Mass tourism has been memorably defined by the writer Neal Ascherson as "an extractive industry mining a non-renewable resource". And with global tourist numbers forecast to double by 2010 to a staggering one billion a year, there is increasing concern about the enormous environmental impact of tourism.
"The tourism industry is BIG. If it was a country, it would be the third-richest in the world," says Tourism Concern, a London ginger group committed to making it more "sustainable". With 250 million employees and revenues of at least $3 trillion a year, the travel trade has become as unstoppable as a juggernaut.
Britain's national heritage secretary, Mr Chris Smith, has warned that if something is not done to control the surge in global tourism, precious sites all over the world will be wrecked. High on anybody's list are such world heritage sites as Venice and Egypt's Valley of the Kings, where the pharaohs' tombs are crudely exploited as a tourism "resource".
In Britain, £350,000 has had to be spent restoring the Oxfordshire churchyard where Winston Churchill is buried because of damage done by visitors. Measures have been introduced to control tourism at Hadrian's Wall. And in Oxford and Cambridge, there are more tourist coaches on their narrow streets than students singing matins in their cathedrals.
As Neal Ascherson sees it, the future seems to promise "organic system tourism - the setting up of circuits which pump huge numbers of human beings from hotel to coach-point through pedestrianised streets and a succession of heritage malls and family entertainment complexes, with some real (i.e. pre-existing) objects of interest on the way".
He was speaking at a conference in his home city of Edinburgh, where people in the Old Town complain that there's nowhere to buy food or have their shoes repaired because most shops along the Royal Mile are given over to selling T-shirts and tartan dolls. As elsewhere, the artificial is rapidly replacing the authentic.
He says: "The result of devising a High Street Heritage Trail lined by epoxy-resin Edinburgh folk will be to drive the real folk out of their homes, as their city is turned into another heritage'n'shopping Disneyland. What the visitor is visiting will eventually not be Edinburgh, but some other thing very much more international and standardised."
But as Green campaigner Jonathon Porritt has said: "It is part and parcel of people's increased material expectations that they will be able to travel more often and further, however unsustainable those expectations turn out to be", although he also conceded that the tourism industry itself was "gradually waking up to the scale of the challenge".
Crudely put, the environment is tourism's raw material. "Who'd want a holiday in a game park with no game, a romantic weekend break in a city where you can't breathe, or a seaside holiday where you end up - quite literally - going through the motions?" asked Mr Porritt.
He is programme director of Britain's Forum for the Future, a think-tank which supports sustainable development. Its council includes Anita Roddick, Pru Leith, David Puttnam, Chris Patten and Richard Branson.
In a recent issue of its magazine, Green Futures, which was devoted to the subject of tourism, Mr Porritt wrote: "Unless you take the kind of holiday where you go by ferry and train to enjoy a walking or cycling tour in the west of Ireland, living off the Guinness and taking nothing but photos, sustainable tourism remains a contradiction in terms."
The missing concept is carrying capacity. "How many visitors can a `destination' absorb without damage being done to the environment or the quality of local people's lives?" he asked. "More is not necessarily better. If you up the numbers and exceed the carrying capacity, there will be a price to pay in economic and environmental terms."
As Green Futures noted, the debate about environmentally-responsible tourism first started about a decade ago. It was "fired by holidaymakers' angst at seeing fragile, traditional cultures commercialised and nostalgia-soaked idyllic locations smothered by garish hotel complexes and visited by - God forbid! - tourists".
Almost 400 million visits were made to national parks in North America in 1991, according to a BBC documentary, Loved to Death. And while the Mediterranean continues to attract over 100 million holidaymakers every summer, less than half the sewage from its coastal towns receives any treatment before it is discharged into the sea.
The latest threatened world heritage site is Victoria Falls. It has become the top tourist attraction in southern Africa, drawing 300,000 visitors a year and potentially a million or more within the next decade. Bungee-jumpers, microlite flyers and thrillseekers in helicopters are destroying its spectacular natural drama and frightening the wild animals.
