Stratigraphy used to be a relatively objective science. The geologist would look at a sequence of rocks that were deposited millions of years ago and define boundaries where the rock type changed. This straightforward description of past events becomes more difficult as the events become more recent. On pp. 16 to 18 of this issue Nicolaas Steenkamp describes the complexity that humans face in defining the Anthropocene, the Geological Age of Humans. The difficulties are not so much those associated with humans being both player and umpire but rather the opposite; what are those signals that can be unambiguously related to human activity rather than to natural processes?
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