In Our Time - The Paleocene-Eocene Thermal Maximum

Episode Date: March 16, 2017

Melvyn Bragg and guests discuss the high temperatures that marked the end of the Paleocene and start of the Eocene periods, about 50m years ago. Over c1000 years, global temperatures rose more than 5 C on average and stayed that way for c100,000 years more, with the surface of seas in the Arctic being as warm as those in the subtropics. There were widespread extinctions, changes in ocean currents, and there was much less oxygen in the sea depths. The rise has been attributed to an increase of carbon dioxide and methane in the atmosphere, though it is not yet known conclusively what the source of those gases was. One theory is that a rise in carbon dioxide, perhaps from volcanoes, warmed up the globe enough for warm water to reach the bottom of the oceans and so release methane from frozen crystals in the sea bed. The higher the temperature rose and the longer the water was warm, the more methane was released. Scientists have been studying a range of sources from this long period, from ice samples to fossils, to try to understand more about possible causes. With Dame Jane Francis Professor of Palaeoclimatology at the British Antarctic SurveyMark Maslin Professor of Palaeoclimatology at University College LondonAndTracy Aze Lecturer in Marine Micropaleontology at the University of LeedsProducer: Simon Tillotson.

This is the BBC. Thanks for downloading this episode of In Our Time. There's a reading list to go with it on our website, and you can get news about our programmes if you follow us on Twitter at BBC In Our Time. I hope you enjoy the programmes. Hello, about 50 million years ago, the Earth's climate changed faster than at any time in our geological record, reaching temperatures much higher than they are today.

That event is known as the Paleocene-Eocene Thermal Maximum, and with the result, it's thought, of vast volumes of carbon dioxide being released into the atmosphere over a few hundred years, along with methane, another greenhouse gas. The Arctic and Antarctic became subtropical with crocodiles where there's now ice. Some life forms went extinct,

others adjusted in the warmer acidic oceans before the earth cooled 100,000 years later. With me to discuss the Paleocene-Eosine Thermal Maximum are Dame Jane Francis, Professor of Paleoclimateology at the British Antarctic Survey, Mark Maslin, Professor of Paleo-Climatology at University of London,

and Tracy A's lecturer in marine micro-pallantology at the University of Leeds. Jane Francis, go with this in context. How much warmer was it then than today? Well, generally through the Paleocene and Eocene geological periods, we know that it was much warmer today, like you said, there were forests in the polar regions. But in this particular spike,

and we are talking about a sort of, spike that we can see in a chemical record. It's a very short interval. What's short in geological? Short when we talk about hundreds of thousands of years instead of millions. So relatively short is a period of overall about 200,000 years, which we can narrow down to a real event about 20,000 years long when, as you say, there was more carbon in the atmosphere. And geologists believe that the world's temperatures increased by about 5 degrees centigrade. So that was quite a big temperature increase in a geologically relatively short time,

which affected many, many systems on the Earth at that time. To put listeners in convenient starting blocks, this was 20 million years after the asteroid hit the planet and eliminated most of the dinosaurs. That was 65 million, and this is, we're talking 55 million now. Yes, that's right. What happened in between? 60 million years ago, 66 million years ago, the dinosaurs died out.

And on the land service, so the earth started recovering from that. So animals started recovering, the system started recovering from what happened at the boundary there. And actually the palisine is not particularly well known. So that's the geological period that came after that. And then suddenly we start beginning to see some warming. And suddenly there's this one big event that happens at 55 million years. And it's a quite short event. And then we go on back into sort of normal geological

climate if you like, but also including warming. This is a once in a geological lifetime. Do you know of it happening before then? Or do you know, it certainly not happened since then? Well, actually, this is a particular period where we've got quite a lot of information because it is geologically relatively young. So the rock record and a fossil record is quite good then, and we can date the rocks quite well at that time.

But I think there have been since a few small peaks of warming in the past. And if we go back in geological time, then it becomes quite difficult in the rock record to see something in such a small scale because, you know, the rock record is quite compressed. And then dating an event that's that short is quite difficult. So this is a particular time when we can see the rocks in great detail. So it's convenient because there's evidence there, although the evidence is rather mixed and a bit confusing. It is. So the cause of the warming is still debated. So we have a whole list of causes of the warming.

Where did all the carbon come from? You know, did it come from volcanoes? Did it come from, you know, burning peat that was around, melting permafrost, or methane that was trapped on the seafloor that was released? So, you know, there's been a lot of work on the causes of it. And there's been a lot of work as well trying to understand why it happened and then what happened afterwards. How did people get interested in this?

We can all understand, especially when we're small, why they're very interested in the wiping out of the non-avian dinosaurs. But how did people get interested in this event? Well, it all started in the late 80s, actually, and it was near Antarctica when there's a fantastic program, ocean drilling. So there's a ship that has a rig on it that drills into ocean sediments, and it was near Antarctica in the late 80s, and it was drilling into the rock record.

