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.