(intense music) - In this lecture, we're talking about, "The Ocean Physics behind Net Zero." And I'd like to start off with a question. Which probably not something
you've thought about very much, but why is the deep ocean cold? You probably think you know
the answer to this question. Well, it must be cold because
the sun can't get down there. All the heat from the sun, is absorbed in the top few millimeters, in fact of the ocean, and the mixing by the
wind only takes it down, top few tens of meters. But if you think about it, that only explains why the
deep ocean isn't warming up. But why is it cold in the first place? And it really is very cold. If we look at a section
through the Atlantic, so this is a sort of south to
north section in the Atlantic, you can see that over most of the depth, so it's about, on average
about four kilometers deep. So this is, remember this
has been exaggerated, by about a factor of a
thousand in the vertical. If I showed you this to scale, it would just be a straight
line on the screen. I tried to do that but it
was rather disappointing. So this scale is 12,000 kilometers. So if we look in the tropical Atlantic, around here over a few hundred meters, you go from surface temperatures, getting on for 30 degrees,
bath temperature almost, you go a few hundred meters down and you reach temperatures
that you'd have to travel, thousands and thousands of
kilometers north to find them. So we have this massive contrast, in the rate at which temperatures
change in the vertical and how they change in the horizontal. And it's even more extreme in the Pacific. Most of the Pacific is almost
at freezing temperature. You see, this is all this purple color, that's down near zero degrees. And you know, in this some
parts of the tropical Pacific, you actually if you were
in scuba diving depth, so in the sort of depth you can get to, with just a sort of regular air tank, you could actually die of hypothermia, before you're not this is recommended, before you die of the bends, so to speak. Because the temperature
contrasts are so great, in the vertical, so be careful, when you're scuba diving off Hawaii. So why is this? By the way, I'm just going
back to this profile, you know, you might think, well it's cold because
it can't be heated up. Well, that's a sort of very, you know, if you don't mind me saying, so that's a bit of an
Aristotelian way of thinking. Aristotle was convinced, that everything had a
sort of natural state. Aristotle was a Greek
philosopher, scientist. Much of science, people call philosophy, lots of, you know, footnotes to Plato. Science is largely tidying
up Aristotle's mistakes. So one of Aristotle's big mistakes was, he thought everything had a natural state. So the natural state of
a cart was stationary or the natural state of
any object was stationary and you had to keep pushing
it to keep it going. Which meant, of course, Aristotle could never understand
how the planets moved, because he couldn't work
out what pushed them. Of course, Galileo
quite a few years later, worked out that actually the
natural state of something, was just to keep going
in whatever it was doing. And the same, you know, unless there's something to slow it down. And the same goes for temperature. The natural state of water is not cold. If it's insulated, the
natural state of water, is just to stay whatever
temperature it's at. So why is it at this
temperature in the first place? And I mean, by the way, water
conducts heat as you know, I mean, if you've ever
got soaked from the rain, you'll feel that. It's actually one of the
best conductors of heat, that's not a metal, that's out there. And so if you hold a nail in a gas flame, again, not recommended, but you'll notice the
temperature equilibrates, you know, evens up along
the nail quite quickly. Within a a minute or two, temperature will be
uniform along the nail. Water, I said it was one of
the best conductors of heat, but it's not nearly as good as any metal. So if you take a glass of water, just say 10 centimeters deep or better still a styrofoam cup, so you don't need to worry
about heat going down, through the walls of the container. It actually would take several hours, for you to warm it up by
conduction from the surface. So that's the contrast between, you know, a metal object like a nail and liquid water, even though
water does conduct heat. But if you, by the way, the depth which temperatures
penetrate by conduction, the speed, the length of time it takes, for you know, a warming at the surface, to penetrate down by conduction, goes with the square on the depth. And that's sort of
again, okay, most of you, before you sort of start,
he's moving into maths, I'm going to tune out at this point, that should be fairly
obvious to you anyway, because what's driving heat along the nail or down through the glass
of water is the gradient, is how fast the temperature is changing, either in the vertical
in the glass of water or along the nail. And the further you go, the less the average temperature
contrast per unit length, the less rapidly temperatures change. So the slower it moves heat along. So you can just, you can do the sum in
your head if you want. If I tell you that it takes several hours, for a glass of water in a 10 centimeters or styrofoam cup of water, to equilibrate temperature, it would take several
thousand years for conduction, to get down through, you know, even just several hundred meters of water. So then you're like, oh wait a minute. Several thousand years ago? Well 20,000 years ago, the
earth was in an Ice Age. Maybe this is because of the Ice Age. Okay, it's not, okay? 'Cause of course, if you go
that route, you might think, maybe sea level rises, all the recovery from the last Ice Age. And then I remember in the last lecture, I commented on the number of people, who write like to write me angry emails, telling me that they've
discovered that climate change, is entirely natural. So no, that's not the reason
