John Tierney thinks Richard Branson's $25 million prize for a technology that removes carbon dioxide from the air is a good back up plan.

Vacuuming up our carbon-dioxide mess sounds improbable now, but so did the idea of precisely determining a ship's longitude in 1714, when the British government offered a prize that led to a revolutionary tool for navigators, the chronometer. Private spaceships seemed impractical a decade ago, but the $10 million Ansari X Prize spurred competitors to spend more than $100 million, and the winning design will soon be taking tourists into space.

If governments and other moguls throw in more money, the new Virgin Earth Challenge may be the start of competitions that ultimately yield nanobots or microbes capable of gobbling up carbon dioxide. As far-fetched as it seems today, removing carbon dioxide from the atmosphere could turn out to be a lot more practical than the alternative: persuading six billion people to stop putting it there.