The airline industry is responsible for an estimated 2 percent of global carbon emissions, but so far no government has enacted emissions standards for aircraft similar to those in place for cars and trucks in many countries and regions.
Even the recent far-reaching Paris climate-change talks included no concrete action on airplane emissions.
But now the United Nations' aviation agency says it has drafted the first-ever emissions rules for airplanes.
Announced in Montreal, proposed rules from the International Civil Aviation Organization (ICAO) would apply to all new airplanes delivered after 2028.
For planes delivered after that date, they call for a 4-percent reduction in emissions of carbon dioxide and other substances compared to 2015 deliveries.
They also set new limits for airplanes already in production that would be delivered after 2023, with emissions reductions of 0 to 11 percent, depending on a plane's size.
Boeing 787 Dreamliner
These airplane emissions standards still need to be finalized, but they're already drawing strong reactions--both pro and con.
Certain environmental groups believe the proposed rules don't go far enough, in particular because they don't address aircraft currently in use.
The proposed standards won't make a dent in airline emissions growth, Vera Pardee--a lawyer with the Center for Biological Diversity--told The New York Times, calling the situation "deplorable."
Some analysts believe airline emissions could triple by the middle of the century because of projected growth in air travel.
The Center for Biological Diversity is pressing the U.S. Environmental Protection Agency (EPA) to draft tougher standards of its own.
But while the Obama Administration has discussed setting emissions standards for airlines, the nature of the industry has made that difficult.
The administration last year issued a legal finding naming airplane emissions a threat to human health because of their contribution to global warming, and calling for regulation of emissions under the Clean Air Act.
Yet because of the global nature of the airline industry, the administration said at the time that it would wait for the international rules to be drafted before issuing its own standards.
For its part, the administration is pleased with the draft U.N. regulations. In a statement, the White House said the proposed rules will cut carbon emissions more than 650 million metric tons between 2020 and 2040--equivalent to taking 140 million cars off the road for a year.
Before they become binding, the proposed rules must be formally adopted by the civil aviation council of 36 member states in June. Then they must be endorsed by the council's assembly in October.
Southwest Airlines Boeing 737 (Image: Flickr user Aero Icarus, used under CC license)
They would then need to be adopted by each member state, although countries would still have the option to enact even stricter rules of their own.
The airplane-emissions standards were actually one half of a two-part negotiation process.
A second phase would focus on making airlines pay for their carbon emissions through a market-based system.
Discussions on that front will reportedly be completed by the end of the year.