On the first Earth Day, 31 years ago, the population explosion was everyone's chief worry. And a book, The Population Bomb, by Paul R. Ehrlich, was the environmentalists' Bible.

An odd thing has happened to the panic over population. It has disappeared. But the problem has not.

The U.S. population in 1970 was 203 million. Since then, it has grown 72 percent to 281 million. If current trends continue, today's population will double in the next 70 years.

So why have mainstream environmental groups abandoned the issue? If anything, the crises associated with population growth have gotten worse: water shortages, suburban sprawl, destruction of natural habitats.

The why of it has been explained in a Center for Immigration Studies paper written by Leon Kolankiewicz and Roy Beck titled Forsaking Fundamentals: The Environmental Establishment Abandons U.S. Population Stabilization.

Today's environmental groups won't go near the subject. The reason? Population growth is now almost entirely fueled by immigration from Third World countries. They are terrified of being tarred as racist.

Back in 1970, the environmental movement had no difficulty grappling with the issue. In the 1950s and '60s, rapid population growth was the result of high birthrates among mostly white, native-born women.

Environmentalists feared that the children of the Baby Boom would do what their parents did go out and have lots and lots of children. (Immigration levels at the time were a lower and largely uncontroversial 200,000 a year.)

In 1969, the Sierra Club urged Americans to commit themselves to limit the total population of the United States . . . and to achieve a stable population no later than the year 1990.

And these ideas resonated with ordinary citizens. In 1970, President Richard Nixon and Congress set up a bipartisan Commission on Population Growth and the American Future.

The commission's chairman, John D. Rockefeller III, wrote that gradual stabilization of our population through voluntary means would contribute significantly to the nation's ability to solve its problems.

Since then, birthrates among American-born women have fallen close to the replacement rate. Had immigration been kept at traditional 1970 levels, the American population would probably have stabilized at 230 million by 2050.

Sierra Club members have tried to bring up the matter, only to come under attack. In 1996, the group's national board ruled that no one speaking in the Sierra Club's name may call for a reduction in the immigration numbers.

The board, however, was not without a population policy. Incredibly, it called on American women to do away with the two-child family. Only one child per family, please. Immigration levels need not be touched.

(Frustrated members have organized their own group, Sierrans for U.S. Population Stabilization. For more information visit www.uscongress-enviroscore.org.)

The Sierra Club's Web site is an exercise in intellectual cowardice. Check out New Research on Population, Suburban Sprawl and Smart Growth at www.sierraclub.org. The piece is heavy on footnotes but never utters the forbidden word immigration.

The command stop sprawl is highlighted at the top. Oh, they're so tough.

Meanwhile, serious environmental problems continue to march under the Sierra Club's nose. Sprawl in congested New Jersey threatens to engulf the last patches of farmland. The state expects to add a million people to its population in the next 20 years.

Texas is suffering a terrible water shortage. It's so bad that some people want to pipe scarce water from the ranch and agricultural areas and into the cities. The state population of 21 million is predicted to double in 50 years.

The environmental movement desperately needs new leadership - people who can talk about population in terms of numbers rather than colors. They can hark back to great liberals who have supported their cause: former Colorado Gov. Dick Lamm, former U.S. Sen. Eugene McCarthy, and the late congresswoman and civil-rights activist Barbara Jordan.

And they must explain the importance of a stable population to all Americans. Whether newly naturalized or native-born, every one of us has an interest in maintaining our quality of life.

This is no easy job. But the time has come for people of talent and courage to step up to the plate. Environmentalists must find forthright advocates.