Aviation contributes to global warming in a number of ways, the most significant of which is the combustion of kerosene (a fossil fuel) in flight. Greenhouse gas emissions from ground airport vehicles and those used by passengers and staff to access airports also contribute, as do emissions generated by the production of energy used in airport buildings, the manufacture of aircraft and the construction of airport infrastructure. First flight, December 17, 1903 Aviation or air transport refers to the activities surrounding human flight and the aircraft industry. ... Global mean surface temperatures 1856 to 2005 Mean surface temperature anomalies during the period 1995 to 2004 with respect to the average temperatures from 1940 to 1980 Global warming is the observed increase in the average temperature of the Earths atmosphere and oceans in recent decades. ... Russian kerosene lamp Kerosene or paraffin oil (British English, not to be confused with the waxy solid also called paraffin wax or just paraffin) is a colorless flammable hydrocarbon liquid. ... Coal rail cars in Ashtabula, Ohio Fossil fuels are hydrocarbons formed from the remains of dead plants and animals. ... Top: Increasing atmospheric CO2 levels as measured in the atmosphere and ice cores. ... Airbus A380 An aircraft is any machine capable of atmospheric flight. ...

The principal greenhouse gas emission from aircraft in flight is carbon dioxide (CO₂), but other emissions include nitric oxide and nitrogen dioxide, (together termed oxides of nitrogen or NO_x), water vapour and particulates (soot and sulfate particles). Other emissions include sulfur oxides, carbon monoxide, hydrocarbons and radicals such as hydroxyl. Top: Increasing atmospheric CO2 levels as measured in the atmosphere and ice cores. ... Carbon dioxide is a chemical compound composed of one carbon and two oxygen atoms. ... The chemical compound nitric oxide is a gas with chemical formula NO. It is an important signaling molecule in the body of mammals including humans, one of the few gaseous signaling molecules known. ... [1] R-phrases , S-phrases , , , , , Supplementary data page Structure and properties n, εr, etc. ... The term nitrogen oxide is imprecise and can be used to refer to any of these oxides (oxygen compounds) of nitrogen, or to a mixture of them: Nitric oxide (NO), nitrogen(II) oxide Nitrogen dioxide (NO2) Dinitrogen monoxide (N2O) (Nitrous oxide) Dinitrogen trioxide (N2O3) Dinitrogen tetroxide (N2O4) Dinitrogen pentoxide (N2O5... Boundaries: Phase, Pressure, Temperature Evaporation/Sublimation Whenever a water molecule leaves a surface, it is said to have evaporated. ... Particulates, alternately referred to as Particulate Matter (PM) , aerosols or fine particles are tiny particles of solid or liquid suspended in the air. ... Sulfur oxide refers to one or more of the following: Sulfur monoxide (SO) Sulfur dioxide (SO2) Sulfur trioxide (SO3) This is a disambiguation page: a list of articles associated with the same title. ... Carbon monoxide, with the chemical formula CO, is a colourless, odourless, and tasteless gas. ... In chemistry, a hydrocarbon is a cleaning solution consisting only of carbon (C) and hydrogen (H). ... // Hydroxyl group The term hydroxyl group is used to describe the functional group -OH when it is a substituent in an organic compound. ...

The contribution of civil aircraft-in-flight to global CO₂ emissions has been estimated at around 2%.^[1] However, when non-CO₂ altitude-sensitive effects are included, the total impact on anthropogenic (man-made) climate change is believed to be significantly higher. Moreover, that contribution is set to rise for the foreseeable future as increases in the volume of aircraft movement outpaces improvements in fuel efficiency.

Aviation's contribution to climate change

Subsonic aircraft-in-flight contribute to climate change in four ways:

Carbon dioxide (CO₂) emissions

CO₂ emissions from aircraft-in-flight are the most significant and best understood^[2] element of aviation's total contribution to climate change. The level and effects of CO₂ emissions are currently believed to be broadly the same regardless of altitude (i.e they have the same atmospheric effects as ground based emissions). In 1992, emissions of CO₂ from aircraft were estimated at around 2% of all such anthropogenic emissions, though CO₂ concentration attributable to aviation in 1992 was around 1% of the total anthropogenic increase, because emissions occurred only in the last 50 years.^[3]

Oxides of nitrogen (NO_x) induced effects

At altitude, emissions of NO_x are particularly effective in forming ozone (O₃) in the upper troposphere. High altitude (8-13km) NO_x emissions result in greater concentrations of O₃ than surface NO_x emissions, and these in turn have a greater global warming effect. The effect of O₃ concentrations are regional and local (as opposed to CO₂ emissions, which are global). For other uses, see Ozone (disambiguation). ... The Troposphere is the lowermost portion of Earths atmosphere. ...

