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All in good time

Stratigraphers have yet to decide whether the Anthropocene is a new unit of geological time.

In western Berlin, Devil’s Mountain rises 80 metres above the surrounding landscape to offer a clear view across the city. Known in German as Teufelsberg, the tree-covered hill looks primeval, but it was not there until 70 years ago. It was constructed as a dump for more than 25 million cubic metres of rubble cleared from the streets after the Second World War. So it is fitting that this artificial hill had a visit last year from a group of researchers assessing the geological imprint of humans on the planet.

The Anthropocene Working Group has a simple name but a very complicated job. These are the people who have to work out whether the world has entered a new slice of geological time — the Anthropocene.

As the group continues to assess the evidence, the rest of the planet has apparently made its decision. Three journals have been launched that are dedicated to research on the Anthropocene. Environmental advocates have heartily adopted the term and all it signifies, and so have many others, including artists and social scientists. And four years ago, Nature recommended that geologists formally accept the Anthropocene, arguing that the term “provides a powerful framework for considering global change and how to manage it” (see Nature 473, 254; 2011).

But although many people have already made up their minds, those whose opinions matter the most have yet to do so (see pages 144 and 171).

The Anthropocene working group is diverse: about half of the three-dozen researchers are geologists, the rest a mix of archaeologists, palaeontologists, climate experts, atmospheric scientists and representatives of other disciplines. Working without pay over the past six years, and communicating mostly by e-mail, they have been sifting through evidence and arguments about when the Anthropocene might have begun, what kind of geological markers might define it, and whether it is worthy of recognition as a separate unit in Earth’s geological history.

Despite the popular appeal of the Anthropocene, decisions relating to the geological timescale must rest with stratigraphers — researchers who study the evidence embedded in rock, ocean sediments, ice cores and other geological deposits. These people must look past the clamour and decide whether the Anthropocene is an appropriate new unit of chronostratigraphy. Their proposal will then be voted on by the International Commission on Stratigraphy (ICS) and the International Union of Geological Sciences.

Stratigraphers must be given time and space to consider the consequences of formally adopting the Anthropocene.

The process remains conservative because the timescale is a tool used by tens of thousands of geoscientists around the world. Changes can create confusion, so the ICS requires strong scientific justification for any amendments. The fundamental question for the working group and for the ICS is whether geologists would find it sufficiently useful to define an Anthropocene unit in the rock record, which is the physical manifestation of the timescale. The Anthropocene would probably be an epoch that would sit after the Holocene, which started with the end of the last ice age, around 11,700 years ago.

If the Anthropocene is under way, then when did it start? Initial suggestions focused on the Industrial Revolution, but momentum has picked up to set the boundary after the Second World War. Since then, the global population has increased by 180%, water use by 215% and energy consumption by 375%. Researchers have called this surge the Great Acceleration, and it has skewed the composition of the atmosphere, warmed the planet, eroded the ozone layer and acidified the oceans. “The last 60 years have without doubt seen the most profound transformation of the human relationship with the natural world in the history of humankind,” says the International Geosphere-Biosphere Programme, which has charted those changes.

It seems obvious that such broad planetary upheavals would warrant recognition on the geological timescale. But they may not be adequately reflected in stratigraphic evidence. In many parts of the globe, the geological record of the past 65 years is thin to non-existent. In the deep sea, less than a millimetre of sediment has built up, and that could be erased as ocean acidity increases. Signs of atmospheric changes are also preserved in recently laid down glacial ice, but much of that record could disappear in coming centuries as a result of global warming.

The working group still faces a considerable amount of work to evaluate whether — and how — to define the Anthropocene. If the committee or upper levels of the geology hierarchy decide against amending the timescale, the Anthropocene will not disappear. Many scientific disciplines and the public will continue to use the concept and word, in much the same way as they use the terms Neolithic era or Stone Age.

In the meantime, it is important that stratigraphers be given time and space to consider the consequences of formally adopting the Anthropocene. Any such change cannot be revisited for at least a decade, so the geological community will have to live with its decision for some time to come.

Related links

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Anthropocene: The human age 2015-Mar-11

Defining the Anthropocene 2015-Mar-11

First atomic blast proposed as start of Anthropocene 2015-Jan-16

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All in good time. Nature 519, 129–130 (2015). https://doi.org/10.1038/519129b

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