The Anthropocene as a geological event or a new geological epoch

The Anthropo-what? The Anthropocene of course! The word is formed from the ancient Greek anthropos (humans) and kaino (new), and it can be translated as "the age of humans".

Originally introduced in the science cummunity, the concept of Anthropocene has entered the public consciousness, in particular because of the media coverage of two global consequences of human activity: life extinction and climate change.

These two planetary phenomena are indeed the results of the activity of a single animal species: humans! By acquiring the capacity to modify our environment on a global scale, we have become the proeminent driver of Earth's surface transformation, ahead of the geological forces (volcanic activity, plate tectonics, meteorites…) that have prevailed so far! The Anthropocene is therefore the period during which humans have become the main power of change on the Earth's surface, and not necessarily for the better...

For the first article of this website, it seemed logical to me to address the definition of the Anthropocene. I had considered a relatively short text... failed! This will therefore be a four-part article. In this first part, I retrace the emergence of the Anthropocene in the field of geology and lift the veil on the fascinating scientific debates raised by this concept.

Questionning the memory of the Earth to know its history

Geologists don't mess with time. They indeed developed an extremely sophisticated time segmentation to describe Earth's history, which they segment according to different units of time (eon, era, period, epoch, etc.). Boundaries between segments generally correspond to major upheavals that have occurred on Earth's surface.

These past changes are assessed by stratigraphy, i.e. by studying the different strata that make up sedimentary rocks. Indeed, the properties of strata are influenced by the conditions that prevailed during their formation by sedimentation. By recording the conditions of the past, sedimentary rocks thus constitute huge natural archives: they are the memory of the Earth.

The first sign of the Anthropocene

The Pleistocene/Holocene turning point is marked by a rapid and significant wave of extinction of terrestrial megafauna species (animals over 45 kg), an event known as the Pleistocene or Quaternary extinction.

Between 70,000 and 10,000 years before the common era, a massive proportion of large animal species have disappeared from the surface of the Earth [1, 2]. If causes such as major climate change or meteorite impacts are invoked [3, 4], the decisive influence of humans on this extinction is more and more recognized [2, 5–8]. The fact is that the Pleistocene extinction coincides very clearly with the colonization of land by our Homo sapiens ancestors.

The extinction of megafauna, and in particular of large herbivores, has induced profound changes in ecosystems and had an influence on a large part of the Earth's surface [9]; it might therefore constitutes the first sign of the Anthropocene. The extinction of the Pleistocene reveals something important in any case: humans did not wait to be 8 billion, to have overpowered machines, the combustion engine, the nuclear bomb and synthetic poisons to impact their environment in depth and on a large spatial scale.

References

[1] R. G. Roberts et al., « New ages for the last Australian megafauna: continent-wide extinction about 46,000 years ago », Science, vol. 292, no 5523, p. 1888‑1892, 2001. https://www.science.org/doi/10.1126/science.1060264

[2] G. Haynes, « North American megafauna extinction: Climate or overhunting », Encycl. Glob. Archaeol. N. Y. Springer, p. 5382‑5390, 2016. https://link.springer.com/referenceworkentry/10.1007/978-1-4419-0465-2_1853

[3] J. M. Broughton et E. M. Weitzel, « Population reconstructions for humans and megafauna suggest mixed causes for North American Pleistocene extinctions », Nat. Commun., vol. 9, no 1, p. 1‑12, 2018. https://www.nature.com/articles/s41467-018-07897-1

[4] R. B. Firestone et al., « Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling », Proc. Natl. Acad. Sci., vol. 104, no 41, p. 16016‑16021, 2007. https://www.pnas.org/doi/10.1073/pnas.0706977104

[5] A. D. Barnosky, P. L. Koch, R. S. Feranec, S. L. Wing, et A. B. Shabel, « Assessing the causes of late Pleistocene extinctions on the continents », Science, vol. 306, no 5693, p. 70‑75, 2004. https://www.science.org/doi/10.1126/science.1101476

