Flying is a heated topic. But if there is no such thing as an 'eco-friendly' flight, is grounding planes the only option?

Does air travel really have a big environmental footprint?

There's no way around the fact that flights are bad news for the environment. It's not just that planes are worse than most other forms of transport in terms of the impact of greenhouse gases per passenger mile. Just as important is the simple fact that flying allows us to travel a far greater number of miles than we otherwise could. Thanks to these two factors, individual trips by air can have a remarkably large carbon footprint – which helps explain why aviation has become such a heated issue in the climate change debate.

What is the total impact of flying on the climate?

As the aviation industry is usually keen to point out, planes account for only around 1.5%–2% of global CO2 emissions. However, this figure is somewhat misleading. For one thing, most flights are taken by the wealthy, so in developed countries the slice of CO2 emissions caused by flying is higher – around 6.3% in the UK, according to Department for Transport figures for 2005. Even this figure underplays aviation's environmental footprint, however, and not just because the number of flights has risen since 2005. There are at least three other reasons why 6.3% is likely to be a strong underestimate.

First, the total global warming impact of each flight is thought to be around twice as high as the CO2 emissions alone (see 'What's an aviation multiplier?', below). Second, the figures are skewed in favour of British travellers. The standard way to account for the emissions for an international flight is to allocate half to the country of departure and half to the country of arrival. But UK residents take up two-thirds of the seats on the average plane landing at or taking off from a British airport. This means the official statistics are effectively offloading the emissions of British holidaymakers and businesspeople on to the countries they're visiting. Third, the aviation industry causes emissions over and above those of the planes themselves. The processing and transportation of the aviation fuel, and the manufacture and maintenance of planes, airports and support vehicles all create extra carbon dioxide.

There's not enough data to say for sure, but it seems likely that aviation's true impact in the UK is around 13%–15% of total greenhouse gas emissions. If that still sounds fairly low, compared with the massive amounts of attention heaped on aviation by climate change campaigners, bear in mind that most people in the UK don't regularly fly. The average British resident takes a short-haul leisure flight only every two years, and a long-haul leisure flight only every five years. In other words, the air travel of a minority of regular flyers causes a substantial slice of UK emissions.

Is the UK government's aviation policy compatible with its carbon targets?

It's very hard to reconcile the British government's plans for increased aviation capacity with its plans for carbon cuts. The UK is seeking to reduce its emissions by 80% by 2050, relative to 1990 levels. At the same time it predicts a rise in the number of flights sufficient to use up more than half of the remaining 20% of emissions.

What about greener planes?

A number of technologies designed to reduce the environmental impact of flying have been researched, tested and implemented. However, compared with greener cars, where the technologies are proved and the carbon saving huge, the potential for eco-friendly flying looks rather limited. There will be some further gains in engine efficiency over the coming decades, and larger planes with more seats will allow slightly lower emissions per passenger. But there is nothing in the pipeline with the transformative potential of the electric car.

The problem is that electric motors can't produce enough power to get a plane off the ground, so the only alternative to regular kerosene-based aviation fuels are special kinds of biofuels. These aren't an ideal solution, since biofuels can be environmentally problematic in themselves, and anyhow it would take a huge chunk of the world's arable land to grow enough crops to fuel all the world's planes. (A back-of-the-envelope calculation suggests it might require as much as a fifth of all cropland.)

What can individuals do?

For anyone concerned about their contribution to global warming, cutting back on air travel is an obvious goal. This might mean giving up flying altogether or it might mean taking fewer flights and picking destinations that are closer to home. It's true that short flights tend to be more harmful to the climate per mile travelled than long-haul flights are (because they have more empty seats, and because taking off and landing burns more fuel than cruising) but this doesn't change the fact that the further you travel, the greater the emissions that will result.

If you do fly, you can in theory make some small difference to the carbon impact by favouring day-time flights. This at least means that any contrails (see 'What's an aviation multiplier?' below) caused by the plane will reflect some sunlight away from the Earth in addition to locking warmth into the atmosphere. Also consider limiting your luggage.

Finally, you might want to consider which airlines you use. People often assume that budget flights are somehow more eco-unfriendly than expensive ones. In fact, the opposite tends to be true. Budget airlines pack more passengers on each flight and typically have younger, more fuel-efficient fleets than longer-established airlines. Indeed, the least eco-friendly tickets of all aren't the cheapest but the most expensive. Business-class and first-class seats take up more space on the plane, thereby reducing the number of people who can fit on each flight.

Is it really greener to go by train?

As a rule, taking the train instead of the plane will substantially reduce your carbon emissions – perhaps by a factor of five to ten on a domestic trip. The benefits will be somewhat reduced as the journey gets longer. That's partly because shorter flights are more polluting per passenger mile than longer ones, but it's also because long train journeys usually necessitate sleeping onboard. Sleeper cars usually carry fewer passengers than regular carriages, so their emissions per passenger are higher. If, as is common in some countries, the train is powered by diesel rather than electricity, then the emissions will typically be higher still. Indeed, an old diesel sleeper train travelling a long distance might emit nearly as much CO2₂per passenger as a plane. Even then, the train will typically be greener once you consider the plane's non-CO2 warming effects, but the fact remains that long-haul rail is not by any means inherently eco-friendly.

Unfortunately, almost every long-distance train journey will cost you far more than flying would. Indeed, the difference in price is often so great that for some unavoidable trips it would arguably make sense to take the plane and spend the savings on something more environmentally beneficial than a train ticket, such as insulation at home.

What's an 'aviation multiplier'?

The impact of planes on the climate is complicated and not perfectly understood. The CO2 emissions are straightforward enough, but plane engines also generate a host of other "outputs", including nitrous oxide, water vapour and soot. At flying altitudes in the upper troposphere and lower stratosphere, these outputs produce a range of climatic effects, multiplying the plane's environmental impact. For example, nitrous oxide causes the formation of ozone — a greenhouse gas that warms the local climate — but at the same time undergoes reactions which destroy methane, thereby removing another greenhouse gas from the atmosphere.

Even more complicated is the impact of soot and water vapour, which together can cause contrails (vapour trails) and in cold air can lead to the formation of cirrus clouds. The science surrounding this topic is not yet rock solid, but researchers believe that contrails add to the greenhouse effect – especially at night, when their tendency to stop heat escaping from the Earth isn't offset by their tendency to reflect incoming sunlight.

Today, most experts favour an aviation "multiplier" of around two. In other words, they believe that the total impact of a plane is approximately twice as high as its CO2 emissions. The exact multiplier, however, will always depend on the individual plane, the local climate and the time of day.

• This article is adapted from The Rough Guide to Green Living by Duncan Clark.