In Our Time - Ice Ages

Episode Date: February 14, 2013

Jane Francis, Richard Corfield and Carrie Lear join Melvyn Bragg to discuss ice ages, periods when a reduction in the surface temperature of the Earth has resulted in ice sheets at the Poles. Although the term 'ice age' is commonly associated with prehistoric eras when much of northern Europe was covered in ice, we are in fact currently in an ice age which began up to 40 million years ago. Geological evidence indicates that there have been several in the Earth's history, although their precise cause is not known. Ice ages have had profound effects on the geography and biology of our planet.With:Jane Francis Professor of Paleoclimatology at the University of LeedsRichard Corfield Visiting Research Fellow in the Department of Earth Sciences at Oxford UniversityCarrie Lear Senior Lecturer in Palaeoceanography at Cardiff University.Producer: Thomas Morris.

This BBC podcast is supported by ads outside the UK. Thank you for downloading this episode of In Our Time. For more details about In Our Time and for our terms of use, please go to BBC.co.com. I hope you enjoy the programme. Hello, 20,000 years ago, much of Northern Europe was covered in thick ice. A prehistoric visitor towards now the British Isles would have found much of the north covered by an ice sheet, while even the south would have been barren and freezing cold. This episode from our past is frequently called the ice ice.

age, but that's not strictly accurate. In fact, the latest ice age began millions of years ago and we're still in it. We're going to emerge from it until the ice caps in Greenland and the Antarctic melt sometime in the distant future. Ice age is a period of dramatic global cooling. Earth has experienced several of these episodes in its history, some of them hundreds of millions of years ago. They've had a huge effect on the way the planet looks and on the organisms living on it. But how do we know about ice ages, what causes them, and why do they come to an end? with me to discuss ice ages are Jane Francis, Professor of

Paleo-Clamatology at the University of Leeds, Richard Corfield, a visiting research fellow in the Department of Earth Sciences at Oxford University, and Carri Lear, Senior Lecturer in Paleo-Oceanography at Cardiff University. Richard Corfield, I've just mentioned in the introduction, the term

ice age, is often misused in the notes. I've had ice age, ice epochs and all the stuff. Could you clear all that up, now we can get going? The phrase ice age is a sort of early 20th century term which reflects the fact that people thought there was only one or a couple and they were in the relatively recent geological past.

What's happened in the 70 years since then is that people have realised that in the more recent geological past by which I mean the last 2.7 million years there have been many ice ages, many more than the four that were originally thought. And also we now recognise that there have been intervals in the history of the earth, the 4.5 billion year history of the earth, which we call ice-house earths, as opposed to greenhouse earth. The point being that there are five intervals in the history of our planet, which have been conducive to the growth of ice at the

poles. And superimposed on these five major periods are intervals of much shorter frequency, glaciales and interglacials, which occur on a time scale of a few tens, hundreds of thousands of years, and then on even higher frequencies, there's stadials and interstadials which occur on timescales of tens of thousands of years. So since 4.5 billion years ago, roughly, there's been ice around coming and going, sometimes massively, and inside these great epochs, there have been shorter, bursts of ice ages which have come and gone and periods between ice ages which have been

where the Antarctic has been full of forests and that sort of thing. So the whole thing has been coming and going for about 4.5 minutes, that's sort of about it. That's right. If you think of it as a percentage, 15% of the history of the earth has been

ice house conditions. The other 85 has been greenhouse earth conditions. Have you any idea how many, can you tell the of this as how many ice ages that have been because we don't want to throttle them with figures at the start, but one or two of them are intriguing. Okay, well, there's these five glacial epochs,

these ice house earths. And then within that, there have been hundreds of glaciation's and interglacials. Tell us, what do you mean by a glaciation? A glaciation is when there is ice on one or both poles. Like now? Like now, because we're actually in an interglacial now, but only 18,000 years ago was the last glacial maximum

when the ice sheets extended down across much of North America and across much of Europe, as you said in your introduction, to northern Germany. So that was the height of the last glacial maximum. That's a glaciation to a geologist. And then in the 10,000 years since about 8,000 years ago, since 18,000 years ago, sorry, we've come out of that.

