Numbers between 1 and about 100 million I can comprehend. 6.5 billion (as reported over at Science Blog) is a entirely new ballpark. Living in Southern California (home to 17 million people in a five county area) I have grown accustomed to knowing there are a lot of folks around and not nearly enough land to hold us all. But economies of scale to accommodate 6.5 billion seems staggering and impossible.
Though the earth has demonstrated it's capacity to provide it makes sense that one day there will not be any more of something. Of course there always been instances of 'not-enough', but for now those problems are due to poor distribution, political struggle, and indifference. It's just amazing to me with 6.5 billion people in the world that there are not already long lists of things that the earth has run out of that until now had been a staple in my life.