Each year, humans use about 18,000 times more copper than the planet puts into new deposits, according to a study in the March issue of the journal Geology. At this rate, the amount of copper available within 3.3 kilometers of the Earth's crust--the likely limit of future mining--will be depleted in just 5,500 years, the study concludes.
This is an alarmingly short span in geological terms, says Stephen Kesler, professor of geological sciences at the University of Michigan and a co-author of the study. Unlike other resources such as forests and fisheries, which can be renewed relatively rapidly through natural processes, mineral deposits like copper form so slowly that they're effectively classified as "nonrenewable," the study explains.
To calculate copper reserves, Kesler and collaborator Bruce Wilkinson of Syracuse University took a new approach to measuring the planet's mineral resources. Past measures relied either on geologic extrapolations of existing mineral deposits in well-explored areas, or on economic estimates of how much of a given resource has already been found. But both of these methods typically provide estimates to depths of only about 1 kilometer, ignoring large parts of the Earth's 50-kilometer thick crust that might contain important mineral deposits.