When Jon Erlandson and his wife, Madonna Moss, were talking about how many children they should have 20 years ago, the exploding world population was front and center in their thinking.

The two anthropologists, now both University of Oregon professors, were teenagers when Paul Ehrlich's "The Population Bomb" became a non-fiction best-seller.

In the three decades between their birth and their baby-making decision, the world's population nearly doubled, picking up more than 2 billion people.So the couple chose to have one child, Erik, who's now 18.

But in the two decades since, Erlandson says, the population issue "has kind of dropped off the map."

"This used to be a big deal; people used to talk about it a lot," said Erlandson, whose work includes studying the effects of ancient people on marine systems. "Now it's not part of any regular dialogue about the environmental problems humans face."

Erlandson is one of 160 scientists and thinkers who've signed up for a "global population speakout" this month. It's the brainchild of John Feeney, a Colorado environmental writer who immersed himself in population issues while fighting a residential subdivision.

The participants say it's time to talk population again. They're worried we won't make adequate progress on the most crucial environmental goals -- reducing carbon emissions, preventing overfishing and decreasing deforestation, among them -- unless we tackle growth and its ever increasing demands on the planet.

But talking population brings up issues that can offend both sides of the political spectrum. Abortion. Immigration control. The effects on economic growth of cutting population growth. Race. The Catholic Church's opposition to contraceptives. Human rights.

CONSUMING MAN

The world's population continues to grow, straining resources. The United States' consumption is generally much higher than elsewhere in the world. Annual levels, per person, for the United States and others:

• CO2 emissions: U.S. 2,789, World 679

• Coffee, pounds: 9, 2

• Fish, pounds: 47, 36

• Electricity, kilowatt hours: 13,636, 2,596

• Televisions: 0.88, 0.28

• Beef, pounds: 90, 20

• Paper, pounds: 655, 120

• Gasoline, gallons: 432, 46

• Beer, gallons: 22, 21

• McDonald's: 0.43, 0.1

• Passenger cars, per 1,000 people: 474, 100

The number of kids you have.

"That's a very personal choice, and I don't think it's something the government should be dictating," said Erlandson, who directs the university's Museum of Natural and Cultural History. "But I do think it should be part of the dialogue. You can reduce your carbon footprint per person, yet if the population keeps growing you're making no progress."

Oregon has seen that dynamic at work. From 1990 to 2004, the state succeeded in slightly reducing its per person carbon emissions, for example.

But the overall level still rose -- by 22 percent -- the state says, thanks to 700,000 new residents.

Recycling rates have risen most years since 1992. But the amount of trash landfilled has still mostly gone up, despite state mandates to reduce it, with population growth and increased consumption to blame.

And the Metro area has become a national leader on containing sprawl. But the expansion area from which Metro plans to pick future urban development -- to accommodate an expected doubling of the area's population by 2060 -- includes some of the state's best farmland and stretches to Sandy to the east, Molalla to the south and nearly to Newberg on the west side.

Population activists have more than just intractable controversies to fight. There's some self-inflicted damage: Ehrlich's famous book was famously wrong in predicting mass starvation by the 1980s.

Affluent countries have seen fertility rates drop. Agricultural yields and energy efficiency -- along with the standard of living in many countries -- have risen even as population has grown, allowing the planet to absorb more people than doomsayers predicted.

Technologies such as carbon sequestration and water desalination could help stave off future crises.

Then again, the United Nations' latest medium-range prediction has the world's population growing to 9.2 billion in 2050, a spike of about 7 billion people in 100 years.

The Global Footprint Network, which calculates the effect humans have on the environment and translates it to acres of land and water, estimates it would take more than two Earths to support human needs for food, raw materials and energy by 2050 if the U.N. projections are right.

The group figures the human population is already above Earth's sustainable limit -- the threshold at which humans can use resources without depleting the environment.

"We know we're not succeeding at that," said Feeney, the Colorado author and father of two teenagers. "We are damaging our habitat, whether it's climate change, deforestation, species extinction or toxic pollution."

The U.N. projects the U.S. population will grow from about 306 million today to more than 400 million by 2050, driven largely by immigration.

Metro, the Portland area's regional government, predicts the population of Portland and surrounding areas, including Clark County, will about double by 2060, from about 2 million people to 4 million.

Growth in Oregon and the rest of the relatively affluent United States packs a double whammy, said Richard York, a researcher and sociology professor at the University of Oregon who's also participating in the population speakout.

That's because increasing affluence, while reducing fertility rates, greatly increases consumption of resources and environmental damage, York says.

But like most other population activists, York and Erlandson steer clear of coercive population policies. That includes China's one-child policy, which officially restricts to one the number of children married urban couples can have.

Plenty of population solutions have potentially wide appeal, they say, including improving women's rights and access to education, improving health care, reducing infant mortality and increasing access to safe and effective birth control.

Also key, Erlandson said: Public -- and private -- discussion.

"My wife and I made a conscious choice about what the size of our family was going to be," he said, adding that they don't judge the neighbors and friends who chose differently. "Everybody's got to make their own choice."

-- Scott Learn; scottlearn@news.oregonian.com