Variability of contrail formation conditions and the implications for policies to reduce the climate impacts of aviation

The contribution of air travel to climate change is significant and growing, but emissions and their effects are not yet regulated. One of the major impacts on climate from the aviation sector is the production of contrails (vapour trails) in the atmosphere and their influence on cirrus cloud formation. Potentially, reducing cruise altitude represents one option for controlling the growing climate impact of aviation. In general, this would reduce contrail and cirrus cloud formation but there are associated penalties, including an increase in the rate of fuel consumption and hence in the rate of carbon dioxide emission. Constraining cruise altitudes also raises operational issues, including increases in airspace congestion and in journey time. Atmospheric variability can change the amount of contrail and contrail-cirrus, so contrails may sometimes be more likely to form at lower altitudes. In these cases, reducing cruise altitude could increase rather than reduce the contrail amount. This paper describes an approach to optimise the balance between the benefits of contrail reduction and the penalties incurred for altitude restriction. The calculations use an air traffic sample for western Europe, with NCEP-II reanalysis data for atmospheric temperature and humidity. A maximum cruise altitude is selected for each six-hour period, according to atmospheric conditions. This altitude provides the greatest reduction in contrail for the lowest increase in carbon dioxide emission. This avoids the contrail and carbon dioxide increases associated with ineffective or counter-productive altitude restrictions. Calculated contrail reductions are presented, along with the associated increases in carbon dioxide. These values compare favourably with previous policy designs based on altitude restrictions fixed on a monthly basis. In addition, potential operational issues associated with a varying altitude restriction policy are discussed. Introduction The consumption of aviation fuel continues to increase, despite efficiency gains from improvements in engine and airframe design. Continued growth in air travel means aviation fuel accounts for a growing share of global fossil fuel use and of anthropogenic CO2 emissions. These emissions represent only one of the mechanisms through which aviation can influence the global climate. At high altitudes, in appropriate atmospheric conditions, the expanding aircraft exhaust triggers the formation of a contrail, which may persist for several hours. These contrails can spread to form extensive cirrus clouds. During the near-complete shutdown of air traffic over the United States following September 11 2001, the contrails from just 6 military aircraft spread to form cirrus cloud coverage over 20,000km [1]. The combination of linear contrail and long lasting contrail spreading to form cirrus cloud are considered to make a substantial contribution to the radiative forcing of climate by aviation. The Intergovernmental Panel on Climate Change (IPCC) special report on aviation estimated the radiative impact of linear contrail to be similar in magnitude to that of CO2 emitted by aircraft, and identified considerable uncertainties in assessing the impact of aviation-induced cirrus clouds [2]. Recent research has improved the understanding of the radiative impacts of both contrail and cirrus cloud, identifying a smaller contribution from linear contrail than estimated by IPCC, but a larger impact from aged contrail spreading to form extensive cirrus cloud. One study suggests this to be at least 10 times that of the CO2 from aviation [3]. In addition to CO2 emissions and the formation of contrail, the formation of NOx in the combustion process also has significant radiative impacts by increasing ozone and reducing methane. Other minor contributors include increases in sulphate aerosol. The total impact of aviation on the global climate arises from the net combination of these impacts, taking into account differences in their spatial and temporal characteristics.