Encouraging in some ways and dismaying in others, this book looks at the large and small ways human activity has changed the world, in the context of which of those changes would reverse themselves and how long it would take if humanity disappeared from the Earth. The author's style flows well and is very conversational and readable, presenting a huge amount of solid information without ever being boring or hard to follow. I was already interested and fairly well informed about a number of these subjects, but some of what he explained in this book surprised (and occasionally shocked) me. An important book for anyone interested in the technological history of humanity, our future, the world's future, or the environment.