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."