Airlines are being strongly urged to submit their emissions-monitoring plans for ICAO's global carbon-offsetting scheme by the end of September 2018, ahead of the formal February 2019 deadline, regardless of whether or not their home state has agreed to participate in the voluntary phase of the market-based measure.

The more data that is available to set the baseline, above which airlines must pay to offset their emissions under the Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA), the more accurate that baseline will be. If the baseline is set too low as a result of airlines under-reporting their emissions, experts warn, carriers will collectively face a higher bill for offsets – particularly in the early phases of the scheme when offsetting obligations are calculated on a sectoral rather than an individual operator basis.

"There is a cost if airline operators are not reporting [their emissions]. If under-reporting occurs, you will have to offset more, collectively," says Stefano Mancini, emissions trading scheme support facility manager at Eurocontrol.

Speaking at the Aviation Carbon 2017 conference in London in early December, Africa Abajas Bermejillo, head of aviation services at Vertis Environmental Finance, emphasised that from 2021 until 2029 the formula for calculating who pays what, in relation to offsetting above-baseline emissions, will be 100% sectoral.

In other words, "it might happen that your [airline] is not growing but because it's sectoral you will have some responsibility for what others do", says Abajas. This "socialist" approach, as she describes it, was "the way to convince many states to be in CORSIA".

Between 2030 and 2032 this ratio changes to 80% sectoral and 20% individual. In the final two years of the scheme, from 2033 until 2035, the balance will switch to a 70% emphasis on the individual operator and 30% on the sector as a whole.

Any airline hoping for a "free ride" during the sectoral phase by reaping the benefits of efficiencies at other carriers while not taking steps to become more efficient themselves is following a "very risky strategy" when the onus switches to the individual operator, says Dr Pedro Piris-Cabezas, senior economist global climate at the nonprofit Environmental Defense Fund.

The baseline for CORSIA will fluctuate on an annual basis as states opt in and out, and will also be affected by factors such as the more widespread use of sustainable aviation fuels, technology improvements and more efficient air traffic management procedures.

To date, 72 states have declared their intention to take part in the voluntary phases of CORSIA. Both the pilot phase, from 2021 to 2023, and the first phase, from 2024 to 2026, will be voluntary.

Participation in the mandatory phase, which runs from 2027 until 2035, will be "more complex", according to Andreas Hardeman, managing director of Hardeman Consulting.

This will include states that have volunteered, as well as other states that meet two criteria: those with an individual share of international aviation activities in revenue tonne -kilometres (RTKs) in 2018 that is above 0.5% of total RTKs; and those whose cumulative share in a list of states, from the highest to the lowest amount of RTKs, reaches up to 90% of total RTKs.

Exempted states include the least-developed countries, small-island developing nations and landlocked developing countries. However, these countries can still volunteer as long as they communicate their decision to ICAO by 30 June in the year before they wish to join.

Regardless of whether an airline's home state is covered by the scheme, all non-exempt operators must monitor and report their emissions, Hardeman stresses.

"All international flights between a state that's included and a state that's not are exempted, as are flights between two states not included in CORSIA, but operators will still have reporting requirements," he says. In cases where an airline based in a non-participating state operates routes between states that are participating, emissions from those flights will be covered by CORSIA.

New-entrant carriers will be excluded for three years or until their annual emissions exceed 0.1% of the total 2020 emissions, but are still required to monitor and report.

Adoption of the standards and recommended practices that will guide states on how to implement their emissions monitoring, reporting and verification (MRV) systems will take place at the ICAO Council in the middle of 2018, Hardeman expects. The formal deadline for submitting monitoring plans is February 2019, but he says that "airlines are strongly recommended to submit their monitoring plans by 30 September 2018".

Airlines are advised to prepare as soon as possible for what could be a long and complex process. Jonathon Counsell, group head of sustainability at British Airways and Iberia parent IAG, is concerned about the high number of airlines lacking experience in monitoring and reporting emissions, and warns against taking a last-minute approach.

"The challenge we've got is there are 1,000 airlines to apply for CORSIA but only 100 who have got experience," says Counsell, noting that the monitoring plan – the core document outlining all processes and procedures – for British Airways alone is between 50 and 100 pages long. Information must be collected on a per-flight basis and verification with an external verifier can take up to three months, he adds.

"Don't wait until the end of the year to start that process," he advises.

Complications can also arise for wet-leased aircraft and aircraft used by various airlines within a group. "Wet-leased flights must be included but the data is difficult to source – make sure in the wet-lease contract that [the wet-leasing company] provide[s] you with robust data. And when aircraft are shared across a group of airlines they must be carefully allocated to the correct operator," Counsell says.

He also cautions against relying on one source of information for calculating the emissions from each flight, noting that in addition to the aircraft technical log, IAG uses data from the aircraft communications addressing and reporting system (ACARS), as well as other operational data.

"Around 95% of that [data] fits between tightly defined parameters, but there is 5% that you will need to manually interpret," Counsell says.

It is essential that airline chief executives are heavily involved in CORSIA preparations and that employees across the organisation are educated on the ins and outs of the carbon offsetting market, according to Counsell. "Allocate resources for MRV. Don't give it all to the poor environment guy to do in a weekend. He will die," he says.

Counsell's advice was echoed by IAG chief executive Willie Walsh in his keynote speech at the Aviation Carbon event. Walsh urged airlines to get ready early for CORSIA and said that IAG was willing to share information with other carriers on how to prepare, "for the benefit of the industry".

"Airlines that want assistance can come to IAG," Walsh says, adding that "the earlier you adapt and participate, the better prepared you'll be for the mandatory phase".