Regarding definitions of an Anthropocene epoch, we disagree with Erle Ellis and colleagues' contention that “Earth sciences long ago moved away from defining precise stratigraphic boundaries to developing records of continuous change” (Nature 540, 192–193; 2016).

Precise boundaries are the basis for defining geological time, a prerequisite for the correlation of abiotic and biotic events and the understanding of the rates and timing of biological and geological processes on our planet. Earth sciences, through the International Commission on Stratigraphy of the International Union of Geological Sciences, continue to this day to define precise global boundaries, which in turn allows scientists to communicate with each other and with the public alike.

Developing records of change (continuous, discontinuous or one-time-only) in the study of Earth's history neither competes with nor detracts from scientists' key contribution in providing tools for unambiguous communication.