NO_x emissions also reduce ambient levels of methane, another greenhouse gas, resulting in a climate cooling effect. This effect does not, however, offset the O₃ forming effect of NO_x emissions. The simplest hydrocarbon, methane, is a gas (at standard temperature and pressure, STP) with a chemical formula of CH4. ...

It is now believed that aircracft sulfur and water emissions in the stratosphere tend to deplete O₃, partially offsetting the NO_x-induced O₃ increases. These effects have not been quantified.^[4] General Name, Symbol, Number sulfur, S, 16 Chemical series nonmetals Group, Period, Block 16, 3, p Appearance lemon yellow Atomic mass 32. ... Atmosphere diagram showing stratosphere The stratosphere is a layer of Earths atmosphere that is stratified in temperature, with warmer layers higher up and cooler layers farther down. ...

Water vapour induced effects

Contrails

Cirrus cloud formation

Aircraft-in-flight emit water vapour, a greenhouse gas, which in turn forms Condensation trails, or contrails. Contrails are visible line clouds that form in cold, humid atmospheres and are thought to have a global warming effect (though one less significant than either CO₂ emissions or NO_x induced effects). NOAA image from http://www. ... NOAA image from http://www. ... Image File history File linksMetadata Download high resolution version (2574x1924, 3344 KB)Cirrus clouds on a very cold day. ... Image File history File linksMetadata Download high resolution version (2574x1924, 3344 KB)Cirrus clouds on a very cold day. ...

Cirrus clouds have been observed to develop after the persistent formation of contrails and have been found to have a global warming effect over-and-above that of contrail formation alone. There is a degree of scientific uncertainty over the contribution of contrail and cirrus cloud formation to global warming and attempts to estimate aviation's overall climate change contribution do not tend to include its effects on cirrus cloud enhancement.^[5]

Particulates

Least significant is the release of soot and sulfate particles. Soot absorbs heat and has a warming effect; sulfate particles reflect radiation and have a small cooling effect. In addition, they can influence the formation and properties of clouds.^[6]

Calculating the total climate change effect

In attempting to aggregate and quantify these effects the Intergovernmental Panel on Climate Change (IPCC) has estimated that aviation’s total climate impact is some 2-4 times that of its CO₂ emissions alone (excluding the potential impact of cirrus cloud enhancement).^[7] This is measured as radiative forcing. While there is uncertainty about the exact level of impact of NO_x and water vapour, governments have accepted the broad scientific view that they do have an effect. Accordingly, more recent government policy statements have stressed the need for aviation to address its total climate change impacts and not simply the impact of CO₂.^[8] IPCC is science authority for the UNFCCC The Intergovernmental Panel on Climate Change (IPCC) was established in 1988 by two United Nations organizations, the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP) to assess the risk of human-induced climate change. The Panel is open to all... The generalised concept of radiative forcing in climate science is any change in the radiation (heat) entering the climate system or changes in radiatively active gases. ...

The IPCC has estimated that aviation is responsible for around 3.5% of anthropogenic climate change, a figure which includes both CO₂ and non-CO₂ induced effects. The IPCC has produced scenarios estimating what this figure could be in 2050. The central case estimate is that aviation’s contribution could grow to 5% of the total contribution by 2050 if action is not taken to tackle these emissions, though the highest scenario is 15%^[9]. Morevoer, if other industries achieve significant cuts in their own greenhouse gas emissions, aviation’s share as a proportion of the remaining emissions could also rise.

Potential for emissions reductions

Modern aircraft are significantly more fuel efficient (and thus emit less CO₂ in particular) than 30 years ago. ^[10]. Moreover, manufacturers have forecast and are committed to achieving reductions in both CO₂ and NO_x emissions with each new generation of design of aircraft and engine.^[11] The accelerated introduction of more modern aircraft therefore represents a major opportunity to reduce emissions per passenger kilometre flown.

Other opportunities arise from the optimisation of airline timetables, route networks and flight frequencies to increase load factors (minimise the number of empty seats flown), ^[12] together with the optimisation of airspace.

In the long-term, potential radical new airspace management techniques could allow aircraft to be routed to avoid climate-sensitive parts of the sky, where contrails would be produced. However, this remains a complex area with many uncertainties, and would not eliminate CO₂.

However, the total number of passenger kilometres are growing at a faster rate than manufacturers can reduce emissions, and at present there is no alternative to combusting kerosene. Aviation is therefore likely to continue to generate an increasing volume of greenhouse gas emissions.

Aviation and the Kyoto Protocol

Greenhouse gas emissions from fuel consumption in international aviation, in contrast to those from domestic aviation and from energy use by airports, are not assigned under the first round of the Kyoto Protocol, neither are the non-CO₂ climate effects. In place of agreement, Governments agreed to work through the International Civil Aviation Organisation (ICAO) to limit or reduce emissions and to find a solution to the allocation of emissions from international aviation in time for the second round of Kyoto in 2007. Kyoto Protocol Opened for signature December 11, 1997 in Kyoto, Japan Entered into force February 16, 2005. ... The International Civil Aviation Organization (ICAO), an agency of the United Nations, develops the principles and techniques of international air navigation and fosters the planning and development of international air transport to ensure safe and orderly growth. ...

