Investor Jeremy Grantham of GMO recently published a startlingly depressing outlook for the future of humanity.

Grantham thinks the number of people on Earth has finally and permanently outstripped the planet's ability to support us.

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).

Basically, Grantham thinks most of us are going to starve to death.

Why?

In part because we're churning through a finite supply of something that is critical to our ability to produce food: Phosphorus.

Phosphorus is a critical ingredient of fertilizer, and there is a finite supply of it. The consensus is that we will hit "peak phosphorus" production within a few decades, after which point our phosphorus supply will inexorably decline. As it declines, we will be unable to feed ourselves. And you know the rest.

Of course, ever since Malthus, a steady stream of doomsayers have predicted a ghastly end to the human population explosion--and, so far, they've all been wrong.

So why is a man of Grantham's intelligence adding his voice to this chorus?

And how real is this threat? Are we all going to starve?