Andrew Revkin, an environmental reporter for The New York Times and author of the paper's Dot Earth blog, warns that the math is pretty depressing. There are about 6.8 billion people on the planet today, a number projected to get to 9 billion by 2050. Americans, the world's greatest per-capita emitters of greenhouse gas emissions, produce about 20 tons of the stuff per person, per year. If we were to cut that in half, as emissions rose with the quality of life in much of the Third World, and everyone on the planet met around 10 tons per person, per year, simple multiplication says we'd collectively emit 90 billion tons of carbon dioxide annually come 2050.

That's three times the already problematic current number.

When we start to think about that number, 9 billion, a lot of "cheery suppositions" about what the world can do to curb climate change evaporate, Revkin said (via carbon footprint-minimizing Skype from his desk in New York). He spoke to an event in Washington discussing population trends and climate change, and the media that seldom correlate the two.

The interrelated topics aren't likely to get much talk when global leaders meet in Copenhagen in December for the next round of wrangling over a successor to the Kyoto Protocol. But at least the media could start highlighting the sensitive relationship, as was suggested at the talk hosted by the Woodrow Wilson Center.

A couple of mental roadblocks emerge, central among them the sentiment that, well, there are just too many people on the planet, so what are we supposed to do about it? Any answer trips up against the politically touchy topic of family planning (a distinctly different concept, reproductive-health advocates stress, from "population control").

"The single most concrete, substantive thing a young American could do is not turning off the lights or driving a Prius," Revkin said. "It's having fewer kids." But this is just a thought exercise, he cautions, and no model for the kind of official policy most Americans would want to live with. A recent study, though, by the London School of Economics and the British-based Optimum Population Trust, suggests meeting the world's unmet need for access to reproductive health would be the most effective and cheapest way to start dramatically cutting carbon dioxide.

Each $7 spent on basic family planning between now and 2050 would reduce emissions by more than a ton, the research says. To get the same reduction through alternative energy would cost at least $32 (or, as much as $83 to implement carbon capture and storage in coal plants, $92 to develop plug-in hybrids, or $131 for electric vehicles).

Providing such family planning over the next four decades would be the equivalent of reducing global CO2 by six times America's annual emissions. All of this, though, assumes there's nothing controversial about getting birth control to rural Africa. Not that the conversation has to start with The Pill: Wherever women have been given access to reproductive health around the world, they have tended to opt for fewer children than they would have had otherwise, meaning that access has a controlling effect without being coercive.

Emily Douglas, Web editor at the liberal magazine The Nation and previously an editor at RHRealityCheck, suggested some historical context: World population projections were revised downward after the widespread dissemination of birth control in the West. Officials once predicted the trend would follow as birth control was made available to the Third World.

"But that assumption turned out to be false," Douglas said. And so politicians head to Copenhagen with the most cost-effective solution to climate change (one piece, of course, of a broader menu) just as divisive as any other, inseparable from a web of policy problems that grows more connected to the climate by the day.

(By Emily Badger, Miller-McCune.com / AlterNet, 19/10/2009)