The potential for reducing the impact of aviation on climate

Abstract The paper reviews the technical options available for reducing the impact of air travel on the Earth's climate. The three main impacts come from the emission of CO2 and NOx and the creation of contrails and cirrus cloud. The long-term potential for reducing CO2 emission is considerable, though bounded by the laws of physics. There is still greater potential for reducing NOx emission and contrail formation, although a measure to reduce any one emission can in some cases increase one or both of the others. Ideally, a balance has to be struck which minimises the total climate impact of all three. There is not yet a robust scientific basis for judging this balance. Even so, the paper outlines the technical possibilities for reducing climate impact and offers a conjectural estimate of their potential effect, when deployed in combination, on the future emissions of the world civil aircraft fleet. Keywords: environmentclimate-changeaircraft emissionsfuel-burn Notes Other emissions with significant impact on climate include black carbon and sulphate aerosols, the effects of which are both relatively small and of similar magnitude but opposite sign, and water vapour which, at the cruise altitudes of current subsonic jets, has about half the effect of either of the aerosols (Penner et al. Citation1999). Until we can reduce the impacts of CO2, NOx and contrails substantially, addressing these other emissions has low priority. The fuel burn figures in AERO2k are based on ‘gate-to-gate’ calculations which incorporate a realistic assessment of fuel burned in the LTO cycle and in climb and descent for each aircraft movement. As noted above, in the text below Equationequation (1), the formula includes an empirical constant 0.022 which allows for fuel burned in taxiing, manoeuvring and climbing to cruise at Mach 0.85 at 35,000 ft. It is this additional fuel burn that increases the importance of the climb to altitude relative to the cruise phase for shorter ranges, causing the outer envelope in Figure 1 to have a maximum at 4000 km and a value of zero at zero range. The effect of halving this constant – an unrealistically large reduction – is to increase the maximum payload-fuel efficiency by 10% and reduce the design range at which it occurs to 3000 km. NACRE – New Aircraft Concepts Research. SESAR – Single European Sky ATM Research. NEWAC – New Aero Engine Concept. The Sustained Global Temperature Change Potential is a recently proposed metric related to Global Warming Potential (GWP). It is discussed more fully in Egelhofer, Bickerstaff and Bonnet Citation(2007) and in their references. Like GWP, it integrates the effect of the emission of a particular greenhouse gas over a defined time horizon (100 years in the case of Figure 12), taking account of the decay with time of the concentration of the gas, but estimates actual temperature change of the atmosphere rather than the energy transferred into it. GTP can be calculated for pulse type emissions (PGTP), in which a specified amount of a greenhouse gas is introduced into the atmosphere at a given moment, or sustained emissions (SGTP, as in Figure 12) in which the gas is introduced into the atmosphere at a steady rate over the chosen time horizon.