[Maritime Q-Scout]

they basically said it was a guess, as any deviation in any one of the variables would cause numbers to drastically change.

I liked some of their hypothesis, but again impossible to test.

Assuming HIV/AIDS spreading slows in 2010, and a cure if eventually found

Assuming no major natural disasters, or major events that effect migration, that would almost surely happen.

Assuming no major civil war, or wars for that matter.

Assuming birthrate drastically falls to 1.85 down from 2.83 in 2000. *IF* the birth rate remains the same (which it won't but still) expect to see 134 Trillion Earthlings (and you think you have trouble finding a parking space at the mall now!)

So really it's all guesses, 9 billion does seem low to me, but I'm not a population forecaster.