Mitigating the contrail cirrus climate impact by reducing aircraft soot number emissions

Abstract

Contrail cirrus are a major component of the climate forcing due to air traffic. For a given contrail cirrus cover, ice water content and ice crystal shape, their impact on radiation is dependent on the number and size of ice crystals. Here we use a global climate model to study the impact of a reduction in initially formed ice crystal numbers, as may be caused by reduced soot number emissions. We find that for reduced initial ice crystal numbers the ice water content is decreased and ice crystal sizes increased, leading to a reduction in contrail cirrus optical depth and doubling the fraction of contrail cirrus that cannot be detected by satellite remote sensing. Contrail cirrus lifetimes and coverage are strongly reduced leading to significant reductions in contrail cirrus radiative forcing. The global climate impact of contrail cirrus is nonlinearly dependent on the reduction in initial ice crystal numbers. A reduction in the initial ice crystal number of 80% leads to a decrease in contrail cirrus radiative forcing by 50%, whereas a twofold reduction leads to a decrease in radiative forcing by approximately 20%. Only a few contrail cirrus outbreaks explain a large percentage of the climate impact. The contrail cirrus climate impact can be effectively mitigated by reducing initial ice crystal concentrations in such outbreak situations. Our results are important for assessments dealing with mitigating the climate impact of aviation and discussions about the use of alternative fuels or lean combustion in aviation.

The contribution of aviation NOx emissions to climate change: are we ignoring methodological flaws?

“…[7]). However, they may largely affect contrail properties and a reduction of the number of soot particles also leads to a reduction of the formed ice particles in the contrail, which reduces their impact on climate [8].…”

Contrail cirrus radiative forcing for future air traffic

“…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 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).…”

“…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).…”

“…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.…”