It takes about 10,000 years to come out of an ice age into and interglacial. It takes 100,000 years to go into an ice age, 10,000 years to come out of an ice age. And of these ice ages, these glacial, interglacial cycles within what we call the late Cinazoic glacial age, which is the last 2.7 million years. There have been hundreds of ice ages.

So we have to build in, Jane Francis, when we're looking at the history of the planet, we have to build in a fact that most people don't build, that evolution and so on, there's ice around, affecting it, destroying it, helping it all the time. It's been a big factor in the process. You couldn't call it really development of the planet. But can you tell us what the earth is like between ice ages? Richard's mentioned that we're interglacial between, although technically, according to what you've all said,

because of snow at the bottom and snow at the top, we're in an ice age still. Well, let's look at it in a geological time scale. being a geologist. So if we go back through millions of years to these five periods of, let's call it an ice house, which I define as the times when there is an ice cap on one or both poles on the globe. So there are these five periods. So if we look between those five ice houses, we're in what we call a greenhouse world where there is no ice on earth at all. So there were times, in fact, a lot of earth history is defined by times when the polar regions were free of ice, which is quite unusual actually if you think about it today.

So we're so used to having an ice house world. But this is when, so between ice houses, the polar regions were, like you said, covered in forests, covered in animals, and there was no ice or very small amounts of ice around. Very different world. You use the word defined. Do you mean that helped the planet

because there was no ice at the south or the North Poles? No, it helps us define the time periods when there's ice on Earth. But there are definitely times in a geological past when we have geological evidence for ice in the polar regions, and then there are times when there's absolutely no ice at all that we can see in the polar regions. And instead it looks like global warmth spread right to the polar regions, and it was too warm on Earth for ice to exist.

Let's talk about evidence for one of the fascinating things, and as Richard hinted, evidence with research over the last 70 years, has it creted considerably in different ways. Talk about the geological evidence for ice. ages. Jane, can you tell us something about that? Well, if I go out into the field with my hammer, I can actually collect physical evidence of ice. So we

find rock types that we can see were formed when ice was present on the earth. So one of these is classically called a glacial tillite, and it's a rock that's formed, that was formed by a glacier, grinding its way down a valley, picking up all different kinds of rocks as it went down

a valley, churning them all together, and then dumping them in a big mass. I mean, today you can go to the Arctic and see these big dumps of what we call moraine, but that over millions of years turns to a rock type, which we call tillites. So when we see a glacier of tillite in the rock record, we know that a glacier has been there. We can also see rock platform, so areas where rock surfaces and have been exposed, and we can see great big scratches and trails through on top of this rock platform. And that's where millions of years ago a glacier was sitting there, and it had lumps of rock frozen into its

basin it's ground its way across that rock platform and gouged out long furrows. And then the ice is melting and the rocks were gone. And I just left those furries still preserved as fossil traces of where that glacier moved millions of years ago. You keep using the word millions. Is it possible to define that more precisely? Okay. So of the five ice ages that Richard was talking about, there is a very famous, or two ice ages sort of in what we call the Precambrian. So that's billions of years ago.

And then there was a short ice age in what we call the Ordovician about 400 million years ago. And we know that geological evidence that I've just described, we see evidence for ice. The interesting thing about the Ordovician glaciation, it was quite short. But to see the geological evidence for it, you have to go to the Sahara. So there's a nice paradox. In the Sahara, when you're baking hot, you can be working on rocks that were formed about 400 million years ago that represent glaciation. And that's because at that time, 400 million years ago,

Northern Africa was over the South Pole. And then, so moving forward in time, there was a huge ice age in the southern hemisphere, just in the southern hemisphere called Gondwana glaciation that covered that big landmass of Gondwana. Gondwana was a great landmass, which was Australia, India. All the southern continents were together. And that was covered in ice for about 80 million years,

so a long time. but the northern hemisphere was all tropical. That's quite an interesting time. And then we come on to really today, which we call a Cenozoic glaciation, but actually started about 40 million years ago with the buildup of ice and Antarctica. And then ice sheets were in Antarctica for several million years,

just coming and going, coming and going, and then suddenly Earth got so cold that ice caps started forming in the Arctic, only about two million years ago. And then that's where we have ice sheets on both, worlds and we're in what we call an ice age, which Richard was talking about where we have, we know we can see glacial and interglacials. So let's continue with looking for evidence, Carri Lear.

