#rC3 - Climate Tipping Points

rc3 prerol music

Herald angel: Greeting creatures im
Neuland. In 2015 governments from around

the world met in Paris and agreed to
attempt to limit anthropogenic climate

change to well below two degrees.
Unfortunately, it seems that since then we

have not done enough and the climate
crisis has only gotten more urgent. Our

next speaker, Stefan Rahmstorf, has more
accolades than I have time to tell. He's

published more than 100 papers, including
in the journals Nature and Science, co-

authored four books and won the Climate
Communication Prize from the American

Geophysical Union, the first European to
do so. Please welcome him. And heed his

advice. Here's Stefan.

Stefan Rahmstorf: Hi, everyone, my name is
Stefan Rahmstorf, and I'm thrilled to be

invited to give a talk at the Chaos
Computer Club's remote chaos experience

2020. I want to give you an overview of
climate tipping points, a very exciting

subject that I will try to shed some light
on. But let's first start with some

background on climate change. You probably
know this image. It shows the global

temperature evolution since the year 1880.
Every line is one year. This is the more

conventional way of viewing this time
series. And the last seven years have been

the hottest seven years since record
keeping began in the 19th century. We know

the reason for this warming: it's the
increase of carbon dioxide, which you can

see here for the last ten thousand years.
And if you just look at the end of the

curve, how the increase has accelerated in
ever shorter time spans, we have seen an

ever greater increase in the amount of
carbon dioxide in our planet's atmosphere.

This increase causes what we call a
radiative forcing that is a kind of

heating in terms of energy release per
square meter of Earth's surface. And the

increase in CO2 in the atmosphere until
now is causing heating at a rate of two

Watts per square meter surface. We
understand the energy budget of our planet

pretty well. On the left here in this
diagram, you can see the incoming solar

radiation in yellow. Part of that is
reflected already in the atmosphere by the

clouds, for example. Another part is
reflected by the bright surfaces, that's

the snow and ice surfaces primarily, and
the rest is absorbed. On the right hand

side, and let's zoom into that, you see in
orange the long wave radiation, which is

clearly distinct from the incoming short
wave solar radiation by its wavelength and

this thick arrow of long wave radiation
leaving the Earth's surface basically to a

large extent gets absorbed by the
atmosphere. And the atmosphere itself

emits like anything, any substance, any
matter depending on its surface

temperature, sorry, depending on its
temperature, emits also infrared

radiation. And one thing that few people
realize is that the back radiation coming

down from the atmosphere through the
greenhouse effect, the greenhouse gases,

is actually twice as large at the Earth's
surface as the absorbed solar radiation.

So heating by the greenhouse effect by the
long wave radiation is twice as big as the

absorbed solar radiation at the Earth's
surface. And so it's little wonder that if

we are increasing this natural greenhouse
effect, which actually makes our planet

livable in the first place, if we are
increasing this effect that it is going to

get warmer. We can also quantify this
effect. And if you add in not just the CO2

increase, but other human caused
greenhouse gases and also cooling effects

caused by humans, then you see that the
total human caused warming that we see in

the orange bar is to, within uncertainty,
as big as the observed global warming

since the 1950s. And that means that about
100% of the observed global warming over

the past 70 years is human caused, and the
best estimates of the human caused warming

is actually even slightly more than the
observed warming, which has to do partly

or is consistent with the fact that solar
activity has gone down. So the decrease in

solar activity has compensated a small
part of the human caused global warming.

It's also very interesting, and especially
to me as a paleoclimatologist who studies

natural climate variations in Earth's
history and has done so for more than 25

years, how the modern warming compares
with the changes throughout the Holocene,

and before that, since the last Ice Age.
And this is what we see here based on

decades of paleoclimate research,
countless sediment cores taken at the sea

bottom, ice cores on the big ice sheets
and so on. We have enough data now to form

meaningful global average temperatures.
And you can see here the warming from the

height of the last ice age into the
Holocene, the Holocene optimum, the

warmest period about until about five
thousand years before present. And since

then, we have seen a very slow cooling
trend, which we have bent around due to

human activities. And we have within 100
years more than undone 5000 years of

natural cooling trend, which normally
would have very slowly continued. These

natural variations, by the way, are due to
the Earth orbital cycles, these so-called

Milankovitch cycles. You can easily read
up on those, for example, at Wikipedia.

