With the worldwide population expected to exceed seven billion in 2011, National Geographic magazine offers a 7-part series examining specific challenges and solutions to the issues we face. The magazine introduces the series with its January cover story "7 Billion," offering a broad overview of demographic trends that got us to today and will impact us all tomorrow. The first in-depth story will appear in the March issue, focusing on humans' impact on the planet's geology. Other stories will follow throughout 2011.

Editor's update: in 2050, 70% of the population will be living in "urban areas," not "megacities" as stated in an earlier version of this video