The Anthropocene as a Geological Time Unit A Guide to the Scientific Evidence and Current Debate

Since it was coined in the year 2000 by Paul Crutzen and Eugene Stoermer, the term Anthropocene has taken the world by storm – pretty much in the same way as the phenomenon it describes. Humanity’s impact on the planet has become so all-encompassing that it warrants giving this period a new name. As a colloquial term that is all snazzy, but are we actually leaving a tangible trace in the rock record to signal a transition to a new period?

Several authors have already written thought experiments to try and answer this question, see e.g. The World Without Us and Zalasiewicz’s The Earth After Us. But the real answer lies in the realm of stratigraphy, the geological subdiscipline that studies rock layers. As with many other conventions, to ensure scientists around the globe all talk about the same thing and use the same names, there is an official body for that. The International Commission on Stratigraphy (ICS) formally decides on naming and dating of geological periods and maintains the International Chronostratigraphic Chart, a.k.a. the Geological Time Scale.

Formal acceptance of a new name means clearing a raft of bureaucratic and academic hurdles first. So, in 2009, the editors of the current book got together to form the Anthropocene Working Group to start preparing a formal submission to, ultimately, the ICS. Two large publications followed, and now this edited collection does what it says on the tin, providing the latest update on the evidence and the debate by summarizing a huge body of work.

The first chapter provides a short history of what I have sketched above and, for the reader not versed in stratigraphy, useful basic information on how stratigraphy works, past decisions on defining and naming geological periods, plus a very interesting and relevant section outlining why formal acceptance and definition of the Anthropocene matters. The bulk of the book consists of five chapters examining how humans have tangibly modified our planet, and whether this leaves stratigraphically suitable markers. Depending on your viewpoint, this could be taken as a catalogue of our atrocities or a celebration of our achievements.

The range of impacts covered is comprehensive and includes some eye-opening facts and frighteningly large numbers. We are leaving a stratigraphical legacy by changing natural patterns of sedimentation via erosion and river damming. We construct infrastructure and buildings above and below ground and create many novel types of “rocks” such as cement, asphalt, and concrete (so much so that we risk running out of suitable sand, see my review of The World in a Grain). But we also enrich soils and sediments with fly-ash and soot from burning fossil fuels. And this is before we even talk of the insane amounts of plastics that end up in our environment, now degrading into micro- and nanoplastics that are found everywhere. And then there are what Zalaziewicz and others have dubbed “technofossils”: all the objects that we discard in refuse tips, revealing a stratigraphy all of their own.

Less visible but no less influential are chemostratigraphical changes. That is to say, the release of carbon and methane from (again) fossil fuel burning, nitrogen and phosphorus from synthetic fertilisers, sulfur compounds, metals, organic (in the chemical sense) compounds such as pesticides and fire retardants (your POPs, PAHs, PCBs, PBDEs, etc.) and, lest we forget, radionuclides from atomic and hydrogen bombs. All these have left detectable accumulations in air, water (including ice), and soil. A biostratigraphical signature is detectable as both recent and ongoing extinctions (particularly the extinction of the Quaternary megafauna, though see my review of End of the Megafauna), the rapid spread of invasive species and domestic animals (with broiler chickens being one example of an expected future signal in the fossil record), and the fate of coral reefs. Finally, there is climate change, made visible in changes to ice cover and sea level.

What I casually summarise here in two paragraphs is presented in-depth, providing an overview of a huge body of research. And despite subchapters being contributed by many different authors, the overall flow and coherence of the text are good. Although not the first book to detail humanity’s planetary impact, the question of interest here is which of these would make suitable stratigraphic markers.

So what makes a good marker? Ideally one that is global in extent and that was laid down synchronously, i.e. very rapidly, so that the age of the marker is the same wherever measured. A volcanic ash layer is a good example, and so, of course, is the iridium spike signalling the meteorite impact at the K-Pg boundary (see T. rex and the Crater of Doom).

Not all of the potential markers discussed in this book meet these criteria, even though they reveal humanity’s impact. So, the sudden appearance of so many new long-lasting rock-like compounds and plastics is a good marker. Another one is lead released during the burning of fossil fuels, which shows up in natural archives such as sediments, peat mires, and ice cores. (Plus, there is a precedent here: Greenland ice cores show a lead spike at the height of Greek-Phoenician and Roman mining, see also Environmental Problems of the Greeks and Romans). But the appearance of soils modified by human agriculture is an example of a signal that is too localised and too diachronous (the opposite of synchronous) to be of use. The same is true for the occurrence of stone tools, though modern technofossils such as broken iPhones could be useful.

A similar question is the when. Though some scientists favour the rise of agriculture ~10,000 years ago or the beginning of the Industrial Revolution in the late 1700s as the start of the Anthropocene, the editors here outline how a consensus is forming on the 1950s. This is when human population size boomed and many things basically went into overdrive (see The Great Acceleration – what an evocative term). When plotted on graphs, many indicators considered here show a sharp upward inflexion right around this time.

As with other periods, it is highly likely that a combination of proxy signals will have to be used to define the Anthropocene – many natural archives are either sensitive to disturbance (lake sediments vs. burrowing animals), or record signals with a delay (e.g. isotope signals in stalagmites). For the moment this is all work in progress, and a formal submission to the ICS is still being prepared by the Anthropocene Working Group. Much like the closely-allied IPCC reports are the go-to books on climate change, this book is the most definitive and up-to-date reference work for anyone working on or interested in the geological case for the Anthropocene.