Jane's mentioned the Antarctic. Was that getting colder and colder? And a lot of evidence, as I've read, comes from the sea. And that was a very effective factor in the Antarctic. Could you tell us about that and explain what an oxygen isotopia is and why the evidence from that is useful? Yeah. So the kind of features that Jane were just describing gives really useful direct evidence of these past glaciation. But they've got disadvantages to them as well for paleoclimatologists. And that's because those records tend to not be complete and sometimes they're quite difficult to date as well. But when we turn to the oceans, what we find is that over time there's layer upon layer of sediment constantly accumulating on this. floor. And as this sediment builds up into a big sediment pile, it's basically giving us a really valuable archive of past changes in climate. And one of the most powerful tools we can use to

get the climate signal out of this archive is the, what we call the oxygen isotope paleo thermometer. Can you? Okay. So within this sediment, there are these tiny, tiny microfossils, size of a pinhead called foraminifera. And we can... can analyze their oxygen isotope ratio, the ratio of oxygen 18 to oxygen 16. And that depends on two factors. The first is the temperature that these four aminifera lived in. And the second is the oxygen isotope ratio of the seawater they lived in. And the reason that that's really useful to us is because when seawater evaporates at low latitude, water molecules containing

that light isotope, oxygen 16, are preferentially evaporated. So you have all this water above the oceans enriched in this light isotope and if that water vapor moves to higher latitude some of it will rain out and you'll preferentially lose the heavy isotope and so as you go higher and higher latitudes this water vapor is getting more and more enriched in this light isotope so the end result of this fractionation is the snow that falls on the poles that's going to make up our ice sheets is really enriched in the light isotope auction 16 so if you grow a large ice sheet at the poles you're locking up this light ice sheet. isotope out of the oceans

and that's leaving the oceans enriched in heavy isotope and that is reflected in the composition of these tiny microfossils that we can pick out of the sediments and analyse and the great thing about this is we can get continuous records and we can date these records as well

so that's a good laboratory for ice age is the bottom of the ocean yes what about other evidence is that the main evidence that can be gleaned from oceans or is that other evidence what about Coral reef evidence, for instance. Okay, so coral reefs are really useful because we can use them to look at past changes of sea level. So as you grow an ice sheet, you're locking up all this water out of the oceans, so sea level will fall.

To put this into some sort of perspective, 20,000 years ago at the last glacial maximum, sea level was about 100 metres lower than it is today. And the last interglacial... 20,000 years ago? Yeah, 100 metres lower. The last interglacial, which was 125,000 years ago, sea level was about 5,000. over six meters higher than today. So these are big changes in sea level,

and there's different ways that we can reconstruct these changes in sea level through time. And one of these, as you said, is the coral reef dipstick. And that's because coral reefs grow at sea level. So if we find ancient coral reefs at different elevations to today, we can date the corals using radiometric dating and look at their elevation and work out how sea level has changed through time. So a bit of coral sticking up is like a tidal mark. The river came this high in 1962, that sort of thing.

Exactly. Except it's 1962 million years ago. So that's what you get out of the oceans, because we're looking for where we get evidence. We're talking about geology with Jane, and we're talking about evidence from the oceans now, getting this ice age. So Richard Corfield, can we talk about one or two of the more dramatic things, which gives us an idea of the evidence?

