Climate impact of contrail cirrus

Air traffic has been growing exponentially over the last decades and thus plays an important role for future climate mitigation strategies. Contrail cirrus is regarded to be the largest contributor to aviation induced climate impact on the basis of radiative forcing estimates, exceeding aviation's CO2 emissions. While previous studies revealed a reduced efficacy of linear contrails in forcing the global mean surface temperature, that has not yet been analyzed for the far more important contrail cirrus. In the context of the present thesis, various climate model simulations were performed in order to gain a better understanding about the climate impact of contrail cirrus. The simulations were conducted with the ECHAM5 and EMAC climate models, equipped with a state-of-the-art contrail cirrus parameterization, and were further analyzed by feedback analysis after the partial radiative perturbation method in order to determine the rapid radiative adjustments and slow feedbacks. First, the climate impact of contrail cirrus was assessed on the basis of radiative forcings, determined by fixed sea surface temperature simulations. The effective radiative forcing of contrail cirrus turned out to be largely reduced with respect to the conventional radiative forcings. In contrast, the effective radiative forcing of a CO2 increase simulation, with a similarly sized conventional radiative forcing, deviates less strongly. If directly compared to those CO2 reference experiments, the contrail cirrus effective radiative forcing is reduced by 45 % (58 %) for the EMAC (ECHAM5) model, which already indicates a reduced climate impact of contrail cirrus. The simulations were further analyzed by feedback analysis to determine a full set of consistent rapid radiative adjustments. A negative natural cloud adjustment due to a reduction of natural cirrus cover was found to be the main origin for the reduced effective radiative forcing of contrail cirrus. To determine the actual climate impact, the global mean surface temperature change, induced by contrail cirrus, was directly simulated with the EMAC model coupled to an interactive ocean. For the first time, the climate sensitivity and efficacy parameters of contrail cirrus could be determined. Overall, the surface temperature change due to contrail cirrus turns out to be considerably smaller than for a CO2 increase simulation with a similarly sized conventional radiative forcing. The efficacy parameter, based on the effective radiative forcing, is 0.38, deviating significantly from the theoretically expected value of 1. In terms of conventional radiative forcing, the efficacy parameter is as low as 0.21. Therefore, the global mean surface temperature change induced by contrail cirrus is clearly weaker than suggested by, both, the effective and the conventional radiative forcing. Feedback analysis identified changing low and mid level clouds, in comparison to the CO2 reference case, as main reason for the small efficacy parameter. In conclusion, there is new and distinct evidence that the use of radiative forcing in aviation climate impact assessments may lead to substantial overestimation of the relative importance of contrail cirrus for aircraft induced global warming. Nevertheless, as several key findings of the present thesis are unique, confirmation from other climate models, equipped with an adequate contrail cirrus parameterization, is regarded as highly desirable.Der Luftverkehr ist in den letzten Jahrzehnten exponentiell angewachsen und spielt deshalb eine wichtige Rolle in der Diskussion über zukünftige Strategien zur Reduzierung von Klimaänderungen. Basierend auf Strahlungsantrieben liefern Kondensstreifen-Zirren vermutlich den größten Beitrag zur Klimawirkung des Luftverkehrs und übertreffen damit die CO2 Emissionen des Luftverkehrs. Während frühere Studien eine verringerte Wirksamkeit von linearen Kondensstreifen auf die Änderung der mittleren globalen Erdoberflächentemperatur gezeigt haben, wurde dies für die weitaus wichtigeren Kondensstreifen-Zirren noch nicht untersucht. Im Rahmen der vorliegenden Arbeit wurden zahlreiche Klimamodellsimulationen durchgeführt, um ein besseres Verständnis über die Klimawirkung von Kondensstreifen-Zirren zu erlangen. Die Simulationen wurden mit den ECHAM5- und EMAC-Klimamodellen durchgeführt, welche mit einer Kondensstreifen-Zirren Parametrisierung nach dem neusten Stand der Wissenschaft ausgestattet sind. Die Simulationen wurden weitergehend mittels Rückkopplungsanalyse, nach der "partial radiative perturbation" Methode, untersucht um die physikalischen Ursachen der Temperaturänderung zu verstehen. Zunächst wurde die Klimawirkung von Kondensstreifen-Zirren auf der Grundlage von Strahlungsantrieben bewertet, welche durch Simulationen mit fixierter Meeresoberflächentemperatur bestimmt wurden. Es konnte gezeigt werden, dass der effektive Strahlungsantrieb von Kondensstreifen-Zirren, im Vergleich zu den klassischen Strahlungsantrieben, deutlich reduziert ist. Im Gegensatz dazu weicht der effektive Strahlungsantrieb einer CO2 Erhöhungssimulation, mit ähnlich großem klassischen Strahlungsantrieb, weniger stark ab. Im direkten Vergleich mit diesen CO2-Experimenten ist der effektive Strahlungsantrieb von Kondensstreifen-Zirren im EMAC (ECHAM5) Modell um 45 % (58 %) reduziert, was bereits auf eine reduzierte Klimawirkung von Kondensstreifen-Zirren hindeutet. Die Simulationen wurden darüber hinaus mittels Rückkopplungsanalyse untersucht, um einen vollständigen Satz konsistenter schneller Strahlungsrückkopplungen zu bestimmen. Als Ursache für den reduzierten effektiven Strahlungsantrieb konnte eine negative Wolkenrückkopplung, aufgrund einer Abnahme der natürlichen Zirrus-Bewölkung, festgestellt werden. Zur Bestimmung der tatsächlichen Klimawirkung wurde die durch Kondensstreifen-Zirren verursachte Änderung der mittleren globalen Bodentemperatur direkt, mit Hilfe des an einen interaktiven Ozean gekoppelten EMAC Modells, simuliert. Zum ersten Mal konnten damit die Klimasensitivität und Klimawirkungseffizienz von Kondensstreifen-Zirren bestimmt werden. Insgesamt fällt die Bodentemperaturänderung aufgrund von Kondensstreifen-Zirren deutlich geringer aus als für eine CO2-Erhöhungssimulation mit ähnlich großem klassischen Strahlungsantrieb. Der auf dem effektiven Strahlungsantrieb basierende Wert der Klimawirkungseffizienz beträgt 0.38 und weicht damit erheblich vom zu erwartenden Wert 1 ab. Bezogen auf den klassischen Strahlungsantrieb beträgt die Klimawirkungseffizienz sogar nur 0.21. Somit fällt die durch Kondensstreifen-Zirren induzierte Änderung der mittleren globalen Bodentemperatur weitaus geringer aus als der effektive und der klassische Strahlungsantrieb vermuten lassen. Mit Hilfe der Rückkopplungsanalyse konnten Unterschiede im Verhalten der tiefen und mittelhohen Bewölkung, verglichen mit dem CO2 Referenzfall, als wesentliche Ursache für die geringe Klimawirkungseffizienz identifiziert werden. Zusammenfassend kann festgestellt werden, dass es neue und sehr deutliche Hinweise darauf gibt, dass die Verwendung von Strahlungsantrieben bei der Bewertung der Klimawirkung des Luftverkehrs zu einer erheblichen Überschätzung der relativen Bedeutung von Kondensstreifen-Zirren für die durch den Luftverkehr induzierte globale Erwärmung führen kann. Da mehrere zentrale Ergebnisse dieser Arbeit einzigartig und erstmalig sind, ist eine Bestätigung durch andere Klimamodelle, welche mit einer adäquaten Kondensstreifen-Zirren Parametrisierung ausgestattet sind, sehr wünschenswert

