Okay. I think I'll start. I guess it's true to say that the problem with climate change is, I would say about 90% a problem in physics.
But when we try to apply physical theory and models to try to figure out what's going to happen, let's say in the next ten years,
we realised there are big uncertainties and many things that we don't actually understand about our climate system.
So we can look at the observations, but the instrumental record only goes back about 100 years.
And so we're still left with lots of puzzling questions about how climate works.
So a third option is to go right back into into history and look at paleoclimate data.
And today's speak is is an expert in precisely that area.
So we're very pleased to welcome Eric Wolff from the Earth Sciences Department in Cambridge.
Eric actually spent much of most of his professional life at the British Antarctic Survey
from getting his Ph.D. there through to rising to become the chief scientist of Bass.
But about a year and a half or so ago,
he became one of the Royal Society's research professors and moved to a more academic position in Cambridge, where he is now.
So very happy, Eric, for you to come over today and talk to us about ice cores, climate and science.
Okay. Thank you. Microphones working? Yes, I can hear it. Okay.
So thank you very much for inviting me. And also, it's very nice.
I can see because I know because I met Summers today that there are people from several departments here.
So that's good.
And that's important because actually ice cores themselves are a slightly well, they're not a strange subject, but they're the most important subject.
But ice cold people in some places are in Earth sciences departments, geography departments, chemistry departments.
But actually in quite a lot of places, they are in physics departments and there is a lot of physics in ice cores.
So I hope you'll spot some of it as we go along and see things that might be of interest to you.
I wasn't quite sure where to pitch this talk.
So what I've done is to the first half is more or less a tutorial about ice cores, why they're so important, and what you can learn from them.
If any of you have been to one of my talks before,
and I know there are some people who have you can go to sleep during that part because you've heard it before.
And then the second half is about what one of the things I'm working on at the moment,
which is about the possibility of learning about sea ice from ice cores.
And the third part is about future ice core research could, based on my practice, I very much doubt will get to that.
So ice cores are just one of many paleoclimate sedimentary records that you can look at things like tree rings,
marine sediments, lake sediments and so on. And ice cores are one of them and they have, like all of them, advantages and disadvantages.
So the advantage is that they're pretty well dated, but probably not quite as well dated as you think.
That's to say there are ice cores where we can count on your layers in the same way you can count tree rings or valves in lake sediments,
but we can't do it perfectly. And actually in the cores that are oldest, which are the most interesting ones,
to some extent, you can't count the layers and it becomes much more difficult to date them.
We'll come back to that later. They have atmospheric signals which are relatively simple,
and that's to say they haven't gone through any biology and they haven't gone through lots of water circulation before they get into the ice.
Of course, that's where the physics comes in they turn out to be not that simple at all,
but we like to think of them as simple and you get many variables on the same core.
For example, the carbon dioxide concentration and the local temperature in the same core.
I'm going to go to emphasise one problem with ice cores, which is the geographical limitation,
because although as a polar scientist, I think the polar regions are very important.
I do understand that trying to understand climate without knowing much about the tropics is quite difficult.
So ice cores by themselves, or at least polar ice cores are never going to answer all our questions.
So that's my favourite map projection.
I'm fortunate I come after I set this to this bit of this picture up about 20 years ago and can no longer remember what projection it is.
So it kind of loses the force of it. But with the important areas of the world in white, you can drill ice cores, elsewhere you can drill ice cores.
And people do in the Alps, in the Andes, in the Himalayas, even in Africa, at Kilimanjaro.
They're much more difficult to interpret, and they tend to have more local signals.
Some of the properties that we measure don't mean the same in tropical ice cores as they do in polar ice cores.
And I'm not going to mention them again so that they're important, they're useful, but we're not going to talk about it today.
So the ice cold signal builds up like any sedimentary record by something getting imported into the ice at the surface.
In the snow and then buried by the next year's layer.
In the next year's layer. In the next year's layer.
Again, I apologise to the Earth scientists for whom this is far too simple, but the physicists may not quite have appreciated how these records work.
The difference with ice is the physics that ICE thins as it gets deeper.
So if you really want to get old ice, you do need to go quite near the bottom.
You're not going to get half the layers by drilling half thickness. You need to drill 80 or 90% of the thickness to get half the layers.
And that's what an ice core looks like.
Like to show that near the beginning of a talk, because I think people have a strange idea of what we might be talking about sometimes.
So typically ten centimetre diameters.
And when I talk about a 3000 metre ice core, what I mean is lots of two metre long pieces sort of like this, not 3000 metres laid out somewhere.
Okay. There's it's a Friday afternoon, so we're all entitled to a bit of entertainment.
So here's a video of drilling an ice core to give you an idea what it's like.
This is Dome C in the centre of Antarctica. Very flat.
There's nothing to see except this tent, which is the drilling tent, which has to be tall to accommodate the drill as the drill going into the ground.
I know it's going to would, but that's just to keep snow from falling into the hole.
And then at the top, there's a section that clings on to the hole you've already drilled in order to drill the next bit and then the cable.
So it's a cable drill. There are 2000 metres to the drill, sit in the heated cabinet while the scientists get cold outside.
It's. It's just a rule. I'm sorry. It's the way it is.
And then the drill comes to the surface and you can put it to the horizontal to get the ice out.
