Contrails form behind an aircraft and can persist, spread, and evolve into contrail cirrus when the atmosphere is ice supersaturated. The scientific consensus is that contrails induce a net warming effect, and their global annual mean net radiative forcing (RF) could be higher than the RF from aviation’s cumulative CO2emissions since its inception. Here, we simulate contrails in the North Atlantic with historical trajectory and meteorological data over five years to identify the set of factors that are associated with flights forming strongly warming/cooling contrails. There are significant interannual variability in the annual mean contrail cirrus net RF in this region (204 – 280 mW m-2), and around 12% of all flights in this region is responsible for 80% of the annual contrail energy forcing. Flights with strongly warming contrails are more common: (i) in wintertime; (ii) between 15:00 and 04:00 UTC; (iii) when they are cruising close to the tropopause and above low-level clouds; and (iv) aircraft types with high non-volatile particulate matter (nvPM) emissions per unit distance travelled. Based on these results, we discuss the advantages and disadvantages of potential mitigation solutions to minimise the contrail climate forcing, such as the use of cleaner-burning engines and sustainable aviation fuels, which reduces the aircraft nvPM emissions, and flight diversion strategies that re-route flights around regions where strongly warming contrails are expected to form.

Roger Teoh is from the Centre for Transport Studies, Department of Civil and Environmental Engineering, Imperial College London.