Climate neutrality not just carbon neutrality: how the AIA is working are working to find out more about the non-CO2 effects of aviation

The aviation industry is increasingly under pressure from passengers, environmental groups, governments and policy makers to reduce its climate impact and become a more sustainable industry. The main focus has been on developing new aircraft and propulsion technology to reduce or eliminate greenhouse gas emissions or at least become carbon neutral. However, aviation’s climate impact extends beyond carbon emissions. On clear days, aircraft flying in cold and humid conditions can be seen forming condensation trails, or contrails. In many cases these contrails disappear within a matter of minutes. However, if aircraft are flying in atmosphere where the relative humidity with respect to ice is greater than 100% (ice-supersaturated), contrails can persist for hours and may evolve into cirrus clouds. Like clouds, contrails reflect incoming solar energy back out to space creating a cooling effect, but they also absorb surface radiated energy creating a warming effect. The net effect is usually a warming of the atmosphere. In fact, one recent study suggested that between 1940 and 2018, the net global warming impact of contrails, measured in terms of radiative forcing, may have been as much as three-quarters of that of the total net radiative forcing generated by aviation (Lee et al. 2021) Therefore, although future aircraft operations may become carbon-neutral, they may not be climate-neutral if contrails are generated In recent months, engineers, scientists and industry partners working on the Aviation Impact Accelerator project (AIA) have been trying to estimate the climate impact of contrails now and in the future. But the science of contrails is relatively less well understood and their climate impact is highly uncertain