Can contrails cirrusly influence our climate?
When do contrails form?
Contrails are clouds that form at altitudes of 7,500 to 12,000 metres in the wake of a passing aircraft. Also known as ‘condensation trails’, contrails may form when the warm water vapour from an aircraft engine cools upon mixing with cold, humid air in the upper troposphere.
Typical temperatures for contrail formation in the atmosphere are approximately -40°C (NASA, 2016). Engine exhaust typically emerges at 850°C. As the hot air from the exhaust mixes with its cold surroundings, it rapidly cools and then – depending on how cold and dry the surrounding air is – may condense, freeze and form a contrail. Figure 1 explains the atmospheric conditions required for contrails to form: the hashed region is where ice particles form/persist, the blue area is where the contrail forms/persists. The airplane exhaust is symbolised by air parcel ‘B’, which becomes air parcel ‘A’ by moving along the dashed line as it cools by mixing with the environment. Whilst the air parcel is in the blue shaded area, a contrail exists.
From figure 1, we can see that once a contrail has formed, if the humidity is low the ice crystals will evaporate and the contrail will be short-lived, whereas if the humidity is high and ice crystals can grow, then the contrail will persist. Persistent contrails often evolve and mix with other contrails to form artificial cirrus aviaticus (see figure 2), which can be relatively indistinguishable from natural occurring clouds.
How extensive are contrails?
Contrail cirrus clouds would cover approximately 10% of European skies and around 6% of skies over the east coast of North America if they were the only clouds in the sky (Burkhardt and Kärcher, 2011). Note though that if the history of the contrail is unknown, studies treat the contrail as a naturally occurring cloud, suggesting that estimates of contrail coverage are largely underestimated (Sausen et al., 1997).
What is the impact of contrails on the climate?
Like natural clouds, contrails affect the radiation balance of the earth’s atmosphere (figure 4).
Studies have found that contrails have a warming effect on the earth’s energy budget, as contrails trap outgoing longwave radiation to a greater extent than they reflect incoming solar radiation (e.g. Spangenberg et al., 2013). Following the grounding of all commercial flights in the US from September 11th to 13th 2001, there was an increase in the average diurnal temperature range (DTR) for the period, which Travis et al. (2002) attributed to the absence of contrails. This ties in with the above as it suggests that contrails do indeed trap outgoing heat and reflect incoming solar radiation, thereby increasing the night time minimum temperatures and decreasing the day time maximum respectively.
Quantifying the warming from contrails is difficult as it requires quantifying the thickness of the contrails both temporally and spatially. However, estimates from the Intergovernmental Panel on Climate Change (IPCC) show that cloud cover has increased by as much as 20% in areas of heavy-air traffic (IPCC, 1999).
Re-routing planes to avoid contrail formation
Findings from a study by the University of Reading in 2014 (Irvine et al., 2014) show that persistent contrails can be reduced by re-routing aircraft to avoid regions of supersaturated ice. However, the re-routing of aircraft generally increases the flight length and therefore, the corresponding carbon dioxide emissions. Irvine et al. (2014) go on to calculate the maximum additional re-routing distance which is environmentally friendly (i.e. taking into account the additional CO2 emissions).
Despite showing that this re-routing distance can be of the order of 100 km, government policies are still rather focused on reducing aviation’s climate impacts based on its CO2 emissions, thereby limiting the extent to which re-routing actually occurs.
Contrail formation and longevity depends on the local atmospheric conditions. Contrails are seen as a negative climate impact, aiding atmospheric warming. Whilst it is often possible to re-route aircraft to decrease the overall climate impact when contrails and CO2 emissions are jointly considered, this is rarely done due to government policies being focused only on CO2 emission reduction.