Now let's come to the famous, much feared
tipping points in the climate system. What

is a tipping point? That has been
described in a seminal paper which I'm

proud of having been a part of from 2008
by Tim Lenton and colleagues. And this is

called tipping elements in the Earth's
climate system. And it says that the term

tipping point commonly refers to a
critical threshold at which a tiny

perturbation can qualitatively alter the
state or development of a system and the

different parts of the Earth's system,
which can undergo such a transition, they

are called the tipping elements. This
whole concept is illustrated in the red

line that's shown here: In the horizontal
axis, we see a control parameter and that

could be the greenhouse gas content of our
atmosphere, it could be the temperature,

it could be, if you talk about natural
climate changes, for example, those

orbital changes, the what we call the
Milankovitch forcing, which drives

changes. And on the vertical axis, you see
the response. And if you imagine the

control parameter changing from left to
right in this diagram, you would march

along that upper part of the red curve
here, the branch, until you come close to

a threshold. And at that threshold, the
system will undergo a major change and

reach then this lower part of the curve, a
different kind of equilibrium state. So

it's basically a small change in the
driver causing a very big systemic

response. That is what defines a tipping
point. If we want to be very accurate

here, we can distinguish two different
types of tipping points. The first one is

what I just showed you, is repeated here
on the left side, and it is characterized

by the fact that this red equilibrium line
has one state for every point on the x

axis. So every amount of forcing
corresponds to one particular system

state. And this is some state just makes a
major transition in a smaller range of the

driving parameter around this threshold.
Now, a second, even more drastic or non-

linear type of tipping point is shown in
the right hand side, where the equilibrium

states are somewhat more complex than the
single red line on the left. You can see

here that there is, again, an upper stable
branch and there is also a lower stable

branch, but they overlap. So there is a
region that is shaded here where two

stable equilibria exist. And it depends on
the initial conditions on which of these

branches you are. Now, there is what is
called a bifurcation structure underlying

this with a bifurcation point. There is an
unstable branch which separates the basins

of attraction of the two stable branches.
So if you're in the bi-stable regime and

you start kind of away from an equilibrium
but above the dashed line, you will fall

up onto that upper stable branch; if you
start out below the dash line, you will

fall down on the lower branch. That
actually is pretty standard non-linear

dynamics. It's a whole branch of physics
which investigates exactly this type of

behavior in many different physical
systems. So the second type of tipping

point, the right hand side one, is
corresponding to multiple equilibrium

states, in this case two stable
equilibria. That's why this error range

here is called bistability, two stable
equilibria. It is coming with

irreversibility, so basically, if you
march to the right here on that upper

stable branch at that bifurcation point,
you fall off down onto the lower stable

branch, but you can't just go back up from
there. You have to go all the way to the

left to that second lower blue point there
until you can go back onto that stable

branch. The second type is actually as an
everyday system that behaves like that it

can be easily compared to a kayak: if
you're sitting in a kayak and you lean a

little bit to one side, then you
experience a counterforce. So the kayak is

trying to upright itself, it's resisting
you tipping it. But if you move further

and further and further, eventually you
will reach a tipping point. This is the

point where the kayak stops resisting your
further leaning over and instead it starts

tipping over further by itself and then it
flips right over until it's upside down

and you're falling out. So I have I have
done this quite a few times. So I have a

kayak that is quite narrow where it easily
happens if you don't take care, that you

flip over. Now, this kayak also has a
range of bistability, so once it's flipped

over, it's also in a stable state and it
takes considerable effort to turn it

upright again into the other stable state
when it's vertical, upright rather than

upside down. Now, the whole point is that
systems like this exist also in the

climate system. The kind of first type on
the left hand side corresponds, for

example, to sea ice and on the right hand
side this type of tipping element compares

