Which SkyTeam flight is the greenest? Judges will decide.

The global air transport industry has resolved to, somehow, achieve net-zero carbon emissions by 2050.

And while eco-actions are already on the agenda of almost every airline, the SkyTeam Sustainable Flight Challenge is trying to get its airline alliance members to dig deeper to find ways to reduce aviation's impact on the environment.

The competition pits 16 members of the Sky Team airlines alliance against each other in a friendly but very serious challenge to fly the greenest, most sustainable, carbon-neutral commercial flight.

For the contest, which runs May 1 – 14, most airlines are bundling up their existing eco-efforts, adding lots of new ones, using their newest and most fuel-efficient aircraft and, in some cases strategically flying between airports that offer sustainable support services. Some carriers are entering both a short and a long-haul flight into the challenge. Others are showing off their sustainable ideas on just one flight.

Midway through the competition, TPG checked in with some of the airlines that have already, or were about to, complete their challenge flights. Here is a sampling of some of the sustainable efforts being displayed and tested.

Related: Carbon offsetting: How to calculate your carbon footprint when you travel

Delta Air Lines' challenge entry was May 2 on a flight from Atlanta to Salt Lake City on a 737-900ER, the carrier's most fuel-efficient aircraft. The 400 gallons of sustainable aviation fuel (SAF) in the fuel blend was a record for SAF used on a flight out of Atlanta.

The airline was able to use 100% electric ground service equipment at both ends of the flight for tasks such as loading baggage and fuel. And in addition to recently launched sustainable inflight efforts such as new pillows and blankets made from recycled plastic bottles, Delta made this a zero-waste flight by eliminating disposable packaging, recycling all beverage packaging, and sending all food scraps to the composting service available in Salt Lake City.

Air France's challenge flights took place on a flight from Paris – Charles de Gaulle to Montreal on May 3, on an Airbus A350, and a flight from Paris to Lisbon on May 4, on an Airbus A220. Among its goals, Air France said it was hoping to illustrate the effectiveness of already implemented and proven sustainable practices, such as eco-piloting, the use of electric ramp and cargo transport equipment, and sustainable aviation fuel (SAF).

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As of January 1, 2022, all flights fueling up in France must use at least 1% SAF in their fuel mix, but for the Sky Team Sustainable Flight Challenge Air France used 16% SAF for the flight to Montreal and 30% for the flight to Lisbon.

China Airlines' Sustainable Flight Challenge flight was on May 4 from Taipei to Singapore on a flight the airline dubbed "Plum Blossom." In addition to all the environmental and carbon reduction initiatives used on the flight, the carrier offered passengers a mileage bonus incentive for participation.

Travelers could earn 100 miles for each of eight actions, including pre-ordering in-flight meals, self-service check-in, filling out a flight challenge survey, bringing eco-friendly bags for their duty-free purchases, and bringing their own eco-friendly cups and cutlery.

Related: 9 Ways to Go Green on Your Next Flight

For their participation in Korean Airlines' Sustainable Flight Challenge flight on May 5 from Incheon to Atlanta on an eco-friendly Boeing 787-9, passengers received a small gift of appreciation and the assurance that the airline planned to continue the inflight sustainability efforts, such as recycling, even after the flight.

Italian flag carrier ITA Airways is operating its first Flight Challenge flight today (May 7) from Rome to Amsterdam on an Airbus A320 and a second on May 14, between Rome and New York (JFK) with an Airbus A330. For both flights, the carrier says it will be integrating "extraordinary" activities to its usual operating procedures ranging from the choice of aircraft and alternative fuels to sustainable catering, a digital newsstand, and carbon offsetting.

And KLM (Royal Dutch Airlines), which initiated the idea for the Sustainable Flight Challenge, is sending off its two challenge flights today (May 7) from Amsterdam's Schiphol airport.

KLM's flight to Edmonton, Canada, is operating with a Boeing 787-10; the flight to Porto, Portugal, is on an Embraer 190 and both flights are using a fuel blend with 39% SAF.

In a release, KLM said that more than 50 existing and new sustainability measures are being taken aboard both flights, including a wide array of new onboard weight-reduction measures such as asking World Business Class passengers to choose their preferred meal in advance and using AI (artificial intelligence) modeling to predict inflight water requirements.

The other Sky Team alliance members participating in the Sustainable Flight Challenge include Romania flag carrier, TAROM, Aerolineas Argentina, Aeromexico, Air Europa, Garuda Indonesia, Kenya Airways, Saudia, Vietnam Airlines, and Xiamen Air.

Each airline is documenting their challenge flights and in June a panel of nine judges selected for their expertise and to reflect the geographical diversity of participating airlines will choose the winners. The panel includes Jan Peter Balkenende, the former Prime Minister of the Netherlands; climate change and sustainability experts; Zara Rutherford, the youngest woman to fly solo around the world; and others.

And while competing airlines surely want to take home the prizes that will be awarded across 14 categories, the stated main goal of the challenge is to develop and share sustainability solutions and insights that can reduce the carbon footprint of the aviation industry, on the ground and in the air.