Introduction

As early as the late 19^th Century, several scientists had suggested that humans were starting to influence the physical environment of planet Earth (e.g. Marsh, 1864; Stoppani, 1873; Arrhenius, 1896; Chamberlain, 1897). This idea was resurrected and expanded in 2000 by Paul Crutzen, a Nobel Prize-winning chemist, and the late Eugene Stoermer, a professor of biology specialising in diatoms, who suggested that we had left the Holocene and entered the “Anthropocene” (Crutzen and Stoermer, 2000). As summarised by Steffen et al. (2011) and Wolfe et al. (2013), these iconoclastic scientists were referring to the Anthropocene as the interval of demonstrable human alteration of global biogeochemical cycles, beginning subtly in the late 18^th Century following James Watt’s invention of the coal-fired steam engine, and accelerating markedly in the mid-20^th Century (termed “The Great Acceleration”). Thus Crutzen and Stoermer (2000) argued that the Anthropocene should be an epoch, and for a starting date at the beginning of the Industrial Revolution (Monastersky, 2015).

The term Anthropocene is now regularly used in the geological/environmental literature, appearing in nearly 200 peer-reviewed articles in 2012, and three new journals have been launched over the last few years specifically focussed on this topic, namely The Anthropocene Review, Anthropocene, and Elementa: Science of the Anthropocene. In 2014, the Geological Society, London, published A Stratigraphical Basis for the Anthropocene (Waters et al., 2014), a 321-page volume devoted to the subject. The problem is that the Anthropocene has not yet been formally defined and different disciplines (and even scientists within the same discipline) have different viewpoints as to when the Anthropocene began, if at all (Table 1). In addition, most perspectives on this issue are derived from the Northern Hemisphere, although Brown et al. (2013) and Ellis et al. (2013) and some others have taken a more global viewpoint.

Table 1: Dates or ages proposed in the literature for the start of the Anthropocene, arranged in chronological order.

Adapted from Lewis and Maslin (2015)

How to define the Anthropocene and when it should start

In 2016, members of the International Commission of Stratigraphy (ICS), as custodians of the formal Geologic Time Scale, will decide whether the Holocene epoch has given way to the Anthropocene and, if so, where the boundary between the two should be placed. As well as the issue of timing, the Anthropocene Working Group (AWG, chaired by Jan Zalasiewicz) of the Subcommission on Quaternary Stratigraphy, which advises ICS, is to recommend which hierarchical status the Anthropocene should attain if adopted. If it is to be a geological “epoch” (i.e. at the same hierarchical level as the Pleistocene and Holocene epochs) then it would lie within the Quaternary Period and follow the (terminated) Holocene Epoch. Alternatively, it could be considered at a lower hierarchical level such as “age”, implying it is a subdivision of the ongoing Holocene Epoch (Monastersky, 2015; see also http://quaternary.stratigraphy.org/workinggroups/anthropocene/).

At the same time the ICS will decide whether to formally adopt a proposal to subdivide the Holocene into three sub-epochs (Walker et al., 2012; Gibbard, 2015). This parallel effort to subdivide the Holocene is relevant to the Anthropocene question because it clearly characterises the Holocene as being based on natural climatic/environmental events, thus leaving open the possibility of a subsequent epoch defined entirely by the global signature of significant human impact on the environment.

The Holocene was formally ratified by the ICS in 2006. It is defined as the most recent epoch of the Quaternary Period, being broadly the time since the first signs of climatic warming at the end of the Younger Dryas/Greenland Stadial 1 cold phase (11,700 calendar years before the year AD 2000) through to the present day (Walker et al., 2009). The official boundary of the Holocene is defined in the Greenland NGRIP ice core at 1492.45 m depth (marked by an abrupt shift in deuterium excess values), and selected auxiliary lacustrine and marine records, including sediments in Lake Maratoto in northern New Zealand representing the Australasian parastratotype. Holocene stratigraphic records, as well as providing evidence of climate and sea-level change, geomorphological and hydrological processes, vegetational changes, and faunal migrations, also contain archaeological data that attest to the development of society, and the evolving relationships between people and the environment (e.g. Walker et al., 2012; Roberts, 2014). A key problem in attempting to define the Anthropocene is to distinguish between the detection of human impacts and their distribution in time and space, and the point at which the magnitude of human impacts on the Earth system (key biogeochemical cycles) exceeds the influence of the natural systems and which can be recognised in the context of geological time (Steffen et al., 2011; Wolfe et al., 2013). In a nutshell, the Holocene might be seen as totally adequate to cover the former, and the Anthropocene should perhaps be used to cover the latter.

