Prof. Francine M.G. McCarthy, Department of Earth Sciences, Brock University, Canada, and Anthropocene Working Group, International Commission on Stratigraphy

DOI: 10.65398/IAMH2856

Abstract

Academician and Nobel Laureate Paul Crutzen insisted that we were no longer living in the Holocene Epoch 24 years ago. The term Anthropocene, ἄνθρωπος (human) + -cene (suffix denoting epochs in the Cenozoic Era) resonated with many who were increasingly aware of the impact humans on our planet. Stratigraphers ignored the concept until 2008, when “Are we now living in the Anthropocene?” appeared on the cover of the Geological Society of America’s newsmagazine, GSA Today. After 14 years of investigation, the Anthropocene Working Group (AWG) concluded the Great Acceleration had forced the planet into a no-analogue state, and that evidence could be found in geologic records worldwide. A proposal to erect an Anthropocene epoch with a ‘golden spike’ in varved sediments in Crawford Lake was rejected by the group that had commissioned it, but is this societally relevant? It is precisely because the Earth System has departed so markedly different from Holocene norms in only 72 years that defining the Anthropocene epoch matters.

 

At an International Geosphere-Biosphere Programme meeting in February 2000, Nobel Laureate and Academician of the Pontifical Academy of Sciences Paul Crutzen insisted that planetary conditions had already changed to be well beyond the stability that had characterized the Holocene Epoch. He suggested naming the current epoch after Anthropos, the main agent of change (Crutzen & Stoermer, 2000). Crutzen initially assumed that the Industrial Revolution forced the Earth System into an ‘Anthropocene mode’, but he came to agree with Earth System scientists like Will Steffen (Crutzen & Steffen, 2003) that physical, chemical, climatic and biological changes resulting from the extraordinary fossil fuel-driven outburst of consumption and productivity in the mid-20^th C had fundamentally altered the planet (Steffen et al., 2007; Zalasiewicz et al., 2015; Syvitski et al., 2020). In response to an article published by Ruddiman (2003), Crutzen & Steffen (2003) concluded that “Earth is currently operating in a no-analogue state. In terms of key environmental parameters, the Earth System has recently moved well outside the range of natural variability exhibited over at least the last half million years. The nature of changes now occurring simultaneously in the Earth System, their magnitudes and rates of change are unprecedented and unsustainable. We conclude that there may have been several distinct steps in the ‘Anthropocene’, the first, relatively modest, step can have been identified by Ruddiman, followed by a further major step from the end of the 18th century to 1950 and, from the perspective of the functioning of the Earth System as a whole, the very significant acceleration since 1950.”

Some have argued that the Anthropocene should be viewed as a long and ongoing ‘event’ reflecting human modification of our planet over many millennia, from the ‘Early Anthropocene’ posited by Ruddiman (2003) beginning during the Neolithic to the ‘Great Dying’ and effects of reforestation of the Americas after colonization on the climate system (Lewis & Maslin, 2015; Koch et al., 2019) as well as the Industrial Revolution (Gibbard et al., 2022; Swindles et al., 2023). Rejection of the formal proposal submitted by the Anthropocene Working Group (AWG) by the International Commission on Stratigraphy (ICS) and its parent body (IUGS, 2024) and promotion of the ‘event’ approach to the Anthropocene does not bring an end to the Holocene Epoch that began 11,700 years ago. However, Crutzen’s basic point was that humans had sufficiently altered the Earth System to make ‘Holocene’ an inadequate descriptor of modern geologic and planetary conditions (Crutzen & Stoermer, 2000; Crutzen & Steffen, 2003).

The Subcommission on Quaternary Stratigraphy (SQS) of the ICS established the AWG in 2009 with the mandate of exploring whether there was stratigraphic reality to Crutzen’s claim. With input from members from the Earth System Science community and social scientists who had recognised the Great Acceleration and its effect on Earth Systems (McNeill, 2000; Crutzen & Steffen, 2003; Steffen et al., 2015, 2016; McNeill & Engelke, 2016), the AWG concluded on the basis of abundant accumulated evidence that erecting a new epoch (denoted in the Cenozoic Era by the suffix -cene) was justified (Figure 1). An overwhelming majority of voting members the AWG recommended formalization of the Anthropocene at epoch/series rank at the 35th International Geological Congress in Cape Town in 2016 (Zalasiewicz et al., 2017), recognizing the Great Acceleration as the driver of planetary change from a Holocene state. From 2019, with financial and logistical support by the Haus der Kulturen der Welt (highlighting the broad significance of the Anthropocene, well beyond the geoscience community), the AWG systematically investigated stratigraphic evidence of the Anthropocene epoch proposing it be added to the Geologic Time Scale in 2023 (Waters et al., 2024).