I'm fascinated, as everybody will be, about this, this, manifestation of snowball earth, the idea of the white planets spinning through the universe covered in ice and snow, not so long ago, in terms of the three of you around this table, 400, 500 million years. Can you tell us about that why it happened? Yeah, one thing I think I would quite like to say

is that if we think about the history of the Earth as a 24-hour clock as we've done many times before in this program, the first of these large ice house Earths occurred at about noon, okay, 12, after midnight. The second, the one that you're talking about, Melvin, the cryogenian, occurred at about 8 o'clock at night, i.e. very recently to a geologist, but that's still 800 million years ago. And then the more recent ones that Jane was talking about, the Ordovician and the one we're

in at the moment occurred sort of at 10 o'clock. see the one at the moment is basically at midnight. Another thing to say is that although our ice age has only lasted 2.7 million years, our ice age is the one we're in at the moment. The conditions which led up to it started, as Jane said, about 40 million years ago with the thermal isolation of Antarctica. Now, just a second. The thermal isolation of Antarctica, I meant to pick that up with Karen.

I didn't. Can you just explain that? Yes. The conditions necessary to form ice on the planet, in these five ice houses that we've talked about are always linked to the presence of ice, of land at the poles, and or a blockage of ocean circulation. So there's got to be land under the ice,

under the snow, in order to make this... There has to be land at the poles for the ice to accrete on. And each of these five intervals of Earth history had land on the poles, which allowed the ice to form. Now you asked about the cryogenian, this 800 million year old ice age, which occurred just before the evolution of the shelly animals,

the things that gave rise to us. And they call this the Snowball Earth because it appears that ice extended, not just from the poles down as far as North Germany or down as far as Wisconsin, as is the case with the ice age we're in at the moment. But in fact, encircled the globe.

So if you think about the Jovian moon Europa which is completely covered in ice, the earth would have looked like that 800 million years ago. This is the granddaddy of all ice house earths. And how did we get out of that? That's an epic

movie rather than a conversation, isn't it? Yes, yes. I'm surprised it hasn't been done, actually. But how did we get out of that? Well, the answer is probably by increased carbon dioxide in the atmosphere, which is put there by volcanism. I'm going to come to carbon dioxide.

It's a big subject. Jane Francis, can we talk about the continent called Gondwana, which you mentioned, and the evidence of the ice age there, and the shift in the continental drift, really, is what we're talking about. This was a really massive glaciational glacial, ice house period. And what we, if we go in... I'm sorry to be a ball, but can you give us a data to?

It's about... It must be a maths at school. It does. It's roughly about 350 million years ago and finished roughly about 250 million years ago. So it's almost, it's actually... As if I can get a fix on it.

Yeah, I know. For geologists, that's sort of about the middle of Earth history. It's about half past 10 at night at 24 hours. That's a bit helpful, but still, right. It's a long time. So this is you know, 80 to 100 million years when there was ice across,

Gondwana. So all the southern continents were amassed together to form one big land mass. And what we can see from geological evidence, so geologists going in the field looking at the rocks, we can see that there were ice caps on the cross this big continent. And the really interesting pattern is if you go to South America, you can see geological evidence that ice was there. And then you can go to South Africa. You can see fantastic evidence in southern South Africa for presence of ice millions of years ago. India, which was tucked in next to Africa, Antarctica, and then Australia.

So when we date these rocks, what we can see is that actually these ice caps formed when the continents were over the South Pole. And we're all, we're going to emphasise one. I'm sorry to be so lumpen, but it was a lump. It was a huge lump. Yes, one big mass. It was all joined together. But that one big land mass through that time period that I mentioned was moving across the South Pole.

and as the land went across the South Pole, it got very cold, and ice caps formed. So we can see a continuous pattern of ice that formed from South America, South Africa, Antarctica, Antarctica, India and then Australia. Beautiful evidence for glaciation in the rock record. And so this is a massive ice age, about 80 million years. The really interesting thing is we'd see no evidence of that in a northern hemisphere because all the continents at that time period were all near the equator, so North. America, Europe, you know, where UK was and across to China, we're all in sort of what we would call a tropical belt near the equator. And that's the time when we know about all the

coal forests, all the lush tropical forests that we're living in hot, warm, wet environments, forming all the coal that we have today. And so we don't have really a very balanced earth. There weren't many continents at the North Pole at that time. So it had this big ice cap in the southern hemisphere and warm, lush environments in the, in the tropical regions in the northern hemisphere. But the two were linked because as the ice sheets, these big ice sheets in the southern hemisphere, waxed and waned and they melted and they grew, they affected the sea level. And that affected all the shorelines and the sea level in the northern hemisphere. And we can see that in the rock record as well. So the ocean is a really important, as Carrie was saying, and the ocean is really important, not only for its record, but the connector between ice sheets and the rest of the world.

