In 2006, the last year for which the U.S. Census Bureau has tallied the data, close to 4.3 million babies were born in this country.

That's 11,780 a day, or about 8 a minute.

That's a lot of dirty diapers. A lot of soccer practices. A lot of college tuitions to cover. Above all, a lot of parents' hearts pounding with a sudden avalanche of love for this new creature.

And something else that researchers are beginning to talk about and quantify: a whole lot of planet-warming carbon dioxide emitted into the atmosphere.

This should hardly come as a surprise. At its simplest, adding more people to the planet results in an increase of emissions.

Like other avoidances - not turning on excess lights, not driving excess miles - having fewer children would result in less emissions.

Egad! Is anyone really saying that to be green, you shouldn't have kids?

Who shouldn't have them? The poor? The rich?

And where shouldn't they have them? For instance, a U.S. baby will generate, in all likelihood, exponentially more emissions than a child in a developing country.

And which children shouldn't we have? All the child-rearing wisdom in the world can't guarantee you wouldn't get a humvee-driving shopaholic instead of a bike-riding vegan scientist who figures out how to make electricity with no emissions. Don't we need all the brains we can get to solve the world's problems?

So far, no one is talking about limits a la China's one-child policy.

But last year, with world population at about 6.7 billion, and projected to grow to 9 billion or more by 2050, the United Nations Population Fund concluded that slowing the rate of population growth might give countries breathing room to take other measures. Like generating cleaner electricity.

In June, researchers at Oregon State University concluded that the carbon legacy of having a child in the United States is 20 times more important than other environmental choices, such as getting a car with better fuel economy or replacing single-glazed windows with energy-efficient ones.

But still. Are they really saying . . .

Not quite. A few brave souls have approached the question through the lens of unplanned pregnancies.

Aside from the fact that unplanned doesn't necessarily mean unwanted, the authors of a recent lead editorial in the prestigious medical journal Lancet noted that more than 200 million women worldwide want contraceptives, but lack access. That results, they said, in 76 million unintended pregnancies a year, worldwide.

So maybe author Eric Sorensen was onto something when he included the condom - alongside the clothesline, microchip, real tomato, library book, bicycle, and ceiling fan - in his 2008 book, Seven Wonders for a Cool Planet.

The Lancet editorial coincided, more or less, with a report from the London School of Economics comparing the cost of carbon-reducing strategies. The researchers concluded that basic family-planning programs were the cheapest, costing $6.46 to remove a metric ton of carbon dioxide or its equivalent.

In contrast, removing the same ton via wind turbines would cost $24, or via plug-in hybrids, $92.

Last month New York Times columnist Andy Revkin tossed out in a "thought experiment," as he called it, the notion of carbon credits for people who have fewer children.

Family-planning critics were aghast.

Fewer babies also would mean fewer future adults to pay into Social Security and Medicare, among many other effects. It's clear that environmental concerns inhabit an ever more tangled web. They overlap with national security. And now, it seems, with reproductive rights and women's health and poverty.

About the time I was thinking about all this, a colleague brought his newborn daughter to work.

I got to count her fingers and coo to her and think about how amazing babies' tiny ears are.

She yawned and her little legs flopped over my arm as she decided not to cry after all, opting for a nap instead.

She took my breath away.

So OK, maybe the world's citizens are going to have to think more carefully about population and how to keep it within the bounds of what the planet can sustain without trampling on people's rights.

But as hard as that may be, it's a whole lot harder when you're holding a baby.

Contact staff writer Sandy Bauers at 215-854-5147 or sbauers@phillynews.com. To post a comment, visit her blog at http://go.philly.com/greenspace.