DURHAM -- Gaia, the Goddess of Mother Earth, is sick. For thousands of years, she has maintained stable concentrations of gases in our atmosphere: nitrogen at 78 percent, oxygen just shy of 21 percent and carbon dioxide at 270 parts per million. Her servants are the plants, animals and microbes that do the hard work to keep our planet alive and in balance.

A quick glance at Mars and Venus shows just how different the atmosphere on a lifeless planet can be. Neither planet harbors significant oxygen, and a dense atmosphere of carbon dioxide raises the surface temperature on Venus to 885 degrees F.

The theory of Gaia was the brainchild of Dr. James Lovelock over three decades ago. He recognized that the conditions on Earth were not only unique in our solar system, but also remarkably stable throughout the history of our planet. For instance, early in its life, the sun was about 30 percent less luminous than today, yet the temperature of the Earth has never fallen so low that the oceans have frozen from top to bottom, nor risen so high as to boil the oceans away. Looking at the less favorable conditions on neighboring planets, Lovelock postulated that, through a unique symbiosis, life on Earth is responsible for the stable, favorable environment that we inhabit. Upset the biosphere and reduce its species diversity, and you threaten the persistence of life on Earth.

There is increasing evidence that Gaia is running a fever -- global warming. For the past 100 years or so, the concentration of carbon dioxide on Earth has risen steadily to its current value near 370 parts per million. Most of the increase in carbon dioxide is directly linked to burning fossil fuels, something that humans seem to do with great gusto. The concentration of methane and other trace gases is increasing as well.

For millennia, the biosphere maintained stable levels of these gases, and scientists agree that rising concentrations of trace gases in Earth's atmosphere are certain to warm our planet.

These changes in Earth's physical and chemical characteristics appear to be the work of a single species -- Homo sapiens. Unless we take immediate steps to curb population growth, today's human population of 6 billion will rise to 10 billion within the lifetime of our children. To feed and shelter our population has required us to usurp and manage large areas of the Earth's land surface for agriculture and housing. Our harvest of the sea has reduced the stock of many fishes to less than sustainable levels. Harvest and loss of natural habitat is driving many species to extinction. The human impact reduces the ability of Gaia's servants -- the diverse species on this planet -- to cleanse our effluents and maintain stable conditions for life.

We face a dilemma: each person on Earth wishes to achieve the highest possible standard of living, and our numbers are increasing rapidly. The human pursuit of a better life and the byproducts of this quest now foul the atmosphere and the waters of our planet, denude its vegetation and erode its soils. We can see this trend each day in central North Carolina, and we measure it globally by the rise in Earth's temperature. There are few laws in ecology, but one of the most fundamental predicts the ultimate collapse of a population showing exponential growth in a closed environment.

I am not at all hopeful that our planet will receive an interplanetary delivery of fresh resources. Gaia functions as a closed chemical system, and the persistence of life in that system demands that we manage it well --both for ourselves and for the myriad of other species that maintain the stable conditions on Earth. As economic incentives demand it, we can use energy efficiently and cleanse and recycle many of the waste products of modern society. However, the changes in the composition of our atmosphere suggest that we are failing in our planetary stewardship. We have forgotten that we too are servants of Gaia.

Perhaps never before has a single species -- through it growth rate and its resource consumption -- had such a dramatic impact on the fabric of the biosphere. While the growth of the human population has slowed somewhat in recent years, each day we still add about 250,000 -- roughly half the population of Wake County, to our number.

The United States must provide leadership by reducing our individual resource use and helping to establish better family planning programs throughout the world. Gaia expects nothing less.

-October 17, 2000

William H. Schlesinger is James B. Duke professor of biogeochemistry at Duke University and author of "Biogeochemistry: An analysis of global change."

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