I just came back from a lecture by Jared Diamond who talked about his latest book - Collapse: How Societies Choose to Fail or Succeed. It made me think about what the capacity of the world is if we relied entirely on energy arriving from the sun. The power density of solar radiation is about 1.4 KW/m^2 and the surface area of the earth is about 10^8 km^2. So in 12 hours of daylight we receive about 10^16 MJ. The energy consumption of the US is close to 10^12 MJ per day which amounts to about 10^4 MJ per person. An average person eats less than 10 MJ per day. If the entire world lived as Americans we would use almost 10^14 MJ per day. I may be off by a factor of 10 or so but it looks like we're cutting it pretty close. If the world starts to live as Americans and we convert about 1% of the total sunlight into useable energy, then 6 billion people is at full capacity. I think this is a pretty good argument for us to conserve a little bit more (or we better get those fusion reactors up and running).