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It's not easy to estimate the impact of aviation on our climate today and even more difficult to determine what effect it will have in the future. Up until recently, the aviation industry had little impact on the climate system but, as it is an extremely fast growing energy consumer, it is assumed that air transport will be an important factor governing climate in the future.
Aviation today - high uncertainties
The data from the most recent IPCC report on aviation in 1999 is now rather outdated, since it refers to developments only up until 1995. However, this is our best guess at present. At the moment we are unsure what the trend in aeroplane use will be in the future. From 1993 to 2000, air passenger numbers increased by about 10% per year in the European Union. However, over the past few years, since the terrorist attack in New York on September 11th 2001, and as a result of the SARS virus and the Iraq war, the rate of increase in passenger numbers has slowed.
1. Airbus A320
Source: Ian Britten, FreeFoto.com
In 2002, the global revenue per passenger decreased by 4%, freight by about 8%. In parallel, Europe saw a rapid increase in low budget flights and increases in this market are predicted for the future. Over the next 20 years it is assumed that a 5% growth in passenger numbers will be seen each year globally.
2. Geographical distribution of fuel burned by civil aviation (May 1992).
The amount of fuel used is given for a grid area of 1 x l degree square.
Source: IPCC Report on Aviation 1999.
Many publications measure air traffic in revenue per passenger kilometre (RPK = number of passengers multiplied by the distance flown by the passengers per year). This number grew by 360% from 1970 (551 billion) to 1995 (2,537 billion). Estimates for the future vary.
For 2015 some predictions suggest a RPK value of 5,700 billion, for 2050 a likely range could be 14,000 to 23,000 billions (ICAO / EDF forecasts for medium economic growth). Assuming a world population of about 10 billion in 2050 this means that the average Earth’s citizen will travel between 1,400 and 2,300 kilometers per year by aeroplane.
Aviation makes up about 2% of all carbon dioxide emissions from man-made sources today. The contribution to the radiative forcing was estimated to be 3.5% in 1992. This is not a lot. But if the current RPK value multiplies over the next few decades, air traffic will become an increasingly important factor, contributing 10% or more of the human induced global warming in 2050.
3. Forecast of worldwide passenger aviation demand in 2015 and 2050 in RPK.
From 'The plane truth' (J. Whitelegg / N. Williams) based on IPCC data 1999.
4 a). The annual development of world scheduled passenger traffic and total traffic from 1992 to 2001 (2001 data is provisional).
Source: International Civil Aviation Organisation (Dec 2001).
4 b). A tonne-kilometre is a combined measure of passenger, freight and mail traffic which also takes into account distance flown.
The climate impact
Aeroplanes emit gases and particles directly into the upper troposphere and the lower stratosphere. They alter the concentration of atmospheric greenhouse gases, including carbon dioxide (CO2), ozone (O3) and methane (CH4). They also trigger the formation of condensation trails (contrails) and may increase cirrus cloudiness. All these factors contribute to climate change.
Gas phase processes
As with most other energy consuming processes, aeroplane engines consume fossil fuels and therefore produce CO2 (about 2% of all anthropogenic CO2). Moreover, jet engines also produce nitrogen oxides. These have two major impacts on the upper atmosphere: They form ozone and they destroy methane.
Since the life time of ozone is short, ozone formation is a temporary local process. In 1992 ozone levels were about 6% higher in flight corridors compared to regions without aviation. In 2050, ozone levels could be 12% higher. Methane depletion (about -2% in 1992 and predicted to be -5% in 2050) is spread more evenly across the globe. Both ozone and methane are greenhouse gases and, on a global scale, the opposing effects of ozone increase and methane loss on global warming nearly cancel each out. However, on a local scale, a warming by ozone formation in the flight corridors (which are mainly in the Northern hemisphere), overwhelms the global cooling caused by methane depletion.
5. Aviation has an opposite effect on the two greenhouse gases, ozone and methane, in the troposphere.
Source: Elmar Uherek.
Water vapour, contrails and cirrus clouds
Aeroplanes emit their exhaust gases into the cold region of the atmosphere, near the tropopause between the troposphere and the stratosphere. Since cold air cannot hold much water vapour, the water vapour emitted by the planes easily condenses to form a very narrow cloud known as a condensation trail (contrail). These are similar to ice clouds and can grow into cirrus clouds. These long narrow clouds can cover up to 5% of the sky in flight corridors over Europe and the USA. Globally, contrails are estimated to cover 0.1% of the sky and this value is predicted to increase to 0.5% by 2050. If the contrails grow into cirrus clouds they have a greenhouse effect - they let solar radiation into the Earth's atmosphere but absorb infra-red radiation coming from the Earth. Soot and sulphate emissions may allow condensation of extra cirrus cloud and add to this effect.
6. Contrails and cirrus clouds: Contrails (condensation trails) are formed from the condensation of water emitted by planes. Some studies show that cirrus cloud formation is favoured by the existence of contrails.
Photo: Bernhard Mühr, Karlsruher Wolkenatlas.
Level of understanding
The following diagram shows the different ways in which aviation contributes to radiative forcing (as a measure for global warming). The scientific understanding of these impacts ranges from poor to good depending on the impact. From the diagram it becomes obvious that estimates of the impact of air traffic on climate still have a high degree of uncertainty and that predictions of the future impact of aviation on our climate are, therefore, also very unsure.
7 a). Estimates of the globally and annually averaged radiative forcing (Wm-2) from subsonic aircraft emissions in 1992. Positive radiative forcing is a measure for the contribution to global warming, negative forcing contributes to cooling.
Source: IPCC Report on Aviation 1999.
7 b). Estimates of the globally and annually averaged radiative forcing (Wm-2) from subsonic aircraft emissions for the year 2050. Estimations are based on the moderate IPCC growth scenario Fa1, assuming traffic growth of 3.1% per year and the growth rate of burned fuel to be 1.7% per year.
Source: IPCC Report on Aviation 1999.
Supersonic air transport
At higher altitudes, emissions of nitrogen oxides lead to decreases in the stratospheric ozone layer. This is one reason why supersonic passenger air transport has never really developed. Emissions into the stratosphere spread around the globe and we don't know the likely consequences of this on the atmospheric system and the ozone layer. Concorde, which had her maiden flight in 1969 and flew at an altitude of 18 km in the stratosphere, was the only regularly used commercial supersonic passenger jet. It had its last flight in 2003.
8. Concorde - the only ever commercially used supersonic passenger aircraft.
Source: BBC news.
About this page:
author: Dr. Elmar Uherek - Max Planck Institute for Chemistry, Mainz, Germany
scientific reviewer: Dr. Didier Hauglustaine, LSCE Gif-sur-Yvette, France - 2004-02-18
educational proofreading: Michael Seesing - Uni Duisburg, Germany - 2003-08-07
last published: 2004-04-20