen is 39kWh/kg, three times higher than kerosene. So maybe we can use that to replace kerosene as fuel to propel current airframes. However, hydrogen is even more potentially explosive and flammable than kerosene, so the kit required to store liquified hydrogen needs to be robust and is necessarily significantly heavy.

Sources:

* PS I think the figure of 503 MtCO₂ for 2017 UK emissions in the Sustainable Aviation report is wrong. It appears to be the number for Territorial Emissions, which flatters us by ignoring emissions from aviation! The correct figure for UK CO₂ emissions in 2017 is 784 MtCO₂e, our Consumption Emissions. See The UK's True Carbon Emissions.



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