Details of a global carbon offsetting scheme for planes are up in the air after an inconclusive meeting in Montreal last week.
With four months until the International Civil Aviation Organization aims to finalise a deal, major uncertainties remain on its environmental integrity.
A proposal by Singapore to introduce a “pre-implementation phase” had campaigners worried action on a growing source of greenhouse gas emissions could be delayed.
It is “injecting a lot of confusion”, Brad Schallert of WWF told Climate Home.
The industry had previously agreed to aim for carbon neutral growth from 2020, paying other sectors to cut emissions equivalent to any increase.
While the details have yet to be worked out, Singapore’s intervention could see that start date pushed back.
NGOs argue any pilot phase, to test out reporting methods with voluntary offsetting, must take place before 2020.
“If you don’t start it until 2020, that is just another way you are opening up the emissions gap,” said Schallert.
The Air Transport Action Group, which represents airlines, airports and aircraft makers, agrees.
Executive director Michael Gill said in a statement: “The industry is ready to play its role and we further encourage governments to deliver a deal with concrete parameters that allows us to start implementation from 2020.”
Also unresolved is the question of how to divide responsibility between developed and developing markets.
Draft plans ahead of the meeting exempted up to 40% of aviation emissions, according to NGO analysis, with carve-outs for poor and small countries.
Much of the focus of talks was on who would be obliged to buy offsets and how that was calculated.
Both green groups and industry are calling for wider coverage, saying poorer states can be supported in other ways.
The 36-country Council is set to thrash out the issues at a June meeting, ahead of the September Assembly with all 191 member states.
“The proposal has to show environmental integrity,” said Tim Johnson, director of the Aviation Environment Federation.
“We know that technology improvements and even biofuels are insufficient. Without the market-based measure, it is impossible to close that gap.”
He welcomed a clearer framework for reviewing the scheme every three years, however.
The latest draft states these reviews should consider improvements to support temperature goals in the Paris Agreement: holding global warming below 2C.