that deep oceans are cold. Because even back in the Ice Age, surface temperatures in the Tropics, are only a few degrees
colder than they are now. Certainly not that kind
of temperature contrast. So and you know, the ocean's been around, for billions of years. I'm hoping by now you should
be a little bit puzzled, why is the deep ocean so cold? 'Cause even just conduction
would warm it up. And the answer is a very
sort of important one, a very important one
for our net zero story. I hope you've all, you know, I'm sure you're all aware, that temperature is not the only effect, the factor that affects
the density of seawater. Any idea where this is? The sort of hint is, I
guess, also in the newspaper. That's the Dead Sea. This is somebody doing the classic thing, of reading a newspaper in the Dead Sea. If you have enough salt
in water, it's very dense. And, you know, as another much, much better scientist than Aristotle, as Archimedes point out, that means that you can
float in it at high enough, that you can actually read a newspaper. So the salt content of
water increases its density. So if you want to increase
the density of water, so these lines show you
lines, contours of density, it gets more dense in this direction. You can either cool the water
or you can make it saltier. So both of these things
make sea water denser. So cooling, for example, you know, the wind blowing over
the surface of the ocean, cooling it down, that
helps make water denser. But evaporation and the formation of ice, that also makes water saltier. When ice forms, the salt is left behind. So if you're ever stuck on an ice floe, lots of survival tips in this lecture, eat the ice, don't drink the sea water. Yep? Because it's much fresher. But what happens is, as seawater freezes, the water that's left behind, it becomes saltier and therefore denser. So there you are. Here's water getting, you
know, as you increase, reduce temperature and
increase the salt content, the water gets denser. And so the places where water can escape, to the ocean depths, are places where it's
very dense at the surface, to start with. And those are places where it's very cold and where the formation of ice and the action of wind
blowing over the ocean, act to make water particularly dense. So places like this, that's
ice floe in the background, to give you a hint about where you are. You got the wind, a storm
in the North Atlantic, causing lots of
evaporation, lots of spray. The water that's left
behind is getting saltier. Ice is forming, that's also
making the water saltier. Eventually, that surface
water becomes denser, than the water beneath it and slides away down
into the ocean depths. Now crucially, once the
water's left the surface, there's nothing down
there to warm it up again. There's no equivalent to rain formation or sun in the atmosphere
to warm up different, it's on its own. There's nothing happening down there. So, once it's gone, ocean water remembers the temperature and the salt content it had, when it was last at the surface, until it gets back to the surface again, which may be many, many hundreds of years. So water forms in these isolated regions. So here they are. There's a few regions. The Labrador Sea, the
extreme North Atlantic and this region near Antarctica, where the water's cold enough, to escape down into the ocean depths. So all of the water that you see, in 90% of the world's
oceans has got there, through the so-called deep
water formation regions. So it really is arctic temperatures, below the tropical Pacific, just a few, you know,
50 to 100 meters down. You find arctic temperatures
because it's arctic water. It's come from the Arctic. Because once it's down there, it can't mix with the water above it, because it's so cold and dense and the water above it is
being heated by the sun and made, you know, made
less dense as a result and sits on top. So here's an amazing
graphic courtesy of NASA, which shows you this circulation. Do you see the water flowing
up through the North Atlantic and reaching this black region, that color indicates the density. And in this region where surface
density is at its highest, the water drops away down
into the ocean depths and then it can't mix with
the water above it anymore, because it's so dense,
it's stuck down there. And it travels southwards on average, back through all these sea mountains. Remember this is massively
exaggerated in the verticals, so it's not nearly that spiky in reality. And you can see that this downward, under surface deep, deep flow, coming back down through
past the tropical Atlantic, no mixing with the surface, because you've got this density contrast, the lighter, less dense water above, more dense water below. And goes all the way around the world, back out into the Pacific. And then eventually,
somewhere else in the oceans, it resurfaces again. It's a fantastic graphic. The only thing I don't like about it, is that it sort of implies, you see here the water sort
of popping up at one place. That's actually very misleading. That's not the way the ocean works. There's these few places
where deep water is formed, but there's no such thing, as a shallow water formation region. It's actually much more
like an aquarium pump, but working in reverse. So if you've got a tropical aquarium, you know you've got to have
this sort of pump thing, that sort of circulates the water, pulls water from the bottom
and pushes it out at the top. And so over the rest of the aquarium, you've got a very, very
gradual down-welling of water. And then you've got this one little pipe, pumping it back up to the surface. Well the oceans work in exactly
the same way in reverse. You've got these few points, where you've effectively got a pipe, from the surface down to the deep ocean. And then over the rest of the ocean, it's slowly up-welling, yep? That's the only criticism
I've got of this, otherwise fantastic animation from NASA. Anyway, I think that's all
