I found an interesting statistic today. Your typical tropical rain forest produces about 9000 kcal/m^2/year in net primary production (NPP). Doing some back of the envelope calculations I came up with the estimate that to replace the 84mbpd of oil used in the world would require the net primary production of the amazon rain forest. This was assuming no loss converting the cellulose to a usable energy form. If I understand it right the net primary production does not take into account the other consumers of the primary production ie other animals, bacteria and fungi so there is no way that we could every exploit the tropical rain forest that intensively anyway.
Does anyone have any idea what proportion of the net primary production we humans can take from field crops? What is the most efficient method to take solar energy in the form of biomass. I've read an estimate that says that to replace fossil fuels would require about a quarter of the net primary production of the world. My question is what percentage of the NPP do we actually take from field crops. Presumably there is an upper limit to the percentage of the NPP that we can take off a field since at some point the energy return on the marginal activity is negative (For example guarding the field from racoon competitors is probably not an energy positive activity).
This all ties back to the cellulose ethanol argument. I suspect we really don't have a hope of extracting enough biomass from the planet to be able to replace fossil biomass (oil) with renewable biomass, but I'm open to others saying I'm wrong. What do you guys think?