You can see the ice glistening that we can see the ice glistening there.
You can see some liquid dripping out. Don't worry, it's not melting. That's a drilling fluid.
You can ask me about drilling fluid later if you want.
And although actually I've realised you don't actually see it, the person's about to do the most important thing you could ever do to an ice core,
which is to put a pencil mark with an arrow on it pointing,
which is up because because turning 100 metres 100 years, the wrong way up can create interesting climate records.
That's the ice sitting, waiting. It's actually being cut at this point. You can see it's no longer got the fractured surface.
It's never a cut surface. And then I'll just leave it for another few seconds just to show you how we handle ice in the field.
This is just handling the ice to cut it up for sections to go back to the laboratories and you can just handle it with a band.
So also, if you want to go into the field to do ice cold science, I strongly recommend the course in carpentry.
It's the most important skill you could have and it's cut into lots of different sections and then packed up and sent back to labs,
or it might be analysed in the field in some cases. I think we'll cut the cut at that point in the hope that we might get to the end of the talk.
So signals in our schools, what do we get out of ice cores?
So I like to think of it as having three types of information.
Other people might think of it in a different way, but this is the way I classify it. So the first one is the isotopic content of the water itself.
So that is exactly what you think.
It's the ratio of 18 of water to 60, no water or deuterium to hydrogen, and it's determined mainly by the temperature at the time of snowfall.
Now, I could give you a lecture about why that so and I could give you another lecture about why it's not really so I would get caught out.
I did once get asked, why don't you give a lecture? But actually I've not written it, but I'm just saying that there is potentially a lecture on that.
So you're going to have to take it as read for the rest of the talk that it gives you temperature.
There are ways of calibrating it so that you really can back the temperature out, but they require much more complex analysis.
Then there are lots of soluble and insoluble impurities that are trapped on the surface,
so they're relatively straightforward to think about, but actually very difficult to interpret.
So there are things like the sulphate that comes from volcanic eruptions.
So when a volcano erupts, sulphur dioxide goes up into the stratosphere and you will end up for a couple of years after the big of a big volcano,
you'll end up with an increase in sulphate in across the world,
but in ice cores in particular, which tells you how strong,
something about how strong the volcano was and actually helps us to date the ice cores by matching between different cores.
But there are all sorts of other things, sea salt, which I'll be talking about later dust chemicals like nitrate,
which I've worked on for probably 25 years and still don't understand what it means in articles and all sorts of other things.
And we're just starting to work on various organic chemicals as well that haven't yet been much investigated and ice.
And then finally the thing that ice cores are probably most famous for and which
again requires a lot of physics to really understand as the snow gets deeper,
whereas here in Oxford, when it snows, it probably melts the next day and then refreezes and you've got solid ice.
That's not how you get ice in Antarctica. You get ice by the snow, partly sintering and partly through vapour movement.
So sintering under pressure and vapour movements forming next between the crystals.
It turns from loose snow eventually into a solid ice matrix, but it still has pools in it that finally close off and trap air in them.
So that air is younger. It's only however long it takes to diffuse through a few tens of metres of firn.
That's unconsolidated ice and it is literally a sample of air.
You can crack the bubble open and measure the nitrogen, oxygen, argon, carbon dioxide, methane isotopes of various things and so on.
Anything that stable, not not photochemistry unstable species, but anything else.
However, it does require a lot of physics because first we need to understand the age of the air compared to the age of the ice.
They're not the same because you've got young air being closed in relatively old ice, that maybe 70 metres depth.
And that varies with time. You need to check that there isn't any diffusion.
And eventually when we get down to 800,000 year old ice, there is some diffusion.
You need to check that you haven't lost anything out of the core.
And eventually when you get to ice that hasn't been stored properly, you can lose material out of the core.
So it's it's there's a lot of work involved in these records, but that's just to show you, there really are bubbles in the ice.
That's a finish section, a few millimetres across of ice.
And you can see the bubbles there, typically a few tenths of a millimetre across when they're relatively near the surface.
Actually deep down under pressure, they turn into the the air in the ice turns into collect right hydrate molecules within the ice matrix,
but it can still be extracted more or less as if it was bubbles. So we don't need to know the distinction for at least for what I'm showing you today.
So I'm just going to give you three examples of what we've learned from ice cores.
The first one is from the very recent past, and then we get older.
So from the very recent past, it can be very brief. It's a curve that you have all seen in one form or another hundreds of times.
Probably, but maybe don't know where it came from.
So this is carbon dioxide in ice over the last thousand years.
What you've actually got is I'll have to walk away. But that's okay, I think.
Yes. Well, you've got the blue line. There is measurements made in the atmosphere since the.
All right. I went to China recently and needed to load some software to get at websites.
The Chinese don't like to get out, so that's what that was.
So. That's atmospheric measurements from actually from Manilla in this case.
But it could be South Pole or any of the other sites that was started 30, 40, 50 years ago.
Manilla happened to be the first one back in the 1950s.
And then everything else come on here comes from high schools and shows you I like this way of showing you because it shows you well,
firstly, it shows you that things went unusual in about 1830 and that the increase has been 40% since then.
It shows you that there are variations before that, whether they're natural or unnatural variations, is still a cause of debate in the community.
