As members of the Anthropocene Working Group, we contend that the proposed new geological epoch should reflect a unique stratigraphic unit that is characterized by unambiguous, widespread and essentially permanent anthropogenic signatures in rock, glacial ice or marine sediments. We therefore find the two dates chosen by Simon Lewis and Mark Maslin to be questionable candidates for the start of the Anthropocene (Nature 519, 171–180; 2015).

For the first date suggested by the authors, the short-lived decline of atmospheric carbon dioxide that reached its minimum in 1610 is not an ideal stratigraphic marker for an epoch-scale boundary. It is one small dip of several in the Holocene epoch, which began about 11,700 years ago, and is not outside the range of natural variability — in contrast to the signature associated with industrialization. Associated indicators of colonization of the Americas, such as the worldwide spread of pollen from maize (corn), lasted for centuries and so do not represent near-synchronous markers.

By the time of the authors' other suggested date of 1964, the 'great acceleration' in human activity was well under way (W. Steffen et al. Anthropocene Rev. 2, 81–98; 2015). Also, the year 1964 is later than the near-synchronous upward inflections of many physical and socio-economic trends and their respective stratigraphic signals, which date to around 1950 (J. Zalasiewicz et al. Quat. Int.; 2015).

We need further comprehensive analyses of the advantages and limitations of different proposed markers before we can arrive at an effective starting date for the Anthropocene.