The International Chronostratigraphic Chart (v2020/03), produced by the International Commission on Stratigraphy, is a wonder of consensus. It contains 4.6 billion years of information printed on one side of A4 paper. It is beautifully neat, sectioned, and colored to assist the reader to understand “this is like this, but different to that.” It is a mini book of chapters, names, and numbers recording times of change on Earth, over millions of years, leading up to the present. When reading the history of Earth from oldest to newest, you start with the Hadean eon—which began with the formation of the planet—located at the bottom right of the chart, and work up, then left, down to the bottom and back up, repeating until you reach the present, which is situated at the top left. The chart contains fewer than a hundred names and words, which allow geologists to place everything that has happened in Earth’s history so far in a temporal framework. Its effort toward internationalism is evident in its availability in multiple languages: Holoseeni, 全新统, Holoceno.

The 5- to 6-million-year story of upright walking hominins occurs only in the two most recent periods of the chart, the later Neogene and the Quaternary. Given that the next proposed epoch, the Anthropocene, is deemed to have occurred in approximately the last seventy years—a human lifetime, if you’re lucky—you will need a microscope to estimate its thickness within the final bounding line. The chart is a product of centuries of scientific consensus-building—how else to compartmentalize a restless, evolving planet into a table of agreed-upon, correlateable segments of Earth history? It represents the work of thousands who have sought to question and understand how landscapes and their patterns of rocks in the crust below are connected in time and space. In the past, certain individuals and institutions, as well as entire countries, had hegemonic control on naming and dictating when and what occurred; dig into the history of this global chronostratigraphic pursuit and you can find geographical and historical expediency and hand-in-glove science and geopolitics, but also a genuine human effort to seek consensus. The current International Chronostratigraphic Chart represents a continual work in progress and refinement of planetary history from crustal evidence.

The straightforward way of understanding stratigraphic ages is the “law of superposition,” which states that, in undeformed sequences, old layers are formed and laid down first and younger layers are deposited on top. Scientists’ methodical research into superposition and estimates of time for layers to form, beginning in the eighteenth century, were part of the opening salvo that tore into the biblical and other religious estimates of the age of the Earth that had held sway for millennia. Crustal history is made more complicated by blocks of layered crust the size of entire nations that have been flipped upside down and intensely crumpled. Once geologists iron out these inverted, crushed sequences by looking for “way up structures” and applying other dating tools, nowhere is this stratigraphic law broken. Dissensus regarding chronostratigraphic position is created only by lack of evidence or errors in interpretation.

Look closer at the chart and you will see small golden spikes, each with a numerical value, running down the right-hand side of the columns—these are the Global Stratotype Sections and Points (GSSPs) that have been decided upon to identify the base of a single section of rock at a particular location, representing a point in time that marks these stratigraphic divisions. These GSSPs delineate time boundaries, not just changing geological matrices. They represent a synchronous time horizon that can be determined globally where the right age of rock is present. The start of the Holocene? That was 0.0117 million (or 11,700) years ago. These numbers have been determined by absolute dating techniques developed to calculate the formation of rocks and sediments after decades of detailed scientific testing and stratigraphic research. The margins of error shown for some ages on the chart arise from the radiometric dating techniques used and the mathematical consequences of testing multiple samples but being unable to measure everything. What better way to describe dealing with dissensus than a calculation of uncertainty? The age of the GSSP defining the start of the Holocene is estimated as being within 99 years of 11,703 years before 2000 CE, with more than 95 percent probability.11Walker, M., Johnsen, S., Rasmussen, S.O., Popp, T., Steffensen, J.-P., Gibbard, P., Hoek, W., Lowe, J., Andrews, J., Björck, S., Cwynar, L.C., Hughen, K., Kershaw, P., Kromer, B., Litt, T., Lowe, D.J., Nakagawa, T., Newnham, R. and Schwander, J. (2009), Formal definition and dating of the GSSP (Global Stratotype Section and Point) for the base of the Holocene using the Greenland NGRIP ice core, and selected auxiliary records. J. Quaternary Sci., 24: 3-17. https://doi.org/10.1002/jqs.1227 Uncertainty is acknowledged throughout the GSSP process and as such sometimes only approximate numerical ages are provided.

