# The Water-Enhanced Turbofan as Enabler for Climate-Neutral Aviation
## 4 Engine Performance and Climate Impact Evaluation Methods
### Climate Impact Methodology
#### 4.5.4 Climate Metric  
A typical metric to quantify the impact of emissions is the radiative forcing (RF). However, it is a backward-looking metric that is often used to evaluate the impact of the entire aviation up to a certain date, e.g., in [1]. Another well-established metric that builds upon the RF is the absolute pulse global warming potential (\(APGWP\)), defined as \(APGWP_{H}=\int_{H}RF(t)dt\). It considers the RF of a pulse emission over time, such that the lifetime of the emitted species has an influence. The timeframe \(H\) is arbitrary and has a significant impact on the relative impact of different emissions. A typical timeframe--also chosen in this paper--is 100 years.  
Similarly to the first estimation of the WET concept's climate impact potential in [12], the approach of the Low Emission Effect Aircraft (LEEA) project [50; 51] has been used for this study. The LEEA method uses sensitivity studies, based on a global emission scenario, to determine the change of RF in terms of dependence on emissions and flowin kilometers. The location of emissions is discretized into 16 altitude levels, between Flight Level (FL) 165 and FL 485. No lateral or longitudinal differences were considered. Using a background scenario from flights in 2002, the emissions and flowin kilometers were increased (perturbation) in each of the 16 levels to derive a response surface [52].  
With a radiative transfer model and two global chemistry transport models, the effect of NOx emission perturbations on the atmospheric chemistry was investigated in [50]. A linear relation between the atmospheric response and a small change in emissions was found for NOx, given that the geographical distribution of the emissions was not altered significantly [50].  
A similar approach was followed to assess the influence of contrails, as presented in [51] and described here briefly. To determine the number of flights per altitude level in ice supersaturated regions, meteorological data were merged with the same background  
\begin{table}
\begin{tabular}{c c} \hline
**Parameter** & **Change \(EI_{NOx}\) vs. TF2030+** \\ \hline \(T_{37}\) & \(-45\%\) \\ \(p_{37}\) & \(-16\%\) \\ \(WAR\) & \(-84\%\), \(-92\%\), \(-95\%\) * \\ \hline \end{tabular}
\end{table}
Table 3: Change in \(EI_{NO_{x}}\) in WET concept vs. TF2030+ separated by each influence parameter.  
scenario. At each level, the contrail coverage was assumed to be proportional to the distance flown, multiplied by the probability of ice-supersaturated conditions. By using a normalization factor on the result, the contrail coverage was matched for an area where satellite measurements are available. This normalization factor was used globally afterwards. A linear relationship between the increase in traffic and RF, depending on the altitude, was found. With a simple multiplier, the influence of contrail-induced cirrus clouds was taken into account.  
For CO\({}_{2}\) emissions, due to their long lifetime, the RF per kg was assumed to be constant, independent of the altitude [52]. The \(APGWP_{100}\) was then calculated with the same approach as described in [12], using the formulas in [53], with the equilibrium RF values from [50; 51] and the parameters in [52]. As in [12], the impact of contrail cirrus was included by multiplying line-shaped contrail estimations by five.