The Jan Zalasiewicz Interview



Defining Earth’s Human Age

The overwhelming impact of human beings on Planet Earth is plain to see. Twenty years ago, Dutch chemist Paul Crützen and American limnologist Eugene Stoermer captured it in a single word.

Human population growth, resource extraction and pollution had pushed Earth out of the ‘Epoch’ geologists say we’ve been in for the past 10,000 years – the Holocene – into a brand new one Crützen and Stoermer called the Anthropocene.


This past July, at a conference of the International Commission on Stratigraphy, in Lille, France, a scientific panel confirmed Crützen and Stoermer’s idea, and presented its own proposal for how the Anthropocene should be defined, geologically: when it began; where the chemical and material ‘signatures’ of human activity are best observed in Earth’s rock and sedimentary record, as a reference standard for other spots of the same age around the world, and how the new interval should be ranked in the geological timescale.

True to its own name, the Anthropocene Working Group’s (AWG) ranking choice was obvious. Only Epochs have that cene suffix. Since the start of the Cenozoic Era, 66 million years ago (after that big comet annihilated the dinosaurs), there have been seven: the Paleocene, Eocene, Oligocene, Miocene, Pliocene, Pleistocene and Holocene.

Epochs are periods of dramatic change in surface temperature and ocean level. Holocene conditions, warm and stable, were what made  human civilization possible in the first place. If the Holocene is over, humanity could be in trouble.

The AWG’s Lille announcement culminated fourteen years of intense study and analysis. Set up in 2009 by a subcommission of the International Commission on Stratigraphy, the largest component body of the International Union of Geological Sciences, the AWG began by establishing the stratigraphic ‘reality’ of the Anthropocene. That is to say, whether or not the human species has indeed left an indelible record in Earth’s layered sediments, all around the planet.

Having answered that question, affirmatively, their next job was to decide when Earth’s human time interval began.

Paul Crützen’s original idea was that the Anthropocene began with the Industrial Revolution in Britain, somewhere between 1750 and 1820 – a compelling choice.

But, while atmospheric CO2 levels certainly started rising dramatically around then, the terrestrial impacts of the Industrial Revolution were much less obvious outside Europe and North America, where industrialization wouldn’t emerge until a century later.

In 2019 — a decade of research under its belt — the AWG decided that the Anthropocene actually began around 1950, at the start of the so-called ‘Great Acceleration’. American environmental historian John McNeill coined the term in 2005, inspired by a 1944 work by Austrian economist Karl Polanyi, The Great Transformation: The Political and Economic Origins of Our Time.

McNeill’s idea would be corroborated by a set of ‘Great Acceleration curves’, or graphs, published in 2007 by Crützen, McNeill and Australian Earth system scientist Will Steffen.

There are two sets of Great Acceleration curves– one for human drivers of planetary change: population, GDP, energy use and so on.  The second set charts Earth system responses to these drivers, such as rising CO2 concentrations, mean global temperature; ocean acidification and tropical forest loss. Each graph showed sudden, steep increases in the mid-20th century.

Having pegged the start of the Anthropocene to the Great Acceleration, the AWG had to get specific. They could have chosen a precise date, perhaps associated with a globally significant event (e.g. the first atomic bomb test, Trinity, on July 16, 1945). In geological parlance, this would be called a Global Standard Stratigraphic Age (GSSA).

Instead, in line with stratigraphic convention for the last 540 million years of Earth timekeeping, the AWG opted for a Global Boundary Stratotype Section & Point (GSSP). Popularly known as a ‘Golden Spike’, this would be the thin sequence of layers in a core pulled from the bottom sediments of a lake, bay or estuary, or from a peat bog, ice sheet, coral reef or stalagmite, somewhere on Earth, containing the chemical and material “signatures” of human activity dating to the mid-20th century. That Golden Spike would then serve as a prototype or reference standard for other sedimentary layers of the same age, all around the planet.

