This transcript was generated automatically and lightly edited for readability.

Oliver Strimpel: This is Geology Bites with Oliver Strimpel. There's a lot of debate about the idea that the global changes brought about by humans define a new geological epoch, dubbed the Anthropocene. Should such an epoch be added to the official geological time scale? If so, what aspect or aspects of anthropogenic change should be used? And exactly where do we place the golden spike that will define the base of the Anthropocene? Such questions come under the purview of the International Commission on Stratigraphy, whose current secretary general is our guest today.

Phil Gibbard is emeritus professor of Quaternary paleoenvironments at the University of Cambridge. In 2009, he was a founding member of the Anthropocene Working Group, tasked by the International Commission on Stratigraphy to examine the status, hierarchical level and definition of the Anthropocene as a potential new formal division of the geological time scale. Phil Gibbard, welcome to Geology Bites.

Phil Gibbard: Thank you, Oliver. Very kind of you to invite me to speak.

Oliver Strimpel: It's been 13 years now since you co-founded the Anthropocene Working Group, and you're still an active member of the group. Has it provided some officially sanctioned answers to the Anthropocene debate?

Phil Gibbard: Well, "officially sanctioned" perhaps is not quite the right term, but certainly the discussion has provoked a good deal of new and interesting approaches to the topic of the impact of humans, and its extent in natural systems, over the last 200 years or so. And I think it's driven the research agenda quite effectively, not only in the Earth sciences, which is the principal area that we are responsible for in the International Commission, but also in related subjects such as archaeology, and more particularly in the humanities.

Oliver Strimpel: Let's talk about the geological time scale. How was it established, who administers it, and how is it structured?

Phil Gibbard: The geological time scale is the result of more than 200 years of geological research and is effectively a very substantial accumulation of knowledge, which is boiled down into this apparently rather simple hierarchical structure. The International Commission on Stratigraphy is responsible for administering the divisions that we use today, such as Carboniferous, Permian, Triassic, et cetera, but it's only since the Second World War that the commission was established by the International Union of Geological Sciences to administer these divisions. So the structure we see today is very much a product of the last half century or so of research. We see, of course, when we look at the chart, that it has a very clear hierarchical structure, with very large-scale divisions, such as those I've already mentioned, and then progressively smaller and smaller divisions, which are used to divide geological time. In general, the definition of boundaries within the geological time scale is based on the recognition of indicators in the geological record, and these units are defined by their base, the end of any individual division being defined by the base of the subsequent division. So in the division of time up to the present day, the present day has no end. Of course, geological time is going on, day after day.

Oliver Strimpel: So the common misconception that people have, and certainly that I have, is that the significant indicators that divide the geological epochs are the mass extinctions, for example the end-Permian extinction that defines the end of the Permian. But that's not quite what you said.

Phil Gibbard: No, that's not generally the case. Time divisions are actually based on first appearances, and for most of the record, the Phanerozoic, the last 600 million years or so, it's based on the appearance of certain fossil taxa, nearly all animals rather than plants. In general, it's a first appearance, rather than a last appearance, which is the main criterion for recognizing a boundary. The logic behind that is that we don't actually know when the last member of a particular fossil taxon continued to exist, but we can determine the first appearance of a particular organism. So that's the basis. But not all of the geological column can be divided on the basis of paleontology. Four-fifths of Earth history, the Precambrian, for example, is divided on the basis of more physical evidence: paleomagnetism, geochemistry and so on. There we have to rely far less on the few fossils that are available in that part of the sequence. And as we get towards the present day, we are using not only fossil taxa but also other criteria, measured indirectly, such as climatic changes and the like. We hope that the divisional structure is based on multiple criteria. The more lines of evidence we have, the better we feel about recognizing a specific boundary.

Oliver Strimpel: I'm reminded of a couple of earlier podcasts in which we talked about some of these paleontological markers, particularly with Richard Fortey, when we talked about the trilobites in the Ordovician. What happens if someone discovers an earlier instance of a fossil or species that we'd previously used as the marker at the base of a geological epoch?

