National Geographic is running a series on population this year, as we're expected to top 7 billion by the end of 2011. Clearly population is a big factor in sustainability. Population growth, and the increased consumption, demand, and stress on natural systems that comes with it is a fundamental driver of the funnel metaphor. Some people get too caught up on population and want to treat it as the only driver. It's not - for some of the reasons highlighted in the video below.
In sustainability-academia, one of the classic formulas for representing the interplay of major drivers is the IPAT equation: Impact = Population * Affluence * Technology. (this equation has some limitations, but is a helpful way of conceptualizing the interplay between some of these major drivers).
I'm looking forward to National Geographic's coverage on this - and think these two videos do a great job relaying some sense of the scale of our population, and some of the expected demographic trends: