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.

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.

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.)