And the scientists there realized that suddenly there was a signal, that there was a dramatic change. And a very fast dramatic change, and I think like all scientists, they're very excited about something very new and something very dramatic had happened. Did something ping or did they find a different colour or what happened? Yes, that's right.

They had a rock record that was drilled into the seafloor and when they started working up through these cores of rock, what they saw, two things really, they saw that some of these small shells as a marine organisms that lived in the oceans, particularly on the bottom of the seafloor. Suddenly they disappeared from their cores as they were working through.

So these things called four arms, four amniferous, they went extinct. So geologists are really excited when things go extinct because they want to know why. So that's what, there was a lot of interest in this extinction event. And they also found when they started looking at the chemical signature in the rocks, you know, taking the rocks, sampling them in great detail and looking at the chemical signature, they found suddenly there was a really dramatic change in the chemical signature in these rocks at this particular point. And so that, despite a lot, literally spiked a lot of interest.

And the geologists all around the world started to find their own place to see if they can find their own peak. Mark, Mark, Maslin, what events, can we go back to events that characterise the start of the Pelliosia? And can you keep giving estates? Because I don't want people to think we're talking about yesterday or tomorrow. This is a considerable time ago. No, absolutely.

So geologists are fascinating and have spent the last 250 years trying to split up Earth history, the whole the 4.5 billion years of Earth history. The reason being is because it allows us to understand where massive changes in the Earth have occurred, both environmental but also biological. So if we put the PETM in context... That's what I... The Paleocene, inane thermaxoid.

As scientists, we get... No, I think it's easier if we say PETM. PETM is... You say it again. PETM. That's right. That'll do. That's what we're talking about. The thing is, we should have... As scientists, we should have actually come up with something a little bit more catchy, okay?

because unfortunately we're not very good at that. So what's really lovely is that at 66 million years, we have the death of the non-avian dinosaurs. And I have to admit that my daughter is still most upset because she doesn't accept that chickens are dinosaurs. But that is 65 million years ago, and that is the start of both an eon, which is the Cenozoic, which is the new life period,

but also the Paleocene. changes from dinosaurs to mammals occur, but actually there's very little change in environment. And we then have 10 million years of the Paleocene, and then at the end of the Paleocene, we then have the change to the Eocene, but that is defined by this very warm spike, 5 degrees warming on a relatively warm world anyway,

and then there's a huge change in the mammals. So abrupt changes are what marked geological periods, aren't there? And this is one of the big abrupt changes. So the PETM is a really important. abrupt change and it clearly changes the mammals. So everybody expects that mammals suddenly take over the world from about 65 million years. But it's 55 million years that's absolutely crucial for mammals. Because what we have there is...

So they've been just hiding away? No, they've been expanding and changing, but it's new species that evolve. So we have things like even-toed ungulates, which sounds fantastic. And they actually include camels, cattle, goats, giraffes, and strange enough, whales, porpoises, and of course, dolphins. But most important for us is, of course, primates. Primates and social monkeys evolve at that time, which, of course, without them, we wouldn't have evolved.

So 55 million years ago, and the Palaecieneer is seen Thermal Maxim, is when the whole of our evolution starts off. And it starts up with monkeys, or recognisably monkeys? Well, firstly we have monkeys suddenly becoming social, And part of that is then primates evolve, which are social creatures, and then from primates we evolve, because we are primates, we then evolve in East Africa later on. Do you know why they became social?

They used to bond in pairs and rove around, then they became a gang. Why was that? Well, a lot of people have speculated about this, because you suddenly have this massive expansion of the range that you can have mammals, because you have warmth, sub-tropical temperatures up to Antarctica and the Arctic, you suddenly expand the range.

And what happens is then you get groups of mammals moving into new environments that have no other competition, and so therefore changes in different things can be tried out. And actually, working as a tribe, as a group, as a social group, seemed to be very, very successful, and that suddenly took over. Tracy A's, can you give us, we're still trying to hold a 55.5, and very precise this, million years ago, can you give us an idea what the plan,

what was the disposition of the continents at that time? So if you had a kind of photograph of the world from the PETM, it wouldn't look all that different from what the world looks like today on first impressions. So relatively we have a kind of recognisable Africa and South America and North America, but there are certain things that are quite different. So for example, at this stage we don't have a Himalayan mountain chain because India is yet to crash into that tectonic plate that causes the Himalayan mountain chain. We also have South America still attached to the Antarctic continent

and the Arctic Basin was much more restricted than we see in the modern day. So there were similarities but there were certain key differences that made big differences for the climate state. Well let's start with the Arctic Basin. Why did that make a key difference? So in the modern one of the things that's really important from the Arctic Basin is that we generate our kind of deep waters in the highlands. latitudes. And so we had these cold, salty waters in the high latitudes, which get advected down to the sea floor and they drive ocean circulation. And with an enclosed basin like that in the northern hemisphere, it had less influence on ocean circulation. And we believe that the kind

of background state for the kind of Paleocene was probably that we were generating deep waters in the southern ocean rather than in both of these high latitudes. situations like we see today. So what impact did the disposition of the continents? Was Northern America and South America, were they two different continents at that time? Yeah, there was also a gap. The Isthmus wasn't there.

