A deal to lower airline-related greenhouse gas emissions has been struck in a historic 191-nation treaty, but green groups say it falls short of what's needed.

The International Civil Aviation Organization (ICAO), the United Nations body that oversees the flying industry, created a framework that aims to offset carbon emissions while only costing the sector less than two percent of revenues, Reuters reports. The system will be voluntary from 2021 to 2026 and mandatory from 2027 for nations that have big aviation industries.

But as environmental advocates noted, the structure of the deal ultimately lets airlines off the hook by allowing them to "offset" their carbon footprint rather than reducing it, and by making the program voluntary.

"Airline claims that flying will now be green are a myth. Taking a plane is the fastest and cheapest way to fry the planet and this deal won't reduce demand for jet fuel one drop. Instead offsetting aims to cut emissions in other industries," said Bill Hemmings, director of the European sustainable development advocacy group Transport and Environment, which was an observer to the ICAO talks.

"Today is not mission accomplished for ICAO, Europe, or industry," Hemmings said. "The world needs more than voluntary agreements. Without robust environmental safeguards the offsets won't cut emissions, leaving us with a deal that amounts to little more than adding the price of a cup of coffee to a ticket."

The offsetting scheme will allow airlines to take part in programs that fund "forest areas and carbon-reducing activities," the Guardian reports, adding that aviation emissions "account for 1.3% of the global greenhouse gas total today." The system will use the industry's 2020 emissions as a reference point and offset roughly 80 percent of that until 2035.

Although the U.S. and China have both said they will sign up for the voluntary phase, other big polluters, including Russia and India, have said they would not, on the grounds that it puts an unfair burden on developing countries.

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The ICAO was responsible for creating an aviation-specific treaty, because emissions from that sector fell outside the domestic climate targets of countries taking part in the Paris agreement, which reached requisite signatories this week and will go into effect in 30 days. But according to Lou Leonard, senior vice president of climate and energy at the World Wildlife Fund (WWF), ICAO countries scrapped language that linked aircraft pollution reduction to the Paris climate goals.

"This deal was the world's first opportunity to test whether the new Paris agreement would change the way we do business and rally the world toward its new goals," Leonard said. "Yet just hours after celebrating the Paris agreement's early entry into force, countries at ICAO are sending mixed signals about their ambition to reduce emissions by weakening the link between the aviation mechanism and the temperature goals set in Paris."

"Unless we accelerate our pace, emissions from international aviation will take too much of our remaining carbon budget and will restrict our ability to reach the Paris Agreement’s global temperature goals," Leonard added. "Cutting corners on critical pieces like alternative fuel and offset quality criteria would undermine the entire deal."

Vera Pardee, an attorney for the Center for Biological Diversity, also warned that "This dangerous shell game does little more than help airlines hide their rapidly growing threat to our climate."

"The world needs less polluting planes, not a dubious offset scheme that just passes off the industry's exploding carbon debt to someone else. This weak measure puts new pressure on U.S. officials to take stronger steps to curb aviation's skyrocketing emissions," Pardee said.

And Greenpeace U.K. chief scientist Dr. Doug Parr said, "This agreement is a timid step in the right direction when we need to be sprinting. The aviation industry has managed to get away for years with doing nothing about its growing carbon emission problem, and now it's giving itself even more years to do very little."