Evidence from previous climate model simulations has suggested a potentially low efficacy of contrails to force global mean surface temperature changes. In this paper, a climate model with a state-of-the-art contrail cirrus representation is used for fixed sea surface temperature simulations in order to determine the effective radiative forcing (ERF) from contrail cirrus. ERF is expected to be a good metric for intercomparing the quantitative importance of different contributions to surface temperature and climate impact. Substantial upscaling of aviation density is necessary to ensure statistically significant results from our simulations. The contrail cirrus ERF is found to be less than 50% of the respective instantaneous or stratosphere adjusted radiative forcings, with a best estimate of roughly 35%. The reduction of ERF is much more substantial for contrail cirrus than it is for a CO2 increase when both stratosphere adjusted forcings are of similar magnitude. Analysis of all rapid radiative adjustments contributing to the ERF indicates that the reduction is mainly induced by a compensating effect of natural clouds that provide a negative feedback. Compared to the CO2 reference case, a less positive combined water vapor and lapse rate adjustment also contributes to a more distinct reduction of contrail cirrus ERF, but not as much as the natural cloud adjustment. Based on the experience gained in this paper, respective contrail cirrus simulations with interactive ocean will be performed as the next step toward establishing contrail cirrus efficacy. ERF results of contrail cirrus from other climate models equipped with suitable parameterizations are regarded as highly desirable.
Contrail cirrus has been emphasized as the largest individual component of aircraft climate impact, yet respective assessments have been based mainly on conventional radiative forcing calculations. As demonstrated in previous research work, individual impact components can have different efficacies, i.e., their effectiveness to induce surface temperature changes may vary. Effective radiative forcing (ERF) has been proposed as a superior metric to compare individual impact contributions, as it may, to a considerable extent, include the effect of efficacy differences. Recent climate model simulations have provided a first estimate of contrail cirrus ERF, which turns out to be much smaller, by about 65%, than the conventional radiative forcing of contrail cirrus. The main reason for the reduction is that natural clouds exhibit a substantially lower radiative impact in the presence of contrail cirrus. Hence, the new result suggests a smaller role of contrail cirrus in the context of aviation climate impact (including proposed mitigation measures) than assumed so far. However, any conclusion in this respect should be drawn carefully as long as no direct simulations of the surface temperature response to contrail cirrus are available. Such simulations are needed in order to confirm the power of ERF for assessing contrail cirrus efficacy.
<p lang="de-DE">Auch in aktuellen Bewertungsstudien wird angenommen, dass Kondensstreifen-Zirren den gr&#246;&#223;ten Beitrag zur Klimawirkung des Luftverkehrs liefern. Bisher wurde die Klimawirkung von Kondensstreifen allerdings fast ausschlie&#223;lich anhand von Strahlungsantrieben bewertet. Dabei konnte bereits gezeigt werden, dass der seit einigen Jahren als Bewertungsmetrik empfohlene &#8222;Effektive Strahlungsantrieb&#8220; in diesem Fall erheblich kleiner ausf&#228;llt als bisher verwendete klassische Strahlungsantriebe. Die zu erwartende Temperatur&#228;nderung am Boden sollte demnach ebenfalls deutlich schw&#228;cher ausgepr&#228;gt sein als bisher angenommen.</p> <p lang="de-DE">Hier pr&#228;sentieren wir Ergebnisse von globalen Klimamodellsimulationen mit gekoppeltem Deckschichtozean zur Berechnung der tats&#228;chlichen Bodentemperatur&#228;nderung aufgrund von Kondensstreifen-Zirren. Neben der Klimasensitivit&#228;t wurde damit auch erstmalig die Klimawirkungseffizienz von Kondensstreifen-Zirren bestimmt. Insgesamt f&#228;llt die Bodenerw&#228;rmung durch Kondensstreifen-Zirren erheblich kleiner aus als f&#252;r ein CO<sub>2</sub> Erh&#246;hungsexperiment mit vergleichbar gro&#223;em klassischen Strahlungsantrieb. Die Klimawirkungseffizienz, basierend auf dem Effektiven Strahlungsantrieb, betr&#228;gt nur ca. ein Viertel des erwarteten Wertes. Somit wird die durch den Effektiven Strahlungsantrieb nahegelegte reduzierte Wirkung von Kondensstreifen-Zirren auf die bodennahe Temperatur sogar noch unterboten.</p> <p lang="de-DE">Zur Bestimmung der physikalischen Ursachen der verringerten Temperaturwirksamkeit wurden sowohl die schnellen als auch die langsamen Strahlungsr&#252;ckkopplungen mit Hilfe einer R&#252;ckkopplungsanalyse nach dem &#8222;partial radiative perturbation&#8220; Verfahren bestimmt. In beiden F&#228;llen war die reduzierte Klimasensitivit&#228;t der Kondensstreifen-Zirren vor allem auf eine negative Wolkenr&#252;ckkopplung, bedingt durch die Reduktion von nat&#252;rlicher Zirrusbew&#246;lkung, zur&#252;ckzuf&#252;hren.</p>
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