Yeah, there was a gap between North America and South America, so the Panama Isamuth was open. And that allowed for exchange between the Atlantic Ocean and the Pacific Ocean that we don't see in the modern day either. Just to clarify this a bit, one thing you're drawing attention to is the, way the ocean currents moved and were allowed to move. Why is that so very important?

Because the oceans are a massive regulator of the distribution of heat throughout the earth. And so one of the ways in which we distribute heat in the modern day is by ocean circulation. So pushing warm water from the tropics in kind of near surface currents to the high latitudes and then drawing down these cold waters from the high latitudes to the deep sea and which then up well at the tropics. And that's a kind of major regulator of climate in the modern day. But obviously this was functioning in a very different way in the Paleocene. And one of the things we think happened in the Paleocene-Easine thermal maximum

is this kind of background state where we think we were generating deep waters in the southern ocean, suddenly flipped. And we started generating deep waters in the kind of North Atlantic, which was a change in the state of ocean circulation. and would have happened very rapidly. So it's quite complicated in terms of trying to get a handle on that because trying to understand how ocean circulation worked in the past is difficult. But some of these hypotheses are that ocean circulation changed massively

and that would have had an impact on how we spread and distribute heat. Also, I have quite a – sorry about this, I have quite a difficult getting a handle on how they became deeper. Was the Earth underneath the oceans? the same sort of earth with mountains and all the rest of it. So how did it suddenly become deeper? It's just down there like it was before. Sorry, it's the formation of deep waters in terms of where the deep water is sourced.

So we currently have ocean depth of multiple kilometres, so in some places about four kilometres water depth. And the water that travels along the bottom in a current is generally sourced at the moment from the high latitudes. So it's in the surface and then it down wells. So it kind of sinks down and then moves along the bottom of... Is the bottom current? Is the current along the bottom of the ocean?

Is that the controlling current? Is that the sort of boss current? So that's a tricky question. We don't think so. We think that it's actually kind of a combination of both the surface and the deep. So they all kind of work together in concert. So go back to 55 million years ago. What have we got?

What are we looking at then? So we have, what we think as well is much more restricted basins in general. So we don't have as much connection between our different ocean basins because we have the Drake Passage, which is the connection between South America and Antarctica, is not open either. And that has a massive effect on global temperature, we think, because one of the things that happened when we opened the Drake Passage

was that we allowed for what we call the circumpolar current. So it's a current that seems to kind of aid thermal isolation of Antarctica. So it allows it to stay cool because it kind of stops these warmer waters travelling down there. So we generally have a situation where the tectonic situation is allowing the world to stay kind of warmer than we see in the modern day. Chair, Francis, let's go. Let's dive into the evidence, which is quite complex, really. Anyway, here we go. Do we have to be indirect? What's the main evidence for this happening?

We're talking about temperatures really going up, crocodiles, rainforests, the north and Patagonia, and so on. What's the main evidence for this having happened then? The interesting thing about the PETM is that we can see several, many places actually across the world now where we can see the evidence. And we can see either in rocks that were formed at the bottom of the seafloor at that time or on land at that time. So on land we can look at things like changes in fossil plants. there's a very famous sequence, a really good sequence of plants in Wyoming in the US, in a place called the Big Horn Basin, where they found layer upon layer of fossil plants

that happened before the PETM during the PETM and then afterwards. So that shows evidence of the warming. If we look at the same kind of rocks of the same age in the polar regions, particularly in the Arctic, we can look at the animals, the fossils of animals and plants, as Mark was saying, and that tells us about changes. If we look in the record of rocks from the deep ocean at that time, we see a few things that give us a clue that there were changes. So, for example, in some cores that have been drilled of the ocean floor around the PETM,

we see a change in the colour of the rocks, very dramatic change, from whitish grey rocks to red-dry brown rocks. So it tells us that something is going on. So it tells us really that before the PETM, these grey rocks represent rocks that have a lot of very. lime in them, a lot of carbonate, and then they change and the carbonate disappears. But are these proofs?

I mean, you say, oh, these are changes, but you go behind it and say, these are changes because this happened? It tells us what was happening to the Earth system at that time. And then to find absolute evidence of the temperature, we can look into those marine cores

and we look at the records of what's happening to the carbon and oxygen isotopes. So these are chemical signatures, that are trapped in the shells of the small animals, the four ammns that were formed at that time. And we can use them to work out what the carbon was like in the ocean and what the temperature was like in the ocean.

