Carbon-neutral aviation

July 24, 2009

in Economics, Geek stuff, Politics, Science, The environment, Travel

Watch and red jacket

The climatic impact of aviation

At present, virtually all freight and passenger-carrying aircraft operate in one of two ways: burning kerosene to turn a propeller, generating thrust that the wings partially convert to lift, or generating thrust by burning kerosene in a jet engine. Virtually all of that kerosene is produced by refining petroleum. As such, burning it adds to the stock of greenhouse gasses in the atmosphere. These include carbon dioxide (CO2) (a basic product of the combustion) and other greenhouse gasses (GHGs), like nitrous oxide (NO2). It is also possible that aircraft have an effect on cloud formation (both by producing vapour trails and inducing cirrus cloud formation), but my understanding of the science is that scientists aren’t sure whether that has a net warming or a net cooling effect. The latest IPCC report says:

Moreover, the uncertainties on some aviation forcings (notably contrail and cirrus) are still high, such that the overall radiative forcing consequences of changing cruise altitudes need to be considered as a time-integrated scenario, which has not yet been done. (p. 355)

Helpfully, the report does identify that, if contrails prove to be a significant problem, they “can be easily avoided – in principle – by relatively small changes in flight level, due to the shallowness of ice supersaturation layers.” There is also some uncertainty about the relative emissions of short-lived but potent GHGs like nitrous oxide, compared with long-lived but less potent ones like carbon dioxide. All told, the report does conclude that aviation has a “larger impact on radiative forcing than that from its CO2 forcing alone.”

Carbon neutral possibilities

A couple of logical possibilities exist for making air travel carbon-neutral, though they differ in practicality. Electric planes are conceptually possible, and small versions exist. As I understand it, the big problem is storing enough energy in light enough batteries. My sense is that we are nowhere near being able to do this for large commercial aircraft. Similar issues exist for hydrogen aircraft, in term of storage, and there is the added question of where we get the hydrogen. To me, biofuels seem like the most plausible near-term option. That being said, there are technical issues to be overcome within aircraft themselves, such as the gelling of biofuels at the low temperatures found at high altitudes. While some airlines have tested multi-engine planes with a single engine running on a biofuel/kerosene mix, as far as I know nobody has flown such a plane exclusively using biofuels.

Additionally, not all biofuels are carbon neutral. Ethanol derived from corn might actually represent more greenhouse gasses than an equivalent amount of gasoline, once you factor in fertilizer production, emissions from farming and farm equipment, ethanol fermentation, etc. The same might be true of palm oil derived biofuels, given how their production can lead to the destruction of rainforests that are major carbon sinks.

My sense is that the air travel industry has yet to demonstrate that it will be able to exist in a carbon neutral world, regardless of how expensive tickets become. That being said, it does make sense to displace emitting activities in order from lowest cost to highest cost. If we can replace fossil fuelled ground vehicles with electric vehicles running on renewable power, we should do so first before pouring enormous effort into trying to produce a carbon neutral aircraft. That being said, there does seem to be a strong moral imperative to reduce emissions generally, including by limiting the amount of long-distance travel we undertake.

As usual, I expect any mention of aviation to produce a lively discussion.

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{ 15 comments… read them below or add one }

R.K. July 24, 2009 at 11:53 am

Another option that is at least logically possible is to compensate for aviation emissions by sucking CO2 out of the air and burying it.

Tristan July 24, 2009 at 6:12 pm

Ya, we could just pump pressurised CO2 down old natural gas wells. Nothing bad could possibly happen.

Milan July 24, 2009 at 6:41 pm

Even with very deep cuts in emissions – such as a 95% reduction below 1990 levels by 2050 for high emitting states – producing a stable climate may require removing some of the carbon we have added to the atmosphere and burying it.

Tristan July 24, 2009 at 6:42 pm

There was a nuclear powered aircraft tested by America during the age when keeping bombers in the air constantly was considered desirable. Perhaps that could be the future of air travel. Although, it would probably be even more difficult than hydrogen.

Milan July 24, 2009 at 6:58 pm

An air travel system using uranium and plutonium as fuels might be easier to establish in a carbon neutral way than one relying on hydrogen, but there would be a lot more risks associated: everything from crashes spraying isotopes everywhere to the proliferation risks that would accompany the facilities and technologies.

Tristan July 24, 2009 at 8:24 pm

Wouldn’t it be more straight forward to use nuclear plants on the ground to produce hydrogen?

Milan July 24, 2009 at 8:33 pm

I don’t know if anyone has seriously investigated the logistics and economics of nuclear powered aviation.

I am not really qualified to speculate.

Matt July 25, 2009 at 5:20 am

I think that the only nuclear aircraft that have so far flown weren’t actually powered by the reactor, but they simply lifted a reactor in a conventional airplane to test the concept. I remember reading the crew got a pretty bad dose of radiation from the poorly shielded assembly.

Milan July 25, 2009 at 12:46 pm

I certainly would not bet on nuclear planes being safe or affordable.

Julie July 27, 2009 at 4:48 pm

I’m with Milan on this one when it comes to nuclear planes. Not sure how feasible a lot of these ideas are actually.

You can still use offsets to fly carbon neutral though. I know belgrave trust offers them for commercial and private flight. I think native energy and standard carbon might offer these offsets too. Not sure though don’t use em.

Milan July 27, 2009 at 4:52 pm

Offsets have been a major topic of discussion here before.

In particular, see:

Ranking the quality of carbon offsets

Visiting Vancouver in summer 2009, by land?

. August 2, 2009 at 5:05 pm

NASA Offers $1.5 Million For 200MPG Aircraft

NASA’s Green Flight Challenge is offering up to $1.5 million for an aircraft that can hit 200 passenger miles per gallon while maintaining 100 mph on a 200 mile flight.

Milan December 17, 2009 at 10:39 am
Milan December 17, 2009 at 10:42 am

This flowchart gives a really good at-a-glance understanding of which greenhouse gasses and energy uses are most important.

All of transportation is only about 13.5% of emissions, with all air travel representing 1.6% of the total. By contrast, the cement industry represents 3.8%, deforestation is 18.3%, and unintentional natural gas leakes are 3.4%.

. July 13, 2010 at 5:36 pm

Electric planes
High voltage
Transport: As electric cars make steady progress on land, battery- powered aircraft of various kinds are quietly taking to the air

Jun 10th 2010

RANDALL FISHMAN, a retired jeweller from New Jersey, is a keen pilot who has spent a lot of time hang-gliding over the Hudson river when he should have been working. Despite some 30 years’ experience as an aviator, he yearned to fly in a new and purer way. So in 2006 he converted a powered hang-glider, removing its small combustion engine and replacing it with a battery and an electric motor. He was not trying to be green. “I just did not like all the vibration and noise from a combustion engine,” he says. “With electric power it was like that dream you had as a kid—of soaring and flying silently.”

Mr Fishman is among a growing group of aviators in America, Europe and Asia who are flying under electric power. What began with hang-gliders and microlights has moved on to full-sized gliders and now two-seater aircraft. A dozen or so electric aircraft are expected to turn up at the Green Aviation Show, which will be held at Le Bourget, France, this month. And they will also be out in force at the Experimental Aircraft Association’s annual gathering at Oshkosh, Wisconsin, in July.

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