And is it right that the Antarctic got so very cold because of a particular core? of sea encircled it and cut off its access to any warmer water? That comes later.

Right. So this has gone to one on ice that I was mentioning is about sort of 300 million years

ago. And then the continent's moved away from the pole and so all the ice melted. And we didn't

we were out of an ice age we were into a greenhouse world. And then later on when we come to about

40 million years ago roughly we then go into what we call a Cenozoic ice age which is when Antarctica

was isolated. That one called the present the beginning of the present day. That's the beginning of present day. So can I go to you, Carrie. Can you tell us how that began the present day, the present day, it's 40 million years ago? Well, it's probably best to start off thinking about 50 million years ago,

which is a time that we call the early E.C. What's 10 million? Between climatic optimum. Geologists. Well, we know that 50 million years ago, there was no ice anywhere. So that is this greenhouse world that we've been mentioning. So we have high CO2, Antarctica's vegetated, you know, we've got mangrove swamps along the south of the UK.

Really warm global climate. As a matter of interest, we also have the time, just about the time when we, as it were, what we became, appeared as mammals. Yes? Maybe a warm planet was not a bad thing for human kind. We can leave that. The dead dinosaurs are an even better thing. We can park that. Carrie, you were saying.

Okay, and then we have over the next 10 million years or so, global climate undergoes this pretty steady cooling trend. And towards the end of this trend, we start to see these isolated glaciers on Antarctica and maybe sporadic larger glaciers. Some people have termed this into all the Doubt House world, so we're not really sure about the extent of ice at this point. But then suddenly it's 34 million years ago. This is the transition between the Eocene epoch and the Elycene epoch and the Elysine epoch. Antarctica becomes fully glaciated, and that happens really quickly. Do you know why? there's probably a combination of the gateways being open,

so this currents that you've been mentioning, but primarily CO2. So through this time, CO2 levels have declined, causing global cooling. And what happens is it goes, the climate system probably passed through a threshold. So what that means really is that on Antarctica, the snow that was probably falling in winter

was able to stay all year round so it just didn't melt away again in summer and so if that happens year upon year very slowly these accumulations of snow can build up a large ice sheet and when I say this happened quickly I mean quickly to a geologist that's less than half a million years

very quickly so let's just stick with the present I say we're about 30, 34 million years ago now Richard Caulfield can we not just talk about the causes of ice age, but it would be specific to this one, because we've been, you know, anyway, this one, the cause of, what role and James said quite a deal about this? Is there any more to say about the role of the position of the continents play in this?

Right, well, James explained that once Antarctica isolated itself over the South Pole, it was surrounded by the Southern Ocean, the roaring 40s, and this thermally isolated itself. was not picking up heat from the equator. And as Carrie has said, snow was falling on it and not melting. And actually this is a major feature of ice ages. It's not that it's abrupt. It's just that there's a change in the balance

between how much snow falls and melts. So if you're going into an ice age, you're accreting, as it were, a year by year, more snow. And then, of course, you go into a positive feedback effect because snow is shiny, it has a high albedo, and it reflects the reflectivity of a surface. And it reflects sunlight back into space. So if you've got, as it were, ice caps on the poles north and south,

south at the moment, it's reflecting energy back into space, and so it's getting colder and colder. And the colder it gets, the more cold it will get, because the ice sheet is getting bigger and reflecting more energy. It's a positive feedback loop. until eventually you've cooled the Earth enough so that you're getting ice on the North Pole, which is why the conditions necessary for the current ice age

took about 40 million years to perfect themselves, if I can put it like that. And then only in the last 2.7 million years have we actually had fully fledged glacial interglacial cycles. And I know that you're going to ask me about orbital variations around the sun. And later. Okay, but what I was going to say is that this, that particular type of modulation only manifests itself as ice ages

