"Most natural ecosystems are limited in their growth by the energy of the sun and availability of resources - in other words, nutrients. Every day, the Earth's surface receives 120,000 terawatts of solar energy, or 175 watts per square meter. If you aren't an energy expert (and I am not), these numbers might not mean much to you, but we can put it another way: every hour and a half, the sun strikes Earth's surface with enough energy to power human civilization for a year. The energy we intercept from the sun in 30 days is equivalent to all the world's fossil fuels, combined, both those we have already burned and those still in the world's untapped underground reserves." - Enric Sala - The Nature of Nature, Why We Need the Wild.