They're variations, though. And also there are four different ice cores here which have very different properties,
different temperatures, different snow accumulation rates, different concentrations of other impurities.
So the fact that they all plot on top of each other should give you a very good assurance.
But nothing about enclosing the air in the ice gives you an artefact, because if it is an artefact, you've got to be the same artefact,
despite the fact that all the site, all the properties that might control the artefact are different.
That was it for the recent past, I thought. We're not going to talk about the recent past anymore.
So now we're going to go to the longest ice cold timescale of 800,000 years with the Epica Project,
which I was the chief scientist for for the later years. And Epica was it was a consortium of labs from ten countries in Europe.
I'm sure you all know your European countries, so you can mentally do the exercise of working out which they are.
They were in alphabetical order. That's the clue. And we drilled two ice cores, one of them up here at Running Mode Land,
which was for the last climatic cycle, about 130,000 years have actually been used.
It's actually a bit longer than that. And then the one here, it don't see, which is what you saw in the video.
So that's the oldest ice core that we've got so far and don't see is a very dry place.
That's why the ice is so old, because the amount of snowfall that 25 kilograms per square metre per year, that's 2.5 centimetres of ice.
Equivalent of water equivalent. Sorry. So an inch. Yes.
So it's a desert by any standard. That's what don't see.
Looks like you actually saw in the video just to relate it back to that.
You saw the drilling temples there and the sun shelter that you saw them working in was just behind it there.
That's what's there now. The Italians and French have built a permanent wintering station there and have people there all year round.
And not to show that I was there. If you can't tell I was there, I could have set that up in the studio.
But anyway, I was there. So these are the some of the records.
You've got the water isotopes at the top.
I've actually for the cognition, too, I've added in some numbers so that they know what what they're looking at.
You've got dust represented by calcium.
You've got the electrical properties of the ice, which was what I was actually measuring in the field and so on.
It's not much use to like this because the things you don't recognise on the Y axis and depth on the x axis,
which probably isn't of any great interest. So the next thing you need to know is how we construct the time scale.
Now here comes lots of physics and maths, but I'm not actually going to give it to you.
I'm going to go on to explain it in qualitatively what was done to construct this age scale.
And there are various different strategies depending on the ice core.
What was done here was to start off with a model of how much snow fell each year.
Okay, I should start off by saying if you want to know how old you are at a given depth in any sediment,
what you want to know is how many, how many annual layers are above you.
And to know that you want to know how thick each of them is. So you know how thick each of them is.
Your first bet is I know how thick it was when it fell as snow because I know how much
snow falls at the surface and I know how thick it is now because I know how from physics,
I know how it thins to this depth.
So you use models of those two things to make a first estimate of the thickness with that would give you the age with that.
But that would be very imprecise because you don't really know those things actually.
So you then refine it with an inverse method where you add in lots of known ages and I won't talk about all the known ages we are today.
And some of them were the ages of volcanic eruptions, which you can see in the South record.
Some of them were from parameters in the ice core that are controlled by milenkovic cycles, the orbital cycles of the earth around the sun,
which again we could discuss whether it's legal to use things like that where you're not quite sure what the relationship with the ice is,
but there are good reasons for doing it.
The only thing I want to do is to convince you about one of the time markers that we put in so that you believe me,
that is 800,000 years at the bottom. And that comes from this.
The brain is much young and magnetic reversal. So that's the last time the Earth's magnetic field reversed, dated and rocks at roughly 780,000 years.
It occasionally shifts by a thousand years or so.
When somebody does a new dating, what happens when the magnetic field reverses is that it's weak for a while.
And so when magnetic fields weak, you get a lot of cosmic rays, cosmic rays produced.
Well, most familiarly they produce carbon 14, but by 17 or 2000 years, that certainly isn't any carbon 14 left.
But they also produce beryllium ten and there's 20 brilliant ten left.
So you go to the section of coal where you think you should see this and you find the largest beryllium ten peak in the record.
So then you say that is 780,000 years. I'm going to fix that now and make the rest of the times go.
So there's lots more goes into the ice scale than this.
There was an entire special issue of this age scale, but hopefully I convinced you that it's 800,000 years at the bottom.
There's actually a new age scale now which uses lots of ice cores rather than just one and puts them all together.
It's a better way of doing it because it uses more information,
but there are people who say it isn't a better way to do it because it obscures what it is you're actually doing.
So we've got a little bit of work to do to convince people that's the best way. So now we now we can produce a record.
This is very old. I should have said a lot of what I'm putting up here is not my work.
I was involved in it, but it's not. I wouldn't say it's my measurements.
So be aware that's an entire communities work. So now this is what you really want to see.
This is well,
this is actually deuterium of the ice on the y axis and depth on the x axis converted to age and the jitter has been converted to temperature.
So there's an uncertainty in that conversion. But it's it's within that uncertainty.
This is the temperature of Antarctica over the last 800,000 years.
And seeing I've got a mixed audience, I have to say, I've done this the probably the physicists way.
That's to say time is going from left to right. Scientists prefer time to go the other way.
But because they're thinking of depth. But physicists and I suppose I have to admit, normal people think of time as going from left to right.
So so time going from left to right here.
But don't worry, it'll flip around during the rest of the talk. So.