Australasian perspective

In this article we present an Australasian perspective, and invite your feedback and comments, which will be compiled and sent to AWG to help advise the ICS. We do not suggest that Australasian evidence should necessarily be at the forefront of defining the onset or formal stratigraphic status of the Anthropocene, but that the evidence from our region should be compatible with, and should inform, any globally applicable definitions. Because Australia (and Papua New Guinea) and New Zealand (and other Pacific islands of Polynesia) have very different Quaternary histories, they are treated separately in the brief overviews below.

Australia and Papua New Guinea

Based on the dating of occupation and burial sites, humans arrived in Australia and Papua New Guinea around 45,000−50,000 years ago (e.g. Roberts and Jones, 2000; Turney et al., 2001a; Bowler et al., 2003; Allen and O’Connell 2014). Around this time there is also considerable evidence for significant changes in fluvial hydrology and lake levels (Magee et al., 2004; Cohen et al., 2015), evidence for charcoal and biological turnover (Turney et al., 2001b; Mooney et al., 2011), and for the extinction of the megafauna (Roberts et al., 2001; Gillespie, 2008). There is still considerable debate in Australia, partly due to the sparse distribution and often poor quality dating of megafaunal sites, whether human hunting and associated practices, including use of fire, or climate change, or some combination of both, caused the demise of the megafauna (e.g. Miller et al., 2005; Gillespie et al., 2006; Brook et al., 2007, 2013; Prideaux et al., 2010; Rule et al., 2012; Murphy et al., 2012; Price, 2012; Wroe et al., 2013; Cohen et al., 2015). During the early to mid-Holocene period there is evidence for plant domestication in the highlands of Papua New Guinea (Denham et al., 2003; Haberle, 2007), but Aboriginals on mainland Australia primarily remained hunter-gatherers.

The first Europeans landed on Australia in the 17^th Century, but large-scale European colonisation did not occur until 1788 with the development of the penal colonies around Sydney and the satellite convict settlements on Norfolk Island (from 1789) and Hobart (1803) (e.g. King, 2003). For the next ~100 years there is considerable evidence from historical and palaeoenvironmental archives that the European settlers had a considerable impact on the landscape, with massive deforestation of the east coast forests especially, leading to changes in the vegetation, increased charcoal in the archives, and large increases in sedimentation in lakes, estuaries, and near-shore environments (Haberle et al., 2006). A significant acceleration of the impact of these and other land-use activities occurred in the 1950s (e.g. Douglas et al., 2010; Gehrels et al., 2012; Macreadie et al., 2012).

Pacific islands and New Zealand

In comparison with the early arrival of humans in Australia, many of the Pacific islands were only discovered and populated by seafaring Polynesians in the Late Holocene, with colonisation of West Polynesia from around 200 BC, central East Polynesia from around AD 900-1100, and Hawaii, Easter Island (Rapanui), and South Polynesia, from around AD 1200-1300 (Anderson, 2015a). The earliest arrival in New Zealand (part of South Polynesia) was not until the 13^th Century, possibly around AD 1280 (Lowe and Pittari, 2014). Why New Zealand was settled so late in prehistory is debated, but, as well as being a remote archipelago, its discovery has been linked to the requirement of sophisticated sea craft and navigation and was possibly associated with a peak in El Nino frequencies (e.g. Anderson et al., 2006; Anderson, 2015a, 2015b), or just general changes in the wind systems during this period (Goodwin et al., 2014). The early Polynesians (who culturally became Maori in New Zealand) brought with them domesticated plants and animals (dogs and rats), although some of the tropical plants were not suitable for the New Zealand climate. Evidence from peat, lake, and marine-derived pollen records, lacustrine sedimentation records, and bones of the commensal Pacific rat Rattus exulans, rat-nibbled seed cases and snail-shells, fire records, ancient DNA, and archaeological studies show that Polynesians had a substantial impact on the landscape from around 700 years ago (e.g. Higham and Hogg, 1997; Wilmshurst, 1997; Ogden et al., 1998; Schmidt and Higham, 1998; Higham et al., 1999; McGlone and Wilmshurst, 1999; Brook, 2000; Anderson, 2002, 2005, 2013; Wilmshurst et al., 2008, 2011, 2014; Lowe, 2011; Perry et al., 2012; Jacomb et al., 2014; Anderson, 2015c). There is also considerable evidence that hunting of the local wildlife in New Zealand lead to the extinction of a species of sea lion and many birds (~ 40 species) (Collins et al., 2014), including several megafauna: all nine species of moa (extinct from AD 1440 ± 20; Perry et al., 2014) and Haast’s Eagle (extinct around AD 1400; Tennyson and Martinson, 2006).

