Exactly where and when did the Anthropocene begin? Scientists are attempting to answer this epochal question in the coming months by choosing a place and time to represent the moment when humanity became a “geological superpower”, overwhelming the natural processes that have governed Earth for billions of years. They could decide the start is marked with a bang, thanks to the plutonium isotopes rapidly blasted around the planet by the hydrogen bomb tests that began in late 1952, or with a shower of soot particles from the surge in fossil-fuel power plants after the second world war. Or they may choose the postwar explosion in artificial fertiliser use and its profound impact on the Earth’s natural nitrogen cycle. Microplastics, chicken bones and pesticide residues may also be among the eclectic signs used to bolster the definition of the Anthropocene. Possible other signs include lake beds in the US and China, Australian corals, a Polish peat bog, the black sediments beneath the Baltic Sea and even the human debris accumulated under Vienna. An international team of almost 40 scientists, who have been commissioned by the official guardians of the geological timescale, must select a place where layered deposits show the clear transition from the previous age to the new one. The team has come up with a shortlist of 12 sites that have now begun a series of votes – but there can be only one winner. Humanity has unquestionably changed the Earth far beyond the stability of the Holocene, the 11,700-year period during which all civilisation arose, and which will end with the declaration of the Anthropocene. The atmosphere, lakes and oceans, and the living world have all been transformed by greenhouse gas emissions, pollution and the destruction of wildlife and ecosystems. Humans also now have a greater effect on shaping the surface of the Earth than natural processes, shifting about 24 times more material than is moved by rivers. Defining the Anthropocene is vital, researchers say, because it brings together all the impacts of humans on the world, thereby giving a platform for holistic understanding and, hopefully, action to repair the damage. From a scientific perspective, a precise definition is essential for a clear basis for debate. The first stage of voting is already underway. The site will need to show “specific physical properties in sediment layers, or strata, that capture the effects of recent increases in human population; unprecedented industrialisation and globalisation; and changes imposed on the landscape, climate, and biosphere”, according to a recent paper in the journal Science by Leicester University’s Prof Colin Waters and University College London’s Dr Simon Turner, the chair and secretary respectively of the Anthropocene Working Group (AWG). But creating a new unit of time is a big decision in geological circles and, in parallel, the AWG has also to achieve a bigger task – persuading geologists that a new epoch is justified at all. Both tasks come down to identifying clear markers of change and hundreds of scientists are doing just that. The broad markers of anthropogenic transformation include rising levels of carbon dioxide in the atmosphere, recorded in trapped air bubbles in ice cores, and the huge change in the populations and locations of species, with human and livestock numbers soaring and spreading as those of wild animals plunge and vanish. But other markers offer the “golden spike” needed for a precise definition and enabling strata to record a sharp, clear rise. Principal among these is the distinctive fingerprint of radioactive isotopes, particularly plutonium, produced by cold war H-bomb tests, the first of which was carried out by the US on 1 November 1952 on the Eniwetok atoll in the Pacific Marshall Islands. Scores of above-ground tests soon followed, with some even rocketed into the stratosphere. The fallout from the tests was fast and global, circling the planet within about 18 months, until atmospheric testing was banned in 1962. “For a short period of time, they tested their new arsenal a lot,” said Turner. “That’s why you have this very unique, time-specific, global marker which is so useful for our work.” Another useful marker are tiny spheroidal carbonaceous particles (SCPs), a type of tough fly ash only produced by the high-temperature burning of coal or heavy oil. “They take off with the sudden increase in numbers of thermoelectric plants after WW2,” said Turner. “They’re good at travelling on a continental scale and you find them globally because lots of continents produced them.” Work done for the AWG has revealed SCPs in Antarctic ice cores for the first time. Plastic pollution is also a marker of the Anthropocene, the scientists said. “The 1950s is when you start to see the majority of the polymers that were familiar with being invented and starting to appear in products,” said Waters, with nylon essentially replacing silk around second world war for example. Plastic waste can now be found from the top of Mount Everest to the deepest ocean trench, giving a global signal. Other scientists found