Climate Impact of Contrail Cirrus

Air traffic has been growing exponentially over the last decades and thus plays an important role for future climate mitigation strategies. Contrail cirrus is regarded to be the largest contributor to aviation induced climate impact on the basis of radiative forcing estimates, exceeding aviation's CO2 emissions. While previous studies revealed a reduced efficacy of linear contrails in forcing the global mean surface temperature, that has not yet been analyzed for the far more important contrail cirrus. In the context of the present thesis, various climate model simulations were performed in order to gain a better understanding about the climate impact of contrail cirrus. The simulations were conducted with the ECHAM5 and EMAC climate models, equipped with a state-of-the-art contrail cirrus parameterization, and were further analyzed by feedback analysis after the partial radiative perturbation method in order to determine the rapid radiative adjustments and slow feedbacks. First, the climate impact of contrail cirrus was assessed on the basis of radiative forcings, determined by fixed sea surface temperature simulations. The effective radiative forcing of contrail cirrus turned out to be largely reduced with respect to the conventional radiative forcings. In contrast, the effective radiative forcing of a CO2 increase simulation, with a similarly sized conventional radiative forcing, deviates less strongly. If directly compared to those CO2 reference experiments, the contrail cirrus effective radiative forcing is reduced by 45 % (58 %) for the EMAC (ECHAM5) model, which already indicates a reduced climate impact of contrail cirrus. The simulations were further analyzed by feedback analysis to determine a full set of consistent rapid radiative adjustments. A negative natural cloud adjustment due to a reduction of natural cirrus cover was found to be the main origin for the reduced effective radiative forcing of contrail cirrus. To determine the actual climate impact, the global mean surface temperature change, induced by contrail cirrus, was directly simulated with the EMAC model coupled to an interactive ocean. For the first time, the climate sensitivity and efficacy parameters of contrail cirrus could be determined. Overall, the surface temperature change due to contrail cirrus turns out to be considerably smaller than for a CO2 increase simulation with a similarly sized conventional radiative forcing. The efficacy parameter, based on the effective radiative forcing, is 0.38, deviating significantly from the theoretically expected value of 1. In terms of conventional radiative forcing, the efficacy parameter is as low as 0.21. Therefore, the global mean surface temperature change induced by contrail cirrus is clearly weaker than suggested by, both, the effective and the conventional radiative forcing. Feedback analysis identified changing low and mid level clouds, in comparison to the CO2 reference case, as main reason for the small efficacy parameter. In conclusion, there is new and distinct evidence that the use of radiative forcing in aviation climate impact assessments may lead to substantial overestimation of the relative importance of contrail cirrus for aircraft induced global warming. Nevertheless, as several key findings of the present thesis are unique, confirmation from other climate models, equipped with an adequate contrail cirrus parameterization, is regarded as highly desirable

