If the Heathrow climate campers misjudge their day of “direct action” tomorrow, they will incur the lasting wrath not just of the Government and police, but of overstressed holidaymakers in one of the world’s most overstressed airports.

This would be extremely foolish. But it would not necessarily make the protesters wrong in their answers to fundamental questions about the future of air travel.

The battle waged around the airport perimeter this week will be over by Monday. A far more serious one will continue – between current data, airlines’ self-defence and passengers’ love of flying on the one hand, and alarming scientific projections on the other. Add into that mix a tax loophole that has benefited the airlines for decades and it is hardly surprising that the outcome remains uncertain.

This much is clear: if the Government is serious about climate change, sooner or later it will have to force airlines to factor their emissions into their prices, and passengers will have to pay or cut back on flying.

Is aviation really a greenhouse gas villain?

Not yet, but it will be if current trends continue. Current estimates of the proportion of global carbon emissions produced by aviation range from 1.6 per cent (IATA, the international air travel lobby) to 3 per cent (the European Commission). This is not much, considering the fuss. But it’s already higher than average in Britain at 5.5 per cent, higher still if inbound as well as outbound flights are taken into account, and is expected to account for a quarter of all this country’s emissions by 2025.

Does it make a difference that aircraft fly high?

Yes. Respectable science indicates that carbon dioxide emitted at a typical aircraft’s cruising altitude of about 30,000ft traps between two and four times as much heat as it would at sea level.

What is the industry doing?

Public relations apart, a good deal. While jet engines have doubled in efficiency in the past 40 years, few believe that a further step change in efficiency is possible without abandoning the technology altogether.

However, new materials and (even) bigger planes may bring major cuts in emissions per passenger. The Boeing Dreamliner 787, with more than 500 advance orders and plastic composite wings, boasts a per passenger carbon imprint up to 27 per cent lighter than the wide-body twin-engined aircraft it will replace. The double-decker Airbus A380 superjumbo claims similar efficiencies with the help of wings that automatically change shape in turbulent air and through simple economies of scale: it will regularly carry up to 550 passengers.

What are governments doing?

Not enough. Because of obstacles to agreeing an international tax regime, aviation fuel for international flights remains untaxed worldwide. This is an extraordinary loophole denied to all other forms of transport that governments continue to defend on the ground that unilateral taxation would disadvantage their airlines in favour of foreign rivals. This is true – but still no excuse for the continued failure of every international body that has tried to reach a deal.

The EU has likewise tried and failed to bring international air travel into its existing carbon trading system, and Gordon Brown has swatted aside Tory proposals for personal air travel allowances without offering convincing alternatives, such as ring-fencing the £1.9 billion raised annually in Air Passenger Duty for mitigation of the industry’s carbon footprint.

The US and India deserve credit for taxing fuel for domestic flights.

Are the no-frills airlines more culpable than others for rising emissions?

Probably not. Ryanair, easyJet and many of the minnows boast modern fleets with the most efficient engines available. More importantly, their online booking, flexible pricing and high load-factors mean lower carbon emissions per passenger than for many full-fare airlines. There is also evidence that, instead of bringing a net increase in the rate of growth of air travel within Britain and between Britain and the EU, they have simply put charter carriers out of business: according to the CAA, the growth rate for both these categories of travel has been steady at 5 per cent for the past decade.

What can I do if I want to keep on flying?

Pick your flights carefully, considering trains where possible. Offset with caution: too many companies are charging for offsets that they cannot prove actually result in emissions reductions elsewhere. EasyJet has resorted to buying “carbon credits” only from schemes backed – and in some cases run by – the UN, and is offering to sell them on to passengers. The example is worth following by other airlines.

Why pick on Heathrow?

Because it is a national economic pinchpoint, desperately overstretched, and has asked for a third runway that would involve bulldozing hundreds of homes and bringing thousands more under an expanded final approach corridor.

Handling 67.7 million passengers on nearly 470,000 flights a year, Heathrow is not just critical to the economy but a major employer in its own right. One hundred thousand people work there. All three figures will grow substantially when Terminal 5 opens in March; the climate campers believe that all three should be cut and air travel growth frozen nationally, even at the price of an economic slowdown.

. . . and why use direct action?

Because, they believe, without using it to force the issue to the top of the agenda, the Government will never seriously confront the environmental cost of mass air travel, being too wary of a voter rebellion from middle-class flyers delighted with their vast new choice of low-cost destinations. On this, the campers may have a point.

How will airports cope with growing traffic?

Terminal 5 will make life easier for passengers, but not air traffic controllers. Heathrow is already operating at full stretch in terms of aircraft handled. If it does not get a third runway, Gatwick, Stansted and Luton will have to take on the bulk of extra traffic passing through the London area.

Changes in air travel patterns may help: Boeing’s Dreamliner, and an Airbus rival in the pipeline, are designed for nonstop “point-to-point” routes between regional airports rather than “hub-and-spoke” travel involving changing planes.

This should spread the travelling public more evenly between airports, and may lower per passenger carbon footprints. But total passenger numbers have risen 50-fold in 50 years. There is little sign of that growth slackening. Boeing predicts there will be 3350 airliners in operation by 2020, and a staggering nine billion passengers inside them.

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