when conditions are optimal for the formation of an ice house earth. But these orbital variations can also be found without ice ages, but they don't manifest themselves as ice ages. You wanted to come in, Joan. Yeah, I was just going to say that we have, from a geologist's point of view, we were beginning to build up now a really quite good picture of the history of glaciation into our present ice age

from about 34 million years ago because one of the problems if you're a geologist and you go to Antarctica and you try and look at the history of the rocks is there's a very large ice sheet on top of Antarctica that's covering everything up and so the ice sheet is its own worst enemy in that it not only erodes its history

but it covers it up so we can't see it so we have to drill off the margins of Antarctica we use as sort of an oil rig and drill into the rock record that's on the edges. But we are starting to build up a really good picture. Obviously we, I must tell this. It's very much you going out there with your ice pick. Well, and quite a lot of other people as well.

I understand it. Happy days drilling away in the rocks in Antarctica. So we've got really quite a good picture now of how the ice she's built up. And it's really interesting because the ice, as Carrie said, the ice appeared. I think there was ice on Antarctica for quite some time before that, small ice caps. suddenly at this particular point

34 million years ago it must have got quite cold the ice expanded and we can see that there were big ice sheets in Antarctica sometimes they shrunk sometimes they grew

so there was quite a long time of well almost 32 million years when there were ice sheets just coming and going coming and going and what's happening is it's cooling the earth

it's like having a big refrigerator a big ice cube on the bottom of the world which is gradually cooling the earth's climate but that's not yet a fully-fledged ice age. That's sorry to interrupt, Joan. It's just pre-conditioning the earth for ice ages. And of course, sorry to interrupt you. But the reason we know this so well is because it's relatively

recent history. The last 60 million years of earth history is a very well-understood book indeed. Sorry. Well, what I was going to say is that the earth gradually becomes cooler and cooler, became cooler and cooler. And then only when the earth was cold enough did the ice sheets build up around the Arctic, which is a very sort of geologically recent history only 2 million years ago. So we've looked at geology and we've looked at the oceans and looked at continental drift. Before we move on to CO2 to carbon dioxide,

is there any other sources of evidence that have been helpful to a study of the ice ages over the last six or 70 years, Carrie? Because we're now turning to, we're going to turn to chemistry. So anything else before we turn to the big CO2 question? Are the geochemical proxies you mean? I suppose one really useful one that's just come out in the last 10 to 15 years would be magnesium calcium calcium paleoamometry and the reason this is really useful for us

if you remember I was talking about oxygen isotopes that contains two signals within it the temperature and the ice volume and the first auction isotope records of these glaciers were published in the 50s and 60s so we've had them for decades but But even though they're these fantastic quantitative records, we haven't really been able to look at them and tease apart these two variables.

But in the last 10 to 15 years, lots of new geochemical proxies have started to come on the scene. And one of these is this magnesium calcium, calcium, paleo thermometer, which basically gives us an independent temperature record, which means all of a sudden we can separate out these two variables in the oxygen isotope record, so we can start to say this is how much the, ice sheet grew at specific times in the past and how that was related to temperature changes.

You want to go in. Yeah, I just wanted to talk to very briefly, I think for the sake of completeness, about quite an old-fashioned way of measuring temperature in the deep oceans. Before oxygen isotopes, people used to measure the species composition of the planktonic phrominaferal assemblages, which Kerry mentioned, which we actually, we measure individual individual forums for their oxygen isotope values. But in fact, you can just look at the different types of species and these vary according to whether or not it's a glacial or an interglacial.

For instance. Well, there was a project in the early 1970s called Climap, C-L-I-M-A-P. And what the two geologists in America, John Inbury and his student were doing, is looking at the different composition of these assemblages and number crunching this composition through a computer and they were able to calibrate that assemblage against the modern day so accurately that it was possible to reconstruct the surface of the Ice Age Earth 18,000 years ago using just the species composition of planktonic foreams

and calibrate that against the oxygen isotope ratio. Can I come now to the atmosphere and particularly capitalise, carbon dioxide, Janfrances, about which we've heard a great deal. What role does that play in the ice ages? Well, it's very important in regulating temperature. We've heard a great deal, I mean, recently, not around this table yet. Right. And it would have done in the geological past.

