Exactly what form MBMs for aviation emissions might take remains to be determined. Have summarily excluded inter alia a simple carbon or fuel tax, the ICAO Council reduced the MBM options to three: (i) global mandatory offsetting; (ii) global mandatory offsetting complemented by a revenue generation mechanism; and (iii) a global emissions trading (cap-and-trade system) (Lyle2012). No preference was set in stone at this stage, although there was a leaning toward offsets without revenues, a choice supported by industry. However, selection of this option would leave open the door to the continuing consideration in the UNFCCC of a tax on international air transport as a contribution to the Green Climate Fund, a concept strongly opposed by both ICAO and industry. It would of course also be inconsistent with carbon tax regimes and cap-and-trade emission schemes increasingly under consideration or in place around the world, notably that of the EU emission trading system. While guiding principles for the design of MBMs were adopted by the ICAO Assembly in 2010 and considerable follow-up was undertaken, ICAO postponed work on the final details, initially until 2013 and now until 2016. Key issues which have yet to be definitively addressed include: the mechanism(s) for implementation; who would be the participants (for example states and/or air carriers); quality control; and MRV. The 2013 Assembly also opened the door to consideration of MBM options beyond the three contemplated by the council.
As mentioned above, a fundamental MBM issue is the perceived conflict in the uniform application/equal treatment provisions of the Chicago Convention and the UNFCCC principle of CBD&RC. Proposals for resolution have been widely discussed within and beyond ICAO (Lyle 2012, 2013). The WWF has also published a very helpful think piece on ways of reinterpreting CBDR 20 years after Rio (Lutes and Vorster 2013;WWF 2013).