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Stop Bristol Airport Expansion! (SBAE) is an alliance comprised of the following groups:

cpreBristol Friends of the Earth

Read more about the SBAE alliance

All content © Stop Bristol Airport Expansion, 2009.


100 months

Planes and Climate Change

Why offsetting doesn’t work

Airlines have been inviting passengers to offset their carbon - and feel better about the climate impacts of flying. But the reality is that carbon offsetting is not a solution to the climate damage caused by aviation. Offsetting pays for someone somewhere else to cut the carbon - while allowing you to continue polluting. There is no guarantee that the carbon cuts made would not have happened anyway, and evidence suggests that some offset projects do not in fact deliver the level of cuts promised. Basically you continue polluting - and you may even be encouraging more pollution elsewhere. The scale of cuts that need to be made require both the developed and developing world to make cuts, not just to pass the buck.

Fuel burn and radiative forcing

Planes burn fuel, normally a type of kerosene, to power their jets. This in turn produces carbon dioxide, water vapour and a variety of other pollutants. The fuel consumption of a modern heavily loaded jet aircraft is such that it produces around 0.1kg of carbon dioxide per passenger kilometre. Older planes or ones with less passengers will produce more.

In the first report on aviation for the Intergovernmental Panel on Climate Change (IPCC), the climate effect of aviation emissions was estimated to be between 2 and 4 times worse than the effect of carbon dioxide alone. This is due both to the other pollutants and the fact that they are emitted high in the atmosphere. This is termed ‘radiative forcing’. A later report clarified this factor to being 1.9 times plus an unknown factor for cirrus cloud formation. However, the latest IPCC report gives a value, which includes the cirrus effects, of 3.05 the effect of carbon dioxide on its own. There are still uncertainties, but it is clear that aviation emissions are considerably worse than the carbon dioxide numbers would imply.

This means that the overall effect is around 0.3kg making it close to the equivalent of driving a large 4x4 with a single passenger - not a Toyota Prius as some airlines claim. Of course, few people would drive similar distances to those flown, or drive them with only one person in the car. Assuming an average passenger loading of 1.5 per car, this makes flying 4.5 times as bad as cars.


Offsetting is the concept by which a company or person can avoid reducing their own emissions by paying someone else to make cuts elsewhere. This has been included in the Kyoto protocol as a way for the developed world to transfer money to the developing world to make cuts in emissions there, rather than at home. There are several problems with this mechanism:

  1. it is hard to prove that the actions in the developing world would not have happened without the offset payments, thus it is not clear if these reductions are actually in addition to those that would have happened anyway
  2. many of the projects are paying industrial companies not to increase emissions, rather than actually cutting current levels
  3. in many cases there is no provable reduction or avoidance of emissions
  4. often payments are insufficient to compensate for emissions at the same rate that they are created. Because it is the overall concentration of gases in the atmosphere that matters and CO2 stays in the atmosphere for a more than 100 years, it is not good enough that emissions today are compensated for over many decades when we need to have real reductions by 2020.

But most importantly, the scale of reductions required is such that both the developed and the developing world need to make reductions, and thus industries here cannot pass the buck to those abroad.


Several airlines have run trials on using biofuels to replace fossil fuel kerosene. These have involved use of oils from jatropha, coconut and other sources including algae. However none of them have used a pure biofuel in all engines of the planes. These trials have been to see if the fuel delivers enough power and does not clog the engines, as well as to generate publicity. In reality, it is at least 5 years before any commercial planes will be flying routinely with a mix containing biofuels, not least because of the infrastructure needed to distribute the fuel.

The volumes of fuel required are huge, and it is extremely unlikely that more than 10% of the world aviation fuel mix will be supplied by biofuels by 2020.

What is more, existing biofuels generally require large amounts of fossil fuel input to create them (due to planting, fertilizers and pesticides, harvesting, extraction, purification and esterification) and they often involve major land use changes which release carbon from plants and soils. The largest source of biofuel oil currently is the oil palm, and the major exporters of this are countries where tropical rainforests are being cut and burnt, and the peat bogs on which they stood are being drained in order to grow the crop. These actions release huge amounts of carbon. It has been shown that palm oil is not carbon neutral, and in fact a given field will require 400 years of fuel production to compensate for the carbon released in clearing it.

Technological improvements

The fuel efficiency of jet airliners has improved considerably since their introduction in the early 1960's, but in fact they are only now reaching the fuel consumption per passenger mile of the last of the propeller planes that they replaced. This is due to the planes flying much faster and consuming more fuel to overcome the air resistance, as well as the immaturity of the engines in 1960.
However, a 60% improvement since 1960 doesn’t mean that a further 60% improvement is possible. In fact the rate of improvement is now much slower, averaging out to around 1% per year, against passenger numbers growing at 5% per year. But this is the rate of improvement for the best new aircraft and does not reflect the average plane deployed in the fleet. As aircraft have long lifetimes, up to 30 years, unless legislation forces planes to be removed from service (as occurred because of excessive noise) then they continue to pollute for a long time.

The improvement in fuel consumption going from the 747-300 in 1982 to the A380 in 2005 was around 25% in 23 years. However the A380 required the use of radical new materials, is not suitable for many airports, and will take a very long time to become a significant part of the fleet.

