Living sustainably is not just about preserving the wilderness or keeping nature pristine. The transition to a green economy depends on cities. For the first time in human history, the majority of the people on the planet live in urban areas. If we are to avert climate catastrophe, we will need our cities to coexist with nature without destroying it. Many places are already investing in the infrastructure of the future--including renewable energy, energy efficiency, mass and personal transit, and advanced sewage and waste management--but the modern city still has a long way to go.

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