Increasing global air traffic is commonly regarded as a climate catastrophe, with the aviation industry alone comprising 5 percent of greenhouse gases produced annually.

With the German Aerospace Center (DLR) expecting jet fuel demand to increase 50 percent by 2030, environmental prospects for the industry are dire.

But what if flying could be carbon-neutral; indeed, climate-friendly? It's a little-known fact that this is possible.

Flying high increases warming

To reach the goals of the Paris climate agreement to limit global warming to well below 2 degrees Celsius, every person on Earth would be limited to producing an average of only 2 tons of CO2 annually over the next 30 years.

However, a roundtrip between Berlin and New York in a relatively efficient Airbus 380 generates greenhouse gases equaling some three tons of CO2 in the environment. That's because, in addition to the direct CO2 emissions (one ton CO2 equivalent), the flight results in increased formation of ozone in the clouds (two tons).

A particular problem for air traffic is emissions is the high altitude. In addition to CO2, production of nitrogen oxide through jet engine exhaust produces ozone, a major cause of global warming.

Contrails, or condensation trails, are also produced, which create clouds made of ice crystals that can also trap greenhouse gases, warming the climate.

Contrails: depending upon temperature and humidity, they can cause clouds to form over the longer term

Overall, the climate impact of jet fuel combustion is about three times as high when planes are in the stratosphere than when they are on the ground.

Climate-friendly flying?

With climate-optimized flight routes, however, the negative effects of flying can also be reduced and can "even go in the opposite direction," according to Stefanie Meilinger of the International Center for Sustainable Development at Bonn-Rhein-Sieg University in Germany.

The greatest potential lies in preventing the creation of clouds. "Condensation trails and fog clouds are formed out of ice crystals in certain weather conditions," Meilinger explained. Climate-optimized flight routes would work by avoiding such weather, hence limiting cloud generation.

Beyond this, such clouds can have either a warming or cooling effect on the climate.

Stefanie Meilinger

That would depend on the substratum and ratio of different reflections, she explained.

"If the solar radiation is reflected by clouds formed by plane flight back into outer space, this has a cooling effect," she said. "But if the Earth's radiation of heat into space is hampered by clouds, the climate is further heated."

This all depends on the route the plane flies. "With our current routes, we have an overall warming effect," Meilinger told DW.

To protect the climate, optimize flight routes

Under the direction of DLR and in cooperation with the German Meteorological Service (DWD) and the German Air Traffic Control (Deutsche Flugsicherung), Meilinger has analyzed Lufthansa flights to assess the possibility of designing air traffic routes that limit climate damage. Ideally, they should even contribute to climate protection.

Meilinger's research team developed software for climate-optimized flight routes which, once combined with weather forecast data, meant planes could avoid regions with warming clouds. Moreover, regions for cooling cloud formation on the flight route could also be targeted.

The scientists simulated climate-optimized flight routes for 40,000 Lufthansa flights, which in models succeeded in reducing the overall climate warming effects. "Even net-cooling air traffic" was realized on some European routes, stated the report.

"On the one hand, there is the possibility to close particularly climate-damaging air routes for air transport," explained Urban Weißhaar, a flight route expert at Lufthansa Systems. "The other possibility is the inclusion of cloud formation in emissions trading."

This, however, would result in some effects on passengers and airlines.

Cloud formation with climate impact could be priced, like CO2 emissions. Airlines with climate-friendly routes could gain an advantage by paying less money for pollution certificates - making any additional costs by a small detour very worthwhile. For climate protection, at least.

Renewable jet fuel

Volker Grewe of DLR is in favor of reducing the climate impacts of air travel.

"Simply speaking, if we can avoid regions in the atmosphere where the so-called non-CO2 emissions have the greatest impacts on climate change, we can significantly reduce this climate change," he explained.

A pilot plant in Dresden, Germany, is pioneering larger-scale production of power-to-liquids

With colleagues from five countries, Grewe is part of an EU project that has been evaluating data from 800 daily trans-Atlantic flights. As a result, he believes that "a significant reduction in the climate impact of aviation is possible at a relatively low cost."

Replacing jet fuel made from fossil fuels with more renewable sources would also be a fundamental step on the road to sustainable air travel.

In this regard, the production of jet fuel from biomass - or even from wind and solar electricity - is widely developed and tested, and is generally possible. In the latter case, climate-neutral electricity is produced with the aid of electrolysis from water hydrogen (power-to-gas), and in a further step, with the addition of CO2 kerosine (power-to-liquids).

The production of this jet fuel on the shores of the sun-drenched and windy countries of the world is "economically cost-effective," said Norman Gerhardt of the Fraunhofer Institute for Wind Energy and Energy Systems Engineering.

However, capacity for large-scale production would have to be significantly built out, and he doubts that produced quantities of such fuel would sufficiently match current demand.

Paying for climate impact

Integrating air traffic into emissions trading could be another important way to achieve more climate-friendly air travel. One proposal suggest that by 2020, all airlines should pay $10 (around 9 euros) per ton of CO2 emitted, but that the price of these pollution permits should increase to $80 dollars per ton by 2030.

From 2025, other climate effects of air traffic such as cloud formation would also be included in the emissions trading price.

Another important instrument for more climate protection is the reduction of environmentally harmful subsidies.

According to data provided by Germany's Federal Environment Agency (UBA), Germany subsidizes air flights to the tune of 12 billion euros per year - mainly by exempting ticket prices from sales tax and jet fuel from energy tax.

The UBA recommends that the government abolish such benefits. This would also level the playing field for other, more climate-friendly modes of transport such as bus, car and train.

If these subsidies were slashed and income taxes reduced instead, every worker in Germany would have more than 270 euros more to take home each year.

To protect the climate, the UBA also recommends that people avoid flying. "Take advantage of alternatives to flying: use different modes of transport, vacation closer to home, or use video conferencing for business meetings rather than traveling," is their creed.