"The Trump administration’s refusal to curb plane pollution is fuelling the climate crisis,” said Clare Lakewood, climate legal director at the Center for Biological Diversity, which along with Friends of the Earth filed a notice of intent to sue the EPA if the agency did not act within six months.

“Airplane pollution is increasing at really worrying rates, but the EPA just keeps refusing to address this skyrocketing threat to our rapidly warming planet," Lockwood said.

Airplanes are the third-largest source of U.S. transportation-related greenhouse gas emissions and the largest not subject to greenhouse gas emissions standards.

By 2020, emissions from global international aviation are projected to be about 70% higher than in 2005 because of rising travel demand. Worldwide, passenger numbers are forecast to double from 2017 levels to 8.2 billion by 2037.

Commercial flying currently accounts for about 2% of global carbon emissions and about 12% of transport emissions.

An EPA spokeswoman said the agency "is working to address this issue in 2020" and agency documents say it intends to issue proposed rules in February. The EPA said last year it would release a proposed rule by September 2019, but failed to do so.

In 2010, environmental groups sued the EPA to force it to set greenhouse gas emissions standards for airplanes. A judge ruled the EPA was required to address aviation emissions.

In February 2016, after six years of talks, the U.N. International Civil Aviation Organization (ICAO) agreed on a global standard aimed at makers of small and large planes, including Airbus SE and Boeing Co, that will apply to all new aircraft models launched after 2020.

The EPA said in agency documents it "anticipates adopting domestic GHG (greenhouse gas) standards that would be at least as stringent as ICAO's standards."

Boeing has said it supports the ICAO rules. Both Airbus and Boeing are working to build significantly more fuel-efficient airplanes.

To achieve carbon-neutral growth after 2020, despite rising traffic, commercial airlines aim to use more fuel-efficient aircraft, find more direct flight paths by improved air traffic control and substitute conventional fuel with more sustainable biofuels.

  

By David Shepardson