Uncook carbon offset. Offset air travel and aviation greenhouse emissions

Contrails from a passenger jet shown clearly against the deep blue sky.

Air travel is the fastest growing source of greenhouse emissions representing about 2% of total global emissions.

Air craft produce so much greenhouse emissions that a single flight can double your households annual greenhouse emissions. Until there are jet liners run on solar power or biofuels, there will be greenhouse emissions associated with every flight and every seat.

If you fly, then offsetting the emissions is the only way to counterbalance these emissions.

Where do aviation greenhouse emissions come from?

Below is the story of the jet fuel from when it is pumped from the ground, processed in a refinery to form jet fuel to when its burnt and released as exhaust gas from a jet engine. Shown below is the process from crude oil extraction to the warming (and dimming) of the atmosphere.

An oil well is the first part of the journey of the carbon molecule in the petroleum oil.

Crude oil is extracted from deep below the ground. Crude oil is made from the remains of marine life pressure cooked over millions of years. Note the oil drum feeding fuel into the oil pump. Producing oil has a significant energy and an environmental cost. Furthermore, oil is running out. Experts speak of 'peak oil' the point at which half the reserves have beenn used up and the remaining oil is increasingly difficult and costly to extract.

When things go wrong: Oil workers struggle to shut off a well head that has been damaged. Crude oil is highly toxic and harms human health as well the environment. This picture is from Kuwait after the first Gulf 'oil' war.

The crude oil is transported around the world from the oil producing countries in huge ships. For the most part, there is little spillage. Of note, however, is the greenhouse emissions produced through burning the fuel oil used to power ships. These greenhouse emissions are not covered under the International greenhouse treaty, the Kyoto Protocol.

When things go wrong: Sometimes, oil tankers sink causing massive local environmental disasters. Here the oil tanker Prestige breaks up off the coast of Spain in 2002 spilling 64,000 tons of fuel oil. The estimated cost of the clean up operation is 2.5 billion Euros. Furthermore, as late as March 2004, oil was still leaking from the hull of the ship and floating to the surface from 4,000 metres below.

The crude oil is then pumped to oil refineries where it is processed and seperated into a variety of components to form petrol, diesel, kerosene, bitumen and other petroleum hydrocarbns.

Modern airliners use a fuel called Jet Fuel which is kerosene with a few additives.

When things go wrong: Major oil installations are vulnerable to fire and increasingly to terrorist attacks. Here, an accidental fire at a UK oil storage facility, Buncefield, 2005 pours 177,000 tons of greenhouse emissions into the sky.

Jet fuel is burnt in jet engines. The jet engine - shown here in cut-away form - combusts the kerosene. As the burning fuel expands, it forces hot gases out of the back, forming thrust to drive the airliner plane forward. Modern jet engines are typically more fuel efficient than older engines.

However, they acheive this efficiency by running at hotter temperatures. Ironically, these more efficient jet engines have a higher greenhouse signature than older, 'cooler' engines becuase of higher amounts of Nitros oxides produced. Nitrous oxide is a powerful greenhouse gas.

The thrust produced by the expanding gases in the jet engine push the plane forward and the movement of air across the plane's wings produces lift which keeps the plane aloft. In the lower stratosphere where jets spend most of their time, at cruise altitude, the global warming effect of the jet exhaust is up to 5 times as powerful as when the jet is on the ground.

When things go right: Along with greenhouse emissions warming the atmosphere, airliners also produce contrails. The picture shows many contrails mingling together forming a fine mist of high level cloud. The water vapour acts as a greenhouse gas, warming the atmosphere, but also shading the earth to enhancing a process known as global dimming.

When things go wrong: When all civil aviation was grounded across the United States following the September 11 attacks, the greenhouse emissions and global dimming contrails disappeared. So much so that atmospheric chemists reported being able to see the sky in its natural blue for the first time in their lives.

Aviation contrails and the climate

Aviation contrails formed in the lower stratosphere are shown in the picture, left. They are composed of water vapour from the exhaust of airliners. Contrails contribute to both Global Warming and Global Dimming. These two global forces are well matched except that global warming is marginally stronger and much more enduring. When aviation was shut down over the United States following September 11, the global dimming effect disappeared and the global warming effect accelerated. See Global Dimming report.

This page is a technical description of the scientific understanding of aviation greenhouse emissions. It is based in part on the Intergovernmental Panel on Climate Change Report, Aviation and the Global Atmosphere Download Summary and other documents.

