Contrail cirrus introduce a short-lived but significant climate forcing that could be mitigated by small changes in aircraft cruising altitudes. This paper extends a recent study to evaluate the efficacy of several vertical flight diversion strategies to mitigate contrail climate forcing, and estimates impacts to air traffic management (ATM). We use six one-week periods of flight track data in the airspace above Japan (between May 2012 and March 2013), and simulate contrails using the contrail cirrus prediction model (CoCiP). Previous studies have predominantly optimised a diversion of every contrail-forming flight to minimise its formation or radiative forcing. However, our results show that these strategies produce a suboptimal outcome because most contrails have a short lifetime, and some have a cooling effect. Instead, a strategy that reroutes 15.3% of flights to avoid long-lived warming contrails, while allowing for cooling contrails, reduces the contrail energy forcing (EF_contrail) by 105% [91.8, 125%] with a total fuel penalty of 0.70% [0.66, 0.73%]. A minimum EF_total strategy (contrails + CO₂), diverting 20.1% of flights, reduces the EF_contrail by the same magnitude but also reduces the total fuel consumption by 0.40% [0.31, 0.47%]. For the diversion strategies explored, between 9% and 14% of diversions lead to a loss of separation standards between flights, demonstrating a modest scale of ATM impacts. These results show that small changes in flight altitudes are an opportunity for aviation to significantly and rapidly reduce its effect on the climate.

One of the proposed ways to reduce the climate impact of civil aviation is rerouting aircraft to minimise the formation of warming contrails. As this strategy may increase fuel consumption, it would only be beneficial if the climate impact reduction from the avoided contrails exceeds the negative impact of any additional carbon dioxide (CO₂) emitted by the rerouted flight. In this study, we calculate the surface temperature response of almost half a million flights that crossed the North Atlantic sector in 2019 and compare it to the temperature response of hypothetical rerouted flights. The climate impacts of contrails and CO₂ are assessed through the perspective of CO₂-equivalence metrics, represented here as nine combinations of different definitions and time horizons. We estimate that the total emitted CO₂ and the persistent contrails formed will have warmed the climate by 17.2 µK in 2039, 13.7 µK in 2069, and 14.1 µK in 2119. Under an idealised scenario where 1 % additional carbon dioxide is enough to reroute all contrail-forming flights and avoid contrail formation completely, total warming would decrease by 4.9 (−28 %), 2.6 (−19 %), and 1.9 (−13 %) µK in 2039, 2069, and 2119, respectively. In most rerouting cases, the results based on the nine different CO₂-equivalence metrics agree that rerouting leads to a climate benefit, assuming that contrails are avoided as predicted. But the size of that benefit is very dependent on the choice of CO₂-equivalence metrics, contrail efficacy and CO₂ penalty. Sources of uncertainty not considered here could also heavily influence the perceived benefit. In about 10 % of rerouting cases, the climate damage resulting from contrail avoidance indicated by CO₂-equivalence metrics integrated over a 100-year time horizon is not predicted by metrics integrated over a 20-year time horizon. This study highlights, using North Atlantic flights as a case study, the implications of the choice of CO₂-equivalence metrics for contrail avoidance, but the choice of metric implies a focus on a specific climate objective, which is ultimately a political decision.

The aviation industry has committed to decarbonize its CO₂ emissions. However, there has been much less industry focus on its non-CO₂ emissions, despite recent studies showing that these account for up to two-thirds of aviation’s climate impact. Parts of the industry have begun to explore the feasibility of potential non-CO₂ mitigation options, building on the scientific research undertaken in recent years, by establishing demonstrations and operational trials to test parameters of interest. This paper sets out the design principles for a large trial in the North Atlantic. Considerations include the type of stakeholders, location, when to intervene, what flights to target, validation, and other challenges. Four options for safely facilitating a trial are outlined based on existing air-traffic-management processes, with three of these readily deployable. Several issues remain to be refined and resolved as part of any future trial, including those regarding meteorological and contrail forecasting, the decision-making process for stakeholders, and safely integrating these flights into conventional airspace. While this paper is not a formal concept of operations, it provides a stepping stone for policymakers, industry leaders, and other stakeholders with an interest in reducing aviation’s total climate impact, to understand how a large-scale warming-contrail-minimizing trial could work.

One of the proposed ways to reduce the climate impact of civil aviation is rerouting aircraft to minimise the formation of warming contrails. As this strategy may increase fuel consumption, it would only be beneficial if the climate impact reduction from the avoided contrails exceeds the negative impact of any additional carbon dioxide (CO₂) emitted by the rerouted flight. In this study, we calculate the surface temperature response of almost half-a-million flights that crossed the North Atlantic sector in 2019 and compare to the response of hypothetical rerouted flights. The climate impacts of contrails and CO₂ are assessed through the perspective of CO₂-equivalence metrics, defined here as nine combinations of different definitions and time horizons. We estimate that the total emitted CO₂ and the persistent contrails formed will have warmed the climate by 16.9 µK in 2039, 13.5 µK in 2069, and 14.0 µK in 2119. Under a scenario where 1 % additional carbon dioxide is enough to reroute all contrail-forming flights and avoid contrail formation completely, total warming would decrease by 4.6 (−27 %), 2.4 (−18 %), and 1.8 (−13 %) μK in 2039, 2069, and 2119, respectively. In most rerouting cases, the results based on the nine different CO₂-equivalence metrics agree that rerouting leads to a climate benefit, assuming that contrails are avoided as predicted. But the size of that benefit is very dependent on the choice of CO₂-equivalence metrics, contrail efficacy and CO₂ penalty. Sources of uncertainty not considered here could also heavily influence the perceived benefit. In about 10 % of rerouting cases, the climate damage resulting from contrail avoidance indicated by CO₂-equivalence metrics integrated over a 100-year time horizon is not predicted by metrics integrated over a 20-year time horizon. This study highlights, using North Atlantic flights as a case study, the implications of the choice of CO₂-equivalence metrics for contrail avoidance, but the choice is ultimately political.

Aviation contrail cirrus has important climate impacts. To construct efficient policies to reduce the uncertain, heterogeneous and short-lived climate impacts of contrail cirrus and balance these against the certain, homogeneous and long-lived climate impact of CO₂ emissions the climate impact of CO₂ and contrail cirrus need to be placed on a common scale. We analyze the social cost of CO₂and of contrail cirrus as well their ratio using an updated version of the Dynamic Integrated Climate Economy (DICE) model with three different discount rates and three different future climate pathways. The social cost of contrail cirrus is less sensitive to the discount rate and less affected by the long-term temperature pathway than the social cost of CO₂. However, the social cost of contrail cirrus is strongly dependent on specific meteorological conditions, which makes the social cost associated with individual flights vary by several orders of magnitude.