A futurist perspective on the Anthropocene

Introduction

The convenience or not of considering the Anthropocene as aformal chronostratigraphic unit, with the same rank than the Holocene epoch, is currently debated. Supporters of the Anthropocene epoch argue that, during the last two centuries, human activities and their consequences have become a global geophysical force with similar or greater influence on the functioning of the Earth system than the major forces of nature (Steffen et al., 2007). Opponents do not discuss this fact, which is widely accepted, but doubt that the Anthropocene fulfils the criteria to be recognized as a formal stratigraphic unit, namely the existence of a Global Stratotype Section and Point (GSSP) through the precise definition of an isochronous datum that marks a critical change in the sedimentary sequence (golden spike) that can be considered the boundary between two epochs (i.e. the Holocene and the Anthropocene) and identifiable elsewhere. Moreover, some critics question the utility of the Anthropocene as a stratigraphic unit in itself (Gale and Hoare, 2012). Several potential golden spikes have been proposed so far but none has been universally adopted. Among the latest proposals, Certini and Scalenghe (2011) suggested the use of soils affected by human activities (anthrosols or anthroposols) as GSSP but Gale and Hoare (2012) disagree with this proposition arguing that these soils fail to meet at least nine of the necessary criteria for a stratigraphic datum. The International Commission of Stratigraphy (ICS), through the Subcommission on Quaternary Stratigraphy, has created the Anthropocene Working Group (AWG, 2012) to address this issue (Zalasiewicz et al., 2011a). Meanwhile, the informal usage of the term Anthropocene has become widespread within the palaeocommunity and beyond. Indeed, the Anthropocene issue has transcended the field of palaeosciences as it has profound implications for other disciplines such as, for example, economy, law, sociology, politics or nature conservation, among others (Ellis, 2011; Kellie-Smith and Cox, 2011; Tickell, 2011; Vidas, 2011).

This paper discusses the issue of the golden spike and the global stratotype, from a general conceptual and theoretical perspective. Emphasis is placed on the concept of diachronism and its relationship with dating precision. The convenience or inconvenience of the definition of the Anthropocene as a formal chronostratigraphic unit is also discussed, not only in terms of past and present evidence but also in terms of future prospects for human existence on Earth and the potential stratigraphic consequences. Terminological issues are also considered under the same perspective.

The onset of the Anthropocene

Before analyzing the topic from a theoretical point of view, it would be useful to briefly introduce the existing proposals for the onset of the Anthropocene. The Anthropocene was originally defined as the geological epoch characterized by the conspicuous imprint of human activities on the Earth’s surface and on atmospheric processes (Crutzen, 2002; Crutzen and Stoermer, 2000). However, as the stratigraphic signal may not coincide with the surface processes that generate it, the key point in defining the beginning of the Anthropocene as an epoch is determining when the consequences of human activities on the planet started to be evident on the stratgraphic record.

The first markers were considered to be the increase in carbon dioxide and methane in ice cores, significant changes in biological assemblages in lake sediments, or the deposition of artificial isotopes produced by nuclear weapons, all of them having occurred during the last two centuries and being related to industrialization (Crutzen, 2002; Steffen et al., 2011; Wolfe et al., 2013; Zalasiewicz et al., 2008). Steffen et al. (2007) subdivided the Anthropocene into two stages: Stage I (ad 1800–1945) or the Industrial Era, and Stage II (ad 1945–2015) or the Great Acceleration, characterized by a sudden increase in human population after the Second World War and a growth in petroleum consumption by a factor of 3.5. However, such a definition of the Anthropocene and its stages is based on chronometric rather than chronostratigraphic indicators and, as such, it cannot be used to formally define an epoch. A very recent review of the evidence from arctic and alpine lake sediments proposes that theHolocene–Anthropocene transition should be placed around ad 1950, almost coinciding with the onset of the Great Acceleration (Wolfe et al., 2013). The authors favour this date, which is approximately one century older that the onset of the atmospheric CO₂ increase due to fossil fuel combustion because sedimentary changes in the lakes studied show a clear stratigraphic marker that would serve as a global datum (the golden spike), as required for the ICS to accept the Anthropocene as a formal unit.

