Can we ever fly green?

Jenny Love, UCL Energy Insitute

With help from Olivier Dessens, UCL Energy Institute & Oliver Sleath, Credit Suisse

A few years ago I decided to stop flying until it becomes eco-friendly. This was because I’d heard that going on aeroplanes is pretty much the worst action you can do in terms of climate change impact – and if you call yourself an environmentalist I think it’s vital to practise what you preach.

The question is, will it ever be possible to fly without causing global warming? This article will look at the climate change impacts of aviation and the changes that need to come about to limit the damage.

As motivation for you to invent an eco-friendly aeroplane, you won’t be able to get rid of me from the UK and its vicinity until you do.

The ways that aeroplanes cause climate change – in order of importance

1.       Contrails and cirrus formation

I’ll start with a surprise – the biggest climate change impact from aeroplanes is not CO2, nor anything else to do with burning fossil fuels specifically…but an effect called ‘contrails’ – condensation trails forming when a plane crosses a mass of air with particular thermodynamic properties making it ready to form ice crystals (technically: “supersaturated to ice”). The plane triggers the formation of the ice crystals by one of several means: changing the pressure, temperature or planting droplets for the ice to form around. The effect of the ice crystals is to trap heat in the earth-atmosphere system – the technical term is ‘radiative forcing’.

A Nature and Climate Change report claimed that, “…net radiative forcing due to contrail cirrus remains the largest single radiative-forcing component associated with aviation.”[i]

Did you know this was the biggest global warming factor? I didn’t.

2.    Greenhouse gases

Aeroplanes emit gases which cause climate change. There are two types: ‘Greenhouse Gases’ (GHG) including carbon dioxide and water vapour, and gases such as nitrous oxides (NOx) which are not GHG but have an impact on the chemistry of the atmosphere and change the concentration of ozone and methane, which are potent GHGs. Again, ‘radiative forcing’ is the most common measure to use when calculating the overall effect, so that people like you and me can just look at the end result and not have to worry about the complicated atmospheric chemistry that  happens between exhaust being emitted by the plane and resulting global warming. However, the radiative forcing metric does not take into account the future effect of long-lived gases, such as CO2, which will stay in the atmosphere for hundreds of years.

Some factors which affect the radiative forcing are: the amount of each of the gases present, their efficacy in trapping heat, their lifetime before decaying into something else, how high the plane is, and where the plane is over the earth (since there are local differences in distributions of e.g. ozone). Concerning these last two – there are some heights at which the resulting chemical reactions will produce more GHG, and some where it will produce less. One thing I learned in my physics degree is that modelling the atmosphere is a tricky business.

Given these problems of contrails and gases which cause radiative forcing, are there any actions which would lessen or eliminate their respective formation and release?

Possible solutions

1. Efficiency increases

Cutting down the amount of fuel used decreases the CO2 and other greenhouse gases emitted into the atmosphere. How could such fuel a decrease come about?

At the moment, efficiency increase is focussed on advances in engine technology. Possible the most groundbreaking technology that will actually be a reality within the next 3-5 years is the Pratt & Whitney geared turbofan: It is claiming 16-20% reduction in fuel burn compared to conventional engines. However, there is normally a trade-off between CO2 decrease and NOx increase, so I’m not sure what the change in NOx is from previous P&W engines.

Potentially the greenhouse gas issue could be helped. Unfortunately, a 20% reduction in fuel burn over the next 5 years will probably be negated by a 5% annual growth rate in demand for aviation (this is called the ‘rebound effect’).

And unfortunately the contrails issue is still with us.

2.  Alternative fuels

(this bit is summarised from this book:

‘Biofuels’ covers a large range of substances. One definition is: “A type of energy derived from renewable plant and animal materials”.[ii]

The principle of using a biofuel in a jet engine is similar to that of kerosene:

Fuel + oxygen = energy + waste gases (CO2, water, etc).

The two fuels most suitable to put in a jet engine are biodiesel and a type of synthetic kerosene, called Fischer Tropsch (FT) Kerosene. We will rule the former out using a measure called ‘energy ratio’ – energy in the finished fuel compared to non-renewable energy used to produce it. For biodiesel, this is less than one. Forget it. For FT Kerosene, it’s 18-44, so we’ll go with this one. You can make FT Kerosene out of lots of different organic materials; what shall we use?

