How can the environmental effect of the ETS for aviation be kept constant, despite changes in scope?

The European Commission has proposed to change the geographical scope of the EU ETS. This would result in fewer emissions under the cap, and consequently a smaller absolute emissions reduction. This note analyses how the cap would need to be changed in order to ensure a constant absolute emission reduction from the aviation sector. It finds that the cap needs to be 15-55% than the one proposed by the Commission.