An American Airlines plane taxis near a group of planes from Alaska Airlines, Monday, Oct. 28, 2013, at Seattle-Tacoma International Airport in Seattle.  (AP Photo/Ted S. Warren)

A United Nations panel has proposed the first-ever standards for carbon dioxide emissions for aircraft worldwide — a historic step, but one that environmental advocates immediately criticized as being inadequate.

The rules, proposed by a committee of the International Civil Aviation Organization, are aimed at large aircraft — which produce the most emissions — but include smaller planes involved in international flights as well. They would apply to new designs as of 2020 and to new deliveries of all such aircraft from 2023 on.

“Our sector presently accounts for under two percent of the world’s annual CO2 emissions, but we also recognize that the projected doubling of global passengers and flights by 2030 must be managed responsibly and sustainably,” said Olumuyiwa Benard Aliu, president of the broader ICAO Council, in a statement. The council will ultimately have to approve the committee’s proposal for it to go into effect.

The White House hailed the standards Monday, saying they “are expected to reduce carbon emissions more than 650 million tons between 2020 and 2040, equivalent to removing over 140 million cars from the road for a year.”

However, green groups instantly raised numerous objections. Existing aircraft wouldn’t have to improve their efficiency under the standard, charged Sarah Burt, an expert with Earthjustice, on a press call with reporters. “By exempting the thousands of aircraft flying today, ICAO has set the bar embarrassingly low,” she said.

“Given the substantial lead time for the standards, along with anticipated fuel efficiency gains for new aircraft types already in development by manufacturers, the standards will serve primarily to prevent backsliding in emissions,” added Daniel Rutherford of the International Council on Clean Transportation by email. “Additional action would be required for the standard to reduce emissions below business as usual.”

Last year, the U.S. Environmental Protection Agency issued a proposed Clean Air Act “endangerment finding” for aircraft emissions, saying it planned to adopt upcoming ICAO standards. That prompted criticism from environmental groups who feel the international body is too weak in its approach.

The move is also significant because aircraft are expected to contribute an increasing amount of global carbon dioxide emissions in coming years, up from about 1.3 percent of the total today.

“If international aviation were a country, it would be a top ten emitter of carbon dioxide (CO2), on par with Germany or the United Kingdom. And it’s expected to grow enormously: with more than 50,000 new large aircraft slated to take to the skies, its emissions are expected to triple or quadruple by 2040,” wrote Annie Petsonk, an attorney with the Environmental Defense Fund, in anticipation of the ICAO move.

There is a second and potentially more consequential step to the ICAO process, which will involve ensuring that the projected growth of the airline industry is achieved in a carbon neutral manner after the year 2020. “We encourage ICAO to immediately begin negotiating the second phase of this program, recognizing that aircraft emit 11 percent of the U.S. transportation greenhouse gas inventory,” said Bill Becker, the executive director of the National Association of Clean Air Agencies.

Aircraft, even more than automobiles, are difficult to decarbonize because they are mobile and require energy-dense fuels. While there is an increasing move toward electric rather than gasoline-powered cars — whose batteries can, at least theoretically, be charged with solar-generated electrons — the possibility of similarly powering aircraft remains far off. Biofuels appear considerably more promising, at least in theory, but the biofuels sector has struggled in its own right.

Aircraft emissions also went unmentioned in the final 2015 Paris climate agreement, although earlier drafts of the agreement did note the need to address emissions from this key sector.

Vera Pardee, senior counsel with the Center for Biological Diversity, which has pushed the EPA to pursue tougher regulations on airline emissions, said that the weakness of ICAO’s proposed standards should force the agency to go further on its own.

“EPA has a duty not just to set any sort of standards, but one that protects public health or welfare,” Pardee said. “The standard at present does not begin to meet that requirement, because emissions will continue to go up.”

The U.S. airline industry, however, praised the move. “We strongly support the aircraft CO2 standards put forth by [the ICAO committee], as they will further support our global aviation coalition’s emissions goals to achieve 1.5 percent annual average fuel efficiency improvements through 2020 and carbon neutral growth from 2020,” said Nancy Young, vice president of environmental affairs at Airlines for America or A4A, the industry trade group.

Observers of the process say the Obama administration was highly involved and active in seeking tougher standards. “The U.S. has been pushing very hard, and a lot has to do with the fact that this president has given guidance to the agencies, EPA and FAA, to pursue a strong climate action for aircraft,” said Margo Oge, the former director of the EPA’s Office of Transportation and Air Quality.