We're getting those crazy yields because of fossil fuel based fertilizer, which could be replaced but only by some other equivalent source of external nitrogen.

We're getting those crazy yields because of fossil fuel based fertilizer, which could be replaced but only by some other equivalent source of external nitrogen. My background is in physics, and in physics we deal with constraints- thermodynamics, conservation of energy, and so on. I'm telling you that no matter what technology we develop certain constraints are inescapable. Crop yields can only increase so much. If we restrict the domain of discussion to the surface of the earth, then we're left with limits. That doesn't necessarily imply a Malthusian scenario! But it means that we need to plan for a time when exponential growth is no longer the rule. I mean, there is nothing normal or sustainable about this wacky, unbalanced moment of time that we live in. I don't know how it ends- with a plateau or a bust- but sometime I would say this century it has to end. Look: if you assume 3% year on year productivity growth per person, in around 700 years every person is producing as much goods and services as all 7 billion of us do today. That's clearly preposterous. Your fingers could not move fast enough to write that much code, compose that much music, and your little human brain could not appreciate it all anyway if everyone was producing that much art, let alone all the physical work. The question is just how long we have left before that growth flattens out, and what happens next.