Abel Tasman was the first known European to reach New Zealand in AD 1642 with evidence of possible contact by further Dutch explorers after AD 1705 ± 5 (Palmer et al., 2013). Following the arrival of Cook in 1769, there is ample evidence for explorers, missionaries, whalers, sailors and traders, with small whaling and sealing stations around the country from the early 19^th Century (e.g. King, 2003). The Treaty of Waitangi was signed in 1840 and large pakeha (primarily British) settlements continued to develop during the 19^th Century. Further dramatic changes in the vegetation occurred during this European era, as observed from historical as well as geological and palaeoenvironmental archives. Pollen records in particular indicate resurgence in forest clearances by early pakeha settlers and accompanying introduction of a wide range of exotic plants. The large-scale development of grasslands occurred in the late 19^th and early 20^th centuries with the so-called “grasslands revolution”, which lead to forest clearance on lowlands through to steeplands, and the widespread and efficacious drainage of wetlands (e.g. King, 2003; Brooking and Pawson, 2011; Cushman, 2013; Brooking and Wood, 2013; Park, 2013). Accelerating soil erosion and measurable environmental geochemical influences are recorded in landscapes and in lake and marine sediments from the 1920s onwards (e.g. Rawlence, 1984; Hume et al., 1989; Page et al., 2004; Augustinus et al., 2006; Gomez et al., 2007; Gehrels et al., 2012; Basher, 2013), with a markedly increased intensification of land use from the 1950s, including the advent of aerial topdressing (from 1948-49), leading eventually to widespread changes (degradation) in water quality (e.g. Pawson and Brooking, 2013).

The Great Acceleration

It is clear that the 1950s was an important decade for both New Zealand and Australia, and it is also an important time globally, with a major expansion of human population after World War II, intensification of land use, and the development of many new technologies and materials (plastic, artificial fertilizers, advent of new breeds of crops such as rice, etc.). This period, as noted earlier, has been called the “Great Acceleration” (e.g. Wolfe et al., 2013; Monastersky, 2015; Steffen et al., 2015). These changes have had a major impact on our atmosphere and climate with atmospheric CO₂ and methane rapidly increasing after the 1950s. It is now estimated that nearly half of the nitrogen in our bodies was produced in a factory using the Haber-Bosch process. In addition, ever-enduring plastic can now be found in all parts of the Earth – the current estimates suggest the ratio of plastic to marine life in the world’s major marine gyres is 6 to 1 by weight (Vince, 2014; Kidwell, 2015; Young, 2015). Steffen et al. (2015) emphasise that only beyond the mid-20th Century acceleration is there unequivocal evidence for fundamental shifts in the state and functioning of the Earth system that exceed the range of variability of the Holocene, and which are driven by human activities. Thus these authors consider that the beginning of the Great Acceleration is “by far the most convincing for a start date for the Anthropocene”.

The atom bomb effect

 The mid-20^th Century was also the start of the nuclear age. The first nuclear bomb was detonated by the United States of America (USA) on 16 July 1945. In the subsequent nuclear arms race, first the USA and Soviet Union, and then the United Kingdom, France, and China, began developing and testing nuclear weapons of ever-increasing size. The first thermonuclear bomb (or hydrogen bomb) was detonated by the USA in 1952: this new type of bomb, which the Soviet Union first tested in 1953, produced much larger yields of fission products and blasted radioactive isotopes high into the stratosphere where they spread around the world and were deposited as nuclear (radioactive) fallout.

It was a New Zealand scientist, Athol Rafter, who first observed that nuclear weapon detonations – particularly the hydrogen bomb tests – were significantly increasing levels of radiocarbon in the atmosphere. In a 1957 paper he coined the phrase “the atom bomb effect” (Rafter and Fergusson, 1957) and outlined how nuclear weapons tests had doubled atmospheric radiocarbon in the Northern Hemisphere and increased Southern Hemisphere levels by 60%. These increases produced a sudden spike of “bomb carbon” in the 1960s (Figure 1).

