In 1999, the owners of a tourist attraction in north-west Thailand appeared in court on charges of running a 'human zoo'. Twelve adults and twenty-one children of the Padaung tribe had been discovered by a British journalist and a Thai human rights campaigner in a compound near the Burmese border. They had been tricked into leaving a refugee camp, then displayed to tourists who came to see their famously elongated necks. They were forced to dance, sing and sell artefacts to the visitors, and those who tried to escape were beaten up. By the time the slaves were discovered, one woman had died, due, her husband said, to a 'broken heart'.

These were the lucky ones. Their human zoo was closed down when exposure forced the reluctant authorities to start an investigation. But in many parts of South-East Asia, slavery is either ignored or promoted by the state. As both tourists and Thai men demand HIV-free prostitutes in Bangkok, brokers and bawds scour the Thai hills for girls to trick or kidnap. Across the border in Burma, the entire tourist industry has been built on slave labour, as hundreds of thousands of men and women have been forced, on pain of death, to construct the roads, airstrips, hotels and golf courses demanded by an industry that neither sees nor cares. Thousands have died of beatings, malnutrition and exhaustion. Yet still the vampire tourists come, purchasing a pound of human flesh with every kyat they spend.

Of course, it's not the tourists who are imprisoning people, forcing them to work and beating or killing them if they refuse. Our ignorance is exploited, our perception of where we should and shouldn't tread is complicated by the knowledge that some tourism can do more good than harm. But we are the ones who buy these slaves: ours, as William Wilberforce first pointed out nearly two centuries ago, is therefore the primary responsibility. It is up to us to discover whether or not our money will ruin or enhance people's lives. Today, you don't have to be evil to be a slave-driver, only unthinking.

Tourism has often been presented as a force for global salvation, bringing people closer together and providing alternative livelihoods for the victims of exploitation. It has also, however, become one of the world's principal sources of oppression and destruction.

Clearances are a common component of national tourist industries. All over South- East Asia, farms, forests, villages, even suburbs, have been destroyed to make way for golf courses. Slums are razed for fear of offending visitors. In many parts of Africa, conservation is used to justify the creation of new parks and reserves for tourism. Their inhabitants are excluded from the lands they have possessed for centuries, and if they dare to re-enter them they do so (in Kenya) on pain of death.

Wherever it occurs, tourism is an extractive industry. It extracts the differences between our land and culture and those of the nations we visit, until they scarcely exist. Remote and romantic beaches become mundane resorts. Remote and remarkable people tailor their culture to suit those who pay for it, until, in the words of a Maasai man, "We have ceased to be what we are; we are becoming what we seem." The exotic, of course, is illusory: as we approach it, it disappears. Tourism will never be sated, therefore, even when it has penetrated the remotest parts of the world.

While organised tours may be most directly responsible for the muffling of diversity, it is the backpackers who blaze the trail others follow. An independent travellers' destination becomes a mainstream resort within a few years. Indeed as travel becomes easier and tourists more adventurous, the distinction between the two groups is breaking down: hundreds of tour companies organise journeys that mimic those of independent travellers. Independent travel itself has had an enormous impact on places such as Goa, the South African coast and several Thai resorts, which have become increasingly unwilling hosts of the European dance scene. Neither category - if they can still be categorised - is blameless.

Whatever happens when you get there, travelling itself has begun to ruin the lives of millions of people. By 2016, for example, the number of passengers using London's airports is expected to double to 160 million, an average of nearly three flights per year for every man, woman and child in the United Kingdom. This, of course, means ever noisier skies. Sedative use increases by eight per cent (over the average) in areas affected by aircraft noise, while people living within ten kilometres of an airport consume 14 per cent more anti- asthma drugs. Around Los Angeles airport, mortality rates are five per cent higher than in quieter places: suicide accounts for much of the difference. One study shows that the reading ability of 12- to 14- year-olds whose schools lie under flight paths is impaired by 23 per cent, while children of all ages are more likely to develop anxiety disorders when routinely exposed to aircraft noise.

The impact on the environment is even graver. The transport specialist Dr Meyer Hillman has shown that every passenger on a return flight to Florida is single- handedly responsible for generating 1.8 tonnes of carbon dioxide. Climate scientists estimate that, if the worst effects of climate change are to be avoided, total emissions per person per year should be little more than half this amount.

Now several companies are vying to become the first to propel tourists into space. It is hard to think of a project better designed for maximum environmental destruction. If the industry takes off as some of its boosters would like us to believe, it will rapidly become the world's primary source of carbon dioxide emissions. In our quest to populate the barren interplanetary wastes, we threaten to lay waste to the only life-sustaining planet astronomers have been able to detect.

None of the ethical questions raised by tourism have easy answers. Tour organisers have justified their work to me on the grounds that it is a form of 'cultural exchange'. Yet what I have seen of their activities suggests that this is not how the transaction actually works. While the visitors get culture, the hosts - if they are lucky - get money. As identity is rooted in place, the tourists have little to offer.

Other people claim that tourism breaks down the barriers between our lives and those of the people we visit. Yet in most cases tourists remain firmly behind barriers, be they the windows of a coach, the walls of a hotel or the lens of a camera. In many parts of the world, tourism has served to compound misunderstanding and hostility, as local people's sensitivities are trampled.

Tourism, we are told, brings wealth to local people. My experience suggests that the opposite is more likely to be true: in most places, tourism makes a few people extremely rich while impoverishing the majority, who lose their land, their resources and their sense of self, gaining in return (if anything) a tiny amount of money.

Even the oldest maxim of all, that travel broadens the mind, is questionable. Tourists are the aristocracy of the New World Order. They are pampered and protected wherever they go; they are treated with deference and never corrected. Indeed, tour companies do their best to provide what the tourists expect, rather than educating the tourists to expect what the country can reasonably provide. For most tourists, the only surprises will be unpleasant ones, when the reality of the countries they visit pricks the bubble in which they travel. At this point the shock of discovery tends to compound fears rather than assuaging them, and thus many people return home more convinced than they were when they left that foreigners are dirty, deceitful and dangerous.

And yet it is also true that some people - those who manage to engage with the place they visit on its own terms - come back from their travels better than they were before, able to see both themselves and the rest of the world in a new light, able to grasp, perhaps for the first time, that theirs is not the only valid way to live. Visiting other countries can help us to understand the impacts that our lives exert on other people's.

Like almost everything else, travelling is much harder to do the right way than the wrong way. We must collectively reduce the number of flights we take, but as flights become cheaper and other forms of travel more expensive, this becomes increasingly difficult. Perhaps we should try to reverse the trends of the past few years, and either travel closer to home or take longer, less frequent breaks, which would enable us both to fly less and to engage more effectively with the people we visit. We should read as much as we can before we go and try to learn the rudiments of the local language, so that when we get there we can discover things for ourselves, rather than allowing intermediaries to tell us only what they want us to hear.

Travelling, like all other forms of consumption, is not a neutral activity. Everything we do affects other people, everything we own is taken from someone else. If you can't travel carefully, don't travel at all.