So what we've got here is the Holocene, the last 10,000 years, it's lost 10,000 years of relative warmth there.
And then the last glacial so the last place you probably think of it as ice sheet down to East Anglia or Wisconsin.
But it was also much. Colder in Antarctica during the last glacial maximum, about ten degrees.
And then the last interglacial which was actually a bit warmer and Antarctic than the present.
I did think of giving a talk about the interglacial here,
which would have been quite interesting cause I think it's a very important time to look at because we know in the last interglacial
both the Antarctic and the Arctic at some point were quite a bit warmer than today and sea level was quite a bit higher than today.
And working out what balance of two ice sheets created a sea level that was 5 to 10 metres higher in the last interglacial.
It's probably one of the most direct ways that we can contribute to understanding what might happen in the future under a warming climate,
but in particular warmer polar regions. Okay, so what we've got here is roughly eight glacial cycles.
This was not unexpected. I mean, from marine sediments, we already knew very well that there would be cycles that looked a little bit like this,
that had roughly 100,000 year cycles of warm and cold.
Interglacial in glacial. And there's a tendency to stronger cycles in the later part of the record.
I've done quite a bit of work since then. On looking at other records to see whether this is true and actually this general idea that the
interglacial the warmer in the last 400,000 years than they were before is some kind of a break.
Here is true in Antarctic temperature and it's true in carbon dioxide and it's true in some Southern Ocean records.
But it's not globally true, actually,
because there's there are there are periods that are weak in the second half of the record in some records and the other way around.
Nonetheless, there's something something slightly different after here.
Into that. Okay. So we've got those periods and the obvious question and the one we didn't know
about before the ice cold came out is what do CO2 do in this changing climate?
And this is the answer. So the plot at the bottom is exactly the record you saw before.
Temperature on the one at the top is carbon dioxide from the same ice cold over the same period.
And I think some of you are further back in the room, so probably they look absolutely identical.
I do know where the bodies are buried, that there are some differences, for example, there,
where the CO2 stays high while the temperature's already gone down a little bit before you get there.
But nonetheless, they are incredibly similar. If you went out and drilled, I won't say marine sediments kill them, might offend people in the front.
Right. So if you drilled to lake sediments ten metres apart and measured exactly the same thing in them, they would not look this similar.
And yet these are two completely different things.
I mean, deuterium and an ice core and carbon dioxide in the atmosphere, they couldn't be more different, but they look incredibly similar.
So if we look at it from one direction, we could say, well, that carbon dioxide, based on just what we know about radiator forcing,
would be responsible for something up to a half of the glacial interglacial warming.
So it's an important factor in causing the glacial interglacial climate change that we see worldwide.
But also the fact that they look so similar also tells you, I think, that the carbon dioxide must be being controlled mainly in the Southern Ocean.
And in other words, by something rather similar to what's controlling Antarctic temperature.
This doesn't look like Greenland temperature, for example.
Greenland temperature would have a lot of other stuff in it, as we'll see in a minute.
And then I always have to remind you that if we want to put the last 200 years on the same plot,
we have to shrink that down so that we can see what happened in the last 200 years.
So there isn't an analogue in this record. CO2 was not anything like as high as it is today at any point in the last 800,000 years.
And the chances of that being something hidden in there that's as high as that are negligible.
Just because I like in these talks to give you a few a few killer facts to use in case you meet climate sceptics or maybe you are a climate sceptic,
just put your hand up. It's the right stuff. So there's always this thing about what happened during the termination coming out of the last Ice Age.
So that's this section here. And did the CO2 lead the temperature or did the temperature lead to CO2,
which actually doesn't really matter which led, but nonetheless, it's an issue that people keep raising.
So it's better to show you what the latest data looks like.
So you don't have to get complicated about it because that's what the latest data look like.
This is two different ways of doing it on the bottom. One, it's the Dome C record that you saw before.
That's the temperature and that's the CO2. And they've been I told you,
there's this correction you have to make for the fact that the age of the air is slightly different from the age of the ice.
And the the correction has been made using the nitrogen isotopes in this case have been used as have been made using the nitrogen isotopes in the ice,
which I haven't talked about, which is a very nice piece of physics about the gravitational column that the
air is diffusing through before it becomes solid ice to make the correction.
And you can see, well, then at the top it's completely different.
What's been done there is to use coastal ice or ice cores closer to the coast of Antarctica where the snowfall rate is much higher.
So that problem of synchronising the air and the ice is much smaller.
The difference, the age difference is much smaller. The uncertainty in synchronise them is much smaller.
And in both cases what you see is that the temperature and CO2 within any reasonable uncertainty start to rise in exactly the same moment.
So we don't have to argue about which came first.
Now, they both went together exactly as you would expect if they were mutually amplifying each other.
Chickens and eggs come to mind, but that there isn't a lag.
There isn't the problem. So that's ice cores for glacial cycles.
But as I said before, I just want to remind you and remind myself at this point that I schools are wonderful, but they don't tell you everything.
So you definitely need to add to them. Everything else around the world in order to understand what's happening to climate change during these cycles.
So this is actually a marine record that, again, looks similar, has the same cycles as a terrestrial record,
a tree pollen record from Greece with the same glacial and glacial cycles on it and so on.
