AMNA NAWAZ: The Memorial Day season is the unofficial start of a big summer travel season, and that means a lot of driving and flying.

When we talk about greenhouse gas emissions and transportation, we largely focus on cars and trucks, since they generate the most.

But the aviation sector is under pressure to reduce its carbon footprint too.

In the second of two reports, science correspondent Miles O'Brien looks at efforts to create greener fuels for the skies.

MILES O'BRIEN: A half-mile outside the fence from Boston's Logan Airport, Carlos Flores is helping grease the skids for an ambitious goal, erasing the carbon footprint of airline travel.

He is at a Wingstop, harvesting used cooking oil, or UCO.

It contains hydrocarbons and can be refined into sustainable aviation fuel, or SAF.

UCO to SAF, from Wingstop to wing tank.

CARLOS FLORES, Mahoney Environmental: Every time I fly back home in Brazil, I think about it.

It's like, maybe I help put some fuel in here, you know?

MILES O'BRIEN: He drives for Mahoney Environmental, a subsidiary of Neste, a Finnish oil refiner that is now a global leader in renewable fuel production, including sustainable aviation fuel.

Dave Kimball is Mahoney's president and CEO.

DAVE KIMBALL, President and CEO, Mahoney Environmental: So the really cool thing about cooking oil is, it's already had one life, and now we're having a second life with it.

MILES O'BRIEN: Mahoney currently sucks about 400 million pounds of grease out of dumpsters nationwide.

It's cooking up plans to retrieve a billion by 2030.

Even though sustainable aviation fuel is two or three times more expensive than the fossil alternative, the airlines are demanding it.

Facing public backlash over its climate footprint, the industry has set an aggressive goal, net zero carbon emissions by 2050, and it has no other short-term alternative to fossil fuels.

DAVE KIMBALL: It's a drop-in fuel, so you don't have to modify anything to use it.

You don't have to build charging stations for airplanes and all those types of things.

So, to me, that's the logical next step.

MILES O'BRIEN: Globally, sustainable aviation fuel production will likely reach nearly a half-billion gallons in 2024, a six-fold increase since 2022, and yet still only one-half of 1 percent of the 99 billion gallon annual burn rate for jet fuel.

In 2021, the Biden administration launched a sustainable aviation fuel grand challenge.

The goal is to produce 35 billion gallons of SAF in the U.S. by 2050.

But to get there, grease is not the only word.

JERRY TUSKAN, Department of Energy Director, Center for Bioenergy Innovation at Oak Ridge National Laboratory: There won't be a silver bullet.

There won't be one commodity that will satisfy the 35 billion gallon target.

MILES O'BRIEN: Jerry Tuskan is director of the Center for Bioenergy Innovation at the Oak Ridge National Laboratory in Tennessee.

He says oil from fryer grease, soybeans and corn can produce a third of that goal, adding hydrocarbons to existing ethanol production can address another third, and the rest will have to come from new crops dedicated to energy.

He says 20 to 40 million acres of land will be needed.

There are about 900 million acres of farmland in the U.S. We can have it all and not have to make a choice between food and fuel?

JERRY TUSKAN: There is enough land potentially available to produce 35 billion gallons of aviation fuel.

It will take a portfolio or a mixture of species geared toward adaptive production in specific regions.

MILES O'BRIEN: The Oak Ridge team is partnered with 17 other institutions, including the University of University of Illinois, Urbana-Champaign, home of the Center for Advanced Bioenergy and Bioproducts Innovation.

Agronomist Emily Heaton is a professor in the Department of Crop Sciences.

EMILY HEATON, Department of Crop Sciences, University of Illinois, Urbana-Champaign: We are at the stage where we're testing the first iterations of making jet fuels from the bioenergy crops that we have today.

MILES O'BRIEN: She gave me a tour of their energy farm, where they grow, tweak and study so-called bioenergy crops.

When you say bioenergy crops, what are we -- what exactly are we talking about?

EMILY HEATON: We are talking about crops that are used to capture carbon out of the atmosphere and use in place of plants that captured carbon out of the atmosphere millions of years ago, which are fossil fuels.

MILES O'BRIEN: The carbon that renewable fuels emit when burned is offset by the CO2 absorbed as the feedstock grows in the field.

Because the cycle does not unearth any ancient carbon, it is called net zero.

One of the leading contenders for sustainable aviation fuel is miscanthus giganteus, a hardy, fast-growing perennial grass plant that thrives on marginal land in cold climates.

Oh, wow, a little -- it's doing well in here, huh?

EMILY HEATON: They're getting pretty big.

It's about time to cut them back.

MILES O'BRIEN: Did you bring the machete?

(LAUGHTER) EMILY HEATON: We actually have several.

MILES O'BRIEN: Inside this greenhouse, they are crossbreeding miscanthus with sugarcane, hoping to add fatty compounds known as lipids to it to make the conversion to aviation fuel cheaper and easier.

So how much growth is this?

How long did it take for them to get this big?

EMILY HEATON: So for a mature plant, this is a single growing season's worth of biomass.

MILES O'BRIEN: It can grow 14 feet high, but that's just half the picture.

EMILY HEATON: You can start to get a feel for what's below ground.

MILES O'BRIEN: There is an equal amount of biomass beneath the surface.

EMILY HEATON: And if you include the avoided fossil emissions, because we're not fertilizing very much, we're not tilling, and it's storing things below ground, it comes back carbon-negative.

MILES O'BRIEN: Not just zero, carbon-negative.

The energy farm is outfitted with a million-dollar network of air, water, soil, and weather sensors to verify the true carbon budget of these crops.

But, ultimately, it will be the budget of farmers that will determine the success of these ideas.

It's a chicken-and-egg problem, as I learned one morning when I visited Emily's parents' farm 20 miles west of Urbana.

JOHN CAVENY, Caveny Farm: So you're going to put a wire in here, here, and here.

MILES O'BRIEN: John and Connie Caveny are focused on pasture-raised beef and lamb.

They know a lot about growing grass.

But, right now, it's not a viable option for most farmers.

The streamlined infrastructure that makes this such a productive place to grow corn and soybeans does not exist for grass production.

If you're thinking about growing grass that ultimately might fuel an airplane, the system isn't set up for that, is it?

JOHN CAVENY: No.

It's a long way off.

MILES O'BRIEN: To entice farmers to grow energy crops, they will need new equipment, financing and crop insurance.

For now, it's a field of dreams, except, if you build it, the market may not come.

JOHN CAVENY: The best use for miscanthus right now is animal bedding.

MILES O'BRIEN: That's it?

JOHN CAVENY: That's it.

We plowed up a lot of it.

MILES O'BRIEN: But this family is undeterred.

Energy crops not only offer benefits for the climate.

They also improve the local environment, reducing run-off and improving soil health, adding diversity.

EMILY HEATON: Getting back to our roots using contemporary carbon to base our society, instead of fossil carbon, is a choice that we need to make if we are to persist on this planet.

MILES O'BRIEN: And still freely travel around it without carrying a lot of excess carbon baggage.

For the "PBS NewsHour," I'm Miles O'Brien in Boston.