8.4 Climate impact from aviation
In general NTM focus on energy use and emissions from transportation. This far has different environmental effects been excluded as they add new complexity and a need for other scientifically pillars than NTM possess. The effects on the recipient changes with regard to time, geography and altitude in a very complex intertwined system.
Aircrafts are a unique emission source since much of the release is made in the upper part of the troposphere and covers areas on a global scale. When released in to the atmosphere emitted compounds react with the surrounding. It creates new, and modifies existing chemical and microphysical processes in the atmosphere. The emissions alter the radiative forcing and thus have an impact on the climate.
Aircrafts emissions results from the combustion of kerosene and air inside the engine. Kerosene contains carbon, hydrogen and sulphur. In the combustion process it mixes with air and for an ideal jet engine it would release carbon dioxide CO2, water vapour H2O nitrogen N2, Oxygen O2 and sulphur dioxide SO2. In real life, the combustion process is not perfect so there are additional emissions of volatile fuel residuals and particulate matter such as carbon monoxide CO, Nitrogen Oxides NOx, hydrocarbons HC, soot (BC Black Carbon). The release of CO2, H2O and SO2 are direct proportional to the fuel consumption. The release of CO, HC and NOx also depends on type of engine and its exact operating environment (i.e. throttle settings, phase of flight, quality of the fuel and how well the engine is maintained.
How the different compounds impact the environment differs depending on where they are released. CO2 has global climate effects independent on where it is released. The environmental impact from NOx varies due to location of release. Close to an airfield NOx increase acidification and as many of the other volatile fuel residuals and aerosols released from engines close to the ground, they have a negative impact on health. When released at cruise level NOx has an impact on the global climate. However, its impact on the radiative forcing can be both positive (ozone production) and negative (methane reduction) depending on different chemical reaction triggered by NOx. The net contribution from NOx emissions to the radiative forcing is positive.
During certain atmospheric conditions, water vapour released from aircrafts can generate contrails. A contrail is a cirrus cloud and when it persists on the sky it will, like all clouds have an effect on both the incoming short-wave radiation as well as the outgoing long wave radiation.
The emitted sulphate aerosols and the methane reduction caused by the nitrogen oxides are the only processes having a negative (cooling) impact on the radiative forcing. All the other aircraft induced emissions CO2, water vapour H20 soot, and contrails has a positive (warming) impact on the radiative forcing and the total net contribution is on the order of 0,05 W/m2. This excludes aviation induced cirrus clouds. The impact from aviation with the knowledge we have today is presented in the figure 1 below.
Figure 1. The climate impact from aviation (Lee, D.S., et al., Aviation and global climate change in the 21st century, Atmospheric Environment (2009).
Given the need for credibility and consensus as well as the NTM emission focus mentioned above and the uncertainty with respect to these climate effects, NTM does not give any recommendation in this issue for the time being.