'Aviation and the Global Atmosphere'
A Special Report of the UN Intergovernmental Panel on Climate Change Available from Cambridge University Press - Also available on CD-ROM

Calculating Aviation Greenhouse Emissions

Note: the information below considers the science behind calculating aviation greenhouse emissions and the method that Uncook uses in its calculator. The below information is based on information provided by Atmospfair and Tufts report on Voluntary Offsets.

Aircraft engines emit a range of pollutants which raise the temperature of the atmosphere directly or indirectly. Carbon dioxide (CO2) is the easiest to describe in terms of its production and effect. It is produced during the combustion of kerosene in direct proportion to the consumption of kerosene. CO2 is used as the basis for calculating climate damage. The various other pollutants and their effects can be summarised via an internationally recognised calculation method so that its warming effect can be converted into CO2 equivalents. The Emissions Calculator first calculates the fuel consumption per passenger and, based on this, then determines the amount of CO2 whose warming effect is comparable to that of all pollutants emitted by the flight together (effective CO2 emissions). This is the amount of CO2 output by the Calculator which is saved by atmosfair in climate protection projects. Aircraft engines emit a range of pollutants which increase the atmospheric temperature. The most important are carbon dioxide (CO2), nitrogen oxides (NOx) and various particles of soot or sulphur. The climate impact of these pollutants has been described in detail by the IPCC, the United Nations Intergovernmental Panel on Climate Change (IPCC 1999). The impact of these pollutants on the climate varies:

Carbon dioxide (CO2)
is always generated during the combustion of fossil fuels (coal, gas, oil). The amount of carbon dioxide emitted is a direct function of fuel consumption: 3.16 kilograms CO2 are produced per kilogram of kerosene on combustion in
the aircraft engine with ambient air. Carbon dioxide is a greenhouse gas which, simplified somewhat, remains in the atmosphere for approx. 100 years after its emission. As a result is can spread over the entire globe, driving
global warming throughout the world. CO2 is regarded as a leading gas in climate science in climate science and is used as a reference variable for comparisons between the effectiveness of different greenhouse gases.

Nitrogen oxides (ozone formation)
Nitrogen oxides are produced in the aircraft engine at high temperatures and pressures by the reaction between oxygen and atmospheric nitrogen. Its production depends greatly on the engine load. It is estimated that approx. 8-
15 grams nitrogen oxides are produced per kilogram kerosene consumed in passenger jet engines when cruising. Nitrogen oxides have two main impacts on the climate: firstly, they reduce the lifetime of the greenhouse gas
methane, an effect that reduces global warming. Secondly, at cruising altitudes of about 10 kilometres they form the powerful greenhouse gas ozone which spreads out along the major air corridors, for example over the North Atlantic, where several hundred aircraft fly daily from Europe to the US and back.

Particles (condensation trails and ice clouds)
Particles in the engines’ exhaust plume are produced by condensation from gaseous pollutants and consequent processes. Water, soot and sulphur are important starting materials for this. Ambient air saturated with moisture can
condense on particles, resulting in condensation trails and high, hazy ice clouds (cirrus clouds). These clouds act like a glass roof over the earth and thus contribute to climate warming. The formation of these clouds depends less on the number of emitted particles than on the fact that the ambient atmosphere is sufficiently humid.

Flight altitude and state of the ambient air
The equivalent climate impact of the emissions and their effects depends on the flight altitude and the state of the atmosphere at the time when the aircraft flies through it and emits the pollutants. This is adequately addressed in that the Emissions Calculator treats the emissions at high cruising altitudes in excess of approx. 9 kilometres above sea level (this is usually the case for flight distances of greater than approx. 400–500 km) as
more harmful than those of short-haul flights.

The equivalent climate impact of the nitrogen oxides and particles is a function of the flight altitude and the state of the atmosphere at the time the aircraft flies through it and the pollutants are emitted.

Nitrogen oxides, ozone
The generation of the greenhouse gas ozone from nitrogen oxides under the effect of insolation is a result of similar chemical smog reactions to the formation of nitrogen oxides from automotive emissions in cities during the
summer months. At high flight altitudes above approx. 9 kilometres, however, the smog reaction is more effective than at ground level. The existing concentration of nitrogen oxides is crucial in this context: if there are few nitrogen oxides available, ozone is quickly formed; if, on the other hand, there is a very high concentration, further nitrogen oxides can even result in ozone being broken down again. It is therefore important to know whether a flight operates on a route which is frequently or rarely flown and whether the aircraft climbs to the critical heights.

