Opinions are divided over whether ICAO's Carbon Offsetting and Reduction Scheme for International Aviation should be the only market-based measure applied to the airline industry to address its impact on climate change.
The industry is adamant that the global agreement struck at the 2016 ICAO Assembly should mark an end to its inclusion in a growing "patchwork" of environmental taxes and schemes in individual countries.
However, some nongovernmental organisations question whether CORSIA alone will be enough to limit emissions from the fast-growing sector and enable countries to meet the targets they committed to under the Paris Agreement – particularly in the latter half of this century.
Meanwhile, the EU is waiting in the wings with a threat to attempt once again to include aviation emissions from outside the European Economic Area in its Emissions Trading System, in the event that it is not satisfied with the effectiveness of CORSIA.
These divisions came to the fore at the Aviation Carbon 2017 conference in London in early December, where representatives from both sides of the argument put forward their views. In a keynote speech, IAG chief executive Willie Walsh said the "risk is very high" that countries could enact their own measures to run concurrently with CORSIA.
Walsh says CORSIA "is designed to be the measure" and he urges states that have not volunteered to participate in the initial phase of the agreement to sign up. "CORSIA is the first global scheme covering an entire industry sector and that is not a thing we should play down," he argues, noting that the cost to airlines of buying carbon offsets under the scheme would be significantly lower than the expense of paying into a web of country-specific measures.
Similarly, Air Transport Action Group executive director Michael Gill says that CORSIA is intended to be the only market-based measure for international aviation emissions, and expresses concern about emerging measures in individual countries. "Within months of CORSIA being adopted we saw proposals in Sweden, the Netherlands and Colombia [to introduce new aviation taxes]," says Gill. "Getting CORSIA successfully implemented is key to avoiding the patchwork [of individual measures] we've been trying to avoid."
But environmental NGOs are concerned about what they see as a lack of transparency at ICAO level on how CORSIA will play out in terms of the quality and integrity of the carbon offsets that airlines will be buying. In addition, with the mandatory phase of the ICAO agreement set to end in 2035 there are worries over accounting for aviation emissions in the second half of this century.
"The problem is this assumes the world stops in 2050 and it doesn't look beyond that. [The] Paris [Agreement] gives a view out to the end of the century... What does aviation look like at the end of the century?" says Tim Johnson, director of the Aviation Environment Federation. Johnson foresees a "fundamental challenge about whether aviation growth can be funded on the carbon markets", and believes "now is the time to focus on decarbonising the aviation sector itself".
Ismael Aznar-Cano, legal officer at the European Commission's Directorate-General for Climate Action (DG CLIMA), warns that if CORSIA "fails to deliver", states will need to implement additional measures to ensure that aviation growth does not put their Paris Agreement targets at risk.
"We have to be aware that CORSIA is just a start – it is not enough for the goals under Paris to decarbonise the economy by the second half of the century," says Aznar-Cano.
The EU is keeping a close eye on CORSIA as it decides whether to put an end to its "stop the clock" moratorium on including outside-EEA aviation emissions in its ETS. Intra-EEA airline emissions are still included in the system but the EU Council Presidency and European Parliament provisionally agreed in October to extend its derogation on including outside-EEA emissions until the end of 2023, to allow the ICAO scheme to take root.
This decision was agreed "to give ICAO more time to dot its i's and cross its t's", and there will be "regular reporting to the Parliament and the Council on the level of participation in ICAO and the laws being put in place by countries", says DG CLIMA adviser Damien Meadows.
Reserving the ability to include international airline emissions in its ETS after CORSIA takes effect provides Europe with a stick to wield over how the scheme is implemented.
"From a European Parliament point of view, we don't believe the agreement is sealed yet... Where we do have influence is with 'stop the clock'," says MEP Julie Girling, the European Parliament's EU ETS rapporteur. "In order to make our Paris commitments, what we can't have happen is that towards the end of the 2020s CORSIA is delivering less climate benefits than the current ETS."
Meadows believes the EU ETS has been "a little bit demonised" and he is keen to highlight its achievements, which he says will include a 43% reduction in the EU's total carbon dioxide emissions by 2030 compared with 2005 levels.
"This system delivers a reduction of 20 million tonnes per year from aviation. Emissions trading allows aviation to offset emissions in other sectors of the economy," explains Meadows, noting that when Ryanair included ETS costs in its ticket prices it resulted only in an increase to individual ticket prices of 25 euro cents.
And, noting that the Paris Agreement's central aim is to ensure the global temperature rise this century is kept below 2ºC, Meadows adds: "To avoid two degrees we have to do a lot more than we're doing now. CORSIA is a good start but more needs to happen."
However, Airlines for Europe (A4E), the lobby group for Europe's carriers, says it expects CORSIA to be "the only measure applicable to international carbon emissions from flights within the EEA as per 2021".
Sofia Zoe Rohner, manager of environmental and aeropolitical affairs at Austrian Airlines, says: "We really want to stress that there can be no double burden for European airlines when they are in the scope of ETS and CORSIA at the same time. Now is the right time for a transition from ETS to CORSIA."
Such arguments, however, fall on deaf ears among those who believe that airlines and those who use them should pay as high a price as possible for their emissions.
"We believe the industry should pay... and those who fly should contribute to decarbonising the sector," argues Andrew Murphy, aviation manager at Transport & Environment, a Brussels-based pressure group. He believes that a continuation "in some scope" of aviation's inclusion in the EU ETS alongside CORSIA would be "an example of a developed country taking the lead" on climate policy.
"My ideal scenario is one where the industry embraces bottom-up, regional approaches," says Murphy, adding that he believes global standards combined with regional approaches is "the future".
Murphy is sceptical about CORSIA and unimpressed by what he sees as a lack of transparency at ICAO when it comes to spelling out the detail of how it will work. He says: "If you're concerned about how transparent [EU] ETS is, wait until you meet CORSIA."