that's really interesting, about just the way the ocean works. But what has it got to do with net zero? Well, it helps us understand, where energy goes in the climate system. Now remember from the previous lecture and we had a sort of 10 minute recap, before this lecture started, for those who weren't there, we emphasized that our climate system, is fundamentally governed
by this flow of energy, in from the sun and the flow
of energy back out into space. A little bit like a bathtub. If you crank up the tap and
increase the flow of energy in, if this is a sort of the
plug hole of the bath, then the heat content of the
climate system increases, until the pressure driving energy out, balances the pressure
of the energy coming in. And we sort of demonstrated
that with that, with our gadget, which
we'll come to in a minute. So where we are now is
we're adding energy, to the climate system, because of the increase
in greenhouse gases, over the past century and a half or so. At about two and a half
watts per square meter, added up over the whole earth surface, which adds up to a lot of energy. And because the world's already warmed up, about one and three quarters
is going back out into space. And the difference
between those two numbers, is three quarters. The maths in this lecture is
all really straightforward. And so the world is warming
up by that much energy, you know, that's per square meter, of the earth's surface we're seeing. But that doesn't tell you itself, how fast the temperatures are going up. I'm telling you how
fast the energy content, of the climate system is going up. But what does that mean
for surface warming? And to understand that,
you need to understand, where the energy's going, how it's distributed
around the climate system. So this is a graph of
where energy has gone, in the climate system since 1960s. 1960 here, to the present of 2018 or so. And this just shows you how heat energy, has accumulated in the climate system. And one of the sort of
most impressive things, about this figure is this
fine red line you see here, which shows, when we use satellites, to work out how much energy, is being taken up by the climate system, as a whole every year and then compare that to
where measuring the oceans and the land and the ice
caps, where energy's gone, we get a very good match. You see how this red line
matches the blue line very well. So we know where the energy is
going in the climate system. Which, you know, you think,
well how hard was that? Really hard, okay? Lots of people had to do
a lot of work to do that. But, you know, it's done. And it's lots of things like this. Because, of course, what we're doing here, in order to work out
what the energy content, this is the pale blue here, is the top 300 meters of the oceans. The slightly darker blue
is the top 700 meters. And then darker still, sorry, this is the top 300
meters above the green line. This is the top 700 and so on. So you can see how
these different regions, are giving you different, you know, how the energy is distributed
through the ocean system. And as a result of that, you see that, you know, in order to just
be able to observe that, we've had to instrument
up the earth oceans, with vast numbers of drifting buoys, to work out how fast
temperatures are going up, in different regions. But this has been done and we can now, there's a lot of uncertainty
sort of pre-1980s, because we didn't have these
buoy networks back then, but we've got it pretty well wired up now, so we know where the energy's going. And this is what's accumulating
in the climate system. The unit here is something
called a zettajoule and you've probably heard
of a joule of energy. That's the energy it takes to warm up, a liter of water by one degree, I think I've got that right or kilogram of water by one degree. So a zettajoule is one
followed by 21 zeros joules. So it's a billion trillion joules. So quite a substantial number. If you want to compare that to, so that's the 0.75 watts meter square. If you want to talk, compare that to number you
might be more familiar with, that's the total amount
of energy we generate, mostly from burning fossil fuels, but from hydropower and
everything else in the world, over the same period. So the actual amount of energy we generate and by the way, 75% of
that is wasted as well. So the actual energy we use, will be quite a small
fraction of that, okay? So that's whole of humanity, compared to the energy
that's accumulating, in the climate system as a
result of past emissions, of greenhouse gases. So people often ask me, you know, does it matter that a power station, is generating heat itself or that the steam coming
out of the cooling towers, of a power station is quite hot. It is hot, it's steam. Is that what's causing global warming? No. Because that energy is tiny, compared to the energy
that's being trapped, by those greenhouse gases, that that power station
might have produced. That's the big impact on
energy in our climate system, not the energy we actually
generate from burning stuff Okay, by the way, notice here there are some
extra stripes down here, that the little beige one is
energy accumulating on land and energy accumulating in ice, mostly by melting ice
is the sort of gray one. And finally the purple one
is the tiny amount of energy, that's accumulating in the atmosphere. And the reason that's quite
small compared to the ocean, is that the atmosphere has
a very low heat capacity, compared to the ocean. It's much easier to warm, it doesn't take nearly as much energy, to warm air up as it
takes to warm water up. And the land one is quite small, because the land only
the soil really warms up, the sort of top few, you're not, whereas in the oceans you're
warming up kilometers of water. So why is the energy being
distributed the way it is, just to point at you? Most of it is in the top 700 meters, more than half is confined to
that top 700 meters of water. A lot of it's in the top
300 meters and the rest, most of the rest is going
