Europe is calling for a considerably less ambitious carbon emissions standard for airplanes than the US in a new global push to reduce aviation’s contribution to climate change, the Guardian has learned.

The standard would mark a turning point for efforts to regulate fast-growing emissions from airplanes, which are not covered by December’s much-hailed Paris climate agreement.

The milestone would apply to new models and existing aircraft put into production after 2020, but the EU’s preferred version would be less stringent than alternative US proposals, which the Guardian has also seen. Both blueprints have been filed with the UN’s civil aviation agency, Icao, ahead of talks in Montreal next month.

“The US proposal is definitely more ambitious,” said Joris Melkert, a former flight test engineer and senior aerospace lecturer at Delft University in the Netherlands. “It would save more emissions, and the difference is quite considerable.”

The gap between the two proposals is greater than the annual emissions of most medium-sized European countries, and privately confirmed by EU officials.

Aviation is responsible for around 5% of the world’s global warming at present and the industry is growing so fast that, on current trends, it could make up 22% of global emissions by 2050, according to a recent European Parliament study.

The Icao talks are pursuing a twin track approach of making airlines pay a cost for their CO2 output under a market-based scheme, and greening aircraft with a new fuel efficiency standard.

The debate on the standard began in 2007 and proposals agreed in Montreal will be sent on to an Icao council for approval next year. “After many years, decision time has finally come,” one EU official said. “I believe that we are going to have an ambitious standard and I hope we can secure it next month.”

Airplanes emit CO2 in the process of burning engine fuel to provide a lift force that can overcome the aerodynamic drag created by wind resistance. For this reason, the Icao proposals focus on measures to improve planes’ aerodynamics, lightness and engines.

Under the technical proposals, a stringency option of ‘9’ suggested by the US would reduce overall aircraft emissions by 37.5%, while the ‘7’ setting favoured by the EU would imply a 33% cut. The 4.5% gap is equal to 350m tonnes of CO2, or slightly more than Spain emits every year.

EU officials privately concede that their proposal is “the second most ambitious of the positions put on the table”. But they insist that it is still “quite close to the US one” and that the agreement’s small print on cargo aircraft and categorisations should not be overlooked.

Environmentalists argue that the involvement of the Environmental Protection Agency and an unshackled president Obama in the US has contributed to the relative weakness of the EU’s position.

But insiders in Brussels say that the sheer variety of complex issues still to be agreed – and the amount of aircraft manufacturing countries involved, from Brazil and Ukraine to India and China – make like-for-like comparisons unwise, and stringency factors of ‘10’ impossible.

“There are so many parameters to be decided that it creates a lot of pieces you can move around on a chessboard,” one EU source told the Guardian.

Europe has proposed a 60 tonne threshold for classifying aircraft, as this could cover 90% of emissions and apply to aircraft of the size of a Boeing 737 or Airbus 320 and above.

But the classification would be further sub-divided between new model types and those in production; jets ferrying 19 people or less, and planes carrying freight instead of passengers. Alternate starting dates of 2020 and 2023 for the benchmark are other options under discussion.

An EU source said: “We need a standard that reflects the state of art of the technology, that is as close as possible to what engineers are capable of doing, without going so high that a number of 60-tonne aircraft types would be put out of the market as that would be environmentally unhelpful, economically detrimental and also quite unfair, as lead-in times for aircraft take about 10 years.”