And one of the interesting sources of information about CO2 in the past are from ice cores. So now we're coming into a very recent history about the last 800,000 years. And so glaciologists can drill ice cores through the big ice caps. They've done that in big projects in Greenland and in Antarctica and drilled hundreds of metres of ice core. And the ice cores record very faithfully the climate almost on an annual ban. So when you find an ice core, it's segmented almost like tree rings. And within each annual band there are tiny bubbles that are preserved in the ice,

which are full of the atmosphere that was present thousands of years ago. And so what the glaciologists do is they slice up their ice cores and then they treat them and release the gas from these air bubbles and can measure the atmosphere millions of years ago and that gives us a record of the carbon dioxide levels and we can see that that's, along with other ideas, we can see that that's a record of the temperature. So the CO2 is giving us a record of the variation of temperature

when we're in an ice house world with ice on earth. What the... I may I turn to Carolinae. What role does carbon dioxide play in the initiation of ice ages? I think it comes down to this idea of thresholds. So the carbon dioxide level will set the main climate state of the world. So if we look at this past 50 million years that we keep coming back to, at the beginning, carbon dioxide levels were pretty high,

probably, I don't know, about 1,000 ppm. And they've decreased, they've declined over this interval. and as they decline climate cools, and they pass through various thresholds for each of the different ice sheets. So the first threshold you pass through would have been 34 million years ago, and that's when we formed the main East Antarctic ice sheet. And then later on, through the San Joseic, they've declined further to the point that we now have Northern Hemisphere glaciation.

Are we talking about calm dark side as the major and crucial determinant in this? Yes. This is in the initiation. But does this, Richard Coughbyle, does this necessarily continue? It's a major initiating factor, you're saying, the level of carbon dioxide. Well, I mean, it's the flux of carbon dioxide, the movement of carbon dioxide in and out of the, there are three big sinks on this planet. There's the atmosphere, there's the ocean, and then there's the rock, the lithosphere. And carbon moves in and out of these three sinks.

And what happens is that if you have an ice age, you have relatively low carbon dioxide, which is about 350 parts per million at the moment. But it was eight times higher in the late Cretaceous, which is 70 million years ago. And we know the late Cretaceous was very warm. And in fact, it's possible to correlate times of temperature with carbon dioxide. throughout the geological history of the world. And in fact, carbon dioxide is the major factor controlling temperature in the last two billion years. So we have another major factor, though, which I said we'd get on to, which is the Milankovic cycles. Your point, Jane's point to Carrie.

This is Carrie's expertise. Can you briefly tell us about the Malankovic cycles and what importance they play? So we now say that carbon dioxide is the major player in this, but the Malankovir's. in initiating it. And then we have the Malankovych cycles which cause it to increase and multiply. Well, you tell me if they don't. Well, Melankovic cycles are cycles in the way that the Earth orbits the sun.

So they're called Melanchovych because a so-beian mathematician called Melankovic during the First World War calculated the effect that these changes would have had on the amount of energy that our planet receives for different latitudes. it affects the distribution of energy, the seasons, and also the total amount of energy. These cycles have gone on through Earth's history, so you can't see that they're a cause of ice ages because they occur during these greenhouse worlds.

But a better term for them is pacemakers. So what they do is they trigger some initial change, and that initial change probably would be, say, a cooling of summers, northern hemisphere, summers, for example. And that might set into play a whole series of feedback systems within the climate system, typically involving levels of carbon dioxide. Right. I mean, the Milankovic cycle affects glacial cyclicity via the mechanism of carbon dioxide, doesn't it? Yes, that's right. Yeah. There's three main cycles.