down into the ocean depths. So why is this? You can sort of see what's happening here. These are temperature
trends in the global oceans. And you can see here in
the Pacific in particular, all the warmings happening at the surface, really nothing happening further down. In the Atlantic it is penetrating down, but only in a few regions
that region of North Atlantic, that region of the southern ocean, okay? Those same regions that we've said, we've seen water penetrating
down into the ocean depths, because those are the regions where, the surface is connected to the deep ocean and we get this siphoning
off of surface waters. And as the surface warms, you get a siphoning off of surface heat. So that's where the
energy penetrates down, into the ocean depths. So this is what takes us
back to our climate model, our Gresham climate model,
which I will wake up I hope. And just to remind you, this so fluid flowing in, represents energy coming in from the sun. I explained in the last lecture, that if I was to make this to scale, because the energy flowing in
from the sun is so enormous, compared to the changes
due to greenhouse gases, we'd need to have it sort
of a hundred meters high and Gresham couldn't quite afford that. So this is scale down the natural flow, flowing in from the sun and flowing back out into space, okay? Space is downwards, okay? Again Gresham couldn't afford, a sort of anti-gravity arrangement but so, but you can see the basic idea, that we have a balance in
energy flowing in from the sun and energy flowing back out into space. If we crank it up to, if we
increase the speed of the pump, we see the level of
water in this tube rises, pushing fluid out into space
energy back out into space, that corresponds to temperature
the surface temperature and it rapidly reaches
a new equilibrium level. And if I increase the speed
by the same amount again, it goes up by about the same
amount again, all right? So far, so good, so far so simple and relatively obvious this would be, the way the climate would behave, if the whole world were
kind of like the Pacific and you just had a sort
of surface of water, completely isolated from
the water underneath. But it isn't and this is why we need to think about, the way the oceans behave, the way the deep oceans behave, in order to understand
how the climate responds. So as I promised in the last lecture, this is the bit where we uncork the ocean. I'll switch this off before I do that. (machine whirs) And now I'm going to drain this quickly. Sadly there isn't a tap at the
bottom of the global ocean, to cool it down again if only, but so what this pipe
represents is the heat content, the sort of fluid in this pipe, is the heat content of the deep oceans and therefore that level
of water in the pipe, is proportional to the sort of, average deep ocean temperature, okay? And this pipe is fatter than this one, because the oceans have
a bigger heat capacity, than the atmosphere. So it takes more fluid, more energy to push up the
temperatures of the oceans, than it takes to push up the temperature, than it takes to push up the
temperature of the atmosphere. Just to start with, to
make things really simple, let's imagine the ocean, that this pipe is
absolutely enormous, okay? So just to sort of imagine first, the oceans are infinitely big, okay? So I just opened this tap,
it's sort of tricking out here. So now the level in there can't change, 'cause I've just opened
the tap at the bottom. If I now do what I did before, so I'll switch it back on again, okay? And wait till it's back
in equilibrium, okay? So it's now back in equilibrium, okay? Now I'm going to increase my
greenhouse gas concentrations, in the atmosphere, increase the rate of turning of the pump, by the same amount that I did
the first time before, okay? By the same speed increase of the pump. Does anybody want to suggest, what is now going to happen
this time to the level of water, to the level of fluid
in the atmosphere pipe, compared to what happened the last time? Remember the last time
it went up to about here, the first time I did it. What do you think is
going to happen this time? Does it, water is
flowing out through here, into the ocean and through
here, back out into space. So there's two out outlets now, whereas before there was one. So just to sort of get your thinking, what do you think's going to happen? So before when I increase the pump speed, it came up to about here, what do you think's going
to happen this time? Yeah, it'll come up a bit lower. Let's do the experiment. (machine whirs) Okay? And it's warmed up but not as much, okay? About half what it would do
if it actually was allowed to, if it was actually just
the tropical Pacific, just isolated from the oceans. And if I do it again, I can see that, again if I increase it
to that faster rate, that I had before again it goes up again. But again, by about
half what it did before. So in the early stages of global warming, where energy's being
carried off into the ocean, as well as energy going
back out into space, you see much less of the
warming you eventually see, when the system comes
back into equilibrium. And this is really important
because it means that, if we were to keep our
concentrations of greenhouse gases, in the atmosphere constant
at today's level forever, we wouldn't just see the
warming we've already seen, it would keep warming. And this is why it's really important, to understand this process, which is what we're focusing
on in these lectures. So there's in equations, that's
the earth's climate system, exactly the same principle. That's the extra energy flowing in, due to the increasing greenhouse gases. And then there's this important parameter, which is the efficiency, we literally earth gets
rid of energy to space, that's the plastic tube
with an open outlet pipe, that was the first one we had. And here's the new one, where we've got two outlet pipes, okay? And you'll see that because
