Earlier this week, the (estimated) total number of people in the world blew past 7 billion.

To put that number in context, for the first 70,000-odd years of human history, the total human population was around 1 million. Then, over the course of the next ~10,000 years through 1800, the population gradually grew to 1 billion. And now the population has increased 7X in the past 200 years.

Many of the resources used to sustain this population, meanwhile, have not grown.

On the contrary, they've shrunk. Because we've used them up.

Oil, for example. And metals. And potassium, which is used to make fertilizer, which is used to grow the food we eat.

Just go to the slides >

Optimists laugh at the idea that we should be concerned about this — ever-growing population and ever-shrinking resources. Science and innovation will save us, they argue. They always have.

("Always," in this context, of course, means the past few hundred years, which is less than 1% of human history).

And hopefully the optimists will be right.

If you were a stock-market investor and you saw the population chart above, though, you'd probably think to yourself, "WHOA — talk about a bubble! Can't wait to short that thing!"

And some smart investors would agree with you.

Jeremy Grantham. Earlier this year, Jeremy Grantham of GMO published a treatise on the root cause of the exploding commodity prices of the past few years. He also offered a startlingly depressing outlook for the future of humanity.

Grantham concludes that the world has undergone a permanent "paradigm shift" in which the number of people on planet Earth has finally and permanently outstripped the planet's ability to support us.

Specifically, Grantham says, the phenomenon of ever-more humans using a finite supply of natural resources cannot continue forever — and the prices of metals, hydrocarbons (oil), and food are now beginning to reflect that.

In other words, Grantham says, it is different this time.

Grantham believes that the trend of the last 100 years, in which the prices of almost all major commodities have steadily declined, is permanently over. And from here on in, he says, humans will be competing more — and paying more — for ever-scarcer resources.

From an investment standpoint, this paradigm shift need not mean disaster: Grantham says the obvious play is to own "the stuff in the ground" (and the ground itself, as the huge boom in farmland prices illustrates). The less obvious but equally compelling play is to own companies and technologies that facilitate resource conservation.

From a societal standpoint, the news is far worse. 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 question, whether we're headed for a resource and environmental crisis, is obviously a critical question, not just for investors, but for humanity at large. We'll be looking into the question in detail here over the next few months.

In the meantime, here's a snapshot of Grantham's argument, along with his key points at the end.