The challenge for the AWG was to apply stratigraphic principles developed since the late 18^th C, largely based on rapid changes in fossil assemblages that can be traced globally (at least, in theory). It succeeded. The AWG coordinated the effort to propose a Global boundary Stratotype Section and Point (GSSP) to define the Anthropocene as an epoch with reference to a corresponding series of distinctive strata (Waters et al., 2018). This initiative involved hundreds of researchers studying multiple proxies (markers) in a variety of geologic sections from a dozen globally distributed sites (Waters et al., 2023).

One of the proxies proposed as a marker for the Anthropocene (Rose, 2015; Swindles et al., 2015) is spheroidal carbonaceous particles (SCPs) that are formed during high temperature combustion of fossil fuels. These tiny anthropogenic fly ash particles, that can be found in pollen slides (McCarthy et al., 2023) as well as in preparations of sediments using standard techniques (Rose, 2008, 2015), increase gradually in abundance beginning with the Industrial Revolution and then very rapidly with the Great Acceleration. This was thus one the many analyses systematically undertaken at the twelve sites vying for the ‘golden spike’ (the colloquial name for a GSSP) (Figure 2).

Because fly ash is mainly transported in the lower atmosphere, SCPs primarily track regional industrial emissions. As a result, their record was slightly diachronous in sections studied for their potential as GSSP candidates, as the first major increase tracks regional expansion of fossil fuel power generation, and the subsequent decline tracks the equally diachronous implementation of air quality standards in various jurisdictions (Figure 3). This slight diachroneity (spanning a few decades at the beginning of the second half of the last century) is typical of proxies of the Great Acceleration, though this would appear geologically instantaneous across any other boundary on the International Chronostratigraphic Chart (Cohen et al., 2013).

Much greater synchroneity is found in markers of the Cold War, i.e. fallout of radionuclides associated with thermonuclear weapons testing (Figure 4). Although plutonium and other bomb radionuclides (e.g., ¹³⁷Cs), and the slightly delayed production of excess radiocarbon by bombardment of nitrogen in the upper atmosphere) is a consequence of a specific technological development that does not directly reflect the Great Acceleration, the globally synchronous nature of the fallout signature broadly coincident with the Earth System responses to the Great Acceleration led to the selection of plutonium as the primary chronostratigraphic marker for the proposed Anthropocene (AWG, 2019). Although plutonium is naturally occurring, it makes up just 2 × 10⁻¹⁹ per cent (by mass) of the lithosphere, so virtually all plutonium on the Earth’s surface is anthropogenic (Hartmann, 2012).

It is the rapid increase in plutonium in organic lake sediments deposited between fall and spring turnover in Crawford Lake in 1952-53 that provides a precise nominal moment for the beginning of the Anthropocene: linked to the detonation of Ivy Mike at 7:15 AM (local time in the Marshall Islands) on November 1, 1952 (Waters et al., 2024; McCarthy et al., in press). The unique hydrologic and depositional setting of this meromictic lake near Toronto, Canada allows inorganic calcite crystals precipitated in surface waters between spring and fall turnover to cap organic matter that accumulates the rest of the year, producing light and dark laminae that – like tree rings – preserve an annually resolvable record of environmental conditions (Lafond et al., 2023; Llew-Williams et al., 2024). These varved sediments are undisturbed due to the absence of burrowing invertebrates, and they preserve a faithful bomb radionuclide record that mirrors the history of detonation of thermonuclear weapons (Figure 5).

The varved sediments from Crawford Lake were selected by the AWG as the most representative stratigraphic record of the transition to an ‘Anthropocene state’ of the Earth System (Waters et al., 2024) principally due to the quality of its sediment record and in part because of its accessible yet protected location in a conservation area where there is archaeological evidence of agricultural settlement by Ontario Woodlands Tradition people between the late 13^th and early 16^th centuries and historic evidence of European colonists beginning in the 19^th century (McCarthy et al., 2023). This underscores the important distinction between local/regional anthropogenic impacts at various times during the Holocene Epoch and the altered Earth System that justifies the stratigraphic Anthropocene. It is important to note that the proposed GSSP was supported by Standard Auxiliary Boundary Stratotypes (SABS) from marine sediments in Japan (Kuwae et al., 2023) and peats from an ombrotrophic Polish bog (Fiałkiewicz-Kozieł et al., 2023) as well as varved lake sediments in China (Han et al., 2023) (see Figures 3, 4) and consistent with findings from all sites examined by the AWG (see Figure 2). The multiple stratigraphic signals that support the definition of an Anthropocene epoch/series are summarised in Head et al. (2022).

It should be emphasized that although the best chronostratigraphic control for the first few decades following the mid-20^th century is from the fallout of bomb radionuclides, such as measurements of ¹³⁷Cs in varved sediments from Crawford Lake (black curve) that reflect the global fallout (dashed red curve in Figure 6), the environmental effects of the Great Acceleration continue. This is exemplified by metrics like global phosphorus application (blue curve in Figure 6) to feed a growing and increasingly affluent human population (with accompanying eutrophication of aquatic ecosystems), illustrating the interconnectedness of Earth’s various subsystems. The Earth System shift to the current no-analogue state of Crutzen & Steffen (2003) is driven in large part by the massive increase in fossil fuel consumption in the mid-20^th century that liberated 36.1 Gt/y of CO₂ to the atmosphere in 2017 CE, compared with 0.2 Gt/y in 1850 CE (Syvitski et al., 2020). This impacts not only the temperature at the surface of our planet (see Figure 1), but also results in acidification of oceans (Hurd et al., 2018) and soils (Oh & Richter, 2004).