Figure 1: Atmospheric radiocarbon abundance, showing the fossil fuel effect and nuclear bomb effect (Gillespie, 1984).

During 1961-62, many high-yield weapons were tested by the USA and Soviet Union, after which both countries signed the 1963 Limited Test Ban Treaty (tests by France, and then China, continued). In both northern and southern hemispheres, peak strontium-90 (⁹⁰Sr) and caesium-137 (¹³⁷Cs) levels were reached in 1964. The two-year delay between peak testing and peak deposition was due to the long residence time of fallout in the stratosphere. In the Southern Hemisphere, peak ⁹⁰Sr and ¹³⁷Cs deposition averaged 40% of Northern Hemisphere levels, with higher levels of deposition in areas with higher rainfall (Matthews, 1993).

Golden spike (Global Stratotype Section and Point: GSSP) or global age (Global Standard Stratigraphic Age: GSSA), or both, for the Anthropocene?

For the Anthropocene to become a formal geological unit it needs to be recorded in the geological, sediment, or ice records (preferably all of these archives) as a stratigraphically significant single global event (or at least covering a significant proportion of the globe), and preferably not time-transgressive (i.e. not diachronous). If we accept that the Anthropocene onset must be (largely) globally discernible, the Australasian evidence outlined above precludes definitions 1-5 (Table 1). If we take definition 6 then the rise in CO₂ in the ice cores is clear, but it is difficult to define precisely. The initial change in concentrations is gradual, reflecting the sluggish and variable spread in the use of coal, starting in northwest Europe and slowly spreading to North America and then globally. So there is no abrupt change in CO₂ or other products associated with the burning of fossil fuels (Lewis and Maslin, 2015), but the marked rise in greenhouse gases nevertheless is evident from the early 1800s (Figure 2). One other suggestion for a golden spike (GSSP) for the early 19^th Century (definition 7), relating chronostratigraphically to this greenhouse rise, has been fallout from the Tambora volcanic eruption (Indonesia), which occurred in April 1815, and resulted in a global cooling and “the year without a summer” in the Northern Hemisphere (e.g. Fischer et al., 2007; Smith, 2014). More importantly for generating a global marker, the eruption produced an instantaneous, synchronous, and recognisable aerosol-derived sulphate spike in the ice cores of both Greenland and Antarctica and in glacier ice in North and South America, and a distinct signal in dendrochronological records (Briffa et al., 1998; Oppenheimer, 2003; Smith, 2014). Probable fallout from Tambora in New Zealand was identified by Gehrels et al. (2008) from measurements of lead (Pb) concentrations and isotope values in salt-marsh sediments in southeast South Island. It is likely, on the basis of recent discoveries of cryptotephra deposits (sparse glass-shard concentrations not visible as layers: Lowe, 2011; Davies, 2015) at distances >7000 km from the eruption source (e.g. Lane et al., 2013; Jensen et al., 2014), that glass shards from the Tambora eruption are identifiable as cryptotephras in terrestrial and marine sediments over wide areas of the Earth’s surface, hence potentially providing a definitive isochron or marker ‘bed’ on a hemispheric scale. Thus it could be argued that the Tambora eruption deserves serious consideration as the golden spike because it is a demonstrably globally synchronous signal that ties in with associated evidence of increasing human impact, namely the atmospheric greenhouse gas rise from the early 1800s (Figure 2) (Smith, 2014). Another idea is that this eruption event could simply be seen as a global marker for the start of the ~150 year transition from the Holocene to Anthropocene.

Figure 2: Changes in greenhouse gases recorded in ice cores in the last 1000 years with the date of the Tambora eruption (1815) indicated (vertical line). Coloured symbols represent data from Antarctic ice cores: red boxes = Law Dome, green circles = Siple, purple triangles = EPICA DML, blue boxes = South Pole; black boxes are data from GISP2, Greenland. Atmospheric CO2 measurements from Mauna Loa are also plotted (solid blue line) for the last ~50 years. Figure from Smith (2014), after Wolff (2011). Reproduced with acknowledgement and thanks to The Geological Society, London, and to Victoria C. Smith.

