Some years ago, biologists coined a phrase they called the Central Dogma. Its few words (which describe the flow of genetic information inside a cell) became the indispensable mental map by which a torrent of new information was organized and understood. In the 1980s the study of environmental trends went through a period of similar ferment and acquired a central dogma of its own. It holds that the solution to the planet's environmental problems lies in something called sustainable development.

The phrase was popularized by a United Nations commission whose 1987 report "Our Common Future" was a surprise international bestseller, though not in the United States. The Brundtland Report, as it is also known, changed the terms of international debate by persuasively demonstrating to a broad audience that environmental degradation is not merely a consequence of industrialization but a "survival issue," especially for developing countries. Although individuals and commissions had been making the same case for years, "Our Common Future" succeeded where others had failed in illuminating the environmental aspects of poverty, hunger, disease, debt and economic growth.

The report provided a bare-bones definition of sustainable development as economic activity that "meets the needs of the present without compromising the ability of future generations to meet their own needs." While that is accurate as far as it goes, it has proven to be surprisingly difficult to expand the definition in terms useful to policy makers.

Sustainable development assumes that economic growth is essential to environmental solutions. Thus it leaves behind sterile growth/no growth debates. It also implies that there are limits to economic growth. The general view today, however, is that these are not the absolute limits whose existence was so hotly debated in the 1970s. Nor are the limits likely to be imposed by a dwindling, finite supply of resources. Instead, the limiting factor is likely to be, as the report put it, "the ability of the biosphere to absorb the effects of human activities." While these effects are obviously related to the number of people on the planet and to aggregate economic activity, the limits are highly elastic. They are determined by the state of technology and social organization, by political will and capital investment. It is the kind, not the amount, of economic growth that determines environmental stress.

In practice, the difficulty lies in distinguishing growth that is sustainable from growth that is not. Doing so requires a long time frame, detailed knowledge of subtle biological relationships and a broad geographic view. Growth that depends, for example, on the export of pollution may look sustainable as long as only the exporters are considered. Growth that rains acid precipitation on Swedish forests may look fine to the Poles, the Czechs or the Germans, but not to the Swedes.

Finding the appropriate time scale is much harder. It turns up, for example, in the controversy about Northern Pacific old-growth forests and the spotted owl. It is possible to grow old trees in these forests in 200 years. But more than old trees are necessary for sustainable old-growth forests. They also require large, hollow logs on the forest floor. Why? Because the trees depend on fungi that live in their roots, the fungi depend on rodents for their reproduction and the rodents need the logs to survive. It takes much longer than 200 years for a tree to grow old enough to fall, and so a sustainable cutting cycle in these forests must also be much longer than 200 years.

While societies wrestle with these sorts of problems, environmental historians are confirming that the choices are not merely theoretical. The evidence extends back to the beginning of literate civilization. For 1,200 years the history of ancient Sumer hinged on its ability to produce an agricultural surplus, which, in the Mesopotamian valley, meant irrigation. Over time, irrigation caused salinization, as it does today. After 600 years, as the soil grew saltier and the supply of fresh land was exhausted, Sumer's crop yields, and its fortunes, began to fall. By the time of the Babylonian conquest, yields had dropped to one-third what they had once been.

The same story is being pieced together in Mexican jungles where Mayan civilization flourished for 400 years and then collapsed. There deforestation and soil erosion, rather than salinization, apparently did the damage.

The beginning of the last decade of the millennium is a natural moment to glance backward. Clive Ponting, a British environmental historian, points out that an accurate account of human history in 30 minutes would devote 29 minutes and 51 seconds to hunter-gatherers, more than 8 seconds to settled agricultural society and a fraction of the last second to the modern, fossil fuel-fired industrial world. "Viewed over the long term," Ponting writes, "human history has been a story of rising numbers and greater pressures on the environment." A crucial difference is that what were once local pressures are now global ones. "Is humanity too confident about its ability to avoid ecological disaster?"

The Brundtland Commission thought it saw an answer. "We came to see that a new development path was required, one that sustained human progress not just in a few places for a few years, but for the entire planet into the distant future." The notion of sustainable development has defined the goal. Figuring out how to achieve it remains a monumental task. The writer, a vice president of the World Resources Institute, writes this column independently for The Post.