Phil Gibbard: Yes, well, that of course is an inconvenient thing to discover, but it has happened. I believe at the base of the Silurian, there's been a problem with the recognition of a particular graptolite species slightly earlier than the officially defined base. But we don't move bases around just on that kind of line of evidence, and that's the danger, of course, of depending on one taxon rather than an assemblage of information. So the boundaries are not moved immediately unless there's a very strong reason to do so, and for every boundary we have, there's a ten-year moratorium following its definition, in fact, when we're not allowed to move the boundaries around. The reason for doing that is that it would cause considerable confusion to the community. The most important thing is that the boundaries stay relatively stable, or completely stable as far as we can manage.

Oliver Strimpel: What about the advent of radiometric dating, from which we now derive absolute ages? The stratigraphically determined ages then get pinned onto an absolute scale.

Phil Gibbard: Yes, but those numbers are not fixed in the record, because the problem with using numerical age-determination techniques is that those techniques are evolving and being modified, and the result is that the numbers can change with time. And that would then clash with our concept of trying to develop international boundaries which are fixed in time. So, if you like, we calibrate the boundaries with numerical ages, and in the younger part of the record we can also use the marine isotope record from deep-sea sediments, where we can recognize the isotopic changes in a cyclic fashion to provide a chronology. But that chronology is then fixed by the boundaries, which we define using the criteria I've already discussed. If you like, the numbers come second to the definition in the rock record.

Oliver Strimpel: That's interesting. I mean, when you think of a time scale, you might naively think that really nailing down the absolute ages of when things happened would be the main purpose. But if that's not really the main point, what is the purpose of the time scale?

Phil Gibbard: Well, the time scale is basically about communication, allowing workers to communicate between themselves, so that everybody knows what everybody else is talking about. One of the analogies I often make is the clock analogy, or the calendar analogy, where we divide our year up into 12 months, or days into 24 hours. These are labels which, as humans, we put on obviously continuous time, in order that we can communicate with each other and have some kind of structure to our days, weeks, months and so on. In essence, what we're doing in the geological column with these names is basically the same. We're just using it for communication. We're not establishing these names because we like to give things names, but, more importantly, because we need to know that we are all talking about the same thing when we talk about a division of geological time.

Oliver Strimpel: OK, let's talk about the Anthropocene. What motivates people to create a new epoch?

Phil Gibbard: The real drive, I think, behind the Anthropocene question is that it was originally proposed by Paul Crutzen and Stoermer, who recognized that we are departing, in terms of atmospheric composition, in other words the gases in the atmosphere, from what might be deemed to be the natural condition of the present interglacial in which we are living, and that this was being driven by human activity, particularly industry, of course, but also other activities. The environmental impact of those changes, which we see increasing through at least the last 200 years, was the main reason for originally thinking that perhaps we should consider a division of time to cover those changes.

Oliver Strimpel: What man-made phenomena have been proposed to define the base of the Anthropocene?

Phil Gibbard: The original one was the beginning of the Industrial Revolution in Europe, which was fueled, if I can use that term, by the exploitation of coal, and therefore the changes in the atmosphere which I've already mentioned as a consequence. But many other criteria have also been proposed, such as, if we go back to the beginning of human impacts, the possible megafauna extinctions, of the large vertebrates particularly, and the production of other gases, such as methane. One particularly important change has been deforestation, particularly during the Neolithic in Europe, but it's something that's still continuing in parts of the world such as Brazil. And most recently, the suggestion of the Anthropocene Working Group colleagues was to use the nuclear bomb tests in the atmosphere, which produced radioactive elements that are then preserved in sediments and ice cores and so on, which happened subsequent to the Second World War, particularly peaking around the early 1950s. So there are a number of other criteria that have also been suggested, and one could even possibly use the use of fire, but that would take us back very, very far into the Quaternary and is not really practical. What really has driven this proposal has been the arrival of the so-called Great Acceleration.