to refers to the Greenland ice sheet or
continental ice sheets, also Antarctica or

the Atlantic Ocean circulation. In terms
of the trends in behavior, and that means

if you if you kind of go through a global
warming phase, you're moving from left to

right in these diagrams, then in that
sense, they don't differ very much because

in either case, you follow a line like
this green line. So on the left hand side,

the green line more or less follows more
or less closely the red equilibrium line

with a certain delay, depending on how
sluggish the system responds. So that's

why the green arrows are not exactly on
top of the red line here. And in the

right hand side case, you have a similar
thing. You are kind of, in theory, in

equilibrium, you would fall off the cliff
at this bifurcation point. But in praxis,

the system has some inertia, it takes some
time. So if you gradually move on the

right towards the right there, you will
also follow a green line, which is very

similar to the one in the left. So in
practical terms, if you're not trying to

go back, but you just going forward,
progressive global warming, the difference

isn't all that big. And the main
difference comes from the intrinsic

timescale of the system. Obviously, sea
ice can respond much more quickly to being

just a few meters thick compared to
continental ice sheet like Greenland ice,

which is about three thousand meters
thick. And that just takes a very long

time to melt. Now, here's an overview of
different tipping elements in the climate

system. A few examples you can see
starting on the left here, the boreal

forest, that are the kind of northern
forests, which typically, like ecosystems,

do have a tipping point, a point of
collapse. The whole idea of these tipping

points and system collapse is very
strongly linked actually to ecosystem

research and the boreal forests, They have
a point where they get too dry, that fires

and pests are weakening the forest so much
that in a hot summer like last year in

Siberia, they go up in flames lit by
lightning. Or the Amazon rain forest. This

is also a tipping element, has been shown
in many vegetation dynamics models, which

is partly linked to the fact that such a
forest generates its own rain to an extent

by storing water in the soil, keeping it
there and then bringing it up again

through evapotranspiration, as we call it,
the tree brings up water to the leaves

then into the atmosphere again, and then
it moves with the winds and maybe 50, 100

kilometers downwind, it falls again as
rain. So it's a kind of perpetual rain

recycling system which keeps the whole
forest nice and moist. But if you stress

that too far and reduce the first of all,
you cut down forests, you make it smaller,

and also you make it more drought prone by
warming up the climate, which leads to

faster loss of moisture, etc. greater
moisture requirements by the trees. Then

you can stress it up to the point where it
gets so dry that even the Amazon rain

forest can go up in flames. Another
example of how you see the top right is

the permafrost thawing. This is when it
gets too warm. There is a very simple

threshold, namely the freezing point. Of
course, that is a tipping point in the

sense of freezing point of water. When the
permafrost thaws, then there is methane

gas escaping to the atmosphere, which then
also can enhance the further warming,

which then leads to more permafrost
thawing and so on. Typically, these

tipping points are associated with such
amplifying feedbacks. I will discuss three

of these in a little bit more detail. The
Greenland ice sheet, which is undergoing

accelerated ice loss, the Atlantic
overturning circulation, often called Gulf

Stream system. And the third one is the
coral reefs, which are suffering from

large scale die-off, which also as a
typical ecosystem response, have a

critical threshold. These examples are
discussed in our paper 'Climate tipping

points - too risky to bet against' which
we published in Nature about one year ago.

And they are also some of these tipping
points interact, they are interlinked. And

one of our quotes there is that the
clearest emergency would be if we were

approaching a global cascade of tipping
points. That is a situation where one

tipping element is triggering the next one
in a kind of domino effect. This is what

we fear most. Now, let's have a look at
the Greenland ice sheet. This is a NASA

video showing based on GRACE satellite
data where the ice sheet is losing mass.

