2,000 government officials gathered in Montreal to adopt the first ever international mechanism to reduce carbon dioxide emissions from aviation

A UN negotiation is inching towards a landmark deal to curb emissions from airplanes, although environmental groups have warned that the plan will not go far enough to help slow climate change.

The International Civil Aviation Organization (ICAO), a UN agency, has gathered more than 2,000 government officials from around the world at its Montreal headquarters to adopt the first ever international mechanism to reduce carbon dioxide emissions from aviation.

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A market-based scheme has been proposed at the triennial meeting whereby emissions from aviation will be measured from a baseline to be set in 2020. Emissions that exceed this level will then have to be offset by all countries, with exceptions for developing nations, unless they choose to volunteer.

So far, 63 countries have pledged to join the scheme from its outset, including the US, China, Japan and Canada. The European Union has also voiced its support. A draft text has been put forward with a final version of the deal expected to be decided upon next week.

Global aviation currently moves 10 million passengers around every day, pumping out enough emissions to contribute 1.3% to the overall total of greenhouse gases emitted from all sources, including power generation and agriculture. Aviation’s emissions share is expected to leap to 22% by 2050, placing growing pressure on the sector to help the world avoid further global warming.

But environmental groups have criticized the ICAO plan as “incredibly weak” and warned that it will do little to limit pollution unless it is significantly ramped up. They argue that more countries need to sign up in order to honor the goals set at the Paris climate agreement last year.

“In Paris, world leaders put great faith into ICAO to deliver emissions reductions from international aviation. Global aviation needs to step up and agree to a plan that matches the demands of science,” said Lou Leonard, interim deputy leader for WWF’s climate and energy practice.

“But without broad participation, the proposed deal represents a low bar for an international climate agreement. The clock is ticking; the best way to strengthen the deal at this point is the added benefit of additional countries opting in.”

There has also been criticism of the structure of the plan itself. Participating countries will have to ensure airlines offset emissions by backing carbon-reducing activities elsewhere, rather than use technology to cut the emissions from aircraft fuel.

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All emissions up to the 2020 level will not have to be offset, despite probably representing the vast bulk of future emissions, and nations that volunteer to take part can opt out with six months’ notice.

Vera Pardee, a campaigner at the Center for Biological Diversity, said: “This is a dangerous dodge that does little more than help airlines hide their skyrocketing threat to our climate.

“Instead of employing the host of technologies that actually reduce aircraft pollution, the aviation industry has sold us a highly dubious offset scheme that simply passes off its exploding carbon debt to someone else.”

The International Coalition on Sustainable Aviation, an environmental observer of the UN talks, has warned that the deal could still allow absolute international aviation emissions to increase until at least 2035, meaning that more work needs to be done “in order to ratchet the level of ambition up”.

But the International Air Transport Association, which represents airlines, said that the offset scheme is a “pragmatic compromise” that will deliver carbon neutral growth from 2020.

Alexandre de Juniac, director general of the trade association, said the deal “along with measures to improve technology, operations and infrastructure, will keep aviation at the forefront of industries responsibly managing their climate change impact”.

The airline industry has a goal to improve average fuel efficiency by 1.5% to 2020. But green groups believe airlines could do much more – a recent study from the International Council on Clean Transportation found that fuel consumption in new aircraft designs could be slashed by 25% in 2024 using new technologies.