In his techno-optimist manifesto, the entrepreneur Marc Andreessen casually voices his dream that we might ramp up clean-energy resources so quickly that everyone on earth could soon use 1,000 times more energy a day than is currently typical for people in developed nations. Just think what people could achieve! Sounds great. Except that a little physics thinking also shows that using that much energy would immediately cause planetary warming about 30 times faster than we’re experiencing today, and we’d all be dead in a few years. Not so great after all.