So you have read the "The Limits to Growth" (which was pretty scary in 1972) looked at what has happened over the last 40 years, and not figured out where it went off the tracks?

Quite simply it is this, extrapolation of closed systems always fails to predict the behavior of those systems. The two most interesting examples are the use of energy and birth rates. Birth rates go down as countries become more prosperous for a variety of reasons, but the effect is that once it gets below replacement rate (as it has in Japan and elsewhere) the extrapolating the population leads it going to 0. We intuitively know that it is unlikely a population will simply die off due to a failure to reproduce. But it is easier some how to imagine a crushing horde of people, too many too feed, crammed into pens. Why? Energy is another area where we've constantly predicted (again via extrapolation) the end of the world as we know it, and again it has failed to materialize again and again.

These are examples of the responsiveness of the system to forcing functions. People don't reproduce as much if they are not worried that their children won't live to adulthood, energy as an expense is minimized by gains in efficency, process changes. The US' carbon 'footprint' has remained flat for years, policy change, people change, the economics change.

So predictions based on extrapolation are fallible if not simply wrong.