"Agriculture enabled population growth and social complexity, but it gradually robbed soils of nutrients. Sailing ships guided with clocks and navigational charts could increase the scope of trade, but building wooden ships (and making charcoal for forging steel) was leading to the deforestation of whole continents. A reckoning with limits seemed to be in store. Then a miracle happened. People who lived in some key centers of global trade started using fossil fuels—energy sources capable of delivering power in previously unimaginable and seemingly endless quantities. Coal, oil, and natural gas enabled the development of transport technologies (steamships, railroads, cars, trucks, and airplanes) that overcame prior limits to the speed of travel and trade, so that products and resources that were abundant in one place could be transported to places where they were scarce. Fossil fuels could be used to increase the rates of resource extraction via powered mining machinery, and to process lower grades of ores as more concentrated ores were depleted. They could be fashioned into plastics and chemicals to substitute for some natural materials that were getting scarce, such as hardwoods and whale oil. And they could be made into artificial fertilizers, which could replace soil nutrients lost due to unsustainable agricultural practices. All these developments together enabled population growth at rates that far outstripped historic trends: human numbers expanded from one billion to eight billion in a mere two centuries. We were, in effect, stretching existing constraints on population and consumption to the point that it was difficult for many people to see that boundaries still existed at all."