Richard Manning, author of Against the Grain (2004), discusses the 'Green Revolution' and the end of cheap oil. It's only about 5 minutes, and well worth the time.

A few important quotes:
"agriculture, because it's catastrophic, must constantly have new land. it also continuously needs new land because it creates excess population... for 10,000 years, agriculture compensated for its weaknesses by taking new land."

" 1960 we ran out of new land, period. We've colonized some new land since 1960, but we've lost an equal amount to things like salinization an loss of water... We were at 3 billion people then..."

"...the result [of the Green Revolution] was a tripling of production of both rice an wheat. The result of that is that something like 75% of human nutrition today is covered by corn, wheat, and rice - three grains. The ultimate result of that was we were able to increase human population, support that extra population, plus ramp it up further. So, in my lifetime human population has doubled from 3 to 6 billion people."

"The hidden fact in all of that was that all of the increased production depended not just on short plants, but on energy - fossil fuels. Because the chemical fertilizers that took advantage of that short plant architecture come from natural gas... It's a straight conversion from natural gas into fertilizer. but, at the same time, we're using enormous amounts of energy to plow those fields with tractors, to process the food... you can't go out an eat a piece of grain like you can a green bean or tomato. It must be process in some way. It must be cooked. And to transport that food.

"The result of all that is that if we look at about 1940, an American farmer was using roughly a calorie of fossil fuel to make a calorie of food. Today, that same farmer uses something like 10 calories of fossil fuel to make a calorie of food.... petrochemicals have become embedded in our food supply."

"We put off the catastrophe of a generation ago with fossil fuels. In other words, we didn't colonize new farmland, we colonized new oil fields, and new watersheds to make irrigation water... that strategy will collapse. We will be at exactly the position we were a generation ago when we had 3 billion people we couldn't feed, although now we have 6 billion - exactly double the number."
The vegan argument - that farmed animals represent an unsustainable use of resources - is absolutely correct. Unfortunately, this argument is a half-baked idea that plays on what's more inefficient while burying the ultimate truth that all farming is inefficient. Implicit in the argument is that farming requires inputs (resources), and as Manning puts it, agriculture is "catastrophic". Embodied in the ultimate roots of paleo (read: a dietary framework informed by evolutionary theory) is that argument taken to its conclusion: all farming represents an unsustainable use of resources. In a full ecological conception of paleo, fossil fuel inputs would not be required.

Agriculture creates excess population. The argument that we need more agriculture to support higher population fails to recognize its inherently circular nature.

I highly recommend Against the Grain: How Agriculture Has Hijacked Civilization. I'd say it's one of the Top 5 most important books of our time. I suspect Manning's most recent book, Rewilding the West: Restoration in a Prairie Landscape is equally great and important, but I haven't read it at the time of this writing.