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.
Now, I'm not as learned as Mr. Grantham, but I disagree. There are substitutes for metals and hydrocarbons, and anyone who has driven across the United States can tell you that we're nowhere near running out of capacity to grow more food.
Then again, the switch to substitutes takes time, as does bringing online new food production capacity. In the mean time, if demand significantly outstrips supply, things could get ugly.