There are songs written about it, entire industries are built around it and millions of people worldwide use it every day.

We can't live without air travel — but it has consequences for the environment.

In 2016, the world's flights produced 815 million tonnes of CO2 — and in February this year, a staggering 4.72 million passengers travelled around Australia on domestic air carriers, up almost 4 per cent on February 2017.

Although the airline industry and governments are racing to find solutions to the emissions problem, they cannot innovate fast enough to keep up with the world's demand for discount air travel.

Air travel accounts for 2 per cent of global man-made CO2 emissions, and the International Civil Aviation Authority (ICAO) forecasts aircraft emissions will quintuple by 2050.

But global warming caused by CO2 emissions from fossil fuels burned in jet engines is just part of the problem.

The warming effect of contrails

Jet fuel, when burnt in a jet engine, produces carbon dioxide, nitrogen oxides, soot, water vapour, and sulphates — all of which are expelled into the atmosphere.

At high altitudes, water vapour can condense around particles expelled in the exhaust gases and produce contrails (short for condensation trails), which sometimes remain visible for many hours.

Contrails: What are they? Jet aircraft emit various gases during flight, including water, sulphur particles, soot and vapour. At high altitudes of 10,000 metres where aircraft cruise, the air is very, very cold. The water condenses on the particles in the jet's exhaust and turns into water droplets. The droplets form ice particles which are seen as contrails. The length of a contrail depends on atmospheric conditions so if there is high humidity, the contrails will continue to grow and can last for hours, extending long distances. Helped by winds, these contrails can form into extensive cirrus clouds. Source: Air Services Australia

"You've seen them in the sky: the contrails, the streaks of clouds, which can develop into cirrus clouds and they have a [net] warming effect," said Bill Hemmings, from the Brussels-based non-profit organisation Transport and Environment.

Studies have calculated that contrails can warm local climates.

So, while airlines are looking to alternative fuels and technologies, another idea is gaining traction: altering flight routes to avoid certain climatic conditions — and thus, avoiding the creation of contrails.

Stefanie Meilinger, professor of sustainable technologies at Germany's Bonn-Rhein-Sieg University of Applied Sciences, is a proponent of 'climate-optimised' flight routes.

In conjunction with Lufthansa and the German weather service (DWD) Dr Meilinger and her team studied the effect of contrails and the impact of flight planning with the environment in mind.

She said clouds and contrails had warming and cooling effects with the overall effect being one of warming.

"They have a cooling effect by reflecting the incoming visible radiation back to the sun, and they have a warming effect by keeping the infrared radiation from the surface of the earth in the earth's atmosphere," she explained.

Using flight planning tools and information provided by the DWD they reviewed 40,000 Lufthansa flight routes, not from a fuel or cost-saving perspective but with the view of minimising contrail formation and CO2 emissions.

"What we found out that this is not feasible in very crowded areas, like over Germany, but it is feasible in areas like over the Atlantic Ocean where you have more space for an aircraft because it's not so crowded," she said.

Dr Meilinger said the idea of climate-optimised flight routes was still not well understood.

Airlines have expressed interest in the concept of climate-optimised routes, she said, but they need "a real trigger" to get them on board.

So what would do that?

"Just include contrails in the emissions trading scheme. I think this would be the push that would put it forward," Dr Meilinger said.

What do airlines say?

For consumers keen to reduce the carbon footprint they create by flying, some airlines offer the option to offset your share of the carbon emissions your flight creates.

About a third of airlines offer some form of carbon offsetting, allowing customers to pay a few extra dollars when they buy their tickets, which go into a scheme that carries out environmental improvement projects.

But this does nothing to directly reduce the emissions created by each flight.

The idea of climate-optimised flights may be a controversial one, but with increasing global air traffic, and growing concerns about aviation's impact on global warming, research on its feasibility is continuing.

Major airlines, like Virgin Australia and Qantas both state they are committed to reducing aviation's impact on the environment.

Virgin explained most of its flights are planned along fixed airways in accordance with regulatory requirements but, where possible, their flight dispatch team proactively plans what are called Flex Track routes and User Preferred Routes (UPRs) in approved airspace.

These routes account for the optimum wind conditions and airspace management considerations, allowing a flight to be more efficient.

"The Virgin Australia Fuel Efficiency Team is focussed on finding new ways to improve our operational efficiency through reducing weight on board the aircraft and changes in how we operate our aircraft," they said.