The 20th Century was a time of spectacular scientific and technological advances, the freeing of the arts by an exuberant modernism and the spread of democracy and human rights. It also was a dark and savage age of world wars, genocide and totalitarian ideologies that came dangerously close to global domination.

While preoccupied with all this tumult, humanity also managed to decimate the natural environment and draw down the nonrenewable resources of the planet with reckless abandon. It accelerated the erasure of entire ecosystems and the extinction of thousands of million-year-old species. Earth's ability to support our growth is finite, but we usually have been too busy to notice.

Now, as the new century begins, we have begun to awaken from this delirium. Increasingly post-ideological in temper, we may be ready to settle down before we wreck the planet.

The bottom line is different from that generally assumed by our leading economists and public philosophers. They have mostly ignored the numbers that count. With the global population at more than 6 billion and on its way to 8 billion by midcentury, per capita fresh water and arable land are descending to levels resource experts agree are risky.

The ecological footprint -- the average amount of productive land and sea appropriated by each person in bits and pieces from around the world for food, water, housing, energy, transportation, commerce and waste absorption -- roughly is 2.5 acres in developing nations but 25 acres in the United States. The footprint for the total human population is just 5.4 acres. For every person in the world to reach present U.S. levels of consumption, we would need at least four more planet Earths.

The 5 billion people living in developing countries may never wish to attain this level of profligacy. But in trying to achieve at least a decent standard of living, they have joined the developed world in exploiting the last of the natural environments and reducing to extinction a large part of the planet’s biodiversity. At the same time, humanity has become a geophysical force, the first species in the history of the planet to attain that dubious distinction. We have driven atmospheric carbon dioxide. In short, we have entered the Age of the Environment, in which the immediate future is a kind of bottleneck.

Science and technology, combined with a lack of self-understanding and Paleolithic obstinacy, have brought us to where we are. Now science and technology, with the wisdom and foresight they serve, must see us through and out.

As a close observer for many years of science and the environment, I believe the change is possible. Empirical knowledge and technical ability are growing exponentially -- in the case of computer capacity superexponentially -- faster even than the crises of population and resources. With this overall advance will soon come an understanding of the biological basis of the mind and human behavior, and therefore a more solid, predictive social science. A sophisticated picture of the global environment and global resources is emerging. And the technology already is available for raising per capita food production while decreasing materials and energy consumption.

This information is coming online worldwide, allowing people everywhere to see the planet as the astronauts see it: small, and too fragile to bear much more careless tampering. A growing number of leaders in business, government and religion are thinking in this more farsighted way.

The goals of lifting a stabilized world population to a decent quality of life while saving and restoring the natural environment are as noble as any in history -- and attainable. They represent the bottom line of global sustainability, the true key to our future and the most principled guide for economic and political policy.