After years of investigation, the AWG came up with a list of twelve candidate GSSPs, in five continents and eight types of environments: the East Gotland Basin of the Baltic Sea; San Francisco Bay estuary; Searsville Reservoir, California; Crawford Lake, Ontario; Sihailongwan crater lake, in China; Flinders Reef, Australia; West Flower Garden Bank, Gulf of Mexico (also a reef); the Palmer ice core, Antarctica; Ernesto Cave, Italy (a stalagmite); Śnieżka peat bog, at the heart of Poland’s “Black Triangle”; Beppu Bay, Japan, and Karlsplatz, Vienna (urban soil).

Scientific teams were established to study these candidate GSSP cores in the minutest detail. Over the course of their research, a host of chemical, material and biological ‘signatures’ of human activity were identified. Key among these — ‘spheroidal carbonaceous particles’ (fly ash) from high temperature coking and coal burning, microplastics, heavy metals, isotopes of carbon and nitrogen associated with fossil fuel burning, and radioactive Plutonium-239 from atmospheric nuclear weapons testing that peaked in the 1950s.

They also honed in on pollen grains and microfossils from invasive species that have spread around the planet.

Having whittled these twelve sites down to nine, the AWG’s twenty-three voting members began casting votes. To win, a candidate GSSP needed to garner sixty percent of ballots. After three rounds of intense discussion and voting, a victor emerged — Crawford Lake, in southern Ontario.

Coring of Crawford Lake sediments. Mike Pisaric (standing), Tim Patterson (kneeling) and Chelsi McNeill-Jewer (lying down). Courtesy Patterson lab.

Crawford Lake, on the edge of the Niagara Escarpment, an hour’s drive west of Toronto, is meromictic: conical, small, but very deep. So its waters don’t mix and its bottom sediments are perfectly preserved. Those sediments consist of many layers, laid down seasonally, year after year, in paired couplets called ‘varves’, all precisely dated.

Those sediments have been building up since the start of the Holocene, 12,000 years ago, and have recorded three phases of human activity.

For about five hundred years, starting in the late 13th century, Crawford Lake sediments recorded the activities of First Nations people living along the lake’s shores, in the form of pollen from corn, beans and squash – the ‘Three Sisters’.

In the mid-19th century, the logging and farming activities of European colonists laid down their signatures.

Then, beginning in the mid-20th century, industrial activity of a more globally pervasive nature appeared: spheroidal carbonaceous particles, or fly ash, wafting over from the blast furnaces at the steel mills in nearby Hamilton – aka ‘Steel Town’.

Then there’s Plutonium-239. Among the world’s meromictic lakes, Crawford Lake sediments are highly unusual. Contrary to what one might expect, those bottom sediments contain oxygen, that keeps the Plutonium in place.

Having chosen Crawford Lake as its proposed GSSP for the Anthropocene, The AWG is now hammering out final details. Key among these, the Anthropocene’s precise start date. Crawford Lake cores are undergoing plutonium analysis to establish where this lies, in the early 1950s. Several ‘auxiliary’ sites must also be chosen to complement the Crawford Lake GSSP, from among the twelve original candidates.   

Some time this Fall, after dotting its I’s and crossing its T’s, the AWG will present a complete proposal to the body that commissioned it, the Subcommission on Quaternary Stratigraphy (SQS). If two-thirds of the SQS’s members agree, the proposal will be submitted to the International Commission on Stratigraphy, where supermajority approval will also be required in order for it to be passed up to the supreme authority on all matters geological — the International Union of Geological Sciences.

The fate of AWG’s proposal is anything but certain. Ranking members of the SQS, ICS and IUGS are on record as opposing the whole idea. Some think it’s political.

I spoke with Jan Zalasiewicz about the history and work of the Anthropocene Working Group. Zalasiewicz, an Emeritus Professor of Palaeobiology at the University of Leicester, was the AWG’s first chair. Listen to our conversation here: