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There is no doubting the British protesters' seriousness or sincerity as they put the final touches to plans for a direct action campaign next month: "We still have time, but not for long - it all comes down to us now."
Sometimes their communications assume a biblical tone: "Should we not change our ways, we'll see forests burn, soils decay, oceans rise and millions of people die." Their methods, including a huge protest camp and co-ordinated civil disobedience, echo those of past campaigns against the Vietnam War, nuclear weapons, and oppressive regimes abroad. But this time, the mission is not to stop wars, bombs or torture, it's to stop people going on holiday.
Thousands of activists are expected to descend on Heathrow Airport for the Camp for Climate Action from August 14 to 21 and the week will climax with a day when demonstrators will try to disrupt the airport as much as possible.
Never before has flying been so controversial. In the space of two years, the environmental damage done by planes has gone from being something quietly discussed by scientists and committed environmentalists, to a headline-grabbing issue no one can ignore. Politicians are pilloried in newspapers for flying to meetings abroad
Even those who fly once or twice a year on holiday can't help but feel a growing sense of guilt.
Now, however, the backlash is beginning. The tourism and aviation industries are mobilising, setting up lobbying groups, and pointing out some awkward facts.
For example, did you know that some ferries emit far more carbon dioxide than some planes? That driving can release twice as much carbon as flying? And at last month's Paris Airshow, Airbus bosses unveiled their own, very different, solution to climate change - promising to "save the planet, one A380 at a time". That's A380, as in the vast double-decker airliner about to enter service.
So who do you believe? One thing on which all sides agree is that aviation is booming. Today, there are around 17,700 commercial aircraft in the world. Over the next 20 years, manufacturers expect to deliver 25,600 new planes, with huge growth coming from China, India and Russia as economies develop and flying is deregulated.
"Aviation is here to stay and will grow faster than people expect," said Praful Patel, India's civil aviation minister. "Remember, India is a country of 1.1 billion people and fewer than 10 million fly even once a year."
Even if everyone in Britain were to stop flying tomorrow, in less than two years the total number of passengers worldwide would still be rising. This year, there will be 2.2 billion air passengers worldwide and the total is growing by 4 per cent a year, according to International Air Transport Association forecasts.
Recent surveys have suggested that 3 per cent of Britons have already stopped flying and a further 10 per cent have cut back, but people seem slower to practise what they preach.
Moreover, with China building two new power stations a week, mostly coal-fired, it's easy to wonder if it's worth agonising about whether you should go for that winter break.
According to last year's report by economist Sir Nicholas Stern, power stations account for 24 per cent of global greenhouse gas emissions, shipping, train and road transport account for 12.3 per cent and flying accounts for just 1.7 per cent. Compare this with deforestation, which accounts for 18 per cent, half of which is attributed to the destruction of rainforests in two countries: Indonesia and Brazil.
Which doesn't mean we should think we're damned anyway, so let's get on the plane and keep partying till the world goes up in flames, but it does put the issue into balance - should we devote nine times more effort to fighting deforestation than flying? And being aware of the balance should steer us away from extreme positions - refusing to fly at all or ignoring the issue completely - towards taking practical, realistic steps to a solution.
A couple of hours of flight, emits around 260kg of carbon dioxide for each passenger. Replace 10 ordinary bulbs with energy-saving ones, and you'll save 380kg. Chuck out your plasma TV and you'll save 404kg. Even turning off appliances instead of leaving them on standby will save 173kg.
"Dark green" environmentalists argue there is a bankrupt logic in this kind of carbon offsetting. You could, after all, take all those carbon-saving steps, and still cancel your holiday.
But that assumes tourism is a frivolous, self-indulgent activity which is as pointless as leaving your TV on standby. Even putting aside the benefits to the tourists themselves, this is clearly not the case. Tourism employs around 231 million people, and generates 8 to 10 per cent of world GDP.
While the campaigners plot their camp at Heathrow, in Kenya plans are being drawn up for a different camp. Looking out from an escarpment over the deserts of Samburuland is a stunning hotel, the Ol Malo Eco-Lodge.
Revenue from the small number of visiting tourists has allowed the large area around it to be transformed from overgrazed cattle ranch to a pristine conservation site, but that is just the start. The tourist-funded lodge provides the infrastructure and backup for a range of vital community work.
About 100 women are employed in the workshop making traditional beadwork for export, and the children come along to paint for fun.