So that is direct evidence, if you like, of what the ocean temperatures were at that time. Well, we'll keep digging, but we'll go to Mark first of four. Mention of shell there, Mike Muslin. What do we learn from the remains of shells? So the shells that we find in deep sea sediments firstly tell us which species are actually existing in the surface of the ocean and on the sediment floor. And what's interesting is, as Jane mentioned, what really got the scientists, particularly Ellen Thomas, excited in the 80s when they were on ship,

was suddenly these fossils changed. And there was a clear extinction that occurred both in the surface and the deep ocean, and then new species appeared. So the fossils firstly tell us something's happening biologically. And the other great thing is, as Jane said, the fossil. shells, they're about the size of a pinhead for the forminifera, which sort of Tracy works on. And what's interesting is that

we can see the different chemicals inside them. And you can see with the carbon isotopes and the oxen isotopes. And also, there's another one which is as they build their shells, sometimes they make a mistake. And instead of putting calcium, they put magnesium. And actually, the amount of magnesium that goes into the shell is related to the water temperature. So you get a ratio of magnesium in these shells

that allows you to have a thermometer to tell you what the temperatures were in the past. So it's great because you get all this environmental data to really understand the temperatures and the carbon cycle of the planet through this wonderful warming spike as well as seeing how the biology has changed. Can you just now tell our lesson

what this, we've talked about this warming spike. What was it? What temperature is outside broadcasting house now? And what was it then? Just give us some rough and ready guide. So global temperatures, on average now, are between about 15 and 16 degrees for the actual whole planet. You are looking at the Paleocene, probably being somewhere more like 17, 18 degrees. And on top of that, we're adding another 5 degrees on ocean temperatures and global temperatures.

So you're looking at an average temperature for the planet of about 22 degrees, which is 7 degrees warmer than the average today. It doesn't seem massive. It's more like a football score than a rugby score, isn't you? If you imagine that is actually quite a big geological change. Can you get to convince us, Jane? Huge impacts. Well, if you imagine now, we've got ice at the poles.

Let's go to the Arctic. We've got ice at the poles now. And there's hardly, you know, we've got polar bears and seals. But if we go and look at the rocks that are 55, 50 million years old in the Arctic, we can see crocodiles. We can see lemurs. We can see hippo-like animals. We can see Florida.

Everglade type conditions. So what we see, what happened during this really peak warming is the Earth responded very quickly. And all of the sort of animals and all of the environments and trees and plants that lived much nearer the equator where it was warm, suddenly they were able to spread to the higher latitudes. And we can track, we can use the fossils and we can track this migration into much warmer latitudes. It's quite distinct at this time. Tracy, we've been talking with, I think Jane mentioned drilling. into the sea bit. What, Tracy, how conclusive are these findings? So I think to put in context,

we've talked a lot about ocean drilling and about the fossils that we find on the sea floor. And to kind of put that in context, we have been drilling into the sea floor since the 1960s with the various different incarnations of the International Ocean Drilling Program. And we have drilled over 400,000 kilometres worth of deep sea sediments in over 1,400 different sites all over the globe. And what they drill into is essentially what we call largely pelagic ooze. So it's fine-grained sediments that are largely made of the shells of planktonic foraminifera and other calcareous nanophosals. And when we drill down through, what we're actually drilling into is an environment where

there's been the shells of these organisms who have been living in the water column settling down through the water column after they've died and depositing on the sea floor and gradually building up over millions and millions of years and in the kind of general coalescence of the deep sea this allows us to get millions of years

worth of unbroken successions so actually the deep sea provides us one of the most kind of continuous records of any geological environment on earth because of the nature of the deep sea being so quiet. Shells come in, Mark? So yes, so I mean, as Tracy said, what we have, it's like physical archaeology.

You have these beautiful sediment column that literally records what's happened in the ocean. And so what we do is we use lots of physical measurements to understand the magnetics of the sediment, the actual type of grain sizes in there, and also the fossils. And so there is so much information packed into these sediment cores that we have huge international teams of scientists working on each particular a time. And it's not just the PETM that we study. I mean, we look at the ice ages over the last two and half million years. We look at the death of the dinosaurs. And each group of scientists have their own pet sort of subject that they want to sort of study. But no, I mean, this has revolutionized our understanding of climate and how climate's changed in the past.

James Francis, I think the big question, the big question for me anyway, is what caused all this. We've talked about evidence. We've talked about it's there. We talked about 55.5 million years ago. and the drilling, massive figures coming from Tracea there. Let's talk about. Evan said it happened is there. I think we've covered that reason of it well. How did it happen? And what do you three think was the cause of all this? Oh, there's a lot of people trying to work that out.

It's not quite resolved yet, but there's a lot of work. And it probably is actually a mixture of several things. So what we do know is the warming was caused by a massive injection of carbon into the atmosphere at that time. How massive? What's massive? Massive. Thousands of gigatons. Yeah, between two and seven petigrams of carbon.

That doesn't help me all that much. Okay, so to put that into context, the amount of carbon that our huge industrial complex in the world puts into the atmosphere every year is about four gigatons. We're talking about 2,000 to 7,000 gigatons of carbon that were injected in a very short period of time. So that sort of gives you a bit of scale. A massive amount.

