Global aviation leaders will be under pressure to deepen efforts to tackle airline emissions as they gather this week under the shadow of protests led by Swedish activist Greta Thunberg.

The 16-year-old, who inspired a 'flight-shaming' protest movement against aviation and sailed across the Atlantic rather than board a plane, is expected to join a march on Friday in Montreal as 193 nations meet at the U.N. aviation agency.

The International Civil Aviation Organization holds its assembly every three years and its 75th-anniversary gathering starting on Tuesday comes at a time of growing concerns about climate change and a six-month-old grounding of the Boeing 737 MAX jetliner.

On Monday, the head of the U.S. Federal Aviation Administration, Steve Dickson, will brief global regulators about delayed progress in approving MAX flights, which were halted in March following two fatal crashes.

The grounding is not on the agenda of the Sept. 24-Oct. 4 assembly but regulators will be anxious to avoid divisions on the sidelines over actions needed to restore the jet to service.

The debate over aviation's impact on the environment will be a major topic for the public side of the talks, however.

Commercial flying accounts for 2.5% of carbon emissions. But with passenger numbers forecast to double to 8.2 billion by 2037, experts say emissions will rise if no action is taken.

At its last full meeting in 2016, ICAO fostered the first global industrial climate initiative with a medium-term scheme to help airlines avoid adding to their net emissions from 2020.

The Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA) requires most airlines to limit emissions or offset them by buying credits from environmental projects.

The industry says around $40 billion in climate financing will be generated between 2020 and 2035.

The move eased the threat of a trade war after the European Union initially imposed its own emissions scheme unilaterally, but environmentalists say it did not go far enough.

The EU, some campaigners and the industry itself want ICAO to commit now to setting longer-term goals at its 2022 assembly - though they may well differ sharply over what they should be.

Urgent Debate

Andrew Murphy, aviation manager at the Brussels-based campaign group Transport & Environment, said climate protests led by Thunberg may put more pressure on the assembly.

"Discussions in ICAO on boosting climate ambitions have been stuck in neutral for years - Greta's presence could inject much needed urgency to the debate," he told Reuters.

ICAO secretary general Fang Liu said she would be open to meeting Thunberg in Montreal.

"Our goal is the same goal as Greta's," she said.

The aviation industry has committed to a target of halving net emissions by 2050, compared to 2005 levels, but there is no comparable long-term climate target for aviation set by countries in any international agreement, including the 2016 Paris accord. The industry and others believe ICAO can help come up with such a goal.

The International Council on Clean Transportation (ICCT), which commissioned research that led to the 'Dieselgate' auto industry scandal, said last week emissions from commercial flights are rising up to 70% faster than predicted by the United Nations.

Airlines insist they can't tackle the problem alone and are pushing for government help for technology such as alternative fuels.

"We can't wait until 2049 to take action," said Michael Gill, director of the industry-led Air Transport Action Group.