Wednesday, May 23, 2007. Remember that date. It's the day the Earth became an urban planet.

Working with United Nations estimates that predict the world will be 51.3 percent urban by 2010, the researchers [demographers from North Carolina State University and the University of Georgia] projected the May 23, 2007, transition day based on the average daily rural and urban population increases from 2005 to 2010. On that day, a predicted global urban population of 3,303,992,253 will exceed that of 3,303,866,404 rural people.

For the first time in history, more people live in cities than in rural areas. This is, in many ways, the single most important indicator of whether we'll survive this century. Here's why:

Urban centers support people more efficiently than do small towns, villages, and the countryside. This isn't just true environmentally or economically; it's arguably also the case when it comes to the kind of intellectual ferment that drives innovation. New ideas are the sparks coming from the friction between minds -- and you get a lot more friction in the city. Urban growth, over time, makes us all stronger.

Cities require complex support systems, however. Complex infrastructure offers plenty of opportunities for failure, whether via natural disasters or human causation. Isolated failures will happen, and not pose a systemic threat. But repeated -- or un-repaired -- system failures would inevitably drive people out of the cities, by choice or by necessity.

As long as the overall proportion of urban dwellers to rural denizens continues to grow, we can reasonably conclude that human civilization is doing a decent job of maintaining its overall system integrity. If that pattern reverses -- if we start to see the proportion of urban to rural edge back towards rural dominance -- it's time to look for signs that civilization's systems are collapsing.