ABSTRACT
Exponential growth is relevant to discussions of material resources because extraction of resources has generally been growing at exponential rates, probably since the late 1700s, and certainly since 1900. In 1900, the rate was 495,000 tonnes per year, and in 2015 it was 19,400,000 tonnes per year – more than 39 times as much – and the annual growth rate was 638,000 tonnes per year, more than 1.25 times the amount in 1900. Predictions were reinforced by the observation that the highest grade and most easily accessible resources are used first, leaving ores that have lower metal content, are buried more deeply, in places that are harder to reach, and are more difficult to process. Although there are exceptions to the general trend toward mining lower grade ores, it remains true and important for many metals. Resources can be subdivided into different categories, reflecting the level of geological knowledge and associated confidence in their existence.
