BALTIMORE — THOUGH human population growth is slowing, demographers are still warning that the earth will not be able to provide room and board to everyone at a standard of living most people expect.
This conclusion, drawn at this week's annual meeting of the American Association for the Advancement of Science here, could help spur a major rethinking of how resources should be consumed around the globe. The world needs to do far more in the way of using less natural resources, scientists say.
''The wealthy third of the world's population has already appropriated virtually the entire sustainable carrying capacity of the Earth,'' warned William Rees, a regional planner from the University of British Columbia at Vancouver.
He added that supporting twice today's population at present Western European standards would ''require the equivalent of five to six additional planet Earths, assuming prevailing technology.''
The good news is that population growth seems to have turned a corner. This is prompting scientists such as Wolfgang Lutz of the International Institute for Applied Systems Analysis in Laxenburg, Austria, to revise their population-growth predictions.
Dr. Lutz now expects the absolute numbers of additional people - the annual difference between births and deaths - to begin declining after 2010. He anticipates about 10 billion people on Earth by the middle of the 21st century. He further expects world population to peak at about 11 billion by that century's end.
But even the reduced population-growth rate won't ease poverty and starvation in much of the world. The amount of water in current use, for example, is unsustainable, says Gretchen Daily, professor at Stanford University in Palo Alto, Calif.
Humans already use more than half of the freshwater runoff available to them plus 26 percent of the rainfall used directly by all plant and animal life, she reported at the meeting. In the future, humans could be consuming nearly 100 percent of freshwater runoff by 2025, given present trends. ''We're coming up against Earth's limits,'' she said.
Dr. Daily also noted that much water could be saved by increasing efficiency and reducing pollution at its source rather than using river water to dilute it and carry it away.
Rees estimated that it would take a five- to 10-fold reduction in rich nations' consumption of energy and materials and waste generation for other nations to be able to achieve higher general living standards. He suggested it would be easier to find ways of doing this if there were ''a whole-scale shift in popular values away from material consumption toward community and spiritual development as a means of self-fulfillment.''