  In 2019, aviation was responsible for 2.6% of the world CO2 emissions as well
as additional climate impacts such as contrails. Like all industrial sectors,
the aviation sector must implement measures to reduce its climate impact. This
paper focuses on the simulation and the evaluation of climatic scenarios for
the aviation industry. For this purpose, a specific tool (CAST for "Climate and
Aviation - Sustainable Trajectories") has been developed. This tool follows a
methodology adapted to aviation using the concept of carbon budget and models
of the main levers of action such as the level of air traffic, reduction of
aircraft energy consumption or energy decarbonisation. These models are based
on trend projections from historical data or assumptions from the literature.
Several scenario analyses are performed in this paper using CAST and allow
several conclusions to be drawn. For instance, the modelling of the scenarios
based on the ATAG (Air Transport Action Group) commitments shows that aviation
would consume between 2.9% and 3.5% of the world carbon budget to limit global
warming to 2{\deg}C and between 6.5% and 8.1% for 1.5{\deg}C. Also, some
illustrative scenarios are proposed. By allocating 2.6% of the carbon budget to
aviation, it is shown that air transport is compatible with a +2{\deg}C
trajectory when the annual growth rate of air traffic varies between -1.8% and
+2.6%, depending on the considered technological improvements. However, in the
case of a +1.5{\deg}C trajectory, the growth rate would have to be reduced
drastically. Finally, analyses including non-CO2 effects compel to emphasize
the implementation of specific strategies for mitigating contrails.
