Before the COVID-19 pandemic, when the global aviation sector was flying high in 2019, it contributed almost 6% of the planet-warming greenhouse gases in our atmosphere. A year later, with the industry crippled by pandemic-related flight cancellations, that figure had dropped by 43%. Last year, it was still 37% lower.

But air traffic has been steadily increasing, according to industry body the International Air Transport Association.

Greenhouse gas emissions are rising too. In response, the European Parliament has announced a proposal to intoduce environmental labels for air travel from 2025. The sytem would serve to inform passengers about the climate footprint of their flights.

Carbon dioxide accounts for only about a of the global warming effect attributed to air travel. Two-thirds is caused by other factors, most significantly the condensation trails, or contrails, aircraft leave behind.

Alternative flight routes could prevent contrails

Contrails — those narrow, white clouds that trace an airplane's path through the sky — are formed when jet fuel, which contains kerosene, burns. At the average cruising altitude of between 8,000 to 12,000 meters (around 26,000 to 40,000 feet), low temperatures cause water vapor to condense around the soot and sulfur left behind by jet emissions. The resulting ice crystals can remain suspended in the air for hours.

Contrails trap heat in the atmosphere, much like in a greenhouse, greatly amplifying the impact of flying on the world's climate. Recent studies have shown that contrails are around 1.7 times more damaging than CO2 emissions, when it comes to global warming.

On the plus side, contrails are relatively easy to avoid. Using satellite data, flight planners can optimize aircraft routes to avoid weather patterns that favor the formation of contrails. Pilots can also fly their jets 500 to 1,000 meters lower, for example, where temperatures aren't as cold.

"It doesn't require much effort to make these changes," said Markus Fischer, divisional director at the German Aerospace Center, adding that it would mean between 1 to 5% more fuel and flight time. However, he told DW, that would result in a 30 to 80% reduction in the warming effect caused by factors other than CO2, he said.

The European Union aims to include these non-CO2 climate effects in future European emissions trading agreements. Airlines will have to start reporting such pollutants from 2025, according to a preliminary agreement in the European Parliament.

Producing e-kerosene with green energy

Burning kerosene derived from petroleum produces lots of CO2 and, at high altitudes, other greenhouse gases such as ozone. The CO2-free alternative is e-kerosene.

E-kerosene can be produced in a climate-neutral way using green electricity, water and CO2 extracted from the air. First, hydrogen is generated using a process involving electrolysis, and then CO2 is added to produce synthetic e-kerosene.

The problem is that to be cost-effective, e-kerosene needs to be made with plenty of solar and wind power — and so far, there isn't enough of this renewable energy. New production plants for green hydrogen, CO2 direct air capture and synthetic fuels must also be built.

The EU is pushing for at least 2% of aviation fuels to be environmentally friendly by 2025, increasing every five yearsl to reach 70% by 2050. The proposal is yet to be passed.

Could planes be powered with cooking oil?

Another option for planes is to refuel with biokerosene, which can be made from rapeseed, jatropha seeds or old cooking oil. Small-scale production plants already exist, but producers would need to greatly expand capacity to keep up with demand. Intensive production of biokerosene is also limited by the scarcity of arable land — the use of which is itself controversial, as it prevents take space needed for growing food.

Under a European Commission proposal, biofuels and e-kerosene would be mixed with conventional fossil kerosene from 2025. The share of biofuels in the mix would then rise by 2% per year, to reach 63% by 2050. The European Parliament has set a goal of 85% by mid-century.

Battery-powered short-haul flights on the horizon

With electric engines and batteries, flights could avoid producing direct emissions or heat-trapping contrails. But current batteries are too heavy and have insufficient storage capacity, limiting planes to short distances of just a few hundred kilometers.

Several companies are in the process of tinkering with battery and aircraft optimization. Israeli manufacturer Eviation Aircraft, for example, is building an all-electric jet with seating for nine passengers. The private aircraft is expected to have a range of 445 kilometers and a top speed of 400 kilometers per hour (about 250 miles per hour).

Norway is aiming to launch the first regularly scheduled electric flight service in less than three years. The country plans to connect the coastal cities of Bergen and Stavanger, some 160 kilometers apart, with a flight served by a battery-powered aircraft with space for 12 passengers from 2026.

Hydrogen aircraft show promise, but not ready

Smaller airplanes that run on hydrogen have recently been in the spotlight. These aircraft use hydrogen fuel cells to generate electricity and efficiently power the plane's propellers. The jet engines on long-haul aircraft can also run on hydrogen, but would be less efficient.

European aircraft manufacturer Airbus is planning to launch a hydrogen-powered passenger plane by 2035. These aircraft could account for more than 30% of global air traffic by 2050, according to a study by global consulting firm McKinsey.

But hydrogen-powered aircraft continue to pose numerous challenges. The volatile gas only becomes liquid at minus 253 degrees Celsius (minus 423 Fahrenheit), and must be stored under high pressure in special tanks. That means extra space and weight requirements for airplanes, and those plans have yet to be developed. In addition, airports will need to develop new refueling infrastructure for hydrogen-powered aircraft.

One sure way to reduce emissions: Fly less

Even in the most optimistic scenarios, air travel won't be completely free of emissions by 2050. Experts believe that if the industry implements ambitious restructuring plans — completely replacing standard jet fuel with green hydrogen and e-kerosene, and rerouting planes to prevent contrails — it could reduce greenhouse gas emissions by up to 90%.

However, a recent study in the scientific journal Nature noted that even a complete switch to e-kerosene would still result in a residual negative effect on the climate. Therefore, avoiding all but necessary flights and giving preference to climate-friendly modes of transportation remains key, said the UBA, Germany's federal environment agency.

Aviation experts have also stressed the need for new, lightweight airplanes with optimized wings, the use of propellers instead of jet engines and reducing airspeed. They point out that these measures could reduce fuel use by around 50%, compared to today.

Integrating environmental costs into the price of airline tickets would help to implement all these measures, said European clean transport campaign group Transport and Environment (T&E). Airlines currently pay nothing to account for their contribution to the climate crisis. Including environmental costs in air fares would be a fair way to promote a restructuring of the aviation industry, and would make it easier to switch to climate-friendly modes of transport, according to T&E.