By Emma Irvine
“How can we fly without damaging the environment?” The 2014 Longitude Prize acknowledges environmentally-friendly flight as being one of the great challenges of our time. The dream of zero-carbon flight is highly ambitious and, Longitude Prize or not, a long way from becoming a reality. There are, however, many international efforts to make flying more environmentally-friendly, which range from designing new more efficient aircraft and engines to advanced air traffic control procedures. Here at Reading, our research has been focusing on better understanding some of the aviation climate impacts, and assessing potential mitigation strategies, such as environmentally-friendly aircraft routing.
Burning fuel produces carbon dioxide emissions in large quantities; the global fleet emitted 0.63 gigatonnes in 2005, which was around 2% of the total global CO2 emissions that year1 and this proportion seems almost certain to grow in coming decades. The aviation sector is unique in that the majority of its emissions are high in the atmosphere, between 9-12 km. Along with carbon dioxide, aircraft exhausts emit large quantities of water vapour, other gases which have an effect on the amount of ozone and methane (two other powerful greenhouse gases). Aircraft flying through very cold and moist air form familiar line-shaped ice clouds called contrails. In Reading, we get a lot of air traffic overflying us at high altitude and an otherwise clear sky can be filled with criss-crossing lines of contrails. If the contrails are long-lasting, they can spread out and lose their distinctive line-shape, so that they become indistinguishable from the naturally-forming wispy cirrus clouds.
Contrails are man-made clouds which add to the natural cloud cover. Like natural clouds, contrails reflect some of the incoming Sun’s energy back to space (which is a cooling effect), but they also trap some of the infrared energy that is being radiated out to space from the Earth (which is a warming effect – you might notice that cold nights are associated with clear skies when there are no clouds to trap some of the heat). Detailed calculations indicate that generally the warming effect wins over the cooling effect, so this additional man-made cloud cover is contributing to a warming of climate. Estimates of how much warming long-lived contrails contribute is still uncertain, but the warming could be roughly the same size as that resulting from aviation CO2 emissions.
Contrails only form when aircraft fly through very cold air, and last only a few seconds unless the air is also very moist (otherwise the ice crystals making up the contrail just evaporate into the much drier air around it). The technical term for these regions is ‘ice-supersaturated’. These regions tend to be fairly shallow (around one kilometre deep) and are often found in ascending air around the edges of high pressure systems2. Recent research at Reading has focused on better understanding these regions and identifying where they typically occur. If we can predict where these regions will be, it opens the door to the possibility of mitigating their effect by routing aircraft to avoid them.
1 Lee et al., 2011, Chapter 4: International Emissions, UNEP ‘Bridging the emissions gap’, http://www.unep.org/pdf/unep_bridging_gap.pdf
2 Irvine, E. A., B. J. Hoskins, K. P. Shine, 2012: The dependence of contrail formation on the weather pattern and altitude in the North Atlantic, Geophys. Res. Lett., 39, L12802