The fuel consumption is determined by the body shape, the weight, and the speed of the aircraft. Without a major change to the shape of the plane, there is very little scope to fly the same sized aircraft at the same speed and to still reduce fuel consumption enough to compensate for increasing flights.

As most planes using BIA are already modern designs, they are unlikely to be replaced by ones with significantly lower emissions.

BIA claim that new aircraft are 20% more fuel efficient than those 10 years ago and that they will be 50% more efficient by 2020. Of course, not all planes flying from the airport are new, and those that are will not be replaced by 2020. It is also very unlikely that this further cut will be possible as it implies a rate of improvement more than twice that seen in the last 10 years.

Emissions trading

Emissions trading is a mechanism by which the total amount of carbon is limited by only allowing a certain number of "permits to pollute" to exist. Various industries are given permits for free from the total, if they then need to pollute more they have a choice between cutting their emissions or buying more permits from others who have managed to make cuts. This is intended to find the most efficient way of making cuts. Unfortunately the existing trading mechanisms have not made any significant cut in emissions, mostly because too many permits were given away free and also the cost of permits has varied with the price of oil, making investment in cuts difficult to justify.

Aviation has so far avoided being forced to cut emissions or join an emissions trading scheme. In the EU, aviation is meant to join the emissions trading scheme (ETS) in 2012 and to start trading permits against a "cap" based on 2006 levels. This does not mean that aviation emissions will not grow, it just means that the growth will force the industry to buy some permits and this will marginally increase costs, perhaps as much as €9 for a long distance ticket. It is clear that the price being paid is insufficient to trigger the required investment to cut emissions within the industry.

More importantly, because the total cuts we need to make are so large (80% by 2050), there is very little scope within other industries to cut emissions further no matter how much aviation pays for permits.

The EU has recently commented that the ETS is not enough to cause big cuts to be made, and in fact a carbon tax is required as well at least to ensure that the cost of permits does not drop too low.

The inclusion of aviation in the EU ETS is also in doubt because American airlines are threatening to take legal action against it. In addition, several bodies representing the aviation industry are proposing alternatives including one that makes biofuels a major alternative to actual emission cuts. This is a continuation of their long battle to avoid paying for their environmental costs.

It appears that the industry wants another 20 years without effective controls on its climate impacts.

Just because a "cap" is imposed on emissions does not mean that they will shrink, it just imposes marginal costs. It certainly does not mean that aviation should be aiming to increase its own emissions. It is also important to remember that there is no mechanism by which the non-carbon dioxide effects are taken into account. This means that even if the total emissions of carbon dioxide from aviation are compensated for by cuts in other carbon emissions, the total climate impact will still increase. Flying is over three times as bad for the climate as just burning the same amount of fuel on the ground, and using offsets or emissions trading to balance out the nominal carbon dioxide will still leave twice the impact of the original fuel burn.

Necessary emissions

It is argued that flying is necessary for the vitality of the local economy, but this is certainly not true when we look at the destinations leading to the highest emissions from BIA.

Destination % of Total*
NEW YORK 6.03%
MALAGA 4.10%
FARO 2.99%
Total 33.37%

*% of total CO2 emissions from commercial passenger flights

Fraction of emissions

It is often repeated that aviation contributes only 2% of global carbon emissions, and it is therefore implied that it is not worth constraining it. However this is not an up to date or fair representation of the facts.

The 2% figure came from an assessment by the Intergovernmental Panel on Climate Change (IPCC) in 1999. However, since then several things have changed. Firstly, aviation has expanded considerably, but on top of this the scientists have a much better understanding of the effects of aviation upon the climate.

The original 1999 assessment stated that the overall impact was between 2 and 4 times as much as that of the carbon dioxide. This has now been clarified to be 3.06 times as much as the carbon dioxide, including cirrus clouds.

The overall effect is that aviation is 4.9% of global climate impact.

In the UK, aviation emissions are around 6.5% of the carbon dioxide emissions and around 13% of the climate impact, according to the Government, although this used a lower figure for the effects of other gases and excludes cirrus clouds, so it is in reality higher than this.

Struggling with arithmetic

The Government has committed the UK to an 80% cut in emissions by 2050, but it has also a White Paper that plans for aviation to increase by 2.5 times by 2030. As aviation already amounts to 13% of the UK climate impact, it is hard to see how it is possible to cut the total to 20% of the current total when one sector is to grow from 13% to over 30%. This would mean that the only sector of the UK economy that could emit carbon would be aviation, and even then it would be too much.

In fact the arithmetic makes it clear that increasing aviation emissions and cutting the total by 80% do not fit together.

Terminal changes

As a large user of electricity, BIA is obliged by law to make plans for reducing its usage. Its commitment to 20% of the extra power to be from renewables is in fact a very small amount and no more than they are required to do. As the terminal is planned to double in area, the total power consumption is likely to double, and thus this commitment amounts to the climate changing emissions from the terminal increasing by 80% rather than 100%.

The use of the wind turbines is largely irrelevant. They are expected to generate just over 1kw each, although how effective this will be is limited by their siting having to avoid the runway where winds are highest.

The total annual output would be 150MWh, but the airport's consumption in 2007 was 15,350MWh, that is the turbines would generate less than 1% of the total used in 2007 and the amount used in the much bigger terminal would be far higher.