A physical metric-based framework for evaluating the climate trade-off between CO2 and contrails-The case of lowering aircraft flight trajectories

Aviation climate impact is composed of short- and long-lived climate effects. When evaluating mitigation options, policy design requires to consider trade-offs apparent under different metric options. We present an evaluation framework to demonstrate the impact of individual physical metric choices on the preferred mitigation strategy, using the example of a generic trade-off situation in aviation. The concept of a turning point is established, which indicates the point in time where the mitigation of a short-term effect (e.g. line-shaped contrails) at the expense of a counteracting long-term effect (e.g. carbon dioxide) becomes preferable. In our generic situation, some physical metrics (e.g. average metrics, physical cost-benefit metrics) are better suited than others to trade off short- and long-lived climate effects for obtaining a robust policy recommendation. The preferred mitigation strategy depends particularly on the evaluation horizon, over which climate impacts are to be minimised (cost-benefit approach) and the selected aviation emission type (pulse, sustained, scenario). At any stage, value judgement must guide the required policy decision on metric options. Including contrail cirrus in the assessment, however, has the potential to dominate the policy recommendation in favour of contrail mitigation so that normative decisions become secondary. (C) 2012 Elsevier Ltd. All rights reserved.