More than 43 million Americans are expected to fly over the winter holidays, and among them will be many conflicted eco-conscious travelers. Airplanes release huge quantities of carbon dioxide into the atmosphere, which contributes to global warming.

“I do fly, and I feel very guilty about it,” said Nadine Mellor, 44, of San Francisco. “I try to lead a low-carbon lifestyle — we don’t own a car, and as a family we’re making a conscious effort to reduce our carbon emissions. But my family lives in the United Kingdom, and there’s no getting away from flying if I want to see them.”

For reluctant fliers like Mellor, the recession has brought a bit of good news. It has pushed airlines to find ways to save money on jet fuel, by far their largest operating expense. That’s led to scores of initiatives to make planes more fuel-efficient, from investing in biofuels, to using just one engine to taxi as they prepare for takeoff, to switching to lighter seats, meal carts and carpets to reduce the weight in the cabin.

“It’s not often that economic interests and environmental interests align, but airlines are very focused on minimizing fuel burn,” said Nancy Young, vice president of environmental affairs at the Air Transport Association, the industry trade organization for the leading U.S. airlines. “All of the airlines are very focused on this.”

The metric most experts use when talking about greenhouse gas emissions and transportation is “gallons of fuel consumed per passenger mile.” When it comes to air travel, that measurement is influenced by numerous factors, from the kind of aircraft flown to baggage weight and the number of passengers onboard.

That makes comparing the emissions from a plane to a bus, train or car difficult. But aviation experts say every gallon of burned jet fuel emits about 3.1 gallons of carbon dioxide, and U.S. airlines consumed 1.4 billion gallons of jet fuel in September, according to the Air Transport Association.

“The challenge for the aviation industry is the fuel question,” said Deron Lovaas, transportation and energy project co-director of the Natural Resources Defense Council, who has noted that a high-speed electric train emits just a tenth to a quarter of the greenhouse gases of a commercial airliner. “Cars and trucks can run on natural gas, and we’re now seeing electric cars,” said Lovaas. “But substitutes for jet fuel are harder to come buy and harder to engineer, and so the big question is still, ‘How do you move to more-sustainable fuels?'”

The only California-based airline, Virgin America, whose hub is San Francisco International Airport, says one way to make air travel more green is to use newer planes. It has just 34 planes in its fleet, and they are all newer Airbus A320s.

“We have the youngest fleet of planes in the industry,” said spokeswoman Abby Lunardini. “In terms of fuel efficiency, an older plane is like flying a Cadillac, whereas a newer plane is a Lexus Hybrid.”

Virgin America has built numerous sustainable business practices into its operations, from serving cage-free eggs in meals to making sure its new terminal at SFO, currently under construction, is LEED (Leadership in Energy and Environmental Design) certified.

Virgin has also partnered with CarbonFund.org to allow customers to buy “carbon offsets” during flight. Customers use a touch-screen entertainment monitor to buy a $5 or $10 carbon offset, which helps reduce the environmental impact of the flight’s carbon dioxide emissions by investing in projects — like capturing methane at dairy farms — elsewhere in the United States.

“A lot of our fliers are focused on sustainability,” said Lunardini, adding that the number of customers buying offsets, while small, is growing. Virgin America’s parent company is the Virgin Group, which has pledged to reinvest all of its profits into renewable fuels research.

Southwest Airlines, which uses Boeing 737s, has installed “winglets” on the tips of the wings of all of its planes to better harness wind power.

“If you look out the window at the tip of the wing where it goes up, that’s the winglet,” said Southwest spokeswoman Marilee McInnis. “It makes the plane more aerodynamic.”

Among the 540 planes in Southwest’s fleet is a “Green Plane,” an experimental jet that tests lighter materials in the cabin, from seat covers to life vests. There’s no way to book a flight on the Green Plane, but it can be identified by a green decal on its nose and the tail number WN222, McInnis said.

“Our aircraft have 137 seats on board, and the Green Plane is saving about five pounds per seat,” she said. “That’s significant fuel savings.”

The airline industry is also eager to move from a ground-based air traffic control system to space-based global navigation satellites, which Lovaas of the NRDC said would lead to more efficient flight routes and better flight patterns. A lot of fuel is currently wasted as planes circle crowded airports to land. But the cost of the effort, known as “NextGen,” is estimated to be about $40 billion.

McInnis of Southwest has one piece of advice for consumers who worry about the environmental impact of their flight.

“The single most important thing that people can do is pack lighter,” she said. “Just pack one pair of shoes.”

Contact Dana Hull at 408-920-2706.