The forest is so thick that you enter the city without knowing it is there. Then shadows deepen and, looking up through the leaves, you see a stone tower blazing in the sun. The buildings might be cliffs or hills, except that here is a stairway smothered in roots, a mossy statue, an inscription. You walk all day, yet never leave forest or city. The one seems to grow from the other -- trees from buildings, buildings from trees -- as if they have always stood together, as if the stones were cut and raised by the jaguars who make their dens in dripping vaults, as if the writings told the epics of the bats.

People of our time have come and dug beneath the streets and floors, asking, What happened here? Where did the builders go? What silenced the writers? Why were the astronomers, who could trace the planets far into the past and future, blind to their own catastrophe? And if not blind, why powerless?

The fall of Classic Maya civilization in the ninth century has long intrigued archeologists. Now answers are emerging that should worry us all. Tikal, the greatest city, seems a Manhattan of art deco pyramids (Maya architecture influenced the modern style) presiding over a conurbation of 120 square kilometres. It took 1,500 years to reach that size, yet all of Tikal's skyscrapers were built in its final century, an extravagant flowering on the eve of collapse.

Copan is less grandiose, with exquisite sculpture, the statues of its kings radiating order and refinement. Yet excavation has shown that this city, over centuries, smothered the rich soil from which it grew. The best land was paved, the hills were stripped for farms and timber. The ruling class (revealed by their bones) grew tall and fat; the peasants became stunted. The end was an agony of ecological and social chaos, a scramble for resources in a top-heavy, shrinking world. Diggings at Dos Pilas have exposed a final moment, the people huddling in the city centre, tearing stone from the temples to throw up barricades.

Earth is full of dead cities. Civilizations, like individuals, are born, flourish and die. Except ours. Ours, we believe, is different, the beneficiary of all the rest. The sunny afternoon in which we thrive will stretch ahead forever. In this belief, we carry on our lives against the evidence of time.

Civilization (I use the word in the anthropological sense to mean complex, populous societies) is, in round figures, a 10,000-year experiment that began with the invention of farming in key areas -- the Near East, Southeast Asia, Mesoamerica and Peru. Farming led to towns and cities, to specialists and priesthoods, to the rule of many by few. With this came wonderful things: most of art, literature, music, science. Civilization also displaced other ways of life, often forcibly. There are now no viable alternatives, no blank spaces on the map. There is no going back without disaster. As we climbed the ladder of progress, we kicked out the rungs below.

Ten thousand years may seem long enough to declare an irreversible success. But it is less than 1 per cent of our career on Earth. Even our modern subspecies, homo sapiens, has existed 10 times longer than civilization. The settled way of life we regard as normal today is not the life by which, and for which, we evolved. So why were there no civilizations anywhere until 10,000 years ago, when they spring up independently on nearly every continent?

The date is significant, and possibly ominous. Studies of ancient climate show that the world's weather has been unusually stable since the end of the last Ice Age. We couldn't have invented farming earlier, even if we'd tried. Now we face evidence that civilization itself is destabilizing the long run of good weather in which it has grown.

Civilizations rise because they find new ways to exploit natural and human resources, to tip the balance between culture and nature. They feed on their local ecology until it is degraded, thriving only while they grow. When they can no longer expand, they fall victim to their own success. Civilization is a pyramid scheme.

The cusp between rise and fall is a matter of scale, of demand outrunning natural limits. Nomadic bands of hunters and gatherers could live in the Near Eastern marshes and floodplains indefinitely. Their impact was slight; their numbers waxed and waned according to the abundance of prey and vegetation. Then, slowly, they began to plant and reap. All went well for a long time, bringing surplus and stability to food supply. On lands where a few once roamed, thousands toiled in wheat fields. At slow times in the year, they were kept busy with public works and monuments. This was the end of Eden, the beginning of our world.

But there was a cost, a debt to nature accruing so gradually it was seldom observed, let alone understood. Woods dwindled and receded, denuded land became prone to drought and flood (including, perhaps, the Flood), irrigated fields turned sour. The cities of the plain turned their surroundings into a saltpan. The desert in which their ruins stand is a desert of their making.

In the past, these cycles were regional. As Rome fell in the Mediterranean, the Maya rose in Central America, and so on; the setbacks were local, the overall experiment kept going. But now the 10,000-year bets all rest on a single throw. We have one big civilization, feeding on the whole Earth at such a rate that we can observe the exhaustion of natural capital within our lifetimes, whether it be the loss of wildlife, water, coral reefs or rain forests. We are razing forests everywhere, we are irrigating everywhere, we are fishing everywhere, and no corner of the biosphere escapes our hemorrhage of waste. Even if we ceased this minute, our dominion over Earth would still appear in the fossil record as a blight like the impact of an asteroid.

There's a saying in Argentina that each night God cleans up the mess the Argentines make by day. This seems to be what all of us are counting on. During the 20th century alone, our population multiplied by four, while our consumption grew by 40. Yet the number in abject poverty today is as great as all mankind in 1900. Is this progress? Can the stock market be trusted to run the world? Or is our consumerist boom the illusory wealth of wastrels blowing an inheritance -- by no means only their own? Is the promise of prosperity for six billion the Big Lie of our time?

History will soon answer the paramount question: Will the 10,000-year experiment turn out to be a failure? In my dystopian satire, A Scientific Romance, I pictured a verdict we may hear. Our busy cities fall as silent as Tikal. I hope I am wrong. What is certain is that we have one last chance, at best, to get the balance right. There is no more room for mistakes, no room even to do nothing. If we fail to limit our numbers and our impact, if we do not replace our gold-rush economics with a rational sharing of what the earth can yield, this new century will not grow very old before we enter an age of chaos and collapse that will dwarf all dark ages in our past.

History shows that such a reformation is unlikely. So do current events. According to United Nations figures, the three richest individuals have a net worth equal to the poorest 48 countries. As I write, the greatest power that has ever existed is planning to spend $60-billion on a new arms race, a sum that could provide the world with safe drinking water and leave $20-billion in change. The typical response of the mighty is to go on building higher pyramids while the storm clouds gather, like those long-dead Maya kings.

Ronald Wright's books include Time Among the Maya and Stolen Continents. His latest, the novel A Scientific Romance, won Britain's David Higham Prize for Fiction.