Oliver Strimpel: When you mention the nuclear bomb, and the radioactivity from the fallout around the world, it reminds me a little bit of the iridium layer that one gets when you have a meteor impact. It's something a bit like that.

Phil Gibbard: Yes, it is actually very similar. But the question is, how is this radiogenic signal preserved in sediments? Is it going to be stable over the long term? One of the good things about it is that it's truly global.

Oliver Strimpel: And I think that part of the definition of the base of an epoch is that it can be clearly recognized all over the world. I mean, that's the communication point that you made. But some of the markers you talked about appeared in different places at different times. So how can we deal with that?

Phil Gibbard: This is the nub of the problem, actually. You're quite right that if we are going to define a base, it has to be, as we say, isochronous: it has to be the same age everywhere on the surface of the planet. That's why people prefer to use something like the bomb testing, because we certainly can pin that down to specific years, and possibly even specific days, in fact. It's very similar, of course, to volcanic eruptions; the very largest volcanic eruptions can be pinned down in very much the same way. But the problem, as you say, is that human activity is not something that happened instantaneously. It's developed over centuries, and in fact thousands of years. This is the nub of the problem: this diachronism, as we call it, which means that by drawing any fixed boundary, especially if we draw one in the 20th century, we are leaving out a very substantial proportion of time during which humans were certainly modifying the environment, but which would not be included in a fixed epoch of the type that's being advocated by the working group at present.

Oliver Strimpel: So how would you address that problem?

Phil Gibbard: There is a simple way to do that, and that is to invoke a concept called an event. Events in geological history can be anything from a single raindrop falling into some clay, perhaps to form a little raindrop pit, or they can be very much more extensive: the really substantial events in Earth history. I mean, many things are events, of course, like a volcanic eruption, or the iridium resulting from the meteor impact, as you've said already. But the longest is probably the Great Oxidation Event, which is seen in the Precambrian, when we have the first appearance of substantial volumes of oxygen in the atmosphere, which allowed the development of plants and animals. Another example might be what we could call the greening of the land, which happened in the Devonian, and happened throughout the Devonian. That's a substantial amount of time. If we look at the Devonian, it began about 416 million years ago and it ended about 359 million years ago, so you've got effectively 57 million years. So the idea of proposing an event would allow us to include all that time which we mentioned earlier, which would have been cut out by defining a fixed boundary. It would allow us to say that human activity and modification of the landscape and the environment in general began at different times in different places. That seems to me to be a practical solution to an otherwise difficult problem of drawing a fixed boundary. And that event, by the way, will be continuing even as we speak.

Oliver Strimpel: So that would mean that we're not really establishing a new epoch at all, in effect.

Phil Gibbard: No, I agree. I don't think we are really, because we are living in the Holocene, and the Holocene began 11,700 years ago. That base is fixed in an ice core, the NGRIP ice core in Greenland. And one of the reasons for separating the Holocene from the Pleistocene is the presence and activity of Homo sapiens. Now, you could say, well, why do we separate the Holocene from the Pleistocene? And that would be a very valid question, because in every other respect, apart from the activities of humans and the products of our activities, the Holocene is really no more than a typical interglacial in the Pleistocene, and the Pleistocene contains about 50 interglacials, which occur between glacial cold events. So they're temperate events, and the Holocene is just another one of these interglacials, and we might reasonably expect, therefore, that in the future we will go back into glacial conditions once again. Those conditions may be delayed as a consequence of human activity, but the conditions which brought about glaciation haven't changed. So we might expect that the Holocene is simply an interglacial in the Quaternary.

Oliver Strimpel: So what would advocates of establishing an Anthropocene epoch say in response to that?