A contrasting view has been provided by those who locate the beginning of the Anthropocene at the early (c. 8000 yr BP) or the middle (c. 5000 yr BP) Holocene, respectively, based on increases in methane and carbon dioxide as byproducts of human activities, such as widespread rice cultivation or pre-industrial land clearance (Ruddiman, 2003). However, the possibility of these changes being a consequence of natural forcings has also been suggested (e.g. Joos et al., 2004; Stocker et al., 2010). A recent The Holocene special issue dedicated to ‘The early-Anthropocene hypothesis’ discussed these points in detail (Ruddiman et al., 2011).

Diachronism and dating precision

Synchronism and diachronism may be a matter of scale. Geological ages ultimately rely on radiometric methods, although geochronological correlations use a variety of intermediate well-dated markers. Biostratigraphy, based on the First Appearance Datum (FAD) and Last Appearance Datum (LAD) of index fossils, is the more widely used tool to chronologically correlate stratigraphic units (Sadler, 2004). Both radiometric and biostratigraphic dating, however, are subject to inherent errors. Radiometric errors are linked to methodological constraints and are expressed statistically as confidence intervals. Biostratigraphic errors are due to the intrinsic diachronic nature of FADs and LADs, because of lags in biological processes and phenomena, such as differential dispersal ability, which influences FADs, or environmental tolerance leading to local extinction, which affects LADs. These biological errors are rarely mentioned in correlation studies and occur at similar timescales (typically centennial to millennial), no matter the boundary under study. The magnitude of methodological and biostratigraphic errors, in relation to the absolute dates measured, is critical for these dates to be accepted. For example, errors that are millennial in magnitude are very welcome when dating the Cretaceous/Paleocene boundary (e.g. Batenburg et al., 2012) at ~65 Ma, but they are unacceptable if the dating target is the Pleistocene/Holocene boundary, approximately 11,700 yr BP (Walker et al., 2009). Large-scale correlations based on such errors follow the same rules.

In the case of the Anthropocene onset, as currently defined, the dating precision should be remarkably lower than a century (ideally, less than a decade), which is difficult to achieve. In addition, most recent sediments are still unconsolidated and are poorly packed, which hampers the discovery of well-defined lithostratigraphic correlation boundaries (Zalasiewicz et al., 2011b). Exceptions to these rules are the special cases of laminated sediments, ice cores, corals and tree rings, whose precision might attain the seasonal scale. However, this situation could be generalized to any boundary of the chronostratigraphic framework. In other words, the same problems may be encountered when trying to date, for example, the Miocene/Pliocene boundary (~5.3 Ma) just a couple of centuries after its occurrence (if this were possible). With time, the sediments are progressively accumulated and consolidated, the boundaries better defined and, what is more important, the errors required for sound dating and correlation greater. It follows that, with time, the eventual Holocene/Anthropocene boundary will be better dated – and the corresponding stratigraphic markers correlated – than they are today (Zalasiewicz et al., 2011b). This does not mean that the respective boundaries will be less diachronic, just that this diachronism will be less significant with respect to the dating errors. Therefore, satisfactorily dating the onset of the Anthropocene may be only a matter of time. Meanwhile, efforts such as those of Wolfe et al. (2013), who attempted to identify this boundary over vast regions and to propose better potential markers (as, for example, radioisotopes from atomic weapons testing, which have high precision and global distribution), are worth making. Laminated sediments are especially well suited for this purpose because, despite their scarcity, they are relatively well distributed for attempting global correlations (Ojala et al., 2012). Thus far, the thin sedimentary layer reflecting human impact does not appear to be easily discernible from an anomaly (Zalasiewicz et al., 2011b), especially if the Holocene/Anthropocene boundary is located around ad 1950 (Wolfe et al., 2013), but time will confirm whether it attains the rank of a distinct, widespread and chronologically consistent sedimentary unit.

Therefore, under a futurist perspective, the Anthropocene seems to be able to fulfil the requirement of a suitable golden spike, to be properly defined.

