Issues and uncertainties affecting metrics for aviation impacts on climate

Metrics such as radiative forcing and global warming potential have proven to be useful tools in climate policy-related studies, including evaluation of the effects of aviation on climate, to relate different emissions to one another in order to maximize the application of mitigation policies and their benefits. In order to be an effective tool for policymakers and their communication with scientists and industry, a metric should be easy to use and as scientifically well grounded as possible. Thus, the best metrics will be simple and will include uncertainties that reflect the state of knowledge in order to give users confidence in their scientific quality. A concern with developing new metrics is the need to weigh their applicability against the ease of understanding the results. Radiative forcing is commonly used in analyses of aviation effects on climate and is integral to other metrics, but it has known deficiencies. Well-recognized metrics like global warming potential and global temperature potential are dependent on radiative forcing but also have their own advantages and recognized limitations. Simplified integrated assessment modeling may eventually represent a useful alternative to such metrics. The objective of this study is to examine the capabilities and limitations of current climate metrics in the context of the aviation impact on climate change, to analyze key uncertainties associated with these metrics and, to the extent possible, to make recommendations on future research and development of metrics to gauge aviation-induced climate change that could potentially affect decision making, including aircraft design and operations.