With so many people being moved, fed, housed and entertained increasingly in remote and environmentally-sensitive locations, it might seem that the growing trend towards eco-tourism - defined as "responsible travel to natural areas that conserves the environment and sustains the wellbeing of the people" - would offer some respite.
But Dilys Roe, co-author of Take Only Photo- graphs, Leave Only Footprints, a study for the International Institute for Environment and Development, said the report had shown it would be a mistake to assume that eco-tourism was inherently sustainable. "Our study suggests that eco or wildlife tourism does more harm than good."
In many parts of the world, such as some game reserves in Africa, luxury hotels designed to cater for wildlife enthusiasts have a negative impact on the environment and may even deprive local people of scarce resources like drinking water, which ends up being diverted to watering lawns or filling those classic azure swimming pools.
Green Futures said tourists were to blame for stress-related illness which has caused a 30 per cent fall in the cheetah population of Kenya's Masai Mara national park since 1993 and for the fact that iguanas in the Galapagos Islands "have abandoned their territories and, in some areas, lost the ability to find food for themselves".
About 70 per cent of the profits from tourism in developing countries is "lost" because of foreign ownership of the industry, according to Sue Wheat, of Tourism Concern.
In the Annapurna region of Nepal, local people who provide accommodation for 40,000 trekkers every year receive only 20 cents from each $3 per day charge. Neither have the Masai benefited much from the $18 million a year generated by Kenya's big game parks, though the tribe lost much of its ancestral lands when they were fenced off. According to Tourism Concern, many have migrated to urban slums while others hang around selling souvenirs or posing for tourists' cameras.
Not surprisingly, as Ms Wheat noted with approval, communities in holiday resort areas from Gambia to Hawaii and the Solomon Islands have formed coalitions of locally-owned businesses in order to gain more control over the tourism sector, both economically and environmentally. Without this, outsiders will remain in the driving seat.
Meanwhile, St Lucia and five other eastern Caribbean islands have introduced a $1.50 "voluntary levy" on all incoming tourists, with the proceeds going to badly-needed waste management programmes. Yet the pressure for new resort development is immense, and fragile habitats are still being destroyed by tourist infrastructure.
According to Conor Skehan, a Dublin environmental consultant, it is the visitor's eternal quest for "unspoilt" areas that makes management for sustainable tourism almost impossible. "This happens because the visitor seeks the `authentic experience' which, by definition, is an area without the infrastructure to sustainably absorb tourism.
"Damage follows. Infrastructure is put in place to mitigate the damage, but additional revenue is now needed to sustain the new infrastructure, so more visitors must be attracted. The original visitor is discouraged because it is no longer an `authentic experience'. They go somewhere else, even more peripheral, and the story begins all over again."
On the tropical island of Lombok, according to Tourism Concern, the Indonesian government has been "tearing down homes to make way for development and the tourism frontier is moving so fast that communities have no time to prepare and adjust". Villagers in Pagan, Burma, have also had to "pack up and leave" in the cause of tourism.
As Sue Wheat discovered, tourists and business travellers "find it quite shocking to hear that their holidays and business visits might be using up much of the local water supplies, polluting seas, rivers and land with waste, commercialising fragile traditional cultures, or stopping local people from having access to their beaches for fishing".
Tourism Concern is campaigning to put the environmental and ethical issues associated with "sustainable tourism" at the forefront of the travel trade's agenda. But, although it has received a positive response from some tour operators, other sectors of the industry - hotels, passenger carriers and travel agents - are "generally hostile".
Many see sustainable tourism as "nothing more than a niche product for environmental enthusiasts, rather than something to be applied to tourism as a whole".
But a survey conducted by Tourism Concern with the UK Institute of Travel Management found 84 per cent of business travellers wanted to learn more about these issues.
"The climate does now seem to be changing", Ms Wheat said. "Like all industries, the travel and tourism industry is increasingly facing questions from consumers and the media about the way environmental and ethical issues are being tackled."
Meanwhile, the atmospheric pollution caused by rapidly increasing air travel cannot be ignored. Although this is believed to be responsible for up to 10 per cent of worldwide emissions of the greenhouse gases blamed for causing climate change, the airline companies are still exploiting a loophole which means they pay not a penny in duty on aviation fuel.