Yesterday's post ended by suggesting that a single-minded obsession with population actually distracts people from the difficult realities of the quest for sustainability in this century. Lest this sound like an attack on a straw man (doesn't everybody know that economic and political change is also on the table?), let's look at an example of this kind of rhetoric.

It comes from one of my favorite sources of population-reduction manifestoes, Britain's Optimum Population Trust. On the front page of its website the Trust touts a report that puts a climate-change spin on its cause. By preventing 500 million births over the next 40 years, it says, humanity would send 34 billion fewer tons of carbon dioxide into the atmosphere by 2050. So contraception, the OPT argues, is the most effective technology we have against catastrophic global warming.

If you download the report from the site (you can have the pdf from here), though, you might notice a fact about its methodology that the press release omits: The paper, by Thomas Wire, a graduate student at the London School of Economics, "assumes carbon dioxide emissions per capita to be constant for each country,'' as Wire told me. In other words, to find the tons of carbon-dioxide caused by each new baby in 2035 or 2050, it assumes that baby will have the same footprint as one born today.

The IPAT formula postulates that people's environmental impact is a function of population, lifestyle and technology. The OPT forecast holds lifestyle and technology constant, making population the only variable that is allowed to change. That's like asking a mugging victim to pick out his attacker from a one-person line-up: When there were no other candidates, of course population growth bears all the blame for future carbon impact.

Because his assignment was to compare the cost effectiveness of carbon-mitigation versus the effectiveness of contraceptives, Wire told me last year, he had to assume that $1 spent on birth control was a $1 not spent on green tech. If he had allowed for progress on renewable energy, hybrid cars, more efficient appliances and so on, he'd have been counting the same dollar twice. There was nothing wrong with his methodology, but given its assumptions, the report doesn't prove population growth will drive a huge rise in atmospheric CO2. Instead, it presumes this is so.

Wire knows his scenario is only one of many possible outcomes—food for thought, not prophetic truth. "For more insight, the same model can work with other variants, but no single projection can be thought of as a `prophecy,' '' he pointed out in an email. He is a statistician so, of course, "for me, it is quite natural to think of future events in terms of probabilities.''

For an advocacy group, though, what is quite natural is to strip a projection of its context and meaning, and dress it up as a scientific certainty: Do what we want, and you'll save this much money and that many acres. The OPT certainly didn't invent this game. But they, like other groups that play this way, are perpetuating the notion that there is a single key to global problems—just when survival depends on realizing that there is no one key, because all the world's environmental, economic and political troubles are intertwined.