Role of climate metrics in aviation climate assessments

Quantitative assessments of CO₂ and non-CO₂ climate effects of aviation emissions require the choice of a physical climate metric. In consequence estimating mitigation potentials or integrating non-CO₂ effects in legislation or cost-benefit analyses require the choice of a metric. However, since various metrics are currently in use and estimates and numbers vary over different climate metrics, it is desirable and necessary to have conversion factors available which allow to convert from one physical climate metric to another. Hence, we introduce an approach how such climate metric conversion factors can be calculated and present an initial set based on the climate response model AirClim. These conversion factors can be used for various applications. These include, for example, converting results from models that only calculate radiative forcing into a climate metric and scaling model results calculated with different metrics to the same one for one-to-one comparisons.

In addition, the conversion factors can be used for convenient analyses of the influence of metric choice on the results of a climate assessment. To this end, it is shown here how the ratio of non-CO₂ to CO₂ differs depending on the choice of metric. The metrics GWP, EGWP, GTP and ATR are analysed, each with a time horizon of 20, 50, 100 and 500 years.  The choice of the temporal emission curve is also analysed and it is shown exemplarily why a sequence of pulse emissions does not provide the same climate metric result as constant emissions.