It was a massive amount in terms of earth history to cause that warming. So the question is, where does that carbon come from? And in particular, it's what we call light carbon, so of the isotopes, the different forms of carbon. We know that there must be specific sources. So the ideas are, first of all, that it may have come from coal. So we know that in the Paleocene, there were probably a lot of peat deposits around on Earth's surface

because we know there's a lot of coal seams of that age. and one of the ideas is that that coal was burning, burnt, and then it would have put carbon into the atmosphere. Another idea was that maybe there was permafrost around in the high latitudes and it melted. So if you had a little bit of warming, then maybe things started warming even more and that put a carbon in.

Also, there may have been volcanic eruptions at that time and we know that in certain parts of the globe there were plate tectonic activity, in the North Atlantic, in the Atlantic region, which may have added CO2 to the atmosphere and caused more warming. But one of the big ideas that it released methane into the atmosphere.

So methane can be trapped on the seafloor. It's almost as frozen as nodules. My excuse you, for it. I've already in one set of notes that I got from it, methane was massively more damaging, if I can use that word, than come. 27 times? About 30 times more potent in terms of it.

How many times? times more potent in terms of its greenhouse potential. So how much heat it can trap than carbon dioxide? And eventually it changes into carbon dioxide and is a really powerful warming agent. So how did the methane? Where was the methane? I interrupted you. Sorry. Where was that? Where was that going? It tends to get, a lot of it is stored. It comes from sort of rotting animals and rotting corpses and plants on the land and get stored. A lot of it gets stored.

in the oceans, it's frozen as nodules on the seafloor. I mean, it's frozen as nodules on the seafloor today. And if you have a little bit of warming or you have an event that disturbs the seafloor, these frozen nodules are then released into the water and they melt basically, and the methane is released. So that's quite a powerful way we could have warmed the world. At the moment, the jury's out on whether there's one single cause. And I think probably we are looking at a mixture of all those.

of those ideas. Which brings us to the burps of death theory, Mark. Yes, so the methane idea, which had we done this program 10 years ago, we would all have been saying, yes, it's definitely methane. The burps of death, unfortunately, came from a Channel 4 program that they made about the PETM and future climate change, and they did call it the burps of death. But it's these methane, or they're methane hydrates, which are really interesting. So as you build up that sediment column,

what happens is in the deep sediment bacteria break down the organic matter for food but because they don't have any oxygen they can't actually oxidize fully down so you get methane and because it's warm in the deep sediment because of course you're getting closer to the centre of the earth what happens is the methane bubbles up but the actual layer of sediment near the actual sea floor is chilled by the ocean and so you have a cold zone where this methane reaches up to and what happens is is the water freezes round it and you get these clathrates or cages

and literally you can if you get them on ship which I've done you can put a piece of ice on your hand you can light it get this beautiful blue methane flame coming up out of ice and water dropping it is absolutely

go on to YouTube you can see beautiful films of it it's quite amazing but you can store at the bottom of the ocean because remember the oceans cover 70% of the world surface, if you just allow for 10 million years slowly building up this methane, you suddenly have this huge reservoir of organic matter and methane that suddenly can be released. And we think the warming at the end of the Paleocene just tipped it over and suddenly all this methane erupted. And

the reason we call it burps of death is because if you do it slowly, and what happens is the methane goes into the watercolumn and oxidizes and just dissolves in the ocean. We know that, Strange enough from Deep Horizon, the actual BP disaster, methane was released, but it just disappeared into the ocean. You have to do this explosively. You have to really basically release all of it, and it gets through the water column and bursts out into the atmosphere. And that allows you to get thousands of gigatons of carbon into the atmosphere

and get this incredible piece of warming. Do we have any trace of... Sorry about that. evidence of that in the plant life. Yes. Which Jane referred to, rotting corpses and rotting corpses. Charming vision of the world. Seven the seal vision of the world, really.

So plants are really brilliant for kind of making interpretations about past climates if you have fossil records of them. So the fossil record of the PETM and the plants is not as extensive as the kind of marine fossil records. But the kind of things we look at in our fossil plants are the kind of assemblage that we have so all the different types of species that you have and what those species look like and some of the things we can look at

is the edges of leaf margins and how big leaves are and based on what we see in the modern and what we know modern tropical plants So we have a, it's called leaf margin analysis and it's essentially a paleo tool for figuring out temperature like we were talking about with the carbon and oxygen isotoponitis is telling us about the past. But if you have a more jagged edge on a plant leaf,

that is associated with cooler temperatures than you have with leaves that have smooth edges. And we know that from the modern. And so when we look to the past and we see assemblages of plants that are dominated by jagged leaf edges, we can know that temperatures would have been cooler than assemblages that are dominated by smooth edges. and also size is something else that we look at.

So the size of the leaves can tell us about precipitation. So when we see big leaves, we know that there was higher levels of precipitation at that time as well. So, yeah. We can use those leaves to actually work out. Compared to modern day, we compare things how climate and plants respond to modern day. But we can use those things in quite detail to work out what the temperatures were like. So I mentioned this place in Wyoming, which is really quite critical to this story. because it is the one place on earth where we have a fantastic section

of what life was like on land at this particular time. And it's actually through the PETM, which is really quite unusual. And the geologists who've worked on that have been able to look at layers of leaves in some detail or before the PETM and then during the PTM, there's a particular layer they know they've dated as a PETM and then afterwards. And one of the things that they do see is they see that sort of the, you know, central North America, the Wyoming area, before the PETM in the Paleocene, had conifers in it, and it probably was like the Florida Evergrade, something like that.