You can see increasing blue colors here
that the Greenland ice sheet is indeed

losing mass. You can look up at the NASA
Vital Signs website, which has very good

indicators of various vital signs of our
planet, including the data on Greenland

ice loss, constantly updated. Now, the
point with the Greenland ice sheet is that

it does have a stability diagram like the
schematic one that I showed you earlier

with the bi-stable range. And this is
shown, I think it was shown for the first

time by my colleagues, Calov and
Ganopolski in 2005 in this article where

they used the three dimensional ice sheet
model coupled inside a global climate

model with ocean atmosphere and so on and
on the x axis is basically increasing

amount of heating going on, in this case
because they were interested in the

paleoclimate question, it is this driving
force by the orbital cycles and

Milankovitch cycles. You don't need to
understand the numbers, but on the

vertical axis, you see the response of the
ice sheet, the size of the ice sheet, in

million cubic kilometers. And you can see
that upper branch in the blue line, we're

actually moving towards the right here in
this model simulation experiment. And you

can see you stay on that upper branch
until you reach this value on the x axis

of around about five hundred. And this is
where the tipping point is. There the ice

mass declines, melts away, away very
quickly. And you then end up at that lower

branch with no ice on Greenland. And they
played this game. They ran the simulation

out to more than 550 watts per square
meter. And the light blue line is what

happens when they return, when they turn
down the heat again. You move towards the

left on this diagram, but you don't go
back up the same way as the dark blue

line. You have to go to much lower
radiation values until the ice sheet

starts to grow again and comes back. The
dots, by the way, are points where this

has to has been run for many thousands of
years really into an equilibrium just to

show that there are really for the same
value on the x axis, two very different

equilibrium states with and without
Greenland ice sheet. And the fact that we

now and in the Holocene in the last ten
thousand years have the Greenland ice

sheet and it actually is stable in the
Holocene climate is only because of the

initial condition, because we came out of
an ice age. If you took away the Greenland

ice sheet now, then in the current climate
or the Holocene or pre-industrial climate,

it would never grow back. What is the
positive feedback? The most positive? We

don't mean that it's good. That's actually
quite bad and positive feedback. We mean

and amplifying feedback and the key
amplifying feedback here is what is called

the ice elevation feedback. The Greenland
ice sheet does not melt because it's very

cold at the surface, mostly below
freezing. And why is it so cold? Because

it is very high up in the atmosphere, this
ice sheet of three thousand meters thick

after all. So it's like in a high mountain
area where it is quite cold. If you took

away that ice sheet, though, the surface
then would be down at sea level or even

below if you did this quickly because the
the bedrock is depressed, but the surface

would come up to sea level, but down there
it's much warmer than up at three thousand

meters altitude in the atmosphere. And
there it is actually too warm to keep any

snow on the ground year round, which would
be required to regrow a new Greenland ice

sheet. And that's why you'd have to go
back to a much colder climate than the

Holocene to get the Greenland ice sheet
back once it were lost. This is a typical

example of this amplifying feedback, which
leads to a self stabilizing system. It can

either self stabilize in the upper branch
here when you start there or it self-

stabilizes in the lower branch with no ice
when you start there. This is what makes

it a bi-stable system. To summarize, the
Greenland ice sheet is melting as another

data the great satellites show, but also
other data sets. It has a tipping point

due to the ice elevation feedback. What I
haven't shown, but it's come out in study

with many climate models, simulation
experiments going through more than two

hundred thousand years of simulations from
the past through the Eemian interglacial

period where we know how much the ice
sheets shrank back. And we could use those

data from the past behavior of Greenland
to calibrate the model. And so we know the

tipping point for the complete loss of the
Greenland ice sheet is somewhere between

one degree and three degree global
warming. We're already at one point two

degrees global warming. So we have started
to enter the danger zone where we crossed

that tipping point. It doesn't mean that
it suddenly starts to melt very fast also

because it has its own intrinsic slow
response time. But what that crossing,

that tipping point means is that even
without further warming, the Greenland ice

sheet is doomed and will continue to melt
until it's gone, and this will lead to

seven meters of global sea level rise,
drowning most of our big coastal cities

and to many island nations. Here is a look
at the future from models, simulations

from Ashmont and from NASA. And you can
see a nice view of what the surface looks

like. And here's what the what it looks
like in the ice sheet model. You can see

the ice flowing. You can see it
retreating. So in purple, that's bedrock

that is exposed where the ice sheet has
withdrawn in this simulation. And so it's

as much as ice of ice that you would lose
in the coming three hundred years, a

substantial fraction of the Greenland ice
sheet. Now, let's look at another kind of

tipping element, and that is the Gulf
Stream system or the North Atlantic

current. And I can't really introduce this
topic is one of my favorite topics, which

