Aviation and the Global Atmosphere

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7. Issues for the Future

This report has assessed the potential climate and ozone changes due to aircraft to the year 2050 under different scenarios. It recognizes that the effects of some types of aircraft emissions are well understood. It also reveals that the effects of others are not, because of the many scientific uncertainties. There has been a steady improvement in characterizing the potential impacts of human activities, including the effects of aviation on the global atmosphere. The report has also examined technological advances, infrastructure improvements, and regulatory or market-based measures to reduce aviation emissions. Further work is required to reduce scientific and other uncertainties, to understand better the options for reducing emissions, to better inform decisionmakers, and to improve the understanding of the social and economic issues associated with the demand for air transport.

There are a number of key areas of scientific uncertainty that limit our ability to project aviation impacts on climate and ozone:

• The influence of contrails and aerosols on cirrus clouds
• The role of NOx in changing ozone and methane concentrations
• The ability of aerosols to alter chemical processes
• The transport of atmospheric gases and particles in the upper troposphere/lower stratosphere
• The climate response to regional forcings and stratospheric perturbations.

There are a number of key socio-economic and technological issues that need greater definition, including inter alia the following:

• Characterization of demand for commercial aviation services, including airport and airway infrastructure constraints and associated technological change
• Methods to assess external costs and the environmental benefits of regulatory and market-based options
• Assessment of the macroeconomic effects of emission reductions in the aviation industry that might result from mitigation measures
• Technological capabilities and operational practices to reduce emissions leading to the formation of contrails and increased cloudiness
• The understanding of the economic and environmental effects of meeting potential stabilization scenarios (for atmospheric concentrations of greenhouse gases), including measures to reduce emissions from aviation and also including such issues as the relative environmental impacts of different transportation modes.