Aviation and emissions trading

As part of that process the ICAO has endorsed the adoption of an open emissions trading system to meet CO₂ emissions reduction objectives. Guidelines for the adoption and implementation of a global scheme are currently being developed, and will be presented to the ICAO Assembly in 2007,^[13] although the prospects of a comprehensive inter-governmental agreement on the adoption of such a scheme are uncertain. Emissions trading is an administrative approach used to reduce the cost of pollution control by providing economic incentives for achieving reductions in the emissions of pollutants. ...

Within the European Union, however, the European Commission has resolved to incorporate aviation in the European Union Emissions Trading Scheme (ETS).^[14] The Commissions plans to make legislative proposals by the end of 2006, with a possible view to incorporating aviation into Phase II of the ETS from 2008. However, a number of design and implementation issues are yet to be resolved ^[15] and some within the airline industry have expressed doubts about whether this deadline can be achieved. ^[16] The European Commission (formally the Commission of the European Communities) is the executive body of the European Union. ... The European Union Emission Trading Scheme (EU ETS) is the largest multi-national, greenhouse gas emissions trading scheme in the world. ...

References

1. ^ IPCC, Aviation and the Global Atmosphere: A Special Report of the Intergovernmental Panel on Climate Change (1999), Cambridge University Press
2. ^ [1] Sausen R et al, Aviation radiative forcing in 2000: an update on IPCC (2005) Meteorologische Zeitschrift, Vol. 14, No. 4
3. ^ Aviation and the Global Atmosphere: A Special Report of the Intergovernmental Panel on Climate Change (1999), Cambridge University Press
4. ^ Aviation and the Global Atmosphere: A Special Report of the Intergovernmental Panel on Climate Change (1999), Cambridge University Press
5. ^ [2] Sausen R et al, Aviation radiative forcing in 2000: an update on IPCC (2005) Meteorologische Zeitschrift, Vol. 14, No. 4
6. ^ European Commission, Questions & Answers on Aviation & Climate Change (2005)
7. ^ IPCC, Aviation and the Global Atmosphere: A Special Report of the Intergovernmental Panel on Climate Change (1999), Cambridge University Press
8. ^ The Future of Air Transport White Paper (2003), HMSO "The aviation industry is encouraged to take account of, and where appropriate reduce, its contribution to global warming...The impact of aviation on climate change is increased over that of direct CO₂ emissions alone by some of the other emissions released and their specific effects at altitude".
9. ^ IPCC, Aviation and the Global Atmosphere: A Special Report of the Intergovernmental Panel on Climate Change (1999), Cambridge University Press
10. ^ IATA/ATAG, Aviation & the Environment (1999) "Aircraft fuel efficiency has improved by some 50% over the past 30 years"
11. ^ Advisory Council for Aeronautical Research in Europe (ACARE) Strategic Research Agenda (2002) "These objectives include, inter alia, a 50% cut in CO2 and 80% in Nox emissions" [for new aircraft introduced in 2020 relative to new aircraft introduced in 2000].
12. ^ International Civil Aviation Organization Operational Opportunities to Minimize Fuel Use and Reduce Emissions (2001)
13. ^ ICAO news release 30 November 2005 "ICAO is also considering market-based options to address engine emissions through the participation of aviation in emissions trading schemes and the use of emissions levies related to local air quality. Guidelines for Contracting States wishing to implement such measures are being formulated and should be completed in time for the next regular Session of the ICAO Assembly in the Fall of 2007, when direction for future action will be set."
14. ^ European Commission, Reducing the Climate Change Impact of Aviation (2005)]
15. ^ C E Delft Giving Wings to Emission Trading Report for the European Commission, DG Environment No. ENV.C.2/ETU/2004/0074r (2005)
16. ^ Oral evidence to the UK House of Commons Environmental Audit Select Committe enquiry Reducing Carbon Emissions from Transport (2006) Dr Andrew Sentence, Chief Economist and Head of Environmental Affairs, British Airways "We have made a lot of progress in getting emissions trading on the agenda for aviation. It is now being actively supported by the European Commission which is developing a proposal which will be put forward later this year. While it may not be possible to hit the deadline of 2008 for the second phase of emissions trading in Europe, hopefully something will be in place in Europe not long after that."

External links

• Centre for Air Transport and the Environment
• International Civil Aviation Organization
• Advisory Council for Aeronautical Research in Europe (ACARE)
• Sustainable Aviation
• International Air Transport Association
• Air Transport Action Group
• Aviation Environment Federation
• AirportWatch
• GreenSkies Alliance
• UK climate policy and aviation