Particles, ice clouds:
Long-lasting condensation trails and high hazy clouds of ice can only form if the air through which the aircraft is flying is sufficiently humid. Near the equator this is generally only the case at very high altitudes of about 12-16 kilometres above sea level. Since even modern civil jets rarely fly at such altitudes, the formation of condensation trails and ice clouds here is rarer than at more moderate latitudes and in the polar regions of the earth where these clouds can form at depths of as low as 5 kilometres. The humidity in the air is also generally a function of the season, as a result of which this too influences the likelihood of such aircraft-generated clouds being formed.

Radiative Forcing
Calculating the CO2 emissions from jet fuel fuel burned on flights is relatively simple but the overall warming impacts of air travel are much more complex and difficult to calculate. Therefore, and to allow comparisons of varying types of emissions, the concept of radiative forcing is used. Radiative forcing measures the rate at which a given atmospheric gas alters radiation that is entering the atmosphere. A positive value denotes warming; a negative number signifies cooling (IPCC, 1999).

The main greenhouse gases emitted from aircraft are carbon dioxide (CO2), water vapor, nitrogen oxides (NOx), and methane (CH4). Aircraft travel at altitudes of 9 to 13 kilometers (approximately 5.6 to 8 miles). At these altitudes, the effect of the emitted gases is considerably different than on the ground level and in many cases still incompletely understood21. Aircraft also emit water vapor during flight. When emitted in the stratosphere, H2O can cause the formation of ice clouds, called contrails. Where contrails persist, cirrus clouds begin to form which have an additional impact on global warming. Clouds can have a double effect on radiation: they warm the earth by reducing the amount of radiation from the earth that escapes into space but also cool the earth by reflecting the sun's rays back into space. However, contrails lead to a net warming (William, Noland and Toumi, 2002; IPCC, 1999).

The IPCC has estimated total radiative forcing of air travel to be 1-5 times larger in the stratosphere than in the troposphere and calculated the average for full radiative forcing to be a factor of approximately 2.7 (IPCC, 1999.) Therefore to estimate the impact of an airplane trip a multiplier should be used on the CO2 emissions from jet fuel to account for full radiative forcing.

Unless the growth of the air travel industry is slowed, it is estimated that by 2050 air travel will be contributing at least 6% of the total radiative forcing from human activities (RCEP, 2003; Bows, 2005). Although more research is needed to fully understand the chemical processes in the stratosphere, the research used by the IPCC is very robust.

Note: Uncook uses the average IPCC radiative forcing value of 2.7 in its aviation calculator.

Variables Flight Distance
The rate at which fuel is burned is proportional to the drag which is the force of resistance that must be countered by the force of the engine’s propulsion. During the take-off and landing, the engine is at full thrust and more fuel is consumed during take-off and climbing. Shorter flights therefore have a lower overall fuel efficiency; ie. use more fuel per mile than long-distance flights (RCEP, 2003). As the aircraft climbs and begins to cruise - that is, above the altitude of 3000 feet - drag and therefore rate of fuel use decreases (IPCC, 1999). On longer flights (those over approximately 994 miles) the amount of fuel used during take-off is less significant compared to the whole. This efficiency gain is partly offset on long distance flights by the added weight of the fuel that an airplane needs to carry on such long trips (RCEP, 2003.)

To more accurately calculate emissions, some of the companies’ carbon offset calculators distinguish between
short, medium or long flights. Often, airplanes do not take the most direct route and having to change airplanes is very common. This leads to additional inefficiencies.

Note: Uncook uses distances calculated from great circle routes and applies one of three carbon emissions factors according to route length using data from DEFRA.

Occupancy Efficiency

At full occupancy an aircraft will fly at maximum efficiency. Therefore a flight that is at maximum payload
burns less fuel per passenger than a flight that is at less than its maximum payload. On average, international
flights fly at 78% of maximum payload and domestic flights at around 65% (RCEP, 2003).

Note: Uncook uses standardised asumptions based on figures produced by DEFRA.

Business vs. Economy
Business and first class seats are larger and take up more room. Therefore, a passenger traveling in business or first class is responsible for more emissions because they have effectively excluded additional people from traveling on that same flight (IPCC, 1999).

Note: Uncook assumes all flights are economy unless seat type is specified.

Type of Plane
Type of plane also effects efficiency. The size, number of seats, engine types and other characteristics all influence the emissions of a flight. In general, older airplanes are less efficient than newer models. Most calculators use an average based upon all planes or choose just one typical commercial plane (IPCC, 1999).

Note: Uncook assumes all planes are the hybrid plane modelled in the DEFRA report unless the plane type is specified.

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