More impressive still is the Ol Malo eye project. Up to 80 per cent of adults in the area suffer sight loss, caused by the infectious and preventable disease Trachoma, so the Ol Malo Trust runs regular surgical camps.
In January, the camp gave 102 people back their sight, and final plans are now being made for another camp this autumn. "It's very simple - all of our visitors fly here," said Julia Francombe, the founder. "If they stopped coming, it would kill us."
"Our message to all air passengers is to stop feeling guilty about flying," said Captain Mervyn Granshaw, chairman of the British Airline Pilots Association, unveiling a study conducted by the union last month.
Launching the 82-page report, Granshaw pulled out one key point: "Passengers going by high speed train [from Britain] to the south of France would be responsible for emitting more carbon dioxide than if they had flown there."
I rang the union to check and was directed to a section of the report quoting Roger Kemp, professor of engineering at Lancaster University. I then rang him. "No, actually, that's completely untrue," he said. "France generates about 80 per cent of its electricity using nuclear power, so if you wanted to go to the south of France, by far the best way to go is by train.'
But Kemp goes on to say that plane travel is not always automatically the worst choice, environmentally. "The worst way to get to the south of France is to take a car ferry then motorail, where you can end up with a diesel engine hauling a huge train with cars on wagons."
A full plane can sometimes compete with a car, too. Paul Upham, a research fellow at the Tyndall Centre for Climate Change Research, calculated that travelling from Manchester to Guernsey on a full Saab 200 turbo-prop plane produced 103kg of carbon dioxide for each person, compared with 226kg for a Nissan Micra carrying one person the same distance. He was quoted as concluding: "Planes aren't the evil things relative to cars that people imagine."
The reality is that analysing how various modes of transport compare is fiendishly complex. Some trains are far worse than others, while diesels can emit more than twice the carbon dioxide of electric trains. Some high-speed ferries use double the fuel of conventional ships, making them several times worse than modern planes for carbon emissions.
However, as the pollution from planes is emitted high in the atmosphere, its effects are far worse, and vapour trails (or "contrails") lead to the formation of cirrus clouds, which stop heat escaping from the earth.
Most scientists agree that this "radiative forcing" effect is real - and point to the significant cooling in America after all planes were grounded on 9/11 - but few agree on the scale of its effects.
Current estimates are that before comparing a plane's emissions to those of a car or train, you would first have to double or triple them. "The big problem is that there is no consensus on this and people seem to be becoming split along ideological lines, with NGOs accepting the multiplier and industry not," said Upham.
Some of Upham's calculations have made him the unwitting poster boy for the pro-aviation lobby, but his views are very different: "Taking into account the contrails, flying is usually about nine times worse than taking the train, and three times worse than a car with two passengers."
Given the world's apparently insatiable appetite for flying, and accepting it is seriously damaging for the environment, it becomes crucial to develop new and less polluting aircraft. Already, there is some progress: the new Boeing 747-800, which will enter service in 2009, is 16 per cent more fuel efficient than its predecessor, and the 787 "Dreamliner", which enters service next May, uses light carbon composites to cut fuel use by 20 per cent compared with the 767, and 70 per cent compared to the 727 launched in 1963.
Airbus' claim that it can save the world with the A380 may be far-fetched, but its "gentle giant" plane is far more efficient and quieter than those of 20 years ago. Next year, Virgin is even planning to test-fly a 747 on biofuel.
Moves to reform the air-traffic control system so planes are not stuck in circling patterns are also vital. But some environmentalists scorn these advances, saying such measures are a delusion, "like holding out for cigarettes that don't cause cancer".
"The aviation industry is prone to vastly overstating the gains that can be made from technological improvements but sadly a climate-friendly plane isn't on the horizon," says Emily Armistead of Greenpeace. "The only way to deal with aviation's impact is to limit its expansion."
So should we stop flying? If no one set foot on a plane again, it would undoubtedly help to stop climate change - though at the expense of killing off the tourism-based economies of many of the world's poorest countries.
But in the real world, with the US and the developing world demanding thousands of new planes, surely we have to take a more sophisticated approach: to choose airlines with greener, newer fleets, and thus encourage plane-makers to prioritise environmental performance; to travel to destinations that help local communities rather than destroy them; to reduce carbon emissions at home; and, above all, lobby politicians to tackle deforestation and to switch to green forms of energy.
Do all this, and we can start to cancel flights in the knowledge that it really will make a difference.
- Observer