You can imagine it wet and tropical and humid. And then suddenly they see in the PETM layer, they lose all those kind of plants, and they suddenly see things that you might see in Mexico. So they see really intense drying. And a lot of the plants look like they were relatives of things like beans and things like that. So there's a distinct change in the types of plants they get there. And then afterwards they go back to these sort of Everglades type, humid fossils. So really, the plants are really helpful in telling us what life was like on land.

Mark, how were mammals adjusting to the change in the climate? There were mammals with, what was it, 274? All these meaningless statistics of my own concerns. 275 million years have been mammals, but then they got crushed by dinosaurs and they started to peep out of the holes again after the dinosaurs were crushed. Yes, in some way. I mean, so, yeah, you're right.

Mammals evolved originally 225 million years ago and were oppressed by the dinosaurs for about 120 million years. Saves to the dinosaurs. Well, not quite. I mean, there is a wonderful fossil from China of what can only be described as a killer badger. And inside, it has, in its stomach,

it has baby dinosaurs and eggs. so we did actually occasionally get one back on the dinosaurs. But again... You mean you're associating yourself with the badger? Your friend, the badger. You should see how I insult the students. Yeah, we are just rodents and made it through the dinosaur era. But what is lovely is that there aren't any extinctions on land that we've found.

What happens, though, during the PETM, just the environments expand, and so evolution just goes mad. And so the mammals go, hey, we've inherited the earth from the dinosaurs, 55 million years, it's really warm, it's really humid, this is fantastic, bam. And so you get all these wonderful new mammals, from whales all the way through to camels, primates, as I said. And again, it's shame because everybody focuses on the KT boundary and the death of the non-avian dinosaurs.

But really, for mammal evolution and our history, really everything kicks off at sort of 55 million years. Before we move on to all that, can we use? So do you think the three of you, briefly before me, do you think you, what would you, have you nailed why it happened? Do you think, all right, is it, are you still circling around the black hole? No, I think we don't. I think we, you know, we've got all the signals about the potential and the possibilities. So we know the processes which we can get all this carbon into the atmosphere. So that, you know, for the cause of it. But it's going to be really hard to nail it down. I think we need to have a little bit more sophisticated techniques and we need to sort of have a look. And probably, and you know, a lot of these things, it doesn't. have one, when we look at the earth, it doesn't have one single cause. I think it's a mixture of a lot of things happening and they're all coincident. So they're happening at the same time and I think that's a critical key here. But we are talking about something that had an

enormous effect. We've got to keep that in the minds of myself and other listeners. And this was colossal, even though the number of degrees involved aren't as spectacular as we'd like them to be. And it was worldwide as well. So sometimes we see events in the geological past, you know, that people make a lot fuss about, but actually it becomes quite regional in the end, and you don't see it on the other end of the earth. But we do actually see the impact across the whole earth. And I think although you say that some of the degrees are not that exciting, 5 degrees, I realise it doesn't sound like a very big change in terms of temperature.

If you do actually look at some of the local scale changes, in the tropics, for example, some of the work we did in Tanzania, we were looking at sea surface temperatures there, and they're getting towards 40 degrees, which is, you know, probably about as hot as we recommend you have your bath. So it was really extreme changes in temperature even though the kind of global change doesn't seem so much.

There were, it did get extremely hot. Well, so we've got there. Then how long did it take for the earth? Sorry, Mike, you want to say something? No, no, no. That's right. No, no. So I think the interesting thing about the Ptm is also it really does

help us understand current climate change. And this is why I think people really got excited about it, was because looking at it, it suddenly became an event where nature was injecting huge amounts of carbon, just like we're injecting huge amounts of carbon into the atmosphere over the last 100 years. And so it became a way of actually looking at the natural system.

And there are a couple of things that are really, really interesting. The first thing is, if you leave the natural system, it takes about 100,000 years for all that extra carbon to be taken out by normal processes. I don't want to go into modern so much. We talked about then, but you did say earlier that what we were pumping in over last hundred years was a mite compared with the mass that had been pumped in 55 million years ago.

No, no, that's per year. So we've already put in sort of about 1,000 gigatons and we're looking at by the end of this century putting in another 1,000 gigatons. So why the PETM is so interesting for

looking at climate change is because the scale of carbon emissions that we could put in by the end of the century is at the bottom end of the PETM. So we're actually doing our own natural experiment at the moment, which we can look back at 55 million years and go, well, what does that tell us? So the first thing it tells us is if we don't actually take out CO2 physically, the natural system would take 100,000 years. Why did it start to cool after 55 million years, do you think? So we had the initial injection of carbon dioxide into the system.

that ejection period we think was about 2,000 years. And then... So this was going on for 2,000 years of injection. Yeah, we were pumping loads of carbon into the system. Not 100 years. Over about 2,000 years. So, yeah, that's one of the differences between now and then. Rate of change.

