Evaluation of non-CO2 impacts from widespread use of aviation biofuels
Global air traffic demand is projected to grow at 5 %/yr, leading to strongly increasing emissions. While aviation drives economic and social progress, its contribution to global emission reduction targets is now considered as equally important. Because drop-in biofuels have the potential to offset lifecycle CO2 emissions and would, because of their jet-fuel-like characteristics, not require current aircraft technology to change, their exploitation as a substitute or extender for mineral jet fuel is now considered as a key element in moving towards a low-carbon economy in air transport. Particularly also motivated by the EU Emissions Trading Scheme that will include aviation from 2012 onwards, the aviation industry is now claiming for a priority access to biofuel markets.
The drawback in using biofuels compared to airframe and engine technology improvements is their limitation in mainly reducing CO2. In this study, the impact of global aviation on the climate to 2050 is quantified by considering the whole suite of emissions such as nitrogen oxides and water vapour that play a significant role if emitted at higher altitudes. A linear climate model is used to quantify the perturbations in non-CO2 emissions at different cruise altitudes. Input parameters to the model are fuel burned, NOx emitted and distance flown of global aviation at 16 cruise altitude bands. Secondly, the traditional system boundary is expanded to account for indirect emissions from aviation biofuel and fuel production. In a final step, different climate metrics are used to calculate the climate impact from aircraft location and emissions data.
The biofuel CO2 lifecycle reduction potential is assumed to be 85 % (lignocellulosic BTL). The subsequent simulated biofuel uptake under an idealised global cap and trade system leads into a carbon neutral growth, which is usually recognized as a strong indicator for sustainable development. It is however shown that (i) a ‘sustained carbon neutral growth’ is very difficult to achieve if not perceived as impractical and (ii) carbon neutral growth
must not be confused with climate neutral growth.
The net climate benefit of all simulated years between 2005 and 2050 with a total biofuel proliferation of 29.8 % of fuel used and a total CO2 reduction of 25.4 % is found to be 6.1 % for a 20 years time horizon (TH) and 22.4 % for a 500 years TH, using ‘annual pulsed GWPs’ as the climate metric and the reference scenario with 100 % mineral jet fuel as a basis for comparison. The reduction in climate impact could eventually become negligible for THs lower than 20 years. The perceived net climate benefit from widespread use of aviation biofuels is therefore (i) lower than the CO2 benefit (in terms of percentage reductions) and (ii) highly subject to the chosen TH, which illustrates the difficulties and complexity of comparing CO2 and non-CO2 emissions on a unified scale. In evaluating the viability of biofuels in aviation, policymakers must therefore decide whether or not emission reductions in the near term are considered as more important than emission reductions in the long term.