Passengers flying out of Denver International Airport each contribute up to a ton of heat-trapping carbon dioxide before reaching their destinations as airplanes burn fossil fuels.

But those tens of millions of passengers in the future could fly without worsening climate warming if the oil-based fuels are replaced with new fuels being developed at the National Renewable Energy Lab in Golden.

The new aviation fuels are made from a widening variety of plant and other materials — including municipal and industrial waste, algae and corn crop residues — that don’t require drilling for new oil.

Federal government-backed researchers are working toward a target of producing 3 billion gallons a year of this “sustainable aviation fuel” by 2030 and 35 billion gallons by 2050 – enough to meet the projected U.S. demand as air travel at least doubles. President Joe Biden has declared aviation and shipping fuel must be carbon-neutral by 2050. The Paramount refinery in California already produces non-oil fuel for flights out of Los Angeles International Airport.

Heat-trapping pollution from air travel looms as one of the hardest challenges in containing the climate warming that has led to intensifying droughts, floods, fires and other calamities.  Global aviation pollution contributes significantly to warming – an amount equal to the total emissions from Germany – a share expected to increase as more humans fly.

Every time a super-charged hurricane hammers the southeastern coast “there is increased urgency,”  U.S. Department of Energy mechanical engineer Zia Abdullah said at NREL this week.

The new fuel “will be completely indistinguishable at the molecular level from the fuel that goes into airplanes today. The only difference is that it will not be fossil fuel coming out of the ground,” said Abdullah, the manager of DOE’s bioenergy technology lab programs.

“When we use fossil fuels, we’re taking carbon out of the ground and adding it into the atmosphere, which is causing the global warming problem,” he said.

“If we use renewable carbon, the airplane is still going to put out carbon dioxide. But there’s no net new carbon. That’s where we want to go. We want to set up a cycle of renewable carbon – and not have new net carbon coming into the atmosphere. This will mean that, in the future, we won’t have rising levels of carbon dioxide.”

The global average concentration of carbon dioxide, the main heat-trapping pollutant, has reached 421 parts per million — up from 280 ppm in the 19th Century before industrialization, federal climate scientists say. The last time the concentrations reached this high was 14 million years ago, before human life.

Making the new fuels requires chemical conversions. At NREL labs, a team including researchers from the Massachusetts Institute of Technology and the University of Washington recently has focused on lignin, a widespread material from plants, along with cellulose, as a potential source. They’ve developed a system for removing oxygen from lignin pulp. Aviation fuel must contain less than 1% oxygen.

Flying without burning new fossil fuels also could be done using hydrogen fuel or electricity, researchers say, though these face huge hurdles. And a Dutch team in 2016 demonstrated solar possibilities, flying a solar-panel-laden Solar Impulse aircraft around the world (26,000 miles) in 505 days. The aircraft weighed 2.4 tons and flew at an average speed of 45 mph.

Worldwide, air travel contributes 2% to 4% of human-induced climate warming and requires 106 billion gallons of aviation fuel, (projected to reach 230 billion gallons a year by 2050), according to aviation industry studies and International Air Transport Association data. That’s less than food production, ground transport in cars and trucks, and industrial pollution – yet individual transport decisions bring relatively heavy impact.

Flights originating at U.S. airports put out roughly 179 million tons a year, or 23% of the total global aviation emissions of heat-trapping gases (around 800 million tons).

Denver International Airport ranks 6th among airports worldwide for heat-trapping pollution (4 million tons) from domestic flights (behind the airports in Los Angeles, Atlanta, Beijing, Chicago and Dallas), commercial aviation data shows.

A passenger flying from Denver to Frankfurt contributes a share of roughly 1,635 pounds of carbon dioxide pollution.

From Denver to New Delhi, a passenger contributes 2,451 pounds.

To Mexico City: 458 pounds.

By comparison, the per-passenger pollution from ground travel burning a gallon of gas (covering 25 miles or so depending on vehicle fuel efficiency) is about 19 pounds. Driving from Denver to Mexico City (a 28-hour trip) would release around 1,615 pounds of heat-trapping carbon dioxide — more than three times as much as traveling by plane.

Federal energy officials say electric-powered airplanes likely will be able to handle short commuter flights, such as the route from Denver to Grand Junction. These account for less than 5% of global airline pollution.

Turning plants and other material into fuels fast enough is the challenge, researchers said. Their work with lignin, a widely available aromatic material, has established it could be used to produce fuel that would power current aircraft — essential because new airplanes today are expected to operate for decades.

For municipal landfill waste, gasification plants are required. And corn husks from ethanol production can be converted to fuel.

“We’re not going to be able to make 35 billion gallons a year of fuel from one single feedstock,” Abdullah said. “We need to develop technology that can use feedstock from forestry materials after timber is harvested, and municipal waste, and also now we’re developing technology where we can convert carbon dioxide captured from industrial facilities.”

NREL researchers have been talking with officials at DIA and the Suncor oil refinery in Commerce City. Existing equipment at refineries can be used if sufficient source material can be delivered, Abdullah said. “Airports seem open and welcoming to the fuel. The main challenge is how quickly we can make it.”

Simply not flying, or transporting goods across oceans, is considered unrealistic.

“You have to fly. The issue is how you do that – live your life, meet up with your family,” he said. “How do you do that without making such an impact on our environment?’