Climate forcing of aircraft contrails: uncertainty quantification and mitigation potential

Abstract

Contrails, lined-shaped clouds that form behind an aircraft, interact with earth’s radiative balance and are thought to have a net warming effect. Their climate forcing could be comparable to the cumulative impacts of aviation CO2 emissions, but with a low scientific understanding and large uncertainties. This thesis quantifies uncertainties of the contrail climate forcing and proposes mitigation solutions. Several research gaps were identified in an extensive literature review: (i) contrail models currently assume a constant aircraft black carbon (BC) number emissions index (EIn) although it is a critical parameter affecting various contrail properties; (ii) contrail uncertainties were not comprehensively modelled; and (iii) a fleet-wide flight diversion strategy, recommended by existing studies, can be highly disruptive to air traffic management and increase CO2 emissions. To address (i), a new methodology is developed from the theory of fractal aggregates to relate the BC number and mass emissions. The methodology is validated with various BC combustion sources and estimates the fleet-average BC EIn emitted by flights in Japanese airspace to be 1.37 [1.35, 1.39] ×1015 kg-1. Aircraft BC EIn and meteorological uncertainties are then propagated to address (ii), where contrail uncertainties are quantified using the Contrail Cirrus Prediction Model (CoCiP). Only 2.2% of flights contribute to 80% of the contrail energy forcing (EF) in this region. A small-scale strategy of selectively diverting 1.7% of the fleet can reduce the contrail EF by up to 59.3% [52.4%, 65.6%], with only a 0.014% [0.010%, 0.017%] increase in long-lived CO2 emissions. This addresses (iii) and highlights the possibility to instantaneously reduce aviation’s climate forcing. Over the longer term, a fleetwide adoption of cleaner-burning engines, which reduces BC EIn by 76%, achieves a 68.8% [45.2%, 82.1%] reduction in the contrail EF. A combination of both interventions significantly reduces the contrail EF by 91.8% [88.6%, 95.8%].