Phil Gibbard: Well, they would come back to this idea I mentioned earlier, the Great Acceleration, as it's called, a term coined by Will Steffen in Australia. The Great Acceleration refers to the remarkable increase, particularly in the period since the Second World War, so from the 1950s on into the present day: the enormous expansion in human activity, which has produced greenhouse gases. They were produced before, but they're now produced in very huge volumes. There's human construction, rock excavation, and the production of artificial materials such as concrete, which is now produced in vast quantities, and indeed plastics, which are now seen as a major pollutant both in the oceans and on the land. So identifying this Great Acceleration, the beginning of which broadly corresponds with the nuclear bomb testing that I mentioned before, allows people to say, well, the conditions are no longer similar to those which were present earlier in the Holocene, and therefore we want to draw a fixed boundary to reflect that Great Acceleration.

Oliver Strimpel: Stepping out from the Earth for a moment, the people who search for extraterrestrial life have a working definition of what constitutes a technological civilization. For us, it would be our radio signals, now emanating out into space, which could signal our presence to aliens. Has anyone proposed our first radio emissions as a marker for the Anthropocene?

Phil Gibbard: No. The critical point about geological time and divisions of geological time is that they have to be represented in rocks or sediments. It's a perfectly reasonable thing to suggest that if there was somebody out there listening and they'd identified radio signals, they would realize that there are intelligent beings, we hope, able to produce these signals. But since those signals can't be recognized in the rocks which form our planet, we can't use that type of evidence to support a division of the scheme.

Oliver Strimpel: So after working on the question of the Anthropocene for 13 years, has the official Anthropocene Working Group made a formal recommendation as to whether we should have a new epoch?

Phil Gibbard: No, they haven't. They had, in 2016, a vote amongst themselves, where they decided that they should define a formal unit of time in the mid-20th century, and they felt that the status of this unit should be high-ranking, because it recognizes major changes in the atmosphere and the environment. And so they selected the epoch. But at the moment we haven't received a formal proposal. They have several possibilities, I understand, and we're waiting to see which is selected.

Oliver Strimpel: Selected by whom, and when? And how will that selection be made?

Phil Gibbard: The selection would be made by the Anthropocene Working Group, which is a working group of the Subcommission on Quaternary Stratigraphy, which itself is part of the International Commission on Stratigraphy. And at each point there has to be a vote. So the working group must vote, and in turn, if that passes, it has to go to the subcommission itself, which also has voting members, and assuming that passes, it then goes to the International Commission itself to vote. That's made up of the heads of each of the subcommissions for the whole geological column, of which there are 19. And finally, if it passes that hurdle, it goes to the International Union of Geological Sciences for ratification. And I should point out that at each stage, there has to be a 60% supermajority in favor for the proposal to be accepted.

Oliver Strimpel: I don't know if you're a betting man, but do you think this is ever going to get through, and if so, when?

Phil Gibbard: Well, it's a very good question indeed. It will certainly be accepted by the working group, because that will be their proposal. The next step, however, is less clear. The voting membership of the subcommission is made up of people who work on all sorts of areas within the Quaternary, which is quite a wide spectrum. I know that there's a good proportion who are not terribly comfortable with this proposal, and so it's not clear whether that will pass. And then it goes, as I said, to the International Commission, should it pass, and again I have no clue about how that vote will go. If you asked me to bet, I would be very reluctant to do so, once the proposal has left the working group.

Oliver Strimpel: Has your alternative idea of establishing an event, as opposed to a chronostratigraphic unit, got any traction?

Phil Gibbard: Unlike the formal proposal of a chronostratigraphic unit, an event proposal doesn't require voting by any of the people I've mentioned. It's already published; it's in the community. And if sufficient numbers of the community, and by that I mean the wider community, not just those interested in the Anthropocene but really the entire geoscience community, which even includes geological archaeologists and people of that ilk... If that gets traction, as you say, I think it could well snooker the proposal for a formal unit.

Oliver Strimpel: So, just to boil it all down, at the end of the day, when do you think we might have a decision about the Anthropocene?

