But you'd still have to assume some things in order to make such a projection.
Ya' know, when talking economy, the hardest thing to do is actually identify what's really going on--even for economists! It's energy--always was, always will be. Even back when it was a hunter/gatherer existence seeing as how the energy in the food was the only kind in play...
...until man made fire--with fire, you could do all sorts of things! But make no mistake, all those things have some root basis in our survival--it's just a more luxurious survival today for those of us living in the developed and developing worlds.
Which brings us to an equation of sorts:
Economy = Energy Produced x Efficiency x Distribution
The trick, of course, is to define the terms with real numbers and plot a curve over time to see how it goes--and whether or not it mirrors what we're seeing in the big, wide world. I could be off my rocker, of course, but methinks me's not.
Anyhow, let's let the "x" that we want to solve for be the amount of energy in BTUs delivered to the citizenry of the US on a per capita basis. We can get yearly census data here and world primary energy consumption data here. inserting a formula column with simple division and... voila':
(link to the full-sized version)
The years shown are from 1980 to 2006, by the way. Couple of interesting notes: the big downstroke on the left was compliments of Jimmy Carter and the modest downstroke a quarter of the way from the right was 9/11.
Now, we can continue on and further modify the curve by adjusting it for the rising cost of energy--it's too late in the evening for me to piddle around with that in a serious way, but you should be to intuit what the effect of that's going to ultimately be.
