Are we in a new epoch?

A 2016 review assembled the evidence: radioactive fallout from atmospheric nuclear testing, fly ash from fossil fuel burning, concrete and plastics as novel materials, sharply altered nitrogen and phosphorus cycles, and changes in the distribution of species.

Several of these are sharp, global and appear in ice cores and lake sediments at nearly the same moment in the mid-twentieth century, which is the kind of marker stratigraphy uses.

The review concluded that the interval is functionally and stratigraphically distinct from the Holocene.

A working group spent fifteen years developing a proposal to define an Anthropocene epoch beginning around 1950, with the sediment of Crawford Lake in Ontario as its reference point. In March 2024 the Subcommission on Quaternary Stratigraphy rejected it, 12 votes to 4 with 3 abstentions, and the International Union of Geological Sciences upheld the result over a challenge to how the vote was run.

The International Chronostratigraphic Chart, which is the formal record of these divisions, still ends with the Holocene.

The objections were about scale and timing rather than about whether the signal is there. As the International Union of Geological Sciences summarised them, human effects on the planet long predate 1950; they began at different times in different places, so no single horizon marks their onset; and a unit spanning less than a human lifetime sits uncomfortably on a chart whose other units span thousands or millions of years. Two critics had gone further in 2016, arguing that the concept rests more on direct human observation than on a stratigraphic record, and that the drive to recognise it may be political rather than scientific.

An alternative, that the human interval is better described as an ongoing event rather than as a unit of time, was published in 2022 by eleven researchers, before the working group had submitted its proposal. Their objection is that a series with a fixed horizon and a precise start date “fails to account for the diachronic nature of human impacts”, where an event carries the unevenness in space and time that an epoch has to flatten.

The working group considered the event reading in detail and found it inconsistent with the stratigraphic evidence.

Proposals have included the spread of farming, the exchange of species after the colonisation of the Americas, the industrial revolution and the nuclear tests. Each has a defensible case and a different implication about what the concept is naming.

The working group’s answer was that only the mid-twentieth century carries an array of markers that are effectively the same age everywhere. Its critics had already objected that a line there relegates considerable earlier human change to a “pre-Anthropocene”.