Avoiding Contrails – a Solution to Aviation’s Climate Challenge?
 
The recent COP26 conference has shone a global light on climate and sustainability related issues and with the UK government having established legally binding targets on achieving net zero by 2050, the gauntlet has firmly been laid down. Aviation is portrayed as a major contributor to CO₂ emissions and therefore global warming, but did you know that two-thirds of aviation’s contribution to global warming come from non-CO₂ effects with contrails by far the largest factor[1]?
 
What are contrails and how are they formed?
The various by-products of fossil-fuel combustion engines include CO₂, oxides of nitrogen, water vapour and fine soot particulates. Crucially, jet engines inject the latter two products at high altitude, where the water vapour rapidly condenses and freezes into ice crystals.
 
In drier air, these crystals dissipate through sublimation. However, in more humid air, the ice crystals persist and, in association with the soot particulates, become nuclei for further condensation, permitting contrails to expand over large areas of sky. Persistent contrails mutate into cloud – called contrail cirrus or aviation-induced cloudiness (AIC) – often with the influence of upper winds.
 
What are the implications?
The contribution of AIC occurs because of the interaction of two opposing, daily factors. During daytime, AIC reflects incoming solar radiation and absorbs outgoing infrared long-waves radiated by the Earth: this can have a cooling effect, blocking direct heat from the sun. By night, the incoming solar component drops to zero, but the trapping of outgoing radiated heat from the Earth continues. Though this may suggest the two effects cancel each other out, the trapping of radiation by night outweighs the reflective cooling by day; the net effect is overwhelmingly one of warming. Contrails formed after late afternoon have the greatest potential to trap heat overnight. This blanketing effect is known as ‘cirrus radiative forcing’ and is directly linked to the greenhouse effect.
 
What’s being done?
An Imperial College study of Japanese air traffic found that 2.2% of flights caused 80% of radiative forcing. Therefore, targeting a small proportion of flights could yield disproportionately greater benefits. The same study suggested that altering the path of 1.7% of flights would achieve a contrail reduction of 59% for a sub-one percent increase in fuel burn – but a net-positive climate effect.
 
Testing this in practice came to Europe this year, in the following NOTAM (Notice to Airmen): ‘Maastricht UAC (EDYY) Airspace. In an effort to minimise the impact of aviation on the environment, MUAC will be running a contrail-prevention trial from 18 Jan until 31 Dec 2021 between 1500-0500 UTC’. The trial is a practical study of the ability to reduce persistent contrails and, therefore, contrail cirrus. Daily weather and traffic conditions are assessed, looking at afternoon and overnight flights through ice-supersaturated regions occurring around FL300 (winter) and FL360 (summer). Traffic permitting, the trial aims to offer the next most effective level, respecting fuel, and safety concerns, not applicable to climbs through the affected levels.
 
An easy win?
Achieving net zero will be one of the greatest challenges for the aviation industry. Replacing jet fuel or manufacturing new aircraft will be lengthy, difficult, and expensive. Hydrogen fuel cells may be zero-carbon but will still emit water vapour; combusting hydrogen will also produce undesirable nitrous oxides. However, adjusting flight levels to reduce our time in conditions where contrails form and persist has the potential to vastly reduce the environmental impact of aviation quickly and with low associated costs. There is no need for aircraft to be modified nor for pilots to be retrained.
 
The UK is already at the forefront of both the science and development of forecasting and flight planning tools. We urge the government to exploit this leadership and make the necessary funding and resources available to build on the findings of the Maastricht trial and to implement solutions rapidly.