Flying is one of the most carbon-intensive activities — yet it contributes just 2.5% of the world’s carbon emissions. How does this add up? Well, almost everyone in the world does not fly. Studies estimate that just 10% of the world flies in most years.

Between 1990 and 2019, both passenger and freight demand has approximately quadrupled. More people are flying, and more stuff is being moved around. In 2019, passengers traveled more than 8 trillion kilometers: that’s about the same as a light year.

Airplanes emit around 100 times more CO₂ per hour than a shared bus or train ride, and the emissions of global aviation are around 1 billion tons of CO₂ per year — more than the emissions of most countries, including Germany.

Aviation contributes an estimated 2.4% of global annual CO₂ emissions, most of it from commercial travel. In 2018, there were 4.3 billion passenger journeys recorded.

The COVID-19 pandemic halted global travel and reduced aviation by 45% in 2020, but CO₂ emissions persist for hundreds of years, so all emissions from all past flights are still at play.

Recent disruptions may have slowed warming by about five years, but they’re not all that significant to aviation’s overall climate impacts.

When jet fuel burns, it produces CO₂ as well as non-CO₂ emissions, including nitrogen oxides (NO_x), soot, water vapor and sulfate aerosols.

All of these interact with the atmosphere and have an effect on the climate in different ways and at different time scales, making them complicated to calculate.

To calculate carbon emissions from aviation, we need to know three metrics:

• Aviation demand: How many passenger and freight kilometers;
• Energy efficiency: How much energy is used per kilometer;
• Carbon intensity: What fuel is being used, which tells us the carbon emitted per unit of energy.

Multiply these metrics together, and we get carbon dioxide (CO₂) emissions.

At the same time, flying has become more than twice as energy efficient. Traveling one passenger-kilometer in 1990 used 2.9 megajoules (MJ) of energy. By 2019, this had more than halved to 1.3 MJ.

This efficiency has come from improved design and technology, larger planes that can carry more passengers, and a higher ‘passenger load factor’. Empty seats are less common than in the past.

The carbon intensity of that fuel — how much CO₂ is emitted per unit — has not changed at all. We used standard jet fuel in 1990 and are using the same stuff today. It has not gotten any cleaner. Bio-fuels and other alternatives are just a tiny fraction of global demand.

New research that provides the most comprehensive calculations of aviation’s impact on the climate finds that global air travel and transport is responsible for 3.5% of all drivers of climate change from human activities.

The study, published in the journal Atmospheric Environment, evaluated all of the aviation industry’s contributing factors to climate change, including emissions of carbon dioxide (CO₂) and nitrogen oxide (NOx).

And the effect of contrails and contrail cirrus – short-lived clouds created in jet engine exhaust plumes at aircraft cruise altitudes that reflect sunlight during the day and trap heat trying to escape at night.

For those of us that take regular holidays abroad and travel on business, flying makes up a considerable chunk of our carbon footprint, but are there ways of reducing those emissions?

Everything we do, from the food we eat, products we buy to the way we travel, releases greenhouse gases into the atmosphere, and so has an impact on the planet’s climate. But some activities have a far greater impact than others.

For those of us that do fly, it is likely to make up a significant slice of our personal carbon footprint. This is because, mile for mile, flying is the most damaging way to travel for the climate.

A return flight from London to San Francisco emits around 5.5 tonnes of CO2 equivalent (CO2e) per person – more than twice the emissions produced by a family car in a year, and about half of the average carbon footprint of someone living in Britain.

Even a return flight from London to Berlin emits around 0.6 tonnes CO2e – three times the emissions saved from a year of recycling. And emissions from planes are rising rapidly – they increased by 32% between 2013 and 2018.

While improving fuel efficiency is gradually reducing the emissions per passenger, it is not keeping up with the rapid increase in total passenger numbers, which are projected to double in the next 20 years.

You have fuel efficiency improvements on the order of 1% per year, and flights are increasing 6%. It’s not even close. And it is not just the CO2 pumped out from jet engines that is having an effect.

A single passenger traveling on a domestic flight in Britain, for example, can lead to climate impacts equivalent to 254g of CO2 for every kilometer they travel, according the UK’s Department for Business, Energy and Industrial Strategy (BEIS).

The same calculations estimate a long-haul flight can release the equivalent to 102g of CO2 for every kilometer – a lower figure on average per kilometer because of the huge amount of emissions given off during take-off and landing.

But an intercity train releases the equivalent of just 41g for every passenger mile. Traveling by coach releases even less – the equivalent of just 28g of CO2.

All this means that if a journey is possible to do by coach or train, this is likely to be far more climate friendly than flying.

Even driving is usually less carbon intensive than flying, says Rutherford, provided you can give someone else a lift. Driving alone in a medium-sized petrol car produces about 192g of CO2 for every kilometer you travel, but with passengers that can be shared.

Ground transport can also often be cheaper and faster than air travel for shorter distances once you take into account the time taken getting to the airport, checking in, queuing at security and waiting for baggage.

Sites like The Man in Seat Sixty-One can help with planning long-distance routes by bus, train and ferry by the cheapest route.

And there are other benefits to taking trains: they tend to connect directly between city centers rather than being out of town as many airports are.

They also offer the chance to see and explore new destinations. It is also easier to get up and walk around on a train, and, of course, there is the view.

But there is plenty still to be done to provide better low-carbon travel options.

Rail is already common in Europe, where the overnight sleeper train network is rebounding. But trains can be more expensive than flying on some routes and are often more time consuming.

BBC / ABC Flash Point News 2024.