there's two of these Ks, you need less H for the same F, okay? Because this is a bigger number now. And we can sort of do
exactly the same thing, with the climate system, because we've got this extra term, energy being transported
down into the ocean. We see less heating for the
same amount of increase, in greenhouse gas concentrations. It's also really important to use this. You can understand it's a model, not just of the total amount
of increase in height, in depth of the ocean, but it's a good model of
how it responds to changes, in the incoming flow. So if I change the
incoming flow down again, (machine whirs) that means delta F the
change in F was negative. So my delta H is negative. It falls and it falls quickly, okay? I bump it up again. So it's a good model of
how a change in flow, affects a change in heights. So we can go further than this. Same goes for temperature. It's a great model of how a
change in greenhouse gases, on short time scales causes
a change in temperature. But now we've got to do something, make it a little bit more interesting, because the oceans are not infinite. We know that the energy's
got to go somewhere, it's going down into the deep oceans and causing them to warm up. And that's represented by the increase, in the level of the fluid over here. But let's start off here. Now we are just taking this
back down to its sort of quotes, pre-industrial level, okay? So here we are, it's back
in equilibrium, okay? It's down at the really
this is the natural flow, of energy into the climate system. And now do you see the fluid
is flowing back out into space? It can flow between these
two pipes, but right now, because the level's about
the same between them, it's not flowing, yep? Because it's balancing on both sides. So now this is the hard part. What's going to happen now, if I crank up the speed of the pump? (machine whirs) Someone go for it. - [Female Speaker] Does it rise up? - It rises initially
and it rises initially, just as much as it did before. And then so with the infinite ocean, yep? So it'll rise, I will do it in a minute, but I want you to think
about it before we do it. 'Cause afterwards, one of the things in science is, everything's always leading
obvious when you've done it. Yeah, you never find a
scientist who'll admit, that they never didn't
understand something, at some point in the past, because it's always like, oh yeah, yeah, it's always like net zero, everything's net zero is totally obvious. In fact, 15 years ago it wasn't obvious. I have to keep reminding myself this. Anyway, so do think about what'll happen. It'll jump up to here, but then what's going to happen
to the fluid in this pipe? It's going to start rising. It's going to rise slower
because it's a fatter pipe. And what's then going to happen
to the fluid in this pipe? (machine whirs) Should we just do it and see what happens? Okay, I'll go for the
slightly bigger acceleration. So you can see it immediately pushes up. This is the faster speed
immediately it goes up and then it seems to stop rising. But what's happening now, watch it carefully compared
to the ticks behind it. (machine whirs) So this is constant concentrations, of greenhouse gases in the atmosphere. Remember the wording
of the Rio Convention. We're going to stabilize
atmospheric concentrations, of greenhouse gases at a level, that will prevent dangerous
anthropogenic interference, in the climate system. Those are the words of the
Rio Convention in 1992. So the level of greenhouse
gases in the atmosphere, is constant. The pump is set at a constant rate. What's happening to this
surface temperature? Somebody's close, it's still rising, yeah? It's rising very slowly and
it's very hard to predict, when that rise is going to stop, because in order to know that, you need to know
everything that's going on, in the deep oceans. And as I emphasized, it's quite hard to see everything, that's going on in the deep oceans. So let's take this back down to zero again and cool off our deep ocean. So we can do some more
experiments with it. Here we are back down to where we started. So that's our all right? So that's our new system. Now with the ocean in, let's think about in a
little bit more detail, of what actually happens, when we crank up the speed of the pump. So I'll switch it up to by a factor of, I'll increase the pump
speed by sort of one unit. That was the first time we did. And this is what happens. You see this goes up by that much and then this is going
up at a certain speed. And so this is, it's pushing this up at a
certain slow speed as well. If I do it again, it goes jumps up by the same amount again. And the speed with which this is going up, also goes up by the same amount again. Now you'll probably be relieved to hear, we're going to go back to some maths. The whole point of this is
to make you feel relieved, to see some more maths. Here we are. So the change in depth that
we see in a short time, in a short time interval, a few seconds in that system
is proportional to two things. One thing that we know all about, which is the change in fluid flow, that would be the same even
if the deep ocean wasn't there or even if the deep ocean
was infinitely large, okay? So that's that first term. But there's another term
which is a rate of increase, that depends on how fast
the pump is turning. The pump's turning
quickly, it rises quickly. If the pump's turning slowly, it doesn't, it rises at a slower rate. So you've got one term, which depends on the
change in fluid input flow, over that time interval. And another term that depends
on the average fluid flow, over that time interval. And now what's really interesting is, you can reorganize this, by the way this is
exactly the same equation, just I've replaced this,
there's two constants here, but I can replace them with one constant, that represents the sort of
responsiveness to the flow. That's this kappa thing here. And then this other one which represents, the rate at which it