The Anthropocene and the future

Contrary to other chronostratigraphic units of the same hierarchy – as, for example, the Paleocene and Miocene, for which the onset and the end are reasonably well known and dated – the Anthropocene, as currently defined, is still in the making. But the same applies for the Holocene and this has not been a handicap for its stratigraphic characterisation. As the end of a chronostratigraphic unit is defined by the beginning of the next one (Murphy andSalvador, 1999), the eventual acceptance of the Anthropocene as a formal epoch would implicitly define the end of the Holocene. But what about the end of the Anthropocene? At the present state of knowledge, we can only speculate but, as van Loon (2004) pointed out, the negative attitude towards speculation harnesses scientific progress as ‘both hypotheses and speculations may trigger research that will deepen insight into complex earth–scientific relationships’. By definition, the Anthropocene is based on the recognizable impact of human activities in the stratigraphic record; therefore, its end is implicitly defined by the end of such a scenario. Therefore, a precise definition, age and duration of the Anthropocene will require the replacement of visible human impact by another dominant force that can be embodied in the fossil record, which could happen with or without humans inhabiting the Earth. In the first case, one hypothetical possibility is the coming of the next glaciation, provided it affects significantly human populations and their activities. Age estimations for the next glacial inception under natural variability alone have a wide range of variation, from about 1500 to >10,000 years (Tzedakis et al., 2012; Vettoretti and Peltier, 2011), with full glacial conditions in about 60,000 years (Crucifix and Rougier, 2009). In general, any environmental or human-induced event leading to a significant reduction of human influence on global surface processes has the potential to be a candidate. In the second case, such an event should be catastrophic and would eradicate humanity from Earth. A third possibility is that human influence dominates until our evolutionary disappearance, likely a matter of millions of years (Rull, 2009). Of all these possibilities, the Anthropocene, and the rest of the chronostratigraphic units, will only make sense if humans remain on Earth and use the same geological framework as today.

The Holocene poses (or posed, for those who favour the Anthropocene as an epoch) a similar situation, but its identity is not as strongly influenced by its name. Indeed, the meaning of the Holocene is ‘entirely recent’ or ‘recent whole’, a name that does not implicitly involve any driving force or causal context. The Holocene has been defined in climatic terms as the current warm period and, based on past records, is considered the interglacial following the last glaciation (Walker et al., 2009). This definition is more constraining and implicitly places the end of the Holocene at the start of the next glaciation. This view is not shared by those who propose that the impact of humanity on the Earth’s climate is capable of indefinitely postponing the onset of the next glaciation (Ruddiman et al., 2005; Steffen et al., 2007). If humans were capable of modifying the glacial–interglacial alternation in such a way, the Anthropocene would be the last epoch of the current stratigraphic framework, unless other environmental forces, as for example major tectonic and volcanic events, would overcome the stratigraphic signature of human disturbance. In this case, the Anthropocene would likely last millions of years, and its subdivision should follow the different types of human forcing with a recognizable stratigraphic imprint.

In light of this discussion, perhaps it is still too soon to decide about the acceptability of the Anthropocene as a formal epoch; further evidence is needed to validate this proposal. The time needed to do so seems to be very long in relation to the human life cycle; hence, the issue should be postponed and left to future generations. A premature acceptance would close down the current stratigraphic scheme, with uncertainty over whether it should be opened again in the future.

Terminological notes

There is a general consensus that the world has changed recently because of human activities and that this is reflected not only in the climate and other atmospheric and surface processes but also in a variety of sedimentary markers. The real problem seems to be terminological, i.e. the use of the suffix -cene, which implicitly places the Anthropocene within the current chronostratigraphic framework as an epoch, before an ICS pronouncement. Using quotation marks or recurrently insisting on the informal nature of the term has not helped settle the issue, and the debate continues. One potential solution is to wait for the eventual official acceptance of the term Anthropocene and, meanwhile, use a different expression of a clearly informal nature. Unfortunately, the solution is not so simple. Using either term may have important conceptual implications. One is that the use of Anthropocene, as an epoch, implies that the Holocene has ended, which is incompatible with the definition of the Holocene as the present interglacial. The question here is whether both the present interglacial and the Holocene should be redefined. Alternatively, if the Holocene maintains its rank as the present interglacial, then the Anthropocene would be part of the Holocene, as an age, and their names would be changed by replacing the suffix -cene by -ian to read ‘Anthropocenian’ or something similar. This option implies that the next glaciation will be able to impose its stratigraphic signal over the human one, whatever the reason. In this case, the question is whether Anthropocenian-like stages would be recurrent in each interglacial and, if so, what the correct term would be to refer to them (Anthropocenian-1, -2, -3?). The same would be true for the Anthropocene if the stratigraphic consequences of further glaciations prevail over the human signal.

