Just a day after the historic announcement that the Paris climate agreement will enter into force this year, countries of the world yesterday agreed to a new regime to curb a large source of greenhouse gas emissions not covered under that agreement - those from international aircraft flights.
At an assembly in Montreal, the member states of the United Nations' International Civil Aviation Organisation (ICAO) agreed to a "market-based measure" to reduce the emissions from international fights, beginning on a voluntary basis for countries in 2020 and then entering a second phase in 2027. Emissions would be fixed at 2020 levels, and airlines that exceeded those levels would have to buy credits to offset the additional emissions.
The gist is that while international aviation will grow in volume in the future, its emissions should nevertheless be held constant at 2020 levels.
"Aviation can now claim its Paris moment," said Olumuyiwa Benard Aliu, the president of the ICAO council, in a statement following the news.
While some environmental groups hailed the decision as significant in light of what Paris left unfinished, others suggested the emissions cuts won't be strong enough.
The Environmental Defence Fund called the deal "historic". "We estimate the total tonnes avoided is 2 billion tonnes over the first 15 years," said Nathanel Keohane, vice-president for global climate at the group.
"This represents the first global cap on a global sector, and these emissions are outside the Paris agreement, so we see this as a major step continuing the momentum of Paris."
But not everyone felt the same. "This dangerous shell game does little more than help airlines hide their rapidly growing threat to our climate," said Vera Pardee of the Centre for Biological Diversity in a statement, which objected to the delay until 2020, the voluntary nature of participation after that and the fact that even capping emissions at 2020 would still leave quite a lot of emissions every year.
"The world needs less polluting planes, not a dubious offset scheme that just passes off the industry's exploding carbon debt to someone else."
The agreement would not cover domestic flights - rather, only international ones between two cities. But it is the second major step taken by ICAO this year on emissions - in February, the body proposed a set of engine standards for the carbon dioxide emissions from aircraft.
Aircraft emissions have drawn more attention lately, not only because they had escaped the Paris agreement, but also because without regulation, they are expected to be a major growth area in the future.
"International aviation, if you were to put it all in a single bundle it would be the seventh largest country" for emissions, said Lou Leonard, senior vice-president on climate and energy at the World Wildlife Fund. "So 2 to 3 per cent of global emissions, and it's among the fastest growing sectors. So getting a mechanism in place to deal with it now is very important."