Aviation emissions and climate impacts
Introduction
Air traffic emissions contribute to climate warming in a very complex way. The emitted species alter the atmospheric composition and cloudiness, lead to changes in Earths radiation budget and finally contribute to human-made climate change. This chain of physical processes depends on a variety of meteorological parameters and includes a wide range of time-scales. For example, the climate impact of a today’s carbon dioxide emission prevails for centuries, while contrails may exist for hours only. Here, a short overview is given, showing how emitted carbon dioxide affects the atmosphere, what kind of other, non-CO, effects are caused by aviation, and how these impacts can be assessed by using so-called climate metrics. Finally, examples are given to show how a reduction of air traffics climate impacts might be achieved.
Aviation CO2: from the emission to global warming
The most prominent anthropogenic greenhouse gas is carbon dioxide. It originates from kerosene, i.e. fossil fuel, combustion. Air traffic CO, emissions contribute to the total anthropogenic CO, emissions by about 2.5% (Figure 2.1). Aviation emissions have grown since the middle of the last century, when civil aviation became more important. Major international crises have reduced the growth in emissions. However, air traffic and its emissions have grown over-proportionally after these crises (Figure 2.1).
The carbon dioxide emissions lead to an increase in the atmospheric CO, concentration. This CO, enhancement remains in the atmosphere and decays only slowly. The decay is controlled by several physical processes, such as the oceanic CO, uptake, and hence can only be insufficiently described by a single lifetime; although, for illustration purposes, a lifetime for this perturbation of 80 to 100 years provides a reasonable illustration of the long-term effect of CO, emissions on the CO, concentration.
The relation between an emission of CO, and the corresponding impact on the near surface temperature originates from a sequence of effects which is illustrated exemplarily in Figure 2.2. It shows, in a thought experiment, the effect of an imagined new fleet, which would go into service in 2015.The fleet
-----Anthropogenic CO2 -----Aviation CO2 (x10)
..........Aviation CO2 fraction
Figure 2.1 Growth in CO, emissions in Tg CO, yr-1 (left-hand axis) for all anthropogenic activities (top solid line) and from aviation fuel burn (lower solid line), and the fraction of aviation CO, emissions (%, right hand axis). Note that the aviation CO, emissions are depicted ten-fold.
Source: Figure adapted from Lee et al. (2009).
Figure 2.2 CO, emissions and global warming (thought experiment). Thought experiment showing the relation between CO, emissions and resulting global warming. Non-dimensional temporal development of an imagined fleet scenario to illustrate the principle relation between CO, emissions (long dashed), CO, concentration change and radiative forcing (short dashed) and induced temperature change (dotted). Entry into service is 2015, a lower production rate from 2025 to 2050 and a full stop of service in 2050. For all data, the maximum value is normalised to 1.
Source: Figure adapted from Grewe and Stenke (2008).
size is pictured to increase until 2050 with a change in the amount of produced aircraft in 2025. In 2050 the service of all aircraft of this type is shut down. This leads to emissions from the fleet, which follow the fleet size, peaking in 2050 and being zero afterwards. The CO, concentration change, and equally the impact on radiation, induced by these emissions, also peak in 2050, but the curve is much smoother and concentration changes prevail long after the emissions have stopped.The change in radiation (RF) affects the atmosphere-ocean system and leads to an increase in atmospheric temperatures, which, due to the considerable inertia of the system, peaks around 2080 — long after the emissions have stopped. An impact on climate still exists decades after the last emission happened.
