Air traffic contributed approximately 5 % to the anthropogenic climate forcing in 2005 (Lee et al., 2009), and its contribution is rising due to the large yearly increases in air traffic (ICAO, 2007). Radiative forcing due to contrail cirrus, consisting of linear contrails and the cirrus clouds arising from them, is the largest known radiative forcing component associated with air traffic, and is larger than the contribution of CO 2 accumulated from aviation (Burkhardt and Kärcher, 2011). Contrail cirrus are central for mitigation efforts due to their short lifetimes by, for example, varying flight level, path or timing, using alternative fuels, new engine designs or other technological advances (e.g. Noppel and Singh, 2007; Lee et al., 2010; Newinger and Burkhardt, 2012; Deuber et al., 2013; Burkhardt et al., 2018). Both their large climate impact and their suitability for mitigation underline the importance of investigating contrail cirrus for future air traffic scenarios.

The climate impact of contrail cirrus in the future is determined by a number of factors: the strength and geographic distribution of the increase in air traffic volume, improved fuel efficiency, changes in aircraft emissions when using alternative fuels and the change in the background atmospheric state due to future climate change. Several projections for future air traffic volume and its emissions exist. According to ICAO (2007) and Airbus (2007), the distance flow by passengers is expected to double roughly every 15 years. The air traffic inventory Aviation Environmental Design Tool (AEDT) (Wilkerson et al., 2010) estimates that in 2050 the air traffic volume will have quadrupled relative to the year 2006. The distribution of air traffic, as well as its future increase, is globally very uneven. In 2006, 93 % of aviation fuel was burned in the Northern Hemisphere and 69 % between 30 and 60∘ N. More than half of global aviation CO 2 is emitted over three regions: the United States (26 %), Europe (15 %) and East Asia (11 %) (Wilkerson et al., 2010). Due to historically low air traffic densities in the tropics, the relative increases are expected to be much larger in the tropical areas than in the extratropics.

Lee et al. (2009) estimate that fuel usage is expected to increase between 2000 and 2050 by factors of 2.7 to 3.9, depending on the Intergovernmental Panel on Climate Change's Special Report on Emissions Scenarios (IPCC SRES) while AEDT estimates an increase by factors of 2.7 to 5 between 2006 and 2050 (Chen and Gettleman, 2016). Aerodynamic changes, weight reductions, more fuel efficient engines and an increased operational efficiency lead to increased overall fuel efficiency (Lee et al., 2009). ICAO (2007) expects a fuel efficiency improvement of 2 % yr−1 until 2050. Increasing fuel efficiency of engines leads to an increase in the contrail formation probability and contrail radiative forcing (Marquart et al., 2003).

Measurements behind aircraft (Beyersdorf et al., 2014; Moore et al., 2017) show that the combustion of an alternative fuel, a blend of Jet A and Fischer–Tropsch fuel, induces a decrease in the mass and number of soot particles. This results in a lower number of nucleated ice crystals (Kärcher and Yu, 2009; Kärcher et al., 2015) and in a higher survival rate of ice crystals during the contrails' vortex phase (Unterstrasser, 2016). The change in the ice crystal number after the vortex phase has an impact on the microphysical process rates and the evolution of contrail cirrus (Bier et al., 2017) with ice crystals growing to larger sizes and sedimentation initiated earlier in the life cycle. This leads eventually to a decrease in the mean optical depth and lifetime of contrail cirrus (Burkhardt et al., 2018). This is particularly important in large-scale and long-lived contrail cirrus clusters (Bier et al., 2017) which are responsible for a large part of the contrail cirrus radiative forcing (Burkhardt et al., 2018).

With climate change caused by increasing greenhouse gas concentrations, contrail cirrus formation and properties may change. The increase in temperature may lead to a lower contrail formation probability in particular in the tropics and in summer in the subtropics (Marquart et al., 2003). An increase in atmospheric water vapour concentration may lead to higher contrail cirrus ice water content and optical depths. A decrease in the ice supersaturation frequency (Irvine and Shine, 2015) may result in lower contrail cirrus coverage and associated radiative forcing.

The radiative forcing of line-shaped contrails (the contrails that have retained their initial line shape and are, therefore, easily distinguishable from natural clouds in satellite images) and contrail cirrus for the year 2050 have been studied in a number of publications. Minnis et al. (1999) estimate a radiative forcing due to line-shaped contrails for the year 2050 of 100 mW m−2 when assuming a constant visible optical depth of 0.3. In Marquart et al. (2003), line-shaped contrail radiative forcing increases from 2015 to 2050 by a factor of approximately 1.6, amounting to 15 mW m−2 in the year 2050, or after a suitable correction for a low bias in optical depth, to about 45 mW m−2 (Kärcher et al., 2010). For contrail cirrus comprising of line-shaped contrails and the clouds developing from them, Lee et al. (2009) scaled present-day radiative forcing estimates, from models and observations, to 2050 arriving at a range between 27 and 315 mW m−2 with no best estimate given. Chen and Gettelman (2016) studied the change in cirrus cloudiness due to contrail formation using a model in which contrail formation is treated as a source term for cirrus ice crystals and the microphysics parameterization is applied to a mix of contrail and natural cirrus ice crystals. They estimated that contrail cirrus radiative forcing increased by a factor of 7 from 2006 to 2050, reaching 87 mW m−2 in the year 2050, a factor that is approximately double the factor of increase in air traffic volume. They argued that this is caused by the non-uniform regional increase in air traffic and different sensitivities of contrail cirrus radiative forcing to an increased air traffic volume in different regions.

Our aim is to estimate contrail cirrus radiative forcing for the year 2050 globally and regionally, isolating changes due to the increase and upward shift in air traffic volume, due to climate change and due to changes caused by the use of alternative fuels and changes in the propulsion efficiency. We use the atmospheric general circulation model coupled with a contrail cirrus scheme, ECHAM5-CCMod (Bock and Burkhardt, 2016a; Sect. 2.1), which treats contrail cirrus as an independent cloud class. The model simulates the whole life cycle of contrail cirrus and resolves the competition of the two cloud classes, natural clouds and aircraft-induced clouds, for water vapour. We apply ECHAM5-CCMod to future aviation emission scenarios from the AEDT inventory (Sect. 2.2) and estimate contrail cirrus coverage, optical depth and radiative forcing for air traffic for the year 2050 (Sect. 3). Discussion and conclusions are given in Sects. 4 and 5.