Okay, so that's the second example. And then the third example, I just want to focus on this last 130,000 years and that's Antarctica.
And then I want to put Greenland on as well.
So again, what you see, what you see up here is actually what you saw before so that it says deuterium rather than temperature.
And it's still running left to right time, so we haven't switched it around yet as the Holocene left to last close to maximum and then hit Greenland.
So Antarctica looks sort of a bit boring.
It's it's moving quite slowly. It's doing some interesting things, but it's not doing that much.
Greenland, on the other hand, even at this scale, is going crazy throughout the last glacial.
Just look at Greenland now. These are things called dance. Go to rescue events.
That's really done. Scott and Huntzberger off to whom they're named.
And then if we just focus in on the last 40,000 years so that you can see what we're looking at in a bit more detail.
These are what they look like. It can be very cold in Greenland.
And then suddenly the temperature jumps by something of the order, ten degrees.
Some of these are actually 15 degrees, but ten degrees is a typical number for the jump here.
And that looks instantaneous. It's actually about 40 years when you look in detail in the Greenland record,
and then it sort of slowly cools and then jumps back down to the bottom again.
It stays cold for a little while, for a few hundred, maybe a thousand years, and then jumps up again.
And it keeps doing this. We've got there are 23 numbered events, but there are actually more events than that,
depending on how you count them, where Greenland is doing this strange thing.
Now, I should really emphasise this is Greenland doing this.
So it's not that the world is doing this and it's not even that the Northern Hemisphere is doing this.
You certainly do get similar records from the North Atlantic, from European records,
and you get similar periodicity in other records like Cave Records in China that are recording something different,
not not necessarily recording temperature.
So there's something that's spread around the whole hemisphere and there's a warming in the northern hemisphere.
But it's not a global change in that.
Well, it's a global change, but it's not a global it doesn't globally look like there's nothing I'm trying to say.
So these are very interesting things they've been much written about,
including by other people in the audience far more than me, so I shouldn't label them.
One particular characteristic of them is something called the bipolar seesaw you just saw flashing
up something there to warn you that this this plot does go from right to left just to confuse you.
So what's happening? That's a similar record to what you saw before? It's a different Greenland core, actually, but it shows you the same thing.
And what happens is that when Greenland is cold here, Antarctica is warming.
And when Greenland is warm, Antarctica is cooling. And that can actually be written down as an equation, a rather simple equation.
It's just a one once just the integral of the other effectively.
And it works surprisingly well, just as that equation, which I think is cool, but it's known as the bipolar seesaw equation.
It definitely works. Whatever you think the mechanism is, and most of us think we know what the mechanism is.
But whatever you think it is, the equation works.
And what's suggested is and I don't actually know whether because I think I know where this plot came from.
So I don't know whether it replaced. I put it up or sorry, I put it up in an oceanography department where you hate cartoons like this.
So so as you know, I think in this department is a lot of heat travel through the ocean into the North Atlantic.
And the idea behind this, which I'm sure many of you know about already,
is that this thinking up here gets disrupted by fresh water off the ice sheets at some points during the last glacial.
And that disrupts this whole flow of heat to the north, either takes it further south,
weakens it or makes it shallower, or some combination of the three, and then it switches on again later.
And there are all kinds of variants on this. There are various variants on this.
It certainly requires sea ice to be doing something quite strong.
In other words, advancing during the cold periods and retreating during the warm periods to amplify the effect.
Some people have suggested it may involve the growth and shrinkage of ice shelves somewhere.
I'm working with some people in this room actually on an idea about how runoff into the Arctic may affect this process.
But anyway, the basic principle seems to be seems to be the and the significance of these events are that rapid change that occurred into the past.
In the past, as far as we know, only when there are large ice sheets. But nonetheless,
they should make us just a little bit worried that the climate system isn't going to behave in the regular way that we might think it's going to.
So that was all the general stuff. So if you already knew all about ice cold, you can wake up now.
And if you really didn't know anything about it, I'm probably going to get too complicated now.
So half of you can leave now.
But so now I'm going to talk about something I'm working on at the moment, and it's something where we haven't got a final answer.
So this isn't going to come to some brilliant conclusion that, yes, we've solved everything.
The question is, is sea salt in ice cores a proxy for past sea ice extent?
So what we do in paleoclimate is to measure something as a proxy of something else, water isotopes as a proxy of temperature.
And we've got this idea that sea salt may be a proxy for past the ice extent.
And it's work led by my colleague James Levine. So why are we so interested in sea ice?
Well, I hope I don't have to convince anyone here that it's a very critical component
of climate for various reasons of which I've only written down a few here.
If you don't get the sea ice right, you won't get anything else right.
But unfortunately, we've got very short time series available given the sort of oscillations there are in the atmosphere there.
You know, we've got two or three cycles of some of them and less than one cycle of some others.
In satellite data, it's been 40 years.
There are ship borne and airborne observations that have been used to try and extend back the Arctic record to the 19th century.
For the Antarctic record, you can't do anything like that.
There are some whaling records which are highly disputed and as well it's not something where we
can just rely on the models to tell us the answer because the models have some serious deficiency,
even in the simplest things, the current extent and trends.
And I put this up not in order to embarrass the ice modellers, but just because it's the it's the truth.
The black line is the data for Antarctic sea ice extent in February.
