...the only questionable point with that "exponential growth" narration is, that in exponential functions, that point where "things start going vertical" is only dependent on what unit we are willing to apply to Y axis... You know, this point of a perceived "change" of steepness, could be located as far in the past or in the future, as you like, if we are willing (on the theoretical charts), or able (=in the real world, with limited resources and capacity) to adjust the Y axis units (from TRillions to whatever gadzillions, for example - it would push it into the future. If we change them from trillions into billions - our charts would be already way over the roof, possibly somewhere around the moon's orbit)
So it is not anything to judge by, the point in which these charts start to "bent" upwards. Much more important is, what are the PHYSICAL limits on the Y axis, that we are/will be experiencing?
But in general, I fully agree with CM on the topic of limited energy output (oil supply), that will be main driving factor, and a highly detrimental one. I do not know about ANYTHING that would keep us on the rising energy consumption path - and I was quite passionate about renevables/alternatives and all, for quite a few years now. While theoretically, Sun's power falling on Earth's surface is way more than sufficient to cover ALL our energy needs, and way into the future - but practically, ain't going to happened anytime soon, if ever - and for some quite critical branches of economy (ie transport, and God forbid air transport...) - hardly even an option...