Phil Gibbard: I should imagine that it will be at least two years, and possibly much longer.

Oliver Strimpel: What do you think might constitute the base of the next epoch?

Phil Gibbard: I don't know. I think we're going to have to wait and see. We've seen small-scale divisions, such as those we see in the ice cores from Greenland. There are two particular divisions, one at about 8,200 years ago and one at about 4,200 years ago, and these produced substantial impacts on the climate of the Holocene, which is otherwise seen as rather stable. So a future climate change might well be a reasonable basis to recognize such a division. And, since I mentioned paleontology right at the beginning of our discussion, we might also possibly see some paleontological change, the arrival of some new taxon, if that's possible within the time frame we're talking about.

Oliver Strimpel: What about plate tectonics? If we run the clock forward on plate tectonic models, might that create something that could define a significant change in the future? Obviously it's very speculative, I realize.

Phil Gibbard: The interesting thing is that plate tectonics is driving the whole distribution of oceans and land on the planet's surface, and these in turn control the heat transfer from the equatorial regions to the poles. At the present day, if we look at the map of the world, we see that the poles are basically isolated from the global oceans. The North Pole, the Arctic Ocean, is effectively a big lake with very limited exchange with the oceans further south, and the Antarctic continent sits on the South Pole and is covered by very large ice sheets. So we might reasonably expect that into the future, if we wind plate tectonics forward, which to some extent we can do, because we know about the plate motions, at least in the relatively short term... Clearly, the further we go into the future, the less predictable that becomes. But it might be reasonable to assume that the Quaternary will continue to last for another 50 million years, which means that we'll stay in an ice age, because the geography of the planet will not change so significantly before that time, so heat transfer will stay relatively restricted, with the poles being cold and the equatorial regions being warm. If we contrast that with the situation in the Mesozoic, in the Cretaceous, et cetera, which is often held up as being the warmest period nearest the present day, the distribution of the oceans and the land was very different at that time, with, for example, the Tethys Ocean around the equatorial regions and the Atlantic not yet a significant ocean.

Oliver Strimpel: And so in that situation, you would have more efficient heat transport from the equator to the poles. Is that the point?

Phil Gibbard: Yes, that's exactly the point. And critically, there's another factor here, and that is that the present distribution of land and sea gives a very strongly zoned climate, as I've said, with very warm equatorial regions and very cold poles. As a consequence, that brings the Earth into a situation where it can be effectively influenced by the orbital changes of the planet, the so-called Milankovitch cyclicity, which appears to be the force driving the glacial-interglacial cyclicity we see preserved in the geological record. Once again, I think, long term, that will continue for at least the next 50 million years or thereabouts. Once the poles become more open, if Antarctica drifts off the South Pole or something of this kind, then there'll be much more efficient heat exchange with the poles, and we might expect the influence of the Milankovitch cyclicity to be reduced and the glacial-interglacial climate changes also to fade.

Oliver Strimpel: Of course, I recognize these numbers are very approximate and speculative, but what key configuration would you expect to change in about 50 million years that makes you pick that number?

Phil Gibbard: Well, the question is, will the Atlantic Ocean continue to open? There's some speculation that the Atlantic could begin to close again after a certain amount of time, thereby increasing the exchange, for example, between North America and East Asia. There's also the question of the position of Antarctica.

Oliver Strimpel: What are you working on at the moment?

Phil Gibbard: Apart from an interest in the Anthropocene per se, my main interest is glaciation and the evolution of paleoenvironments during the Quaternary. I've been working in recent times on the extent of glaciation, particularly in lowland Britain, during the later Middle Pleistocene. But I'm also interested in extending that and equating it with events on the near continent, and the evolution of the North Sea and the English Channel.

Oliver Strimpel: Phil Gibbard, thank you very much.

Phil Gibbard: You're welcome. Thank you.

Oliver Strimpel: For more about Geology Bites, as well as pictures and illustrations that support this podcast, you can go to geologybites.com.