adjusts to a constant flow. That was the gradual rate at
which it carried on adjusting, after I made the flow constant. And this is where it gets interesting. We can do exactly the same thing
for the global temperature. The change in global temperature, over a multi-decade time interval, is proportional to one term, which is just the change in
greenhouse gas concentrations, over that time interval. And another term that's the average, greenhouse gas concentrations
over that time interval, multiplied by the length
of the time interval. So you think, well, okay, that
seemed like a lot of work. Where does this get us? Well what would it take? So first of all, it tells you that, I've already mentioned this, that article two of the Rio Convention, which was stabilizing
greenhouse gas concentrations, at a level is a sort of
problematic target to have, because the world will
just continue to warm, for a very long time, many
centuries if that's all we do. But the other really
interesting thing it tells you, is actually, well wait a
minute, before we get onto that, I could just ask you, how well this very simple equation, so I've got this very
simple equation here. Here you are, it may not look that simple, but compared to a climate
model, this is pretty simple. So it's just got two terms in it. The change in the energy flow
at the top of the atmosphere, the energy imbalance at
the top of the atmosphere and the average energy imbalance, at the top of the atmosphere, okay? Just adding those two things up and comparing that to these dots, which are the outputs of the
most advanced climate models, in the world. And you can see, you know, they wobble around because
of variations in the climate, you know, variable weather and so on. But broadly speaking we can understand it. Here's the Met office
model, which by the way is, you'll notice that by the way, you'll say well there's
still some, you know, if you're picky you'd say, well there's a bit of a discrepancy there. But hang on, you know, look at the models which
we're comparing against. And you know, the Met office
model actually doesn't shows, no warming at all up until the present. Then it takes off like a rocket. So, you know, the discrepancies between
this very simple equation and these very advanced
models are smaller, than the differences between
the models themselves. I should of course stress, 'cause there is somebody
from Bayes in the room, that there's many more uses and we don't want to
replace the Met office, with a bunch of plastic pipes. There's many more uses for climate models, than just predicting global temperature. But the point is you can understand, what the climate models tell
you about global temperature, you know, without believing every detail, of what the climate model is simulating. And that's really important
because, you know, people find these climate
models quite bewildering. They're sort of black boxes, there's lots of ways in
which they can go wrong. There's lots of things you
can nitpick over in them. But it's important the
fundamental physics, of what drives changes
in global temperature, is actually very simple. We can capture it with
just this simple equation. Now comes the punchline. Bang on time. What will it take to stop global warming? Okay? So you want delta T the
change in temperature, over a multi-decade time
interval to be, what, if you stopped global warming? Answer is obvious zero, okay? So you want delta T to be zero. This is an equation. So we've got to get this
side to be zero as well. And there's two ways to do that. One way is to make everything
in here zero, yeah? Because zero plus zero is zero. The only way we could do that, is to make the average energy flow, due to extra greenhouse gases, zero. That means getting greenhouse
gas concentrations, back to pre-industrial. That would be hard. Maybe one day in many
thousands of years time, we may get that. But there's another way to make this zero. What's that? There's two terms here and I
want them to add up to zero. One of them has to be negative, okay? Now if this average term, that's the average energy imbalance, due to past increases in greenhouse gases, we're kind of stuck with
that being positive, yeah? Because greenhouse gases have gone up and they're not going
down again anytime soon. But that could be negative. Do you want to say how
much it has to be negative? Well, in order to get it, it's this number is crucial, yep? If the ratio between
delta F and F bar delta T or between the change in flow and the average flow multiplied, by the length of a time
interval is this rate, then this will average to zero. In fact, that's down here. Sorry, I was covering it up. So if delta F on delta, you'll hopefully you can see that, just rearranging this equation,
if delta F on delta T, which is the sort of rate of change, of the fluid flow, divided by the average rate of fluid flow, is equal to negative, this rate so called
the rate of adjustment, to constant forcing, which is about 0.3% per
year, 3% per decade, then we get no further warming. So let's try this experiment. This is a simulation where, I'm going to increase the
flow over 10 seconds or so and then I'm going to gently decrease it. I want you to watch and see what happens. After a couple of seconds, okay? You can see it's starting
to go up over 10 seconds, it's accelerating. Greenhouse gas concentrations, are rising in the atmosphere, okay? And temperature goes up and
now it's stopped increasing. And if you listen carefully, you can hear that it's slowly
slowing down again, yep? Yep? And what's now happening to
the level fluid in this pipe? I'm hoping it's not moving, yep? 'Cause that's the point. Even though the ocean is still warming up and the rate of the pump is going down, that's corresponds to
greenhouse gas concentrations, coming down in the atmosphere, these two effects balance each other and we get no further warming. That's what we mean by net zero. It doesn't mean we've got
