Aviation is an integral part of the global transportation network, and the number of flights worldwide is expected to grow rapidly in the coming decades. Yet, the effects that subsonic aircraft emissions may be having upon atmospheric composition and climate are not fully understood. To study such issues, NASA sponsors the Atmospheric Effects of Aviation Program (AEAP). The NRC Panel on Atmospheric Effects of Aviation is charged to evaluate AEAP, and in this report, the panel is focusing on the subsonic assessment (SASS) component of the program. This evaluation of SASS/AEAP was based on the report Atmospheric Effects of Subsonic Aircraft: Interim Assessment Report of the Advanced Sub-sonic Technology Program, on a strategic plan developed by SASS managers, and on other relevant documents.

The NRC Panel on the Atmospheric Effects of Aviation (PAEAN) was established to provide guidance to NASA's Atmospheric Effects of Aviation Program (AEAP) by evaluating the appropriateness of the program's research plan, appraising the project-sponsored results relative to the current state of scientific knowledge, identifying key scientific uncertainties, and suggesting research activities likely to reduce those uncertainties. Over the last few years, the panel has written periodic reviews of both the subsonic aviation (Subsonic Assessment-SASS) and the supersonic aviation (Atmospheric Effects of Stratospheric Aircraft-AESA) components of AEAP, including: An Interim Review of the Subsonic Assessment Project (1997); An Interim Assessment of AEAP's Emissions Characterization and Near-Field Interactions Elements (1997); An Interim Review of the AESA Project: Science and Progress (1998); Atmospheric Effects of Aviation: A Review of NASA's Subsonic Assessment Project (1998). This report constitutes the final review of AESA and will be the last report written by this panel. The primary audience for these reports is the program managers and scientists affiliated with AEAP, although in some cases the topics discussed are of interest to a wider audience.

Scientists and policy-makers alike are concerned that operation of a fleet of high-speed civil transport (HSCT) aircraft could significantly affect the global atmosphere. HSCT emissions may have a direct effect on the chemistry of the atmosphere, leading to changes in the distribution of ozone; they may also have indirect effects on ozone and on global climate through coupling with radiative and dynamical processes in the atmosphere. An assessment of the atmospheric impact of a fleet of HSCTs thus requires not only an understanding of the chemistry of the natural stratosphere and its possible perturbations by HSCT emissions, but also an understanding of the pathways for transport of HSCT emissions within the atmosphere, and the resulting temporal and spatial distribution of HSCT emissions. The results of NASA's Atmospheric Effects of Stratospheric Aircraft (AESA) project were summarized in a 1995 NASA assessment. The present report looks at that summary and at more recent work to evaluate the state of the science. AESA has made good progress in the past few years. Satellite and aircraft observations have elucidated important aspects of large-scale transport processes. Field campaigns have provided a much better picture of the relative importance, below 20 km altitude, of the major catalytic cycles for ozone destruction. Careful intercomparisons of assessment models have led to reduction of some of the differences among the models. However, a number of uncertainties and inconsistencies still remain.

This Intergovernmental Panel on Climate Change Special Report is the most comprehensive assessment available on the effects of aviation on the global atmosphere. The report considers all the gases and particles emitted by aircraft that modify the chemical properties of the atmosphere, leading to changes in radiative properties and climate change, and modification of the ozone layer, leading to changes in ultraviolet radiation reaching the Earth. This volume provides accurate, unbiased, policy-relevant information and is designed to serve the aviation industry and the expert and policymaking communities.