I have studied since the early 90s,
without showing a clip from the famous

Hollywood blockbuster The Day After
Tomorrow. What about the North Atlantic

current? What about it? The current
depends upon a delicate balance of salt

and fresh water. We all know that, yes.
But no one is taking into account how much

fresh water has been dumped into the ocean
because of melting polar ice. I think

we've hit a critical desalinization point.
Yeah, now that statement about the

critical desalination point is a
completely correct description of the

bifurcation point of the Atlantic
circulation, I'll show it in a minute. And

the statement that nobody has taken into
account the meltwater from the Greenland

ice sheet is also was completely correct
when the movie appeared in 2004. Until

then, the typical climate simulations that
you could see in the IPCC reports,

actually until quite a few years later,
still had not taken account Greenland melt

water because basically at that point in
time, the models, almost all climate

models were just ocean-atmosphere models
plus land surface, but they didn't have

continental ice sheet models coupled into
them. And so in the meantime, of course,

we have better models that include
experiments either with artificially added

Greenland meltwater from data estimates or
fully coupled with ice sheet models. And

from that, an example here being that
nature article by Claus Boening and

colleagues. We know that the meltwater
input from Greenland has a non-negligible

effect on the North Atlantic overturning.
It's probably not the dominant effect, but

it adds to various factors that weaken
this North Atlantic current. And we also

know that this system has a well-defined
tipping point. Actually, I described that

in a nature article in 1996 due to a salt
transport feedback. The basic idea behind

that has actually been known since the
late 1950s or early 60s since work by the

famous American oceanographer Henry
Stommel. But what I showed in my Nature

article in 96 is that it actually works
that way in a complex, three dimensional

global ocean circulation model, not just
in very simplified models. And since then,

this has been shown for a whole range of
different climate models. The sole

transportation feedback is also one of
these amplifying feedbacks, and it's easy

to explain. The overturning circulation of
the Atlantic is called overturning because

it's really a vertical overturning where
water sinks down from the surface to great

depth of two to three kilometers in the
Atlantic because this water is heavy and

it spreads thin in the deep ocean until it
rises up in other parts, mainly around

Antarctica in the Antarctic circumpolar
current area and comes back at the

surface. So basically the whole ocean is
overturned with deep water being renewed

and then coming back to the surface on
very long timescale of about 1000 to 2000

years for complete overturning there. Now,
the whole system is driven by the fact

that the water sinks down where it has the
highest density, and that's in the

northern Atlantic and around Antarctica,
around the Antarctic continent. And it has

the highest density there, not only
because it's very cold, but also quite

salty. This is why you don't have deep
water formation in the North Pacific, in

the Northern Hemisphere. You only have
that in the North Atlantic. And that's

because the North Atlantic waters are
quite salty. And this is because this

North Atlantic current exists and brings
salty water from the subtropics up to the

high latitudes, where normally it isn't
very salty because it gets diluted by

excess rainfall, whereas the subtropics
have excess evaporation and that's why

they're salty. And so it's like a chicken
and an egg situation. The Northern

Atlantic is salty because you have this
overturning circulation and you have this

overturning circulation because it's salty
there. And so you can see the self

amplifying feedback there again, which
means it is a self stabilizing system up

to a certain breaking point, a tipping
point which can be reached if you add too

much fresh water, diluting the northern
Atlantic. And the stability diagram,

again, looks like that second one. You've
seen it for the Greenland ice sheet. As I

said, this has been verified in a detailed
model simulations with many different

models that it really works like that in a
complex 3D situation where you have

depending on how much fresh water you add
into the northern Atlantic, this is the