Contrary to claims by opponents to the proposal to add the Anthropocene as an epoch to the time scale, the laws of physics dictate that the resulting novel state of our climate system will persist for tens of millennia, producing conditions at the surface of our planet not previously experienced by humans, i.e., in the last three million years (Hansen et al., 2023; Summerhayes et al., in press) – it is not a ‘blip’. Similarly, the argument that the change is ‘too recent’ is spurious – it is precisely because the Earth system has become noticeably different from Holocene norms in less than a century that defining the Anthropocene epoch matters (Oreskes, 2024; Turner et al., 2024; Zalasiewicz et al., 2024). The AWG argues that, by formalizing that we have moved beyond the functional state of the Holocene, the Anthropocene epoch would help understand and communicate the scale of this altered Earth System, allowing us to make informed decisions that impact the entire biosphere (McCarthy et al., submitted; Zalasiewicz et al., 2024). Climate models suggest that we do not have the luxury of waiting centuries – or even decades – to address the existential issues that face us (Gulev et al., 2021; Hansen et al., 2023; Esper et al., 2024).

Finney & Edwards (2016) suggested that “the drive to officially recognize the Anthropocene may, in fact, be political rather than scientific.” They rejected the scientific projections that established the impossibility of the novel Earth System state reverting to Holocene conditions, as being irrelevant to stratigraphy. The perspective of the AWG is that it is at least equally political to reject scientific data generated by hundreds of scientists from dozens of sites (available online: Waters et al., 2024) without open discussion and feedback on the proposal. It is particularly disheartening that organizers of the 37^th International Geological Congress (2024) rescinded an invited plenary talk on the Anthropocene “due to the complexities and ongoing debates surrounding the topic, which have made it challenging to achieve a broad consensus.”

The Earth Sciences have much to offer by examining analogs from the late Cenozoic record (e.g., the last interglacial, ca. 125,000 years ago and Pliocene warmth ca. 3,000,000 years ago; see Figure 1) to anticipate the less easily quantified nonlinear changes associated with an ‘Anthropocene state’, such as those associated with the melting of polar ice caps, associated sea level rise and coastal hazards, and large-scale reorganization of ocean circulation that is a major internal control on the climate system (McManus et al., 2004; Broecker, 2010; Dutton et al., 2015; PAGES, 2016; Caesar et al., 2018; Guarino et al., 2020; Box et al., 2022; van Westen et al., 2024). Geoscientists can contribute to better understanding and proposing solutions to mitigate and adapt to changes in the Earth System that have clear social, economic and political implications, particularly given the disproportionate number of people (and infrastructure) in coastal regions (Reimann et al., 2023) and the continued demand for earth materials. It is hoped that the split in the geoscience community over the concept of the Anthropocene, and the contested vote by SQS members rejecting the proposed Anthropocene as an epoch/ series with a GSSP in sediments deposited in Crawford Lake (and a corresponding Crawfordian stage/age that has yet to be addressed), does not detract from the urgency of addressing these challenges (McCarthy et al., submitted). An editorial in the journal Nature reiterated “Are we in the Anthropocene yet?”, paraphrasing Zalaziewicz et al., 2008), concluding that the rejection of the proposal by the SQS “should not detract from the reality that humans are altering Earth systems” (Nature 627, 466 (2024)), urging geologists to “quickly resolve their disagreements”. Arguably, however, the ‘kerfuffle’ over stratigraphically defining the Anthropocene (First Dog on the Moon, July 14, 2023) has brought even more attention to the Anthropocene than adding a geologic unit to the ICS International Chronostratigraphic Chart would have done. The Anthropocene is dead. Long live the Anthropocene, as one reporter put it (Voosen, 2024).

 

Acknowledgments

The ideas in this paper were developed through innumerable email exchanges and collaborations with my colleagues from the Anthropocene Working Group (including Jan Zalasiewicz, Simon Turner, Martin Head, Colin Waters and Julia Adeney Thomas who provided feedback on a draft of this paper), most notably during workshops facilitated by the Haus der Kulturen der Welt and the Max Planck Institute for Geoanthropology. The sampling and analysis of sediments from Crawford Lake could not have been achieved without the dozens of members of ‘Team Crawford’ – particularly those who came through to collect and analyse the last set of cores after funding had run out! – and the supporting organizations (Conservation Halton, Brock University, Carleton University, the Canadian Museum of Nature, and the Royal Ontario Museum) and, of course, funding from the Kulturveranstaltungen des Bundes in Berlin and the Natural Sciences and Engineering Research Council of Canada.

 

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