CIVIL aviation accounts for perhaps only 2% of man-made carbon emissions. Add in other pollutants, such as contrails and nitrous oxide, and the industry’s overall contribution to climate change might be twice that figure. That may not seem much. But the sector is growing rapidly. Since the 1970s, global air traffic has doubled in size about every 15 years. Rising prosperity in developing countries and massive backlogs of aircraft orders means that the growth will continue for decades. Without regulation, the world’s airlines could quickly choke the skies.

Last week, the International Civil Aviation Organisation (ICAO), a United Nations agency, announced a framework for mandatory carbon-offsetting on all international flights. The agreement was backed by 65 countries, which between them account for 86.5% of international flight operations.

International flights, unlike domestic ones, were exempt from last year’s Paris Agreement on climate change. This new tentative accord seeks to plug that gaping hole. The Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA) will come into effect in 2021, when signatories will begin voluntarily monitoring and policing pollution on international flights. By 2027, participation is supposed to be mandatory for all nations. Using 2019 and 2020 pollution levels as a baseline, countries and their carriers will be allocated a set quota of “emissions units” each year. Those that exceed their share will then have to buy additional units, which will be traded openly on a carbon exchange. The revenue generated from these transactions will go directly to UN-approved environmental schemes and carbon-offset projects.

Regulation must walk a fine line. Air transport aids economic growth. Planes, in the memorable words of a colleague, are the “silver needles that sew the world together”. Several emerging economies with large and fast-expanding air transport sectors, including Russia, India and, for the time being, Brazil, have declined to join the scheme in 2021. Their governments care more about the economic benefits of industrial revolution than its environmental costs, just as Western nations did with impunity over the past two centuries. Credit should be given to China, the world’s second-largest aviation market, for joining the voluntary scheme despite grappling with this dilemma. But quite whether the naysayers will change their tune by 2027, when opting out is no longer an option, in theory at least, is open to question. Russia is busily drawing up a rival plan, which must make compliance unlikely. And ICAO has baulked at talk of punitive action against those who fail to sign up.

The offset scheme is just one of four measures that Olumuyiwa Aliu, ICAO’s president, says will help towards the “aspirational goal” of carbon-neutral growth from 2020. Improved engine technology, more efficient flightpaths and the gradual uptake of biofuels must also play their part, he says. (The scientific benefits and commercial viability of the latter are far from universally recognised.)

Such a lofty target will be difficult to hit. But if there is hope it lies with the unwavering support of airlines. Given that they can expect to spend up to 1.4% of their international revenues on the CORSIA scheme by 2035, and that carriers have traditionally struggled to break even, one might assume the industry would be fiercely opposed to the idea of coughing up. Yet airlines are stacking up to pay the carbon tax. Their enthusiasm for a global agreement, says Mr Aliu, reflects the “nightmare” alternative of a patchwork of regional and national schemes—something the European Union threatened to unleash four years ago with its Emissions Trading System. Better to stump up for one costly but simple global mechanism, rather than expend untold energy and money complying with a hodgepodge of smaller ones.

Those grown accustomed down the years of hearing airline bosses scoffing at environmental concerns will be skeptical. Probably the promise of fully carbon-neutral aviation growth from 2020 has too many caveats and omissions to be feasible. But, as a means of rallying political will and forging a new path for a gas-guzzling industry, ICAO’s efforts are laudable.