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Aviation and climate change

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.

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 NOx), water vapour and particulates (soot and sulfate particles). Other emissions include sulfur oxides, carbon monoxide, hydrocarbons and radicals such as hydroxyl.

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.


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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 (NOx) induced effects

At altitude, emissions of NOx are particularly effective in forming ozone (O3) in the upper troposphere. High altitude (8-13km) NOx emissions result in greater concentrations of O3 than surface NOx 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).

NOx 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 NOx emissions.

It is now believed that aircracft sulfur and water emissions in the stratosphere tend to deplete O3, partially offsetting the NOx-induced O3 increases. These effects have not been quantified.[4]

Water vapour induced effects

Aviation and climate change:Contrails
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Contrails
Aviation and climate change:Cirrus cloud formation
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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 NOx induced effects).

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 NOx 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]

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 NOx 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.

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.

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]

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 radiativeforcing 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."


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Climate change | Ecology | Climate change feedbacks and causes | Aviation

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