Choosing alternative informal names is also problematic. When using an informal term, any suffix with accepted meaning within the official chronostratigraphic framework should be avoided. For example, the term ‘Anthropogene’ was created as a synonym of Quaternary, considered as a period, with the same rank as Paleogene or Neogene (Gerasimov, 1979). This option has not been validated by the ICS, but the term should be excluded from the potential options to avoid confusion. Another possibility would be the use of Great Acceleration, but this was originally defined as a stage of the Anthropocene (Steffen et al., 2007) and would also be misleading. Rather than clarifying the issue, creating new informal names would further complicate the picture. An alternative would be to employ the well-known and frequently used term ‘industrialization’, borrowed from the history of economy, including both the Industrial Era and the Great Acceleration of Steffen et al. (2007). An advantage of this term is that it opens the possibility of differentiating among industrial and other types of human impact on the stratigraphic record, thus bypassing the problem mentioned above on the need for human demise to properly define the corresponding GSSP. In addition, this terminological framework would reconcile the current definition of the Anthropocene as restricted to the last two centuries (Crutzen, 2002), with the hypothesis of the early-to mid-Holocene onset (Ruddiman, 2003), which could be referred to using common historical and anthropological terms such as ‘agriculture expansion’ or ‘pre-industrial clearance’, provided there are clear and synchronic stratigraphic markers defining distinct sedimentary bodies. Otherwise, these terms would constitute a parallel chronologic system without any formal stratigraphic value, but would be equally useful.

In summary, terminology is not only a matter of words but also of what the words imply in terms of present and future stratigraphic developments. The compulsive idea of attachment to the official stratigraphic scheme would create more problems than solutions, and some alternatives exist to ease this tension.

Escaping the stratigraphic rules?

Gale and Hoare (2012), among others, question the need for a definition of the Anthropocene as a new epoch, arguing that its usefulness is doubtful and that ‘the global stratigraphic approach as a whole may eventually prove of limited practical use in studies of human environmental impact’ (p. 1494). As in other case studies, such as the recently debated Quaternary (Aubry et al., 2009; Gibbard et al., 2009), the resistance of some geological sectors to include modifications in the chronostratigraphic framework have been interpreted as a negative attitude. This is not the case. For new stratigraphic units to be included in the current ICS framework, the ICS rules should be fulfilled, as occurs in any other normative organization or activity. If, finally, the AWG decides that the Anthropocene should not be considered an epoch following the Holocene, this does not mean that they are denying the influence of human activities on the Earth system; it only means that the available chronostratigraphic evidence is not enough to declare the Holocene officially dead and to define a new epoch after it. If further research eventually provides more conclusive evidence, then the decision could be reconsidered.

The above-quoted sentence of Gale and Hoare (2012) should not be taken as a rejection but as an opportunity to bypass the ICS stratigraphic rules while the ICS discusses the final decision. Indeed, the message is that it is not necessary to formally define the Anthropocene as an epoch to accept that human activities have significantly changed the Earth system processes during the last two centuries. In other words, studies on the changes due to human impact, including stratigraphic changes, are independent of whether the Anthropocene is recognized as a formal epoch within the ICS framework. Therefore, the battle for such recognition does not seem to be very productive at present. As discussed above, the resolution of this issue may only be a matter of time. Perhaps it would be better to conserve efforts and concentrate on the study of the involved processes, the potential consequences and their sedimentary imprint, whatever the current stratigraphic status of the Anthropocene. In the end, all the information gathered in this way will help the ICS make a better decision.

If necessary, the development of a parallel chronological framework without formal stratigraphic value, as suggested above, will suffice. Knowledge advancement should not be constrained by the existence (or not) of formal chronostratigraphic terms to account for it. Such a chronological system, however, should avoid the term Anthropocene because of its implicit formal chronostratigraphic meaning.