In his book The Ultimate Resource 2 (first printed in 1981 and reprinted in 1998), he extensively criticizes the notion of "peak resources", and uses copper as one example. He argues that, even though "peak copper" has been a persistent scare since the early 20th century, "known reserves" grew at a rate that outpaced demand, and the price of copper was not rising but falling over the long run. For example, even though world production of copper in 1950 was only one-eighth of what it was in early-2000s, known reserves were also much lower at the time – around 100 million metric tons – making it appear that the world would run out of copper in 40 to 50 years at most.