The assembly of the International Civil Aviation Organization (ICAO) approved a carbon offsetting strategy to cap international aviation emissions after 2020 in what was hailed as an historic and momentous action. Sixty-five nations representing 85 percent of international air traffic have indicated they will participate in the voluntary pilot phase of the global market-based measure (GMBM) scheme the assembly adopted on October 6.

“This is an historic achievement, and it was only possible because of a remarkable willingness on the part of all ICAO member states to come together in the interests of finding a global solution to addressing climate change in this unique international sector,” said Ambassador Jennifer Haverkamp, U.S. special representative for environment and water resources. The initial level of voluntary participation “doesn’t just demonstrate enormous support for the measure; it signifies a resolute commitment to address one of the greatest global challenges of our time,” she added.

The ICAO assembly’s action in Montreal came two days after the European Parliament in Strasbourg, France, voted to ratify the Paris climate agreement of December 2015—advancing the agreement beyond the minimum threshold of 55 countries representing 55 percent of global greenhouse gas emissions it needed to enter force. (The threshold was officially passed on October 5; the agreement enters force on November 4.) International aviation and shipping were not covered by the Paris accord, but they are considered key to helping accomplish its goal of limiting the increase in global average temperature to 1.5 degrees C above pre-industrial levels.

Aviation is the first global industry sector to commit to a carbon-offsetting scheme; the International Maritime Organization is responsible for the shipping industry. ICAO’s “Carbon Offsetting and Reduction Scheme for International Aviation” is among a number of steps the aviation sector will undertake to achieve “carbon-neutral growth,” or capping carbon dioxide (CO₂) emissions, after 2020. They include technological improvements to engines and airframes, air traffic management operational efficiencies and the use of alternative fuels.

The GMBM scheme establishes a voluntary “pilot phase” from 2021 through 2023, followed by a voluntary first phase from 2024 through 2026. The second phase from 2027 through 2035 will be mandatory. Russia and India, which voiced objections during the assembly meeting, are not among the initial participants.

During a conference call with reporters following the ICAO assembly’s vote, Haverkamp and FAA Administrator Michael Huerta said they could not provide an estimate of the airline industry’s cost of complying with the scheme, which at this point is a broad approach with implementing details still to be worked out. Haverkamp noted that the International Air Transport Association (IATA) and U.S.-based Airlines for America both supported its adoption “with the expectation that they can manage the costs.” Carbon credits under the Paris agreement, which is called the UNFCCC, for the United Nations Framework Convention on Climate Change, “will be eligible provided they meet quality criteria set by ICAO,” Haverkamp said.

Environmental groups had mixed reactions to the long-anticipated ICAO action. The International Coalition for Sustainable Aviation (ICSA), which represents six organizations, contends the GMBM scheme will cover only about three-quarters of expected aviation emissions growth between 2021 and 2035. (The U.S. government estimates that up to 80 percent of emissions growth will be covered over the 15-year life of the scheme, said Kevin Welsh, National Security Council director for environment and climate change.) Further, nations “sent a worrying signal” in Montreal by deleting provisions that would align the scheme with the goal of the Paris accord, the coalition said.

“ICAO’s decision to promote carbon offsets for some international flights doesn’t address the core problem,” said Dan Rutherford, aviation program director with the International Council on Clean Transportation, an ICSA coalition member. “In the long run airlines need to decarbonize, not to pay others to do it for them.”

Another coalition member, the Environmental Defense Fund (EDF), issued a more positive response. “This agreement positions aviation to be an engine for achieving reductions in carbon pollution around the globe, while spurring innovations to reduce the sector’s own emissions,” the EDF stated. “Coming the day after the Paris Agreement passed the threshold for entry into force, ICAO’s action sends a powerful signal worldwide that governments, airlines and the aviation industry, and civil society together are recognizing the urgency of climate action.”

IATA in a statement said the historical significance of the Montreal agreement “cannot be overestimated.” The GMBM scheme “has turned years of preparation into an effective solution for airlines to manage their carbon footprint.” But the association acknowledged the scheme, by itself, “will not lead to a sustainable future for aviation. Along with this global market-based measure, the industry will continue to drive its four-pillar strategy on climate change, comprising improvements in technology, operations and infrastructure” complemented by the GMBM scheme.