A proposed climate accord for airlines is likely to attract support from enough nations to cover about 80 percent of global emissions from international flights, a senior U.S. State Department official said in an interview. The draft agreement adopted Friday by the governing council of the United Nations’s aviation agency would initially be voluntary yet has support from nations responsible for most aviation emissions, including the U.S., China and members of the Europe Union, said the official, who spoke on the condition of anonymity because the negotiations are private.  If approved by a majority of 191 countries this fall, it would be the first global climate accord for a single industry. Airline emissions account for about 2 percent of global greenhouse gases and are forecast to more than triple over the next few decades as flights increase in Asia, Latin America and elsewhere. Airlines were not included in the Paris climate accord because delegates feared the intricacies of divvying up responsibility for international flights could derail the broader agreement.  The UN’s International Civil Aviation Organization has been working for more than a decade on the accord and plans to finalize the agreement at its general assembly from Sept. 27 to Oct. 7 in Montreal. Market-Based System The accord would take effect in 2020. It was brokered last week at a meeting convened by the Civil Aviation Organization’s president, Olumuyiwa Benard Aliu of Nigeria, and attended by the U.S., China and more than 50 other nations. The 15-year deal seeks to cap emissions through a system requiring airlines to purchase credits to offset emissions growth beyond 2020. The draft resolution was adopted Friday by the Civil Aviation Organization’s governing council, said Anthony Philbin, a spokesman for the agency. He declined to provide details of the measure, which has not been released publicly. Earlier versions of the agreement called for mandatory participation for some nations, based on the size of their economies and airline industries. Yet officials struggled to agree on where to draw those lines. Instead, they devised a system that would be voluntary for the first six years. Beginning in 2027, most nations would be required to participate, according to the U.S. official. Annie Petsonk, who has tracked the effort for more than a decade for the Environmental Defense Fund lobby group in New York, said the accord’s success hinges on whether it can draw enough nations to cover 80 to 90 percent of emissions. Unlike other industries, airlines have generally welcomed a global accord limiting their emissions. While the effort could cost companies an estimated $23.9 billion by 2035, a single international agreement would be cheaper to follow than a patchwork of regional accords, according to the Air Transport Action Group, which represents airports, airlines, enginemakers and pilots. The push for a global accord rose to the top of the aviation agenda in 2012, after the European Union said it would require airlines to buy carbon permits for all flights in and out of Europe. That triggered an outcry from China, Egypt, Brazil and other nations that argued the measure was beyond the EU’s authority. Europe agreed to suspend the program and allow the international community to negotiate an international deal. Officials still need to finalize details of the accord. That includes how to balance responsibility between the large slow-growing airlines that emit the most emissions and smaller carriers, mostly from developing nations, that are poised to grow.