There's obliquity, precessions. and eccentricity. And obliquity is probably quite an easy one to think about in terms of the impact on ice sages. So that's the tilt of the axis of rotation of the earth. So if there was no tilt on our axis of rotation, we wouldn't have any seasons because certain times of year

the hemispheres wouldn't be pointing towards or away from the sun. And the amount of this tilt has changed in the past, which affects how extreme seasons are. The important impact this has had in our last glaciation, is that it's effect, as Kerry said, it's a pacemaker. So once we're in an ice age, this is a control that sort of, this is a control that paces into glaciers and glasals,

warming and cooling while we're still in an ice house. And we know that this has happened on a time scale of about 100,000 years for the last 800,000 years. So we go into a, we're still in our ice house, our present-day ice age, but we go into a cooler phase, a colder phase, and then 100,000, 100,000 years later we get warmer. and we get warmer and then cooler and warmer and cooler.

So there are places warming and cooling, which we call glacial and interglacials. Can I come, Richie Cough, with the onset of the current ice age, how has it affected life on Earth? And let's talk about us, because there's too much life to talk about here. I mean, human life, humankind. It seems that when the death of the dinosaurs allowed small mammals to emerge, became us, but it was a time of warmth, of warming.

Yes, yes. Three hundred million years ago, there was warmth again, and the development of what became us was increased and people left Africa and started to move right. So in both cases, global warming benefited what became us here now. That's true. Maybe that'd be a bad thing,

but that seems to be what happened. Well, I mean, there is this idea that at the height of the last glacial maximum and even before, somewhere within the cycles of the last 2.7 million years, the environment changes were sufficiently stressful, that it drove natural selection towards increased brain size and the development of some kind of society and hunter-gathering, the ability to work together became highly adaptive,

useful in terms of evolutionary advantage. So it would seem that, as it were, extreme environmental variability at the last 2.7 million years when we were swinging in and out of these sequence of ice ages drove natural selection faster than normal, which resulted in a particular species of naked, bipedal, ape, us, having above-average cranial size. And that, of course, has resulted in us. But I think you can find the roots of human evolution, not necessarily in warmth or necessarily in cold, but enhanced climatic variability. Yes, I wish we'd another because of some,

I'm skipping like mad here, and it's annoying the heck out of me on my notes. Never mind. We've got very little time. What's going to happen? As far as you three geologists are concerned, we're in an ice age, we're coming to the air,

and you're calling it becoming interglacial between ice ages here. As you see it, from a geological point of view, what's happening next? Because we've seen dramas happen without human interference for millions and even billions of years. and now of course

the story is that human interference is going to exacerbate or influence modify those dramas so can you give us a steer on that starting with you Jane well we know from as I mentioned in the ice cool records and the carbon outside records that when the earth for the last 800,000 years

has been going in these glasals into glacial there's always been a window of CO2 levels up to 280 parts per million so even it's got warm cool warm cold glaciers of waxen-wains, it's always stayed within this envelope of 280 parts of million of CO2, fairly stable, warm-cooled, warm-cooled. Where we are now, of course, the CO2 levels are at 394 parts of millions. So we're way, way outside of that envelope.

So to get to those carbon dioxide levels, we have to go back and look at the record millions of years to find that. So I think we're outside of that envelope where we are likely just to go back into a smooth glacial phase. We're way outside that. So we're probably heading into a proper greenhouse climate. And I think a lot of the we'll see glasses on an earth melt.

But a greenhouse climate, of course, is the natural condition for the earth. 85% of Earth history has been greenhouse. 70 million years ago, carbon dioxide levels were eight times higher than they are at a moment, which made them 2,400 parts of a million. Before that, they were 12 times.

higher. The only certainty is that climate change is a natural part of the earth. And as a species, we may have been the result of climate change. We may now be altering it, but anyhow, we'd have to deal with it. So I think we are going to have to geoengineer our own climate to deal with it. Nothing wrong with that. Carrie, could you have a final short word, I'm afraid? Well, just looking at the paleo-climate record, at modern day levels of CO2, the Greenland ice sheet and the West Antarctic ice sheet are not stable. So I think unless we reduce our current levels, they're doomed, I'm afraid. Well, it's been a heck of a story with or without us.

Thank you very much, Jane Francis Carrier and Richard Corfield. Next week we'll be talking about Evelyn Wars' first novel, Decline and Four, published in 1928 and Still a Wonder. There are many more Radio 4 arts and discussion programmes to download for free. Find these on the website at BBC.com.com.uk slash Radio 4. Thank you.