PITTSBURGH (AP) _ The 1990s may be a do-or-die decade for important aspects of the world's environment, an expert on overpopulation says.

''If we don't see some real action in this decade, it will probably be too late to avert some very serious problems,'' Anne Ehrlich, who with her husband wrote the current book ''The Population Explosion,'' told a conference Thursday on solving global environmental problems.

Those problems include inadequate food production, global warming, species extinction and deforestation, said Mrs. Ehrlich, associate director for the Center for Conservation Biology at Stanford University.

''At the moment the rich countries are doing most of the environmental destruction of the planet, but the poor countries are getting to be bigger and bigger players,'' said her husband, Paul Ehrlich, Stanford University professor of population studies.

Carbon dioxide, believed to contribute to global warming, was one measure of environmental deterioration that the Ehrlichs and other scientists discussed at the conference attended by about 1,200 students, teachers, garden-club members and corporate executives.

The United States, with about 5 percent of the world's population, produces 25 percent of the world's output of carbon dioxide, said Peter Raven, professor of botany at Washington University in St. Louis.

''Chicago puts out as much carbon dioxide as China,'' he said. ''It's a peculiar world. The people at the top are consuming far, far more than a vast number of people at the bottom. It's much too unequal, and it's unequal in a way that affects the sustainable productivity of the world.''

Erlich said the current world population of 5.3 billion is 1.8 billion more than in 1968 when he first prophesied the problems of overpopulation in his book ''The Population Bomb.''

At a worldwide growth rate of 95 million people per year, ''we are driving the Earth into impoverishment,'' said George Woodwell, president of the Woods Hole Research Center in Woods Hole, Mass.

''We are living on its capital,'' he said. ''We're eating up the standing stocks of trees and nutrients and soil in the process of feeding the current 5.3 billion people on Earth. In doing that, we make the Earth less capable of supporting people in the future.''

The most serious population problem in the world is in the United States, Erlich said. One American causes 20 times more destruction than one person in the Third World, and one rich American causes 1,000 times more destruction, he said.

''It's not how many people you have but how those people behave,'' he said. ''The most common misperception of the population problem is that it's a problem of poor Indians who don't know how to use condoms. Actually, the problem in the world is that there are too many rich people.''

The Ehrlichs based their figures on 1987 statistics compiled by the United Nations on per capita commercial energy consumption, an index used by environmentalists to measure damage to the Earth.

Citizens of highly developed nations can help stop the spiral by practicing conservation and, when necessary, accepting a lower standard of living, Ehrlich said.

But that does not mean the citizens of wealthy nations have to resort to living like peasants, he said.

In highly affluent Sweden, for instance, the average person uses about 60 percent of the energy as consumed by the average American and enjoys a comparable standard of living, he said.

''The idea of perpetual economic growth is insane, and yet it's an insanity that grips our entire culture,'' Ehrlich said. ''When you hear people say we have to favor economics over the environment, that's like saying we've got to favor eating over breathing.

''There is no way you can keep the size of the economy growing without destroying the environment and killing off your society.''

But, Ehrlich said: ''The bright side is we know societies can change very rapidly when the time is right.''

Frederic Krupp, executive director of the Environmental Defense Fund, said he sees signs Americans are beginning to rally behind the ''green movement'' and demand biodegradable products, energy efficient appliances and sound environmental policies.