constant concentrations, of greenhouse gases in the atmosphere. It's now switch itself off again. It doesn't mean we've got constant, concentrations of greenhouse
gases in the atmosphere, it means we're reducing the concentration, fast enough to balance
this accumulation of heat, in the deep oceans. And how we get concentrations
falling at that rate, is of course the subject of
our carbon cycle lecture, which will be coming up in
a couple of lectures time. So there you are, this is, if you like, a good enough model for climate policy, changes in temperature over
decade to century time scales, are just how much do you change, the concentrations of greenhouse
gases over that period, plus what level are they at? For fast changes, this dominates. For slow changes as your
approaching equilibrium, this becomes important as well. And if you want to know what it
takes to get delta T to zero, you have to get these two to balance, which means you have to have
declining concentrations, of greenhouse gases in the atmosphere. And by the way, for the enthusiasts, here's the gory details, of the whole set of
equations that runs this. And I'm not going to
go through all of that. But and for those of you here, this is the sort of stuff
you'll do when you're doing, actually you probably won't even do, couple differential
equations in further maths. You might do, I'm not quite
sure, I can't quite remember. And that's the this term is
that additional energy flow, to the deep ocean, due to the fact that the
systems out of equilibrium. And by the way, if you think about it, this exact same set of equations, is the set of equations, that governs the fluid in these pipes. So it's a nice example of how you can do, coupled differential
equations with fluid in pipes. Finally, there's one more term, which is just an important one, which I sort of add for completeness. This is here, it's the
additional energy flow to space, that results from the system
being out of equilibrium, for this model to be truly realistic, the length of this pipe
would have to somehow adjust, to the pressure difference
between these two tubes. And we couldn't work out
how to do that, okay? So, but if you can maths to the rescue, you can actually rearrange this and discover that satisfies
the same equations anyway. That's left as an exercise. So and by the way, if you're
really interested in this, indomitable nerds can go and
read a paper free online, net zero science origins and Implications, which has all the maths in it. Although I have been told by a colleague, that the maths is somewhat gratuitous and it's perfectly possible
to follow the paper, without the maths. He was sort of slightly
teasing me about the fact, that I put all these questions in there, he felt rather unnecessarily. Anyway that's the slow adjustment term, because result as the world warms and heads back to equilibrium, the pattern of warming changes, which changes the efficiency, with which the planet gets
sheds energy back into space. This is a really important term, sorry, I should remember to switch this off. Now that it's done its thing. This is a really important time, 'cause it adds to the
considerable level of uncertainty, we have in where the warming will stop, if we were to hold concentrations
of greenhouse gases, at today's level forever. But fortunately, there's another option, if we can get greenhouse
gas concentrations falling, fast enough we never need
to worry about all this. And that's really important. So the ocean physics behind net zero. First of all, how the
circulation of the oceans, keeps the deep ocean cold? So the deep ocean is, it's not
sort of called by accident, it's kept cold by the fact that, all of the water down in the deep oceans, comes from the Arctic. And this slow, we call it
the thermohaline circulation, provides that multi
century timescale response, that's represented in our little model, by the fat pipe over on the right. And just thinking about these two, this sort of simple pipe model, you can realize that just two quantities, the so-called transient
climate response to forcing, that's how much it jumps up, when I crank up the speed of the pump and the rate of adjustment
to constant forcing, that's the rate at which
it continues to rise, when I stop changing
the speed of the pump, are enough to understand
global temperature changes, on decade to century time scales and to understand what it
takes to stop global warming. Thank you. (crowd applauds) - Thank you so much Professor Alan. I've got a couple of questions online, before we go to the room,
if that's all right. The first one is, does the temperature of the earth's core, have any influence on
deep ocean temperature? I eg rather like the way
temperature rises in a deep mine. - There's a yeah, great question. There's a very gentle trickle of energy, into the bottom of the ocean, from geothermal heat in mid ocean ridges. You know columns of boiling water, coming out of mid ocean ridges. You've probably seen movies of that, but compared to the volume of
the ocean itself, it's tiny. So and you'll so, if the ocean circulation wasn't happening, it would eventually warm the oceans up. So you've got to be careful
with numbers like tiny, because you've got to
think, well in what context, you know over billions of years, that would warm the oceans up, but it's the oceans are kept cold, by that ocean circulation anyway. - Second question I've got from online is, this makes it all seem quite simple. Is it really that simple? And does everyone agree that it is simple? - If you want to understand
global temperature, then it pretty much is that simple, at least so far the important
proviso there so far, the response of global temperature, to global changes in
greenhouse gas concentrations, has been remarkably simple. And by the way, this again wasn't obvious. I remember when I started
in this field, you know, when I was doing my doctorate, back in the sort of