Amounts might be similar, but rates of change are very different. And the way we get cool is by removing the carbon dioxide from the atmosphere. And there's kind of two ways that we can do that, and there was two ways we think that happened in the PETM. On very kind of short-term time scales, so like thousands of years, we're looking at absorbing all that carbon dioxide into the oceans. So it gets absorbed into the oceans and the oceans become more acidic.

And on the kind of longer term, so the hundreds of thousands of years time scales, one of the things that happens when you put lots of carbon dioxide into the atmosphere and you make it warm is that you increase weathering rates, so the kind of the breakdown of rocks on land. And as a product of that weathering, we use carbon dioxide. And then the weathered rocks get flushed into the ocean via rain and rivers. and then we put that carbon into the ocean. And then that carbon that's then in the ocean

gets used by organisms that make their shells out of carbon and then gets locked on the seafloor. Jane, Jane Francis, has it ever been as hot again as it was in the period about which we're on which we're concentrating at the moment? Well, actually, yes, it has. Because if you remember we mentioned that through the Paleocene and into the Eocene, we're actually going through a longer warming trend in which this spike of warming sits.

So after the PETM, the temperatures dropped a little bit, but then they started to rise a little bit, again, but much, much more slowly. So the Eocene is the next geological period after that. And what we see is quite a dramatic warming, but much slower, but still quite warm. And then again, we have evidence for that in the chemical record, in the plant record,

in the ocean record. When you say dramatic warming, I'll be back to no ice caps and that sort of thing, or not as dramatic as that. Yeah, no, I think, in the Eocene it was warm. enough that there was probably no, certainly no major ice caps that we have evidence of.

There may have been small ones on Antarctica in the middle of the continent, but certainly no major ice caps because right at the polar regions we have very evidence of warm, we have alligators and crocodiles at the poles, that kind of thing. So although it was a sort of big bang, it wasn't a

one-off then, Mark? It is a one-off because it is a really short, sharp event in geological terms, which almost happened in an instant, whereas the warming at the end of the Eocene took millions and millions of years to slowly creep up to that temperature. So it is a very short event, which is interesting because it has a dramatic effect on a short period of time, and therefore has a major effect on evolution.

We do actually see a couple of less significant what we call hypothermals, which is what the name we give to the Pellocene-eosine-thermal maximum, in the early Eocene. So there was probably maybe two or three other short-lived warming events. that didn't have the same kind of impact or magnitude as the PETM. The PETM seems to be kind of the first and most significant and biggest in terms of effects it had on the Earth system and also the biggest spike that we see on our chemical records. But there are a couple of other little ones that seem to follow in the early Eocene.

And then after that, we really see the end of the warming. And then at the end of the Eocene at about sort of 34 million years, we start seeing the first signs of major ice caps on Earth. So it really is a quite amazing change. So we go from this very, very warm Earth at about sort of 50 million years in the early Eocene, and then gradually temperatures dropped until it became so cold. And other things happened around Antarctica, this warm current, the cold current that Tracy mentioned, that we'd see the first evidence of glaciers and ice sheets at sea level in Antarctica.

And that takes us into the beginning of our current world with ice sheets on the earth, very, very different kind of place to live. Well, I think one of the things that people don't seem to realize is that we're in a very cold period of time in Earth history. We have ice at both poles. We have about 70 metres of sea level potential locked up in that ice. And actually, if we look back through the whole of geological history, it's actually very difficult to find any other period of time where we have ice at both poles. And so this is interesting that we've gone from a really incredibly lush, warm period of the Eocene, and we've dropped down into incredibly cold. period, even though we feel that this is a nice warm interglacial compared with, say, the last

ice age. But actually, it's a very, very cold period of time. And you think that how long are we going to stay in this state? You've got eight seconds, Jane. I don't think we're going back into a glacial for a very long time. The carbon dioxide levels are high now, and it's going to take a long time for the earth to work out what to do with all this carbon in the atmosphere. If ever. Well, thank you very much, Jane Francis, Tracy A's and Mark Maslin. Next week, We'll be discussing the Battle of Salamis, 480 BC, arguably one of the most important battles in history. Thanks for listening. And the In Our Time podcast gets some extra time now with a few minutes of bonus material from Melvin and his guests.

Did we miss any major areas? I think we didn't really talk about ocean acidification. I think ocean acidification. And an extinction of the forums at the bottom of the ocean. That's quite a big signal. Because one of... Tracy works on that. Yeah, so I think a couple of...

really important things that we didn't pick up on. So it was one of the major signals of the PETM was actually this extinction of 50% of the benthic fouraminifera. So these are single-celled organisms that live on the sediment or in the upper part of the sediment on the seafloor. 50% of those went extinct. And in terms of that group, that's the most significant extinction event we'd seen for about 90 million years. They kind of sailed through the Cretaceous boundary extinction event relatively unscathed, and yet this event 55 million years ago had a massive impact on them. And that's probably to do with a number of factors.

