Im not any sort of expert but as I understand it the huge increase in food production over the past 50 years has been driven by artifical fertilizers and the mass production of these is reliant on abundant cheap energy (in the form of hydro carbons such as oil and gas).
George Monbiot been has been writing some intersting stuff on this -
"In 2003, the biologist Jeffrey Dukes calculated that the fossil fuels we burn in one year were made from organic matter "containing 44 x 1018 grams of carbon, which is more than 400 times the net primary productivity of the planet's current biota". In plain English, this means that every year we use four centuries' worth of plants and animals."
http://www.guardian.co.uk/Columnists/Column/0,5673,1659036,00.html
Yes this is clearly unsustainable - especially if cheap energy proves to be a (historically speaking) short term blip - we're using up 100s of years worth of energy and resources at a rapidlly accelrating rate which is fueling global warming and actually makig the planet less fertile - squeezing the planets porodcutivity for short term financial gain.
Yes you can set up perma-culture and sustainable living projects but I cant see how they come close to providing the essentials for billions unless the gross global inequalities in power and wealth are seriously tackled - and the welathy and powerful are never going to allow that to happen unless they are forced to (evidence? all of recorded history).
The whole discourse of 'sustainable development' is a smokescreen - it suggests that with a few reforms and smarter, 'greener' consumption then we we will be able to mitigate the negative effects of industrilaisation and economic growth. But - as I think most people here accept - its consumption itself, and the whole captialist system which demands it must always grow at an ever faster rate - that is the problem.
i.e hydorgen or electric care - yes they have zero carbon emissions - but large amounts of emergy are required to produce the hydrogen or charge the batteries (not to mention the production costs of producing enough of these cars - whcih are further increased by the market requirement that people 'upgrade' their vehicle every 18months or so ). All thats happening is that the energy and emmisions costs of 'green' cars are passed to another part of the economy.
Given all this, one likely outcome is that the rich and powerful continue to consume finite resources at an ever increasing rate whilst protecting that 'right' by force - and it is when this happens that you will see a revival of malthusian, social darwinism and - yes - fascist ideas in order to justify what will effectively be planet wide genocide.
Has anyone got any convincing evidence that 'sutainable' farming etc can provide the essentials for 6 billion people? I love that to be the case - but I am not convinced that it can realisticaly replace the present - and clearly unsustainable and finite - cheap energy boom.