Credit: © ISTOCKPHOTO

The human love affair with metals began in earnest at the end of the Stone Age, when our ancestors stumbled on a material that was far more amenable than rocks for making tools, weapons and jewellery. Metals continue to be central to human civilization and they have pervaded practically every aspect of our lives. In addition to the perennially useful iron and copper, we are exploiting a plethora of metals as exotic as niobium and tantalum. No wonder then, that the extraction, purification and use of metals by humans have had a profound effect on their biogeochemical cycling.

Human modifications of the carbon and nitrogen cycles have been the focus of considerable research, because of their importance to ecosystems and climate. Global cycles of metals have, by contrast, received much less attention. Now, Jason Rauch and Jozef Pacyna have taken stock of the key reservoirs for a range of important metals from silver to zinc, and analysed their global flows (Glob. Biogeochem. Cycles, 23, GB2001; 2009). The analysis reveals that the movement of metals due to human activities is a substantial fraction of the total flow.

The researchers quantified natural cycles and reservoirs as well as the human ones. Natural reservoirs include the solid Earth and the biosphere, and natural mobilization occurs mainly through water movement, biosphere growth and decay, and by winds. In contrast, anthropogenic reservoirs include, for example, metal stocks held by governments and scrap metal. As robust estimates are available for only some countries, Rauch and Pacyna scaled up and interpolated their data.

The end result is a series of flow charts for each metal showing that, compared with only a few thousand years ago, twice as much metal flows in the global cycles because of human activities. In particular anthropogenic mobilization of copper is much greater than that induced by natural agents. Sediments on continental margins are accumulating metals as agricultural activity and urban run-off sweep discarded metals into the rivers and eventually the oceans. At the same time, the amount of metal in human-made objects is increasing at the expense of that in natural ores.

In light of the rapidity and extent to which humans have modified metal cycles in the recent past, we would do well to monitor the anthropogenic alterations of these cycles on a continuous basis — and recycle where we can.