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SPENDING time at London’s Heathrow airport in August is generally a miserable experience. So the protesters who are passing several days there in order to make a point might be considered heroic in their disregard for personal comfort. On Monday August 13th a small bunch of campaigners against climate-change set up camp at the perimeter of the world’s busiest international airport. Organisers say over 2,000 will take part in a “day of action” later this week. For beleaguered travellers this could mean another of Heathrow’s famous days of inaction.

It is not hard to bring Heathrow to a stop. Strikers, terrorists and bad weather have all done so before. The campaigners hope that by disrupting an airport that serves some 200,000 flyers a day they will further their cause. “Plane Stupid”, an anti-flying group, gives a flavour what the protesters want, demanding “airport expansion plans scrapped, a tax on aviation fuel and an end to short-haul flights”.

Specifically, the campaigners object to plans for a third runway at the airport to accommodate growing numbers of passengers. And Heathrow is not alone. Many airports in Britain and elsewhere have plans to expand.

Airports want to grow because more people want to fly. Last year passengers took some two billion trips on scheduled airlines. In 2010 that number is forecast to have risen by a quarter. Much of the recent growth is attributable to short-haul low-cost airline travel. But cheap flying has come with costs attached. By most measures aviation generates 2-3% of man-made emissions of carbon-dioxide, the main greenhouse gas.

The contribution to global warming could be more severe. Jet engines also pump out nitrogen oxides, soot and water vapour at high altitude. Scientists disagree about the added consequences but a recent report commissioned by the British government suggests that this might double the warming effects of carbon-dioxide emissions from planes.

Most environmentalists think that the only solution is to stop people flying. Making air travel more expensive, say through hefty fuel taxes, would put off price-sensitive leisure flyers. Airlines are accused of having a free ride in terms of air pollution because they pay no tax on the fuel used for international flights. But airlines say that protesters have it in for them because they are an easy target. In fact, they say, the airline industry produces far more benefits than ills. Some studies suggest that aviation contributes as much as 8% to global GDP by transporting tourists, business travellers and cargo around the globe.

Planemakers also point out that they are greener than they used to be. Better technology means that planes are around a third more fuel efficient than they were 40 years ago. And that trend is continuing, driven by airlines’ demands for planes that are cheaper to run. Boeing’s 787 Dreamliner, set to enter service next year, uses composite materials to keep weight down. But aircraft fleet will be updated to cleaner models only slowly.

Airlines are using planes more efficiently too. The 787 is a smaller plane that can fly directly between smaller airports rather then taking passengers to hubs for onward flights. Fewer onward flights means fewer fuel guzzling take-offs and landings. On the other hand, Airbus’s giant A380, which enters service this year, takes many more people between hubs than previous aircraft, using less fuel per passenger than smaller planes. Plane and engine developments that cut carbon dioxide, nitrogen oxides and other emissions are set to continue.

But improvements will be small, incremental and relatively expensive compared with those possible in other sectors of economies where changes might come more cheaply and easily. There is no alternative fuel to kerosene. Aircraft can make small savings by changing how they operate, such as being towed to and from runways rather than taxiing under jet power. Making airports bigger will lessen the need for keeping planes aloft in holding patterns at busy times.

Governments and international bodies are intent on encouraging the industry to make greater efforts. A global emissions-trading scheme, drawn up under UN auspices, is set for consideration later this year. The European Commission plans to introduce trading in 2011 for all carriers based in the European Union. Later the scheme might, possibly, extend to all airlines operating there. The introduction of carbon-trading is a welcome step by governments that are not yet willing to consider a carbon tax. The air-travel industry should have to stump up for the pollution it causes. And anyone priced out of a cheap holiday in Spain might like to consider a week-long camping break near Heathrow.