So that's the minimum. February's the minimum because it's summer in Antarctica.
The black line is the data from the satellites and these are the IPCC class models, what they say is happening.
So firstly they're all over the place in the extent and secondly they all say
that sea ice extent is decreasing and it hasn't decreased in Antarctica yet.
So they're clearly doing badly now. It's not this is not a criticism of sea ice modellers.
It's probably not the sea ice that's wrong in these models, actually.
It's the fact that there are biases in the in the temperature of the ocean is one is one issue.
And there are other things to do with the winds that are driving the sea ice around.
So it's probably not actually the sea ice that's wrong. But the end result if the sea ice is wrong.
What was the solid, right? Uh, the solid red is I think the MULTI-MODEL mean, I think.
I'm not absolutely certain now. I'm sorry. I'd have to look that up. Yes, it must be the Multi-model mean, because it declines.
I'm pretty sure it's the Multi-model mean, sorry, the models are terrible, but on average they're good.
I don't know. I don't know what that tells us, though.
They're good. They're good in extent. But they do. They do have the decline, which hasn't been seen.
So. So there would be a huge benefit to getting a sea ice proxy, a really good sized or a really good set of sea ice proxies.
Apart from anything else, when you're doing paleoclimate modelling, trying to look at the climate of the last glacial,
it would be really good to be able to check that we hadn't sea ice roughly, right?
Because if we had the sea ice from, we'll certainly have everything else wrong on this issue.
With the sea ice across the dance go to rescue events is one example of that.
So the obvious way to do this is for marine sediments and they do there are sea ice proxies in marine sediments and they're very, very nice.
They give time sequences of sea ice presence or absence at particular sites, mainly through biomarkers or biological proxies.
And you can combine the sites to give a spatial picture. I'll show you that in a minute, but it would be very nice.
That's very painstaking. You have to measure at every site and then put it together to get a spatial picture.
And it'd be very nice to complement that with a regional picture from an ice core proxy.
And there are two candidates of which I'm going to talk about.
One, but just to give you an idea what I mean by that, so this is the best it's not quite the best now because it's is a few years out of date.
But this is the the compilation most people show for sea ice at the last glacial maximum in and around Antarctica.
And what's been done here is to draw lines on with the last glacial maximum winter stretch of maximum winter.
That's the WAC. You can see where the data points are.
So you can see why there are question marks there where there's no data and then the summer weather, just a few odd bits of line here and there.
What we'd like to complement that with, I mean, people can go and get more cores and that would be great.
But what we'd like to complement that with is an ice core that would tell you not how much ice there is as an individual place,
but how far did it extend in a sector extending away from the continent?
Away from my site. So why sea salt?
Well, sea salt over most of the world comes from the ocean spray and bubble bursting.
And so if if that was the case, then what you would imagine. I'll give you one of the punch lines first.
What you would imagine would happen is that when you had more sea ice, the ocean's getting further away from you.
So you would think that the amount of sea sea salt reaching your core decreased.
But actually, every time we look, if we look at winter against summer or at the last glacial maximum against the present,
what you find is the other way around, that when there's more sea ice,
when we know there was more sea ice, we actually get more sea salt in the ice cores.
And models can't explain that at the moment. So what we think is happening and we've done a lot of work to make us think that,
is that a number of observations suggest that there's a source from the sea ice itself, from the surface of the sea ice itself, a source of sea salt.
And there are various things, too, to two of them are what I just said, this counterintuitive idea that when you've got more sea ice,
you actually get more sea ice, despite the fact that what you thought was the source is now further away from you, a lot further away from you.
The second one is, is a chemical a very distinct chemical signature, which is what we call negative non sea salt sulphate.
And I'll show you what that is. What it is is that if you plot the amount of sulphate in snow or aerosol against the amount of sodium,
what should happen is that it should have a ratio of the point 25 to in weight from the seawater itself and then
it should have a bit more that comes from other sources like volcanoes and biogenic emissions from the ocean.
And so if you calculate this quantity here, it should always be positive because if there was only seawater and nothing else, it would be zero.
What you actually find is that as you get more and more sodium, especially in the winter, it becomes more and more negative.
This quantity, which means some of the sulphate that was in the seawater, has disappeared somewhere.
And you find that everywhere you measure it around and around coastal Antarctica.
Actually, if you go into central Antarctica it's harder to spot it because there's a very big background of other things,
but you can spot it by doing size segregated aerosol sampling.
So what we what is happening,
what we're sure is happening is that the sulphate is being depleted because mirabilis
it's being precipitated out of rubber like sodium sulphate precipitates at minus eight.
So it's not unreasonable, it's being precipitated out.
And of course the amount of sulphate when it freezes out precipitates out is a much higher proportion of the sulphate in the water than sodium is.
So it gives you the impression of a depletion and the only mechanism that anyone can think of for how that happens,
where you can freeze out metabolite and then separate the sea salt aerosol from the light left
behind somewhere else is by that happening on the sea ice surface before the uplift of the aerosols.
So the idea is ice freezes out on the sea ice surface, but then the meribel precipitates out as well.
So calcium carbonate actually precipitates out first, but then meribel like and then walks uplifted as aerosol one way or another is missing.
The meribel item therefore is depleted.