We would argue, however, that the definition(s) that relate best overall to the evidence in the Australasian region are definitions 8 and 9, respectively the “Great Acceleration” combined with the “Nuclear Age” at around AD 1950 (i.e. between AD 1945 and 1965). It is clear that globally there have been many changes in land use and technology and other developments from the mid-20^th Century that have led to fundamental shifts in the functioning of the Earth system to a point where many biophysical indicators now exceed limits of Holocene variability. There is now an unprecedented “emergent, planet-scale coupling, via globalisation, between the socio-economic system and the biophysical Earth System”, according to Steffen et al. (2015). The global fallout of bomb-test radioisotopes has the potential to create a truly global marker horizon for the Anthropocene (Zalasiewicz et al., 2011; 2015). The nuclear weapon detonations introduced a range of radioactive isotopes that can be traced in soil, sediment, ice, tree-ring, and coral archives. Caesium-137 and strontium-90 were first detected in soils in 1952, while there is evidence that bomb radiocarbon in geological archives peaked in 1965 in the Southern Hemisphere, slightly offset by a couple of years from the Northern Hemisphere peak in 1963 and that of the tropics in 1964 (Hua et al., 2013; Zalasiewicz et al., 2015). Unfortunately, not all of these bomb isotopes will be useful for the tracing this event in the geological future because ⁹⁰Sr and ¹³⁷Cs have very short half-lives (28 years and 30 years, respectively), while radiocarbon is slightly longer with a half-life of 5,730 years. However, several other isotopes, such as plutonium-239 and iodine-129, have longer half-lives of 24,110 years and 15.7 million years, respectively, and hence the latter should therefore be evident in the geological record effectively indefinitely (Hancock et al., 2014).

Others have argued that the date of around AD 1950 postdates the upward inflection of atmospheric CO₂ from fossil fuel combustion in ice cores by more than a century (Fig. 2). However, it is clear from the records that there is also an acceleration in the increase of atmospheric CO₂ at this time (Steffen et al., 2015), with evidence that this is the first interval when anthropogenic greenhouse gas forcings dominated over natural climate forcings (Hansen et al., 2008). Thus this ~AD 1950 date fits with the postulated definition of the advent of the Anthropocene as being the point at which the magnitude of human impacts on Earth system (key biogeochemical cycles) exceeds the influence of the natural systems (Wolfe et al., 2013). Taking a rather different, namely microbiological, viewpoint, Gillings and Paulsen (2014) suggested that the ‘Great Acceleration’ be assigned a formal starting date of 1953 (when the structure of DNA was first published) on the grounds that human manipulation of the biological information flows from DNA, to RNA, to protein, and thence to phenotype (a process known as the ‘Central Dogma’ of cell biology), is a major development in the course of evolutionary history. Because such manipulation “has the potential to expand the power of gene technology to whole-Earth scales”, Gillings and Paulsen (2014) concluded that “DNA technology will be a powerful force in our future, and for dealing with the Anthropocene.”

Finally, some have proposed that a GSSA alone may be more appropriate at this time to define the start of the Anthropocene because the community is still fleshing out the full range of physical, chemical, and biological phenomena associated with the postulated Holocene-Anthropocene transition (Zalasiewicz et al., 2011; 2015; Wolfe et al., 2013), and hence a GSSP can wait. However, other scientists completely disagree with all these proposals and believe that we are still in the Holocene and that the “anthropocene” should remain an informal unit (e.g. Autin and Holbrook, 2012; Gale and Hoare, 2012; Smith and Zeder, 2013; Gibbard and Walker, 2014; Ruddiman et al., 2015). In this case, the name would continue to be used in the same way as such archaeological terms as Neolithic and Bronze Age (Monastersky, 2015). Another view is that we are in the transition towards the Anthropocene and need a much longer perspective to assess “the character of the fully developed Anthropocene” and it should be left to future generations to decide (with hindsight) when the Anthropocene began (Wolff, 2014). For example, Ruddiman et al. (2105) suggested that future changes – such as species extinctions and ocean acidification (e.g. Drake et al., 2014) – are projected to be much larger than those already seen but are difficult to predict.

Issues to address include:

• Should the Anthropocene be formalised as part of the Geological Time Scale?
• If adopted, when should it start?
• If adopted, what status should a formally defined Anthropocene have in the hierarchy of the Geological Time Scale: epoch, age, or something else?

Acknowledgement

Mike Walker (UK) is thanked for inspiring HCB to lead the writing of this article after a talk and discussion at the INTIMATE meeting, Zaragoza, Spain 2014.

Author affiliations

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