Constructing a chronostratigraphy is challenging. Sequences of rocks and deposits have to be pieced together from spatially scattered evidence, due to the continual churning and destruction of evidence by our active planet. Plate tectonics mean that tracing a sediment unit horizontally sometimes requires a transoceanic voyage. The longest continuous cores drilled into the deepest ocean sediment basins only penetrate into the furthest left column of the chart, down to the Cretaceous—the word “only” not really doing justice to the logistics and scientific research required to extract and analyze tens of meters of ocean mud from beneath thousands of meters of water. All this evidence has been obtained through millions of measurements (themselves the product of methods developed and used by consensus) and picked over by specialists dedicating lifetimes of work to understanding comparatively small amounts of Earth history. Even the most expensive and illustrious research is compared with previous work and subsequently tested. The more dramatic the conclusion of one piece of work, the more it acts like a magnet drawing in nonbelievers to question and test it. The shoulders of giants not only are stood on to see farther but are sometimes stepped off of when discovered not to be so tall after all.

What if we were to repeat this work in progress, starting entirely from scratch and ignoring all previous research, with the most globally inclusive scientific program of stratigraphic research? Many of the epoch names would surely change, obliviating most of the Eurocentric names for periods where many rocks were first found and described in the nineteenth century. We don’t actually need to start again, however, to see how this process would likely pan out; we can simply compare where the GSSPs are currently located and the eponyms of their associated periods. For example, the Devonian, named after a county in southwestern England, now appears incongruous with the Central European, Asian, and North African GSSPs that define its passing. Similarly, GSSPs for the Cambrian (named after another place in the UK, Wales) are found mostly in China, with one in the US state of Nevada. Even without such a clean-slate assessment, the current chart will continue to evolve and change as scientists develop new analytical techniques and as unseen evidence emerges. This is the exciting world of geological dissensus that is hidden by the straight lines of the table. The current chart is different to the one I fastidiously copied down as an undergraduate thirty years ago; in another thirty years, it will be different again. Even in the time of writing and editing this article, a new iteration has been published: v2021/05. What new geological evidence will Anthropocene-accelerated shrinking glaciers, rising sea levels, and future excavations for mega-constructions reveal?

With geologists and other scientists currently exploring the stratigraphic mid-twentieth-century Anthropocene, we have the option to bring incredible certainty into the International Chronostratigraphic Chart. Through some of the environmental archives being proposed as sites for the Anthropocene GSSP, we have the potential to identify annually deposited layers from the last few centuries. Little dissensus arises in dating a stratigraphic sequence when, for example, individual annual layers of coral growth or lake seasonality can be counted back in time from the present, as with tree rings. This is even before validation by independent dating methods, such as the identification of known historical volcanic ash layers (e.g. in annual-layered ice cores) and using radioactive decay rates after burial of natural, short-lived radioisotopes (e.g. ²¹⁰Pb dating). A GSSP stated to be an absolute known year, perhaps even a season of a year, would signify a unique chronostratigraphic occasion on the chart; this is leaving aside, for now, what that would represent in terms of human activity. Using the near synchronous and global fallout of radionuclides—released from atomic weapons testing in the middle of the twentieth century—as a marker for the Anthropocene GSSP could provide exact timestamps, e.g. 19:15 GMT, October 31, 1952, signifying the detonation of the Ivy Mike thermonuclear bomb test. Environmental archives, however, do not function as a global network of digital recorders. Instead, a time lag between the event and its sedimentary record is present, depending on distance, responsivity, or both, of the recording medium.

The chronostratigraphy of our planet may effectively be devoid of human activity but our understanding of it is not. Ending the Holocene epoch by placing the Anthropocene GSSP in the mid-twentieth century is a continuation of this work in progress. It is not a celebration of some predestined journey of human thought and activity; it is rather recognition that, in recent history, a near globally synchronous change in the Earth system can be recognized in global stratigraphy, resulting from accelerated and unparalleled human activity. Not everyone in the geological—let alone global—community is convinced. The exercise in consensus building undertaken to produce an agreed-upon GSSP candidate, by the collaboration of the Anthropocene Working Group, Haus der Kulturen Welt, and Max Planck Institute for the History of Science is no guarantee that this GSSP will be accepted by consensus in the upper levels of the International Commission of Stratigraphy or the International Union of Geological Sciences, or recognized by the International Science Council. What then? Are we then left with a geologically determined consensus that planetary conditions and stratigraphic deposits of the last seventy years are in the same chronostratigraphic framework as the last four thousand? I feel tremors of geological dissent.