And so we think that the negative non sea salt sulphate is a really clear signature of a source coming from the sea ice.
We do know we've been to sea ice surfaces and measured it and it is depleted on the sea ice surface because of this metabolite precipitation.
So what's the source? I'll come to that in a minute. I actually I've only put this slide in because it's such a nice picture.
I shouldn't have covered it up with the writing. But so this actually came from Shackleton's expedition.
And although his ship, he thought was about to sink at this point, he still took the time to take pictures of frost flowers for people like me to use.
These are frost flowers in Antarctica on Antarctic sea ice, which are one of the possible sources of this depleted sea salt.
We're also interested in this depletion because of the impact it has on a process called called the bromine explosion,
which leads to depletion of the ozone in the boundary layer.
But I'm not going to talk about that today. So the idea was that if the sea ice is actually the source of much of the sea salt going into Antarctica,
then maybe the sea salt is actually a proxy for sea ice, but in the opposite sense to the one we would first have thought of.
So when it's cold, there's more sea salt because there's more sea ice.
So now let's explore it. What we're actually doing here in about the next 10 minutes is to do what we
normally do over five or ten years in a proxy development where we have an idea.
We think it's brilliant because it fits everything that we're thinking about.
We then start to think what the mechanisms are and realise it's pretty damn complicated and it probably shouldn't work.
And finally, we come out the other end with something that partly works,
and we're sort of somewhere in between stages two and three at the moment, I would say.
So what is the sauce on the sea ice? Well, it could be. There are four possible sources.
I think the first one is the brine layer on the sea ice surface, but certainly the brine layer on the sea ice surface.
And it certainly is depleted in metabolites, but it's extremely unlikely that just blowing wind across the ice can lift up lots of lots of brine,
lots of this very thin layer of brine into the atmosphere in order to make sea salt aerosol.
So we're going to grade that out because PowerPoint is so clever that it can do that.
Then there are leads within the sea ice phone, so the little bits of open water within the sea ice zone,
but they won't work because they wouldn't be fractionated. They are just seawater.
They've gone. Then there are frost stars, which are what you saw on the photo they form on new thin sea ice.
They're actually ice that grows on the surface and then whips up salt.
I've written a lot of papers in my time about Frost Flower suggesting that they are really important.
I'm not quite sure that they are. I'm not sure they're ubiquitous enough.
And we've also written has done some work which shows that they're not very easy to mobilise with wind.
I'm not sure that works right because I'm not because Antarctic frost that we haven't done that Antarctic frost we've done it on Hudson Bay.
Frost clouds and laboratory frost clouds. And I'm not sure Antarctic frost flowers are the same in terms of how strong they are.
But I don't think they're ubiquitous enough.
So the fourth proposal is what is causing this salt to become aerosol less fractionated salts become aerosol is actually salty snow on the sea ice.
So on the sea ice, the snowpack and that gets mobilised as blowing snow.
So these things that definitely happens there is definitely snow on sea ice and it definitely gets mobilised of blowing snow.
And then in the atmosphere that would sublime, leaving not naked but nearly naked aerosol particles.
And this was suggested by Shin Young in 2008. So the idea now is that this snow on sea ice is salty.
It's salty partly because of flooding, but covers that puts a lot of ocean water onto the top of the sea ice surface.
But within the snow pack, which then gets whipped up, it becomes blowing snow, which the blind skip, see.
So terrorists will be transported to ice cold sites. So what Shenyang did was to make a model which had various parameters in it,
and the most important ones are probably the salinity of the snow that can be blown, the threshold wind speed that mobilises it into blowing snow.
And then the sites break from which which determines the sublimation rate.
Oh, it's one of the things that determines the sublimation rate. And what we've done is two approaches.
One is to make measurements and the other one is to do some modelling using P Tomcat, which is a chemical transport model.
I think you can probably already see from the parameters that it's going to be tricky to make this into a quantitative sea ice proxy.
So I already said this is going to be tricky. Now it seems to work.
But why it works. That's what exactly what I was just saying. Why it why it works now is a bit of a mystery to me.
But we'll explore it a bit anyway. So here's the cruise where we took some data.
My colleague Marcus Frey went on this cruise. It was in the sea ice from here out to here somewhere.
And we had lots of instruments on the ship. We had instruments for measuring the snow particles or the amount of blowing snow,
the size distribution of blowing snow instruments for measuring the aerosol forms.
Afterwards, we had instruments on the ship.
So on the crow's nest of the ship. And then we also had instruments on the ice itself.
And you might say, why do you take a photo when it was dark? It was always dark on this cruise.
So it was winter. There are some interferences when you're doing this kind of field work.
I'm assured that the penguins didn't get anywhere near the samples, but, uh.
So we measured various things under control.
You can so we measured the salinity of the snow that was on the in the snowpack and we
measured the salinity of the snow that was in the air and we measured the snow particles.
So this is a blowing snow event here, starting here, when the wind speed reached a certain point, the wind speed there is in green.
When it reached a certain point, suddenly the snow starts blowing.
And then we measured the aerosol.
So one thing you can already see on here, actually, is that at least in this blowing snow event and it was generally true,
although we didn't have a huge number of blowing snow events, I don't want to overemphasise this.
Sometime after the blowing snow started, we started to see lots of aerosols.