We think that due to changes in ocean circulation, we probably reduced oxygen content on the bottom water. We think that the oceans became very acidic, both in the surface and in the deep sea. and as these were calcifying organisms, so organisms that made their shells out of calcium carbonate, they'd have found it more difficult to calcify as well. But that was kind of one of our major signals of the PETM. I also think that you could have asked more about why this is so important

to understanding future climate change. I mean, I understand why the programme didn't go down that route. But remember, I mean, scientists around the world, we are spending millions of pounds trying to study this what seems to be an obscure period of time, 55 million years ago, who cares? But it has some profound implications for future climate change and the work that we do on it.

The first one is we know from the amount of carbon that was released during the PETM and the warming we see, we can link the two. What's lovely is we can look at the computer models for future climate change, the predictive ones, and they're about the same, which gives us real reassurance that our models of the future are correct.

The second one is that ocean acidification, which we're currently worried about, could have profound implications for the marine ecosystem, because unfortunately, marine systems are very stable. The pH doesn't change much. So as soon as you start changing it, the organisms have no ability to adapt or to change. I also think that the whole thing about how long will CO2 take to come out of the atmosphere naturally is really important. So if we have, look at the PETM, it's about 100,000 years it takes for the CO2 to come out. And I have to say many of us would argue that it never quite ever gets back to the pre-event levels, which is also why.

So if we want the CO2 to drop in the atmosphere, we are going to have to actually physically take it out of the atmosphere ourselves. Nature is not going to do it quick enough because we can't wait 100,000 years. So there was some really profound ideas of kind of. come out of the PETM, which makes it incredibly important to study because it allows us to really understand future climate change and reassure us that we've actually got the science right. Well, you're right, because this goes to probably more... We've got over 2 million people listening, but this thing seems to rather more than that.

Jane, you want to say what we missed out. No, but I think one of the things, the interesting things about studying the PETM is that we've had this spike, but we've also seen how climate has changed afterwards. So the fact that we can look in the geological record and see the end of it, if you like, is quite important so that we can see how the processes in the natural

earth system can deal with all this carbon. And trying to understand how that system works is quite important for understanding what we're doing to the climate today. But just it's ability to look back in the rock record and be able to see the beginning of something, the peak of something and then the end of it.

and so that we can actually understand all the cycles that go on naturally that's quite important. It just seems to me, I mean, I read as much as I can of the evidence of the climate genes. It just seems to me that compared with what we've been talking about earlier, it's a long way off. You said we were in the cold, one of the cold phases. There's still ice caps and so on. So, yeah, that's one of the complications with the PETM really and why it's not a direct analogue to the modern.

So for a start, we were inputting about the same volume. of carbon dioxide into the ocean atmosphere system as we expect to from kind of industrial activities over the next few centuries but the rate of change is very different so we're talking about hundreds of years in comparison to thousands of years and also we were inputting that carbon dioxide onto an already greenhouse world so an already warm world that we are not in today so we're we're kicking this system in a similar way but not a directly analogous way but that doesn't mean that we can't learn things from that as a kind of kick to the climate system.

What does the climate system do when we see a massive injection of carbon dioxide and how long does it take to disappear via natural systems, as Mark has alluded to? So it's not directly analogous, but it is definitely something. It's one of the only things we can learn from, because this hasn't happened that many times in the past. One of the other things is by studying the past, we suddenly realized that perhaps methane-hydrate. would be really important in both past climate changes. And suddenly we realised that we hadn't actually studied them in the modern system and perhaps they could be a threat lurking underneath the oceans.

And in fact, there has been some evidence in the last year 2016 where they've actually found evidence for methane hydrates releasing methane in the North Atlantic. More burps. Yeah. More burps of death. Yeah. So at the moment, I think the research has shown that, as Mark said, the methane was coming up slowly enough that it's actually been trapped at seawater and when they tried to measure methane

above the oceans to see if it was coming out. They didn't see a massive amount but in the future as we continue to warm the ocean there could be more that actually comes and escapes from the seafloor into the seawater and then into the atmosphere. So it's circular and the warmth

to make the more. Yes, that's right. There's so many cycles in the earth system and one into, they're like cogs in a wheel and one interacts with the other. So trying to understand a very complicated cogged wheel system is what we're trying to do. And we're hoping that was the calculation. So about 10 years ago, people were very worried about the clathrates in the bottom of the ocean. We now think that in the next couple of hundred years, they will actually remain quite stable. But this has actually then shifted the focus

on to all of the actual methane hydrates in the Arctic underneath the tundra, because the tundra is melting and all the permafrost, then we're really worried about that methane. And the problem there is we have no way of really estimating how much methane is stored there. At the oceans, we've got ways of actually making some estimates, sampling, but we have no idea what is lurking underneath the Arctic. Well, thank you very much. I think the producer wants to make his ground entrance. Simon. I'm working outside the door of the Arctic. We would like tea. We'd like coffee. Cheap please.

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