The Frankfurt Auto Show just wound up today, and most of the concept cars focus on lowering carbon dioxide emissions. Volkswagen, for instance, launched a two-seat "1-Liter" concept car that gets a remarkable 170 miles per gallon.

But suppose all the money we're investing in better gas mileage and auto emission controls could be used to cut greenhouse gases more productively? What would we spend it on?

The London School of Economics suggests that condoms and other forms of birth control would be a good first step. A new report, Fewer Emitters, Lower Emissions, Less Cost, suggests that family planning should be seen as a primary method for curbing greenhouse-gas emissions.

Turns out that family planning services are remarkably cost-effective at cutting carbon emissions. Contraception and family planning can cut 1 ton of carbon for $6.70, the report says, against more conventional low-carbon technologies (including more fuel-efficient cars) at up to $31.70 per ton.

It's pretty obvious, really. As Roger Martin from the London School of Economics puts it, with British understatement: "Total emissions depend on the number of emitters as well as their individual emissions."

Or: The fewer people there are, the fewer of them need cars, which lowers total emissions.

United Nations data indicates that 40 percent of global pregnancies are unplanned, and projects that universal access to family planning services could reduce global population growth by 500 million people by 2050.

Not to worry about the planet emptying out, though. That still leaves a projected 8.64 billion humans alive in 2050. If current trends are any indication, they'll all want cars.

Toyota has already sold more than 80,000 Prius hybrids in Japan despite only predicting 100,000 sales for the year

Toyota has already sold more than 80,000 Prius hybrids in Japan despite only predicting 100,000 sales for the year

[Telegraph; Washington Post; condoms photo by Flickr user xmasons]