in 2019 that plastic was being deposited into strata and suggested the stone age and iron age was being followed by the plastic age. However, the sharpest rise in plastic pollution comes a couple of decades after the plutonium isotopes from the H-bomb tests, though both have the advantage of never having appeared in the geological record before. Some scientists have suggested broiler chicken bones as a marker of the Anthropocene, with their production soaring from the second world war onwards. Furthermore, agricultural breeding means their skeletons and genetics are clearly different to those of their wild ancestors. “Chickens are now far and away the biggest population of birds on the planet,” said Waters. “But also two-thirds of the mass of large mammals on the planet are domesticated species - cows, sheep, pigs etc. That is clearly a big change to the populations of species, particularly given the diminishment of natural species.” WWF estimates an average 70% reduction in the population size of wild animals. These biological changes are large, but more gradual than other markers, Waters said. Invasive species introduced by humans to new regions can also be markers, the scientists said. The inadvertent import of alien species in the ballast water of ships arriving in San Francisco from Asia transformed the bay. “There was a point where 98% of the mass of all of the animal species in the bay were actually invasive,” Waters said. Pollen from introduced plant species, such as the trees used in commercial forestry, can also record change. Chemical and metal pollution show up in sediments too, said Turner: “The Green Revolution was based on artificial fertilisers and pesticides, and so you see that in sediment cores. The whole cocktail of industrial chemicals just exploded postwar.” Whether the chemicals persist in the environment long enough to be markers of the Anthropocene remains to be determined. The 12 potential locations for the site that will define the new epoch all display some of the markers, but are very varied. “Because the Anthropocene has not been formally accepted, we’re still trying to prove to people that this is not something localised, it is something you find and correlate in a whole host of different environments,” said Waters. “They all illustrate this dramatic Anthropocene transformation very well. But the sites which really stand out are the ones where you can actually see an annual resolution of layers,” said Turner, including some of the lake, coral and polar ice sites. “It’s quite astonishing that these sites detail planetary changes at annual resolutions.” All have pros and cons. The 32-metre-long Palmer ice core from the Antarctic Peninsula is the longest record of the Anthropocene, but its remote location means the trace of some of the markers is often faint. The Baltic Sea sediments switch from pale to black as the Anthropocene starts. This is caused by pollution-fuelled algal blooms sucking all the oxygen out of the water. But the sediments do not have annual laminations. The archeological site in central Vienna gives a 200-year record, dated by artefacts, but has gaps in the record because of redevelopments. The choice of site, and therefore the official time and place for the dawn of the Anthropocene, is in the hands of the 23 voting members of the AWG, but it will then have to be passed by the Subcommission on Quaternary Stratigraphy, then the International Commission on Stratigraphy and finally be ratified by the International Union of Geological Sciences. There is a deadline too: theinternational geological congress in South Korea in 2024, when the mandate of the AWG expires. “It’s been pretty much stated that we’ve got until then to get this done,” said Waters. Prof Naomi Oreskes, at Harvard University and a non-voting AWG member, said: “As geologists, we were trained to think that humans were insignificant. That was once true, but it no longer is. The evidence compiled by the AWG demonstrates beyond any doubt that the human footprint is now in evidence in rocks and sediments. The Anthropocene is primarily a scientific concept, but it also highlights the cultural, political, and economic implications of our actions.” UCL’s Prof Mark Maslin, who co-authored The Human Planet with Prof Simon Lewis, said: “I think the Anthropocene is a critical philosophical term, because it allows you to think about what impact we are having, and what impact we want to have in the future.” Maslin and Lewis previously proposed 1610 as the start of the Anthropocene, representing the huge and deadly impact European colonists had on the Americas and consequently the world. But Maslin said agreeing a definition was more important than precisely where it is placed. “Up until now, we have talked about things like climate change, the biodiversity crisis, the pollution crisis, as separate things,” he said. “The key concept of the Anthropocene is to put that all together and say humans have a huge impact on the earth, we are the new geological superpower. That holistic approach then allows you to say: ‘What do we do about it?’”