late 1980s, early 1990s, I remember going to lectures where, you know, people were saying, the university I was at or the university I'm still at actually, people saying, oh, you
know, we understand chaos. We understand the things
are very complicated and it's only these sort
of naive geographers, who think that when you ramp
up the greenhouse gases, it'll be very simple and it'll follow. Anyway, turns out on that score, the geographers are quite right. It basically, you crank
up gene greenhouse gases, world warmed up in a very
predictable way so far. We shall see what happens in the future. - I have one more question from online, before turning to the
room, if you'll forgive me. The deep sea is kept cold by the Arctic. Does the Antarctic play
a role role at all? - Sorry, yes, I should have. I should understand Arctic regions, we tend to use the
phrase to refer to both. Deep water is formed around the Antarctica and also in the North Atlantic. So both regions play an important role. - [Male Audience Member] Thank you. You've talked about
politicians obsession with, stabilizing greenhouse emissions and the difficulty of
explaining climate science, to politicians is that just a, another tragic feature
of the two cultures, that politicians learn humanities and journalists then
don't understand basic, scientific concepts? - To be fair, the political community got on board, net zero remarkably quickly. I mean, you know, we did this work, we established that it wasn't enough, to stabilize concentrations. You actually had to get
concentrations going down. That work was done in the late 2000s. And the Paris agreement, which essentially acknowledges that and moves on from that
stabilization framing of Rio, was signed in 2015. So, you know yes it's a
perpetual struggle communicating, but it can work. And on that occasion, I think the politicians did
take this on quite quickly. That said, we have sort of
keep reminding people of this, because as we'll come onto
in the carbon cycle nature, there's an increasing tendency
these days for countries and companies and so on, to sort of declare they got to net zero, when they're no longer, when the sort of balance
in the atmosphere, when effectively their activities, would stabilize greenhouse
gas concentrations, of the atmosphere. You have to remind 'em no, that that's not what we meant by net zero. You need to go beyond that. We'll I'll explain more about that, in the carbon cycle lecture, but it's still topical, therefore. - [Female Speaker] I
understand that temperatures, in Kodiak Island in Alaska
in 1st of January, 2021, reached 19 degrees C. The Antarctic has been
apparently recorded, as having temperatures at
30 degrees above average. How is this affecting deep water, cold water formation and the
the hairline circulation? - Yes, yeah. - [Female Speaker] How quickly. - Yeah, no so obviously, the changes in the
temperature in these regions, have a particularly importance, because the ocean will be
remembering those changes, for hundreds of years. If you warm up in a given year, if you warm up the Labrador Sea, then the water that enters the deep ocean, from the Labrador Sea will
remember that temperature, for centuries. So these changes really matter. Even if they only happen
in a given season, the earth will remember
them for a long time. So and they're not, you know, they're happening more and more often, so exactly it's building up. So yeah, no, it's very important. This of course is why we need
complicated climate models, as well as, you know, fluid in pipes, in order to understand how
those changes in these regions, actually affect the penetration of heat, into the ocean depths. And of course a big worry and a point that I will look
at later in these lectures or we may have to delve
into it more later on, in the, because it's
involves even more maths, is the danger that if you
mess with the system enough, then you could actually affect
the ocean circulation itself. And for example, if you were to shut down the
entire ocean circulation, you'd have a huge impact on climate, which would be very difficult to reverse. That's not predicted
to happen anytime soon. But as I say, you know, just 'cause it doesn't happen in models, doesn't mean it can't
happen in the real world. It's certainly a worry and
one which we're watching. And one of the reasons, why people are particularly concerned, about these huge changes we're
seeing in arctic regions, is 'cause we know not only are they, a very sort of sensitive indicator region, but they play an absolutely vital role in important in what
controls our global climate. So, you know, the fact
that every liter of water, in the earth's oceans, over the vast bulk of the depth, has probably got there via the Arctic, tells you just how important
changes in the or Antarctica, tells you just how important
for future climate, changes today in the
Arctic and Antarctica are. - Thank you so much, professor Allen. Professor Allen's next lecture, is going to be on the 7th of March. - Yes and in view of the fact that, so in a slight shift to the program and earlier we were going to sort of, work through the science and then come back to the
policy in the final lecture. But because we got an energy bill, in front of parliament at the moment and also because I felt having
got through this lecture, that you might want to
break from the algebra and get back to politics. We'll talk about what
the UK is trying to do, to get to net zero and what we could put
into the energy bill, to make it happen in the next lecture. So we'll sort of get stuck into, some of the policy implications, of the need for net zero
next time and then we'll, but don't worry the plastic
pipes will make a reappearance, in the follow in the last two lectures, when we get onto
understanding the carbon cycle and what it'll take to get, atmospheric concentration
of the greenhouse gases, going down fast enough
to stop global warming. So see you on 7th of March. - Thank you so much Professor Allen. (crowd applauds)