Estimating the Effective Radiative Forcing of Contrail Cirrus

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

Towards Determining the Contrail Cirrus Efficacy

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

Radiative forcing and rapid atmospheric adjustments induced by contrail cirrus

The sustainability of worldwide air traffic forms an important issue due to its expected large growth rates in the coming decades. Contrail cirrus is regarded to be the largest contributor to aviation climate impact and thus plays an important role in considerations towards limiting aviation induced climate change. Here, we present results from global climate model simulations, designed to determine the adjusted radiative forcing (RFadj) and the effective radiative forcing (ERF) of contrail cirrus. For a 2050 air traffic scenario a RFadj of 160 mWm-2 was determined, which corresponds to an increase by a factor of more than 3 compared to 2006 values (49 mWm-2) and thus highlights the largely growing impact of air traffic in a future climate. However, as has been indicated by earlier studies, the efficacy of RFadj of linear contrails in forcing surface temperature is significantly reduced and it stands to reason that this might hold for contrail cirrus as well. For this reason we also performed ERF simulations which account for further rapid radiative adjustments in the atmosphere, not included in RFadj, and thus may form a better metric for estimating surface temperature changes. ERF of contrail cirrus is found to be severely reduced by between 50 and 75% (best estimate about 65%), compared to RFadj. In a subsequent feedback analysis the rapid adjustments, which are physically responsible for the reduced ERF, have been determined. A large negative cloud adjustment, due to a decline of natural cirrus cover, is found to be the main driver of the substantial reduction. For a CO2 doubling simulation, the reduction of ERF in comparison to the RFadj is found to be much smaller

Contribution of Contrail Cirrus to Aviation Induced Radiative Forcing and Surface Temperature Change

The net impact of global aviation on climate originates from CO2 as well as from non-CO2 emission components. Recent assessments, based on calculations of conventional and effective radiative forcings, have suggested contrail cirrus to make the largest contribution. Here we provide evidence from a consistent set of scaled sensitivity simulations, which indicates a change of this ranking if the contrail cirrus and CO2 response is determined from a coupled atmosphere-ocean GCM that allows to directly calculate the surface temperature change. Fixed sea surface temperature simulations are used to derive the conventional and effective radiative forcings, while corresponding interactive ocean simulations are used to determine the surface temperature response and climate sensitivity. Resulting climate sensitivity parameters of both forcers indicate an exceptionally low efficacy for contrail cirrus to induce global mean surface warming. If combined with radiative forcing best estimates for air traffic at year 2018 (as given by Lee et al., 2021), the climate impact - in terms of global equilibrium surface temperature change - turns out to be larger for aviation CO2 emissions than for contrail cirrus. An extensive analysis of global radiative feedbacks allows the causes of the remarkably small efficacy of contrail cirrus to be traced back to their physical origin. For both short time-scales (rapid radiative adjustments) and longer time-scales (slow radiative feedbacks), the natural cloud response is found to act quite differently (even in sign) for contrail cirrus and CO2. Together with contributions from a deviating lapse rate feedback it forms the main reason for the low contrail cirrus efficacy to effect surface temperature on the global scale

Climate impact of Contrail Cirrus: From conventional and Effective Radiative Forcings to Surface Temperature Change

Global temperature change and climate sensitivity in response to an external radiative forcing are known to be modified by radiative feedbacks. The net impact of global aviation on climate originates from CO2 as well as from non-CO2 emission components. So far, calculations of conventional and effective radiative forcings suggest contrail cirrus to make the largest contribution. Here, we present results from general circulation model studies indicating that this ranking might change if contrail cirrus and CO2 emissions are determined from coupled atmosphere-ocean simulations to directly calculate the surface temperature change. A set of simulations with fixed sea-surface temperature to derive the conventional and effective radiative forcings and a second set of interactive ocean simulations were performed for contrail cirrus and CO2. Resulting climate sensitivity parameters of both forcers indicate an exceptionally low efficacy for contrail cirrus to induce the Earth's surface warming. If combined with recent radiative forcing best estimates for air traffic, the climate impact - in terms of global equilibrium surface temperature change - turns out to be larger for aviation CO2 emissions than for contrail cirrus. An extensive feedback analysis allows to trace the causes of the remarkably small efficacy of contrail cirrus back to their physical origin. For both rapid radiative adjustments and slow feedbacks, the natural cloud feedback is found to act quite differently (even in sign) for contrail cirrus and CO2. Together with contributions from a deviating lapse rate feedback it forms the main reason for the low contrail cirrus efficacy