[6] F. Saltré et al., « Climate change not to blame for late Quaternary megafauna extinctions in Australia », Nat. Commun., vol. 7, no 1, p. 1‑7, 2016. https://www.nature.com/articles/ncomms10511

[7] M. E. Allentoft et al., « Extinct New Zealand megafauna were not in decline before human colonization », Proc. Natl. Acad. Sci., vol. 111, no 13, p. 4922‑4927, 2014. https://www.pnas.org/doi/10.1073/pnas.1314972111

[8] R. N. Holdaway, M. E. Allentoft, C. Jacomb, C. L. Oskam, N. R. Beavan, et M. Bunce, « An extremely low-density human population exterminated New Zealand moa », Nat. Commun., vol. 5, no 1, p. 1‑8, 2014. https://www.nature.com/articles/ncomms6436

[9] S. Rule, B. W. Brook, S. G. Haberle, C. S. Turney, A. P. Kershaw, et C. N. Johnson, « The aftermath of megafaunal extinction: ecosystem transformation in Pleistocene Australia », Science, vol. 335, no 6075, p. 1483‑1486, 2012. https://www.science.org/doi/10.1126/science.1214261

[10] P. J. Crutzen et E. F. Stoermer, « The “Anthropocene” », Glob. Change Newsl., vol. 41, 2000. http://www.igbp.net/publications/globalchangemagazine/globalchangemagazine/globalchangenewslettersno4159.5.5831d9ad13275d51c098000309.html

[11] M. Subramanian, « Anthropocene now: influential panel votes to recognize Earth’s new epoch », Nature, 2019. https://www.nature.com/articles/d41586-019-01641-5

[12] « Working Group on the ‘Anthropocene’ | Subcommission on Quaternary Stratigraphy ». http://quaternary.stratigraphy.org/working-groups/anthropocene/

[13] M. Prillaman, « Are we in the Anthropocene? Geologists could define new epoch for Earth », Nature, vol. 613, no 7942, p. 14‑15, 2022. https://www.nature.com/articles/d41586-022-04428-3

[14] A. Witze, « It’s final: the Anthropocene is not an epoch, despite protest over vote », Nature, 2024. https://www.nature.com/articles/d41586-024-00868-1

[15] Nature, « Are we in the Anthropocene yet? », Nature, vol. 627, no 8004, p. 466‑466, 2024. https://www.nature.com/articles/d41586-024-00815-0

[16] A. M. Bauer, M. Edgeworth, L. E. Edwards, E. C. Ellis, P. Gibbard, et D. J. Merritts, « Anthropocene : event or epoch? », Nature, vol. 597, no 7876, p. 332‑332, sept. 2021. https://www.nature.com/articles/d41586-021-02448-z

The analysis of the information contained in the strata makes it possible to reconstruct major events that have occurred during the history of the Earth and to date them. For example, when living beings die, they can integrate the layer being formed by sedimentation and become fossilized. In the case of a massive extinction of life, a large number and variety of fossils will end up in the stratum; an a posteriori analysis of the stratum will reveal the extinction event and will serve to its datation. This retrospective reconstruction can ultimately be used to divide up the history of Earth in time.

Stratigraphic analysis has led to the distinction of 37 geological epochs in the 4.5 billion years of Earth's history. We are today and for 12,000 years officially in the Holocene, the geological epoch which corresponds to an interglacial period characterized by a relatively stable climate on a global scale. The Holocene follows the Pleistocene (-2.58 million years to -12,000 years), an era marked by several glaciations, the last of which ended approximately 12,000 years ago. The Pleistocene and the Holocene are currently the two official geological epochs of the Quaternary geological period.

The advent of the Anthropocene

For a little over two decades, scientists have been proposing to define a new geological epoch that would follow the Holocene: the Anthropocene.

