While I found the book interesting, the book itself didn't really give me an answer to the question in the title. As far as I remember (I read it a few years ago) , the book concluded with something along the lines of "wel'll find something else, humans are creative, so technology will solve it" without specifically stating what that could be. And that reminded my a bit too much of Jared Diamond's pondering in his book "Collapse":
"I have often asked myself, “What did the Easter Islander who cut down the last palm tree say while he was doing it?” Like modern loggers, did he shout “Jobs, not trees!”? Or: “Technology will solve our problems, never fear, we’ll find a substitute for wood”?