Bringing a serious note to the debate (thanks Mike WE!)
Global warming, depletion of fossil fuels, food production, scarcity of housing, pressures on transport and future pressures on drinking water all have one driving factor.

Population growth.

The current level of population is sustained by purely technological means. By 2050, global population will grow by 50% to nine billion.

We're already at peak oil as it is. The need for alternative sources of energy is paramount if we stand any chance of feeding those mouths.
(Bicycle Repairman "you're also a bit of a wanker", Fri 9 Jan 2009, 10:18, Reply)