Although they did not invent the term, Paul Crutzen and Eugene Stormer formalized and popularized the Anthropocene concept in an article published in 2000 [10]. The Anthropocene would thus be a new geological epoch during which human activity has become the main force for change on the Earth's surface, ahead of all the geological and natural forces that had prevailed until then.

For years, the concept has been debated in authorized circles. To officially certify a change of epoch is no mean feat: scientific evidence is required, in the form of significant stratigraphic markers, arguing for major modifications on a large spatial scale and over long periods of time. More prosaically, the International Commission on Stratigraphy needs to look into the matter and get its members to agree.

Numerous candidates have been identified as stratigraphic markers of the Anthropocene: artificial radionuclides from the many nuclear weapons tests carried out since the 1950s, pesticide and plastic residues, a marked increase in CO2 concentration, remnants of smoke from the combustion of fossil fuels...

Artificial radionuclides have received particular attention, as they are present on a large scale in sediments [11, 12]. They have been considered the "golden spike", i.e. the key marker for dating the onset of the Anthropocene to the beginning of the second half of the 20th century, a period effectively characterized by a large number of thermonuclear bomb tests that generated massive radioactive fallout.

This starting point also corresponds to the launch of the "consumer society", characterized by a striking rise (a phase called the "Great Acceleration") in the production of various materials (including plastic), in the consumption of natural resources (including fossil fuels) and in waste generation (including greenhouse gases).

The Anthropocene as a singular event in Earth history rather than a new geological epoch

When this article was originally published in 2023, I went a little too far when I wrote: "We would be inexorably approaching official recognition of the Anthropocene [13]. In 2019, a vote by the Anthropocene Working Group, set up by the International Commission on Stratigraphy, produced the following results: 88% of voters in favor of recognizing the Anthropocene as a new geological epoch that would begin in the mid-20th century [12]."

Spectacular turn of events in 2024: on March 20, the International Union of Geological Sciences (IUGS), the final arbiter in the matter, announced the rejection of the Anthropocene proposal as a new formal epoch in the Earth's geological calendar [14].

It is important to note that this decision does not call into question the relevance of the Anthropocene concept, as the vast majority of scientists agree that human activity is having a massive and global impact on the planet's surface [15]. There are, however, several perfectly valid arguments against recognizing the Anthropocene as a new geological epoch.

First, some consider it inappropriate to define a new epoch from 1950 onwards, given that humans began to have a global impact on their environment well before the middle of the 20th century (cf. the Pleistocene extinction mentioned above).

Moreover, if we place the beginning of the Anthropocene in the middle of the 20th century, the time step is very short, whereas a change in geological epoch must be characterized by changes over a long period of time.

The inversion of the logic of reasoning has also been criticized: whereas it is customary to use the analysis of strata to define geological epochs, in the case of the Anthropocene we have sought to define a new epoch whose stratigraphic signal is sought a posteriori.

Finally, while stratigraphic markers are indeed present, the existence of a distinctive large-scale stratum would not yet be sufficiently clear. Some geologists are thus pleading for the recognition of the Anthropocene as a singular geological event rather than as a new geological epoch [14].

Conclusion

In short, the Anthropocene is subject to intense debates within the scientific community! Whether or not the Anthropocene will one day be formalized as an event or a new geological epoch is maybe not so important (geologist friend, if you pass by here, forgive me!). What matters is that scientists now largely agree on the idea that the Anthropocene constitutes a singular time in the history of the Earth, a time during which the surface of the planet is massively influenced (altered would be more appropriate, as we will detail in the third part of this article) by a single animal species: humans.

Henri Cuny

Figure 1: Pile of strata in a sedimentary rock. Image source: Pixabay.

Figure 2: Map of nuclear explosions that occurred between 1945 and 2006. Image source: Radical Cartography, Bill Rankin, 2007 (http://www.radicalcartography.net/index.html?nuclear).