In the previous blog post, here:, I mentioned ‘Life Cycle Analysis’ – the principle of trying to calculate the total environmental impact of a product, following its production from raw materials. It has been shown[iii] that although certain biofuels cut greenhouse gas emissions by more than 30%, their supply path causes more other environmental damage than petrol – mostly from the resources needed to grow them. The report advises that damage is most limited by using grasses, biogenic wastes and wood to produce the fuel.

However, none of the LCA analysis includes the specific use of the fuel in aircraft – and thus does not include the contrail issue. I’m sad to say that the contrails do not go away with the use of biofuels.

3. Rerouting aeroplanes

Now, as we have seen, the location of the plane in the atmosphere affects both the likelihood of contrails forming and the chemical reactions which occur when its exhaust is emitted. Is it possible for aircraft to modify their paths such as to cause the least radiative forcing possible?

The first thing to point out it that contrail formation is seasonal and affected by weather[iiii]. So when we say ‘modify’ paths of aircraft, we’re not talking a once-for-all shift upwards or downwards, but an adaptive, real-time system of air traffic controllers sending a particular plane higher for a few hundred miles then back down, to avoid the contrail forming regions it comes across. There are academics working on this: I think their logo nicely describes what I’ve been trying to say:

Is this going to be practically possible?

a)      Practically, can an aeroplane fly higher or lower? The current height is ~40-45,000 feet. Take them lower and they burn more fuel and are noisy. Take them much higher and, like Concorde, they need a new type of engine and a new type of wing, both of which are able to deal with the lower air pressure – air being essential for lift force and provision of oxygen in the engine. Theoretically, since contrails are 2000 ft high and aeroplanes could go 5,000 ft higher without more expensive engines, contrail avoidance could be possible. It is then a matter of political will.

b)      Is it possible to predict where the contrail forming regions are before the plane gets there? Yes it is, and subsequently meteorological data could be fed to air traffic controllers, who would then talk to pilots who would react accordingly. Feedback from planes to air traffic controllers could be given via cameras on the back of aircraft which show whether contrails are being produced or not. Inevitably this requires a whole-system change of how air traffic control works – our current flight paths have been set up and left as they are for at least 10 years so the thought of reactive rerouting would be a bit of a shock. Passenger safety is the primary concern of the aircraft operators, not climate change.

4. Alternative holiday destinations?–about/tourist-information

Maybe not. But how about:


There is potential to reduce the environmental footprint of aircraft, by efficiency increases, alternative fuels and intelligent routing strategies. There are still a lot of unknowns in how much damage limitation this will result in, and many things about the system need to change if all three strategies above are to be used. My conclusion to this is that it’s currently best not to fly, especially if you have the choice, and come back to this blog in 20 years to find out what has changed.

Please feel free to ask questions and add comments.

Optional extra

Here are answers to 2 questions people might ask you if you decide, shock horror, not to fly. I don’t expect everyone to agree with my answers.

1)      But going travelling is enriching and makes you a better person.

Quite probably true. But go and spend a week amongst a different social class or religion where you are. The most enriching experience I’ve had recently is spending time on a council estate in Coventry. And not everyone can afford to go travelling so we can’t say it’s our right/you have to do it to be a good person.

2)      Can I just offset my carbon?

I think this is the wrong attitude. Only do this if you don’t have a choice whether to take the flight. Never use it as an influencing factor in a decision, otherwise carbon offsetting could increase demand for flying.

[i] Ulrike Burkdhardt & Bernd Kärcher 2011.  Global radiative forcing from contrail cirrus, Nature Climate Change, 1, 54-58 (2011)



Zah, R., Böni, H., Gauch, M., Hischier, R., Lehmann, M. and Wäger, P. (2007) LifeCycle Assessment of Energy Products: Environmental Assessment of Biofuels, Berne: EMPA

[iiii ]Ficheter, C. SUSANNE MARQUART2, ROBERT SAUSEN, DAVID S. LEE3 The impact of cruise altitude on contrails and related radiative forcing. Meteorologische Zeitschrift, Vol. 14, No. 4, 563-572 (August 2005)


2 thoughts on “Can we ever fly green?

  1. Very interesting Jenny. The ideas about rerouteing aircraft remind me of a mitigation for climate change that I looked at in my Msc dissertation: dynamically adapting the timetables of trains to balance increased energy efficiency with customer journey times. I guess you’d need a clever algorithm to make sure planes don’t cause more climate change because of the extra energy used to fly around contrail areas than they prevent by not producing contrails!

    On another train related note, it is in fact possible to get some way away from the UK without using a plane:

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