Jeremy Grantham has been doing the math, and is convinced that world resources are way insufficient to support the current population.
Grantham believes that the planet can only sustainably support about 1.5 billion humans, versus the 7 billion on Earth right now (heading to 10-12 billion). For all of history except the last 200 years, the human population has been controlled via the limits of the food supply. Grantham thinks that, eventually, the same force will come into play again.
This is where we should be innovating – how do we match the level of resources to the (growing) need? Space travel is the old school sci-fi remedy: let’s go to Mars!