This framework is useful to show where the major uncertainties are. Joos
et al (
2013) find that calculations of the atmospheric CO
response agree within 15%, thus the climate impact of the flightʼs CO
emissions can be calculated with a relatively small uncertainty, given knowledge of aircraft fuel burn. The calculation of the climate impact of the contrail has a much larger uncertainty. T
he uncertainty chiefly arises from two sources: an inability to estimate, a priori, the eventual size and therefore climate impact of the contrail that would be formed, and second the radiative forcing (which has a potential dependence on the time of day, not taken into account here) and climate efficacy of that forcing. Even if the radiative forcing were calculated operationally within a forecast model, there would still be an uncertainty in the size of the calculated radiative forcing due to the radiative forcing codes (Myhre
et al 2009), and also due to uncertainty in the contrail characteristics.
Taking into account the uncertainty in the eventual climate impact of a contrail of 100 km length, the estimate of the maximum diversion distance varies by a factor of 20.
The application of such a strategy in the real world would require highly accurate forecasts of ISSRs where potential contrails form, and the ability to know
a priori the climate impact of a potential contrail, as well as being highly dependent on air traffic control and other operational and economic considerations. In addition, the overall climate impact of the flight should take into account the chemical forcings from aircraft
emissions; detailed calculations of such 'climate optimal' routings are currently being performed by the REACT4C project. We note here that for small horizontal diversions it is possible that the chemical forcings between the two routes would be comparable; however since the impact depends on where the
emissions are advected, small diversions could potentially result in large differences in impact (Grewe
et al 2014). The impact of black carbon and other aerosol emissions may also be important and could be incorporated in more detailed estimates (Jacobson
et al 2012).
Nevertheless, despite the uncertainties, the calculations presented here indicate that once a metric (and time horizon) choice has been made, guidance can be given as to whether it is beneficial to divert to avoid contrails. So for example, adding 100 km distance to a flight to avoid making a contrail would seem beneficial for many of the cases presented here, and other parameter choices, such as the extreme high values in Haywood
et al (
2009), could allow significantly longer diversions.