Abstract
Abstract. Condensation trails (“contrails”) which form behind aircraft are
estimated to cause on the order of 50 % of the total climate forcing of
aviation, matching the total impact of all accumulated aviation-attributable
CO2. The climate impacts of these contrails are highly uncertain, in
part due to the effect of overlap between contrails and other cloud layers.
Although literature estimates suggest that overlap could change even the
sign of contrail radiative forcing (RF), the impacts of cloud–contrail overlaps
are not well understood, and the effect of contrail–contrail overlap has
never been quantified. In this study we develop and apply a new model of
contrail radiative forcing which explicitly accounts for overlap between
cloud layers. Assuming maximum possible overlap to provide an upper bound on
impacts, cloud–contrail overlap is found to reduce the shortwave-cooling
effect attributable to aviation by 66 % while reducing the longwave-warming effect by only 37 %. Therefore, on average in 2015, cloud–contrail
overlap increased the net radiative forcing from contrails. We also quantify
the sensitivity of contrail radiative forcing to cloud cover with respect to
geographic location. Clouds significantly increase warming at high latitudes
and over sea, transforming cooling contrails into warming ones in the
North Atlantic corridor. Based on the same data, our results indicate that
disregarding overlap between a given pair of contrail layers can result in
longwave and shortwave radiative forcing being overestimated by up to 16 %
and 25 %, respectively, with the highest bias observed at high optical
depths (> 0.4) and high solar zenith angles (> 75∘). When applied to estimated global contrail coverage data for
2015, contrail–contrail overlap reduces both the longwave and shortwave
forcing by ∼ 2 % relative to calculations which ignore
overlap. The effect is greater for longwave radiation, resulting in a 3 %
net reduction in the estimated RF when overlap is correctly accounted for.
This suggests that contrail–contrail overlap radiative effects can likely be
neglected in estimates of the current-day environmental impacts of aviation.
However, the effect of contrail–contrail overlap may increase in the future
as the airline industry grows into new regions.
Cited by
8 articles.
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