So there is actually little qualitative evidence that the blowing snow does lead to aerosol formation,
which was one of the first order questions for the grant.
And we measured various spectra of the sizes of the blowing snow and the size of the aerosols, which are what are needed to parameterised this model.
So for him, any conclusion from the field or actually the salinities were lower than we expected, which is sort of a problem.
But we think they may have been atypical. So we're going on another cruise in the Arctic soon to try and get some more data.
The threshold wind speed was similar to what we used before, so we got that right and we did see increased aerosol concentrations during blowing snow.
I think I'm running out of time for when you're expecting me to finish, actually, but I think in the next 5 minutes probably.
Okay. So just very quickly, the Tomcat modelling.
What we did was to include an open ocean source similar to what other authors who've
used both higher order and lower order models have used on this blowing snow source.
And it gives good agreement with air.
Once you've changed it a little bit, it gives good agreement with aerosol concentrations at various sites around the world,
including in Antarctica for the present day.
What we found, and this may sound again negative, is that what controls the amounts of sea salt that reached central Antarctica.
This is how much sea salt reach don't see in this model from year to year was not actually the amount of sea ice as we had hoped.
That was the meteorology, the climatology. And actually it wasn't even just the meteorology.
It was very specific. It was the got to make sure we get this right.
It was the blue there. It was the transporting meteorology.
So it wasn't the meteorology that was causing the blowing snow to be lifted up.
It was the meteorology that transported it in. And what we think is going on.
What we did, we discovered that by running the model with a constant year of meteorology unchanged and a different year of sea ice each year,
or a constant year of sea ice and a different meteorology each year.
What we think is going on is that there are only a very small number of events that transport sea salt to Antarctica each year,
probably of the order, half a dozen or so looking into that at the moment.
And of course, that can vary from year to year. If it's for this year and next year, you'll get a big difference.
But it won't be anything to do with sea ice extent. So this property is not going to work from year to year,
but nonetheless that should average out over a period that I think is typically of the order of decade.
And then maybe the sea ice comes in. And so cutting to the chase, what we did was going to jump that.
What we did was to run the model for present day sea ice and last go to maximum sea ice,
but keeping the same climate which you'll instantly see is mad.
But nonetheless, that's what we did.
And it gives you a 50% increase in the amount of sea salt reaching Antarctica, which is actually exactly what you see.
You see a 50% increase, the amount of sea salt reaching don't see.
So the model says that the sea ice increase could due to the sea sea due to the sea ice or the sea salt increase caused by the sea ice.
Change between the Holocene and the last glacial would give us the result we see for the amount of sea salt in Antarctica.
What we haven't done is the converse experiment of what would the changing climate do?
And that's a very difficult experiment to do, because we know that models don't get the last glacial maximum climate in the Southern Ocean.
Right. We know that people think that the winds moved, but the models say they didn't, for example.
So it's not obvious what climate we should use in order to test this.
But nonetheless, we're trying to do some work to do that,
and that will be a future piece of work if you're willing to suspend disbelief and think that qualitatively
this probably is a sea ice property and I really like the way I presented it makes me think probably it isn't.
But nonetheless, if you did, then what this would be saying is that there was less sea ice, there was more sea ice in the last glacial.
I don't think we're very surprised at that. There was less sea ice in the early Holocene and there was less sea ice,
quite a lot less sea ice than we could quantify it with this model in the last interglacial.
And that would be quite important in understanding why the last interglacial behaved as it did in Antarctica,
and especially if we could get ice cores from all around Antarctica to get each sector separately, which is what would be the ultimate aim.
So I'm just exposing to you the workings of how we construct a proxy or possibly deconstructive proxy after several years,
because I think that's the most interesting physics actually.
I'm going to jump both and give the conclusions that we've now got of the second half of the talk.
That is, we've now got a modelling tool with which we can explore the controls on sea salt reaching ice cold sites.
We've we're improving the parameter ization because of field experiments.
It's this interface between field experiments and modelling that I think is the the only way to deal with this kind of property.
We know that it doesn't work for Interannual sea ice because that's controlled by the meteorology and.
We can explain what happened in the last case to maximum, but we don't yet.
We can't yet put our hand on our heart and say this is the unique explanation for it.
We did some modelling that I jumped over which shows that we can define what the region of influence is for each high school site.
It's probably safe to use this under a climate similar for day, which is why emphasise the interglacial.
I think the changing climate between different interglacial is not likely to be so large as to have a strong influence on our results.
So the only other thing I wanted to say because I know conclusions might you think thank God is about to start.
But actually I've got one more. One more thing I'd like to say, which is in the future, we would very much like to go back further than 800,000 years.
And the reason for that is because if you just look at marine sediments climate for the last one and a half million years,
then there's a very obvious change, which is that instead of having these 100,000 year ish cycles here, we had 40,000 year cycles back here.
And these correspond to different components of the Earth's of the astronomical cycles that control the Earth's orbit and its tilt.
And if we've got an ice core that covered this,
we think we could really make progress in understanding what was going on across this transition from these two different types of cycles.
Now, the problem is we don't know where to drill the ice core. We're sure that it's there, but we don't know where.
And so we've got to do a lot of geophysics first to find out.
And that's a really big international effort involving people on four different continents.
And with that, I will stop.