Unit 5: 1 The reason energy and water sit at the top, ahead of food and poverty, is that addressing them makes subsequent problems easier to deal with. Developing abundant sources of clean, reliable, affordable energy enables an abundance of clean water. An abundance of clean water enables food production and protects the environment. And so forth. 2 In many ways, the 1900s was the century of energy conflict. Now, the dawn of a new energy era is just around the corner — with the prices of solar power going down, and distributed generation and energy efficiency on the point of taking off — and we can foresee a time this century when water replaces energy as the next great challenge for humanity. Getting water right could clear the path to a fully liberated, healthy, and peaceful civilization. 3 But water is complicated. First, there’s no alternative. There are alternatives to coal and petroleum, but water cannot be replaced. Second, water is intertwined with every other sector of society. Energy production requires water for cooling power plants and fracturing shale. Agriculture needs water for irrigation. Industry and cities use trillions of gallons of water for all sorts of purposes. Third, water demands are growing at the same time supplies are fluctuating. Climate change is expected to intensify droughts and floods while shifting where water will be and when. That means humanity will be confronted with decisions about whether to move people to water or water to people.