The only problem about tourism is the other tourists. We see yet another boatload in motorised canoes in the wide waters of the Amazon; yet more louts demanding lager and chips in Mediterranean villages; yet another tourist procession between endangered species in the Galapagos Islands; yet more feet along the ever- widening trails that expose the mountain rocks; yet another cruise liner pouring people like an ant army on to white Caribbean beaches or even whiter Antarctica; yet more Land Rover tracks across the African savannah. We love tourism and we hate it. The problem is relatively new. As with the industrial revolution some 250 years ago, it was the British who started it. In the early days travel abroad was the preserve of the rich, who did it to enlarge their education and enjoy exotic pleasures. With the new wealth generated by business and industry, it moved rapidly down the social scale. While the Duke of Wellington was against railways because they gave the lower orders ideas above their station, Thomas Cook organised the first rail excursion from Leicester to Loughborough for a shilling. But it was only in the lifetime of present generations that tourism became the world's biggest industry, with all the impacts and consequences of industrial development: the generation of wealth and employment, the opening of minds to new horizons, the pleasures of new ways of life, the impact on the environment, consumption of natural resources, production of wastes and pollutants, and not least effects on the cultural attitudes of all in contact with it.

It is worth looking at some of the figures. Tourism accounts for over six per cent of world gross national product, and provides up to ten per cent of total employment. All this arises from the eight per cent of the world's population that travels. Most tourists come from just 20 countries. Last year there were well over five million tourists in the world, and this figure could double in the next 15 years. Some countries have become dependent on tourism for the good health of their economies: they range from Egypt, Spain, Jamaica and Kenya to the islands of the Caribbean and the Pacific. Some of the environment in such countries has been radically changed as a result.

There has been a substantial diversion of resources to tourist use. This is particularly important in countries short of water. Some figures produced by the United Nations Food and Agriculture Organisation illustrate the problem. Fifteen thousand cubic metres of water can irrigate one hectare of high-yielding modern rice; support 100 nomads and 450 cattle for three years; maintain 100 rural families for three years and 100 urban families for two years; or meet the needs of 100 guests in a tourist hotel for 55 days. The cost of meeting tourist needs, whether in terms of clearing land for golf courses, building major works of infrastructure, producing specialised foods, disposing of wastes and pollution and the rest, are immeasurably large. The cultural impact is also beyond measurement. I remember my shock when, as an academic at Oxford, I was the subject of tourist curiosity with flashbulbs. For once I was at one with the Matabele guide or the Amazonian Indian (who reasonably charge for having their photographs taken). Like seeing television or hearing radio programmes from another world, tourists generate unrealisable expectations and consequent frustration in others.

Yet the right to travel has become an icon of liberty, especially for those under oppressive regimes. So it should be. But rights carry obligations, and if tourism is not to be like the Indian goddess Kali, the creator of wealth and at the same time the destroyer of what generated it, we need to see how tourism can be brought into balance, particularly in relation to its impact on the environment.

What can we do? I believe that the first requirement in this, as in so many other environmental matters, is to establish true costs, and make sure that they are met. It has been well said that markets are marvellous at fixing prices but incapable of recognising costs. Recently a brave attempt has been made to establish the true costs of the natural services - ranging from the fertility of the soil to its ability to absorb wastes within the natural ecosystem, of which humans are a tiny part - and, although the results were approximate, they are very interesting: the average works out at about 33 trillion a year, while the world's gross national product is around 30 trillion a year. A good example is the price of coal. In no country does it include the cost of the effects of burning it, whether on human health, on buildings or on the chemistry of the atmosphere. The same goes for transport.

It is not easy to establish environmental costs. The last Chancellor of the Exchequer justified a continuing increase in petrol prices on environmental grounds, and the new climate change levy will fall in the same category. As for tourism, there can be no question that tourists should, in one way or another, bear the cost of the effects of their actions.

A particular perversion is the lack of any tax on aviation fuel. It has become cheaper to convey people as well as materials by air than to use other means of transport, with effects throughout the economy. Air traffic contributes carbon to the atmosphere. It makes up 11 per cent of total transport fuel emissions, and some three per cent of the emissions that can be attributed to human activity. Sixty per cent of air travel is now tourist related.

Another perversion is that very little of the wealth generated by tourism goes to the people who live on the spot. This means that local communities increasingly resent tourism, and in sensitive environmental areas have little incentive to protect and conserve their surroundings. If local people are to identify themselves with the good health of their own environment, then they must see most of the return from it. Individual tourists could make a big difference by spending their foreign currency on goods and services bought directly from the local communities.

At present most fees charged for admission to National Parks or other areas of conservation are derisory. They scarcely cover the most elementary requirements of conservation. In a way, tourists rent other people's environments for brief periods, and should be ready to pay a fair price for them. This, in turn, requires stronger local control. Nothing is more important than control of numbers. This can be done relatively easily in such isolated places as the Galapagos Islands, Machu Picchu in Peru, or Bhutan in the Himalayas, but control elsewhere, notably in our own Lake District or the Scottish highlands, has been non-existent or ineffective. Many feel that the natural environment should come for free, without realising that nothing is for free, above all the impact on the natural world. The Earthwatch programme in Zimbabwe over the last few years is a good demonstration of what can be done. Yet the threat to the natural world continues to increase almost everywhere, and something like a fifth of the world's rich biological diversity could be lost in the next 20 years unless coordinated action is taken to conserve it.

I think that the tourist industry is already well aware of these problems. So far there are few signs that they are being taken as seriously as they deserve. So long as the philosophy is primarily commercial, with the usual stuff about competitive markets, economic growth and promotion of mass movements of people, things are not likely to change, whatever the gloss put on them. True environmental costing has to enter in at all points.

Just as the industrial revolution did much good but created multiple problems for the good health of our planet, so its product, tourism, risks doing likewise. Indeed it is already doing so, and things could get much worse before they get better. There is a simple principle we should always bear in mind: do not kill the goose that lays the golden eggs.