control parameter here, you can move along
that upper stable branch with the

overturning circulation until that Stommel
bifurcation point. And there this

overturning breaks down and you fall down
onto that lower branch without this

overturning. It's labeled here NADW Flow
that NADW stands for north Atlantic

deepwater. It's a, I would say, one of the
favorite water masses of the

oceanographers. Now, let's look at the
Gulf Stream, the surface circulation in a

climate model. This is the CM 2.6 global
coupled climate model ocean atmosphere by

the Geophysical Fluid Dynamics Laboratory
in Princeton. You can beautifully see the

Gulf Stream and dark red here because it's
warm leaving the coast of the United

States at Cape Hatteras there, starting to
meander, breaking up into these eddies, et

cetera. And it actually meets the cold
waters coming down inshore from the north,

which are shown in blue here. And so this
is what this the surface part of the

circulation looks in a global climate
model. And if you add carbon dioxide to

that climate models atmosphere, the
climate warms, of course, but it does show

a peculiar pattern of sea surface
temperature change, which you see here.

And this actually shows the sea surface
temperature change relative to the global

mean. So everything that is blue has
either warmed less than the global average

or even cooled, which is actually the case
south of Greenland. And everything that is

orange or red has warmed substantially
more than the global average sea surface.

And you see a very strong pattern in the
northern Atlantic with this big cold blob,

the blue blob south of Greenland and a
very warm region inshore of the Gulf

Stream along the coast of North America.
And in the climate model, of course, we

are a bit like gods in that sense that we
have complete information about what's

going on there. If we store all the data
at every grid point, we know exactly everything

that's happening and we can analyze the
reasons. And the reason for this funny

pattern in the northern Atlantic actually
is a slowdown of the North Atlantic

overturning circulation. That means that
less heat is transported to the subpolar

ocean south of Greenland there. That blue
area, which makes it cool down and the

Gulf Stream proper at the surface, moves
inshore there is complicated dynamical

reasons for that. But there is already
long before this was shown in this model,

a theoretical underpinning for this. It
has to do with the vorticity dynamics on a

rotating sphere too technical to go into
in such a talk. But it's a well understood

phenomenon. And so we know that this
slowdown of the Gulf Stream system is the

reason behind this peculiar temperature
pattern. And this pattern is predicted by

this climate model for a global warming
situation. And my PhD student, Levke

Caesar, who was the first author on this
nature paper from 2018, she looked at all

the available measurements of sea surface
temperatures since the beginning of the

20th century. And of course, because we
have only limited ocean temperature

measurements, we have only a fuzzy picture
here, not a sharp one like in the climate

model. But you can see a similar pattern
in the North Atlantic in the observations

compared to what the model predicts in
response to a slowdown of the overturning

circulation. And our conclusion here is
that we are actually observing this

slowdown of the circulation. Why do we
take indirect evidence for this like this?

Because we don't, of course, have
measurements going back 100 years or more

about the strength of that overturning
circulation. We have actually only started

to measure this regularly in 2004 with a
so-called rapid array, At twenty six

degrees north in the Atlantic, and what we
reconstructed about the evolution of this

current for the last period where we do
have the direct measurements, agrees well

with what the direct measurements show. We
concluded that the overturning circulation

has declined since at least the mid 20th
century by about 15% so far. There are, of

course, other indirect types of
measurements. You can use sediment data of

various kinds and with various
methodologies to reconstruct the strength

of this Atlantic overturning and a number
of different studies compiled here in this

diagram. And even though, of course, they
differ somewhat in the detail, they all

tend to agree in this overall picture that
the Atlantic overturning circulation has

been quite stable for the previous
thousand years or so before the 20th

century, but then in the 20th century has
showed a clear declining signature. And

one example of the media coverage of this
is that Washington Post article here,

which if you can see the small print of
the most read articles there on that, they

actually made it to number three or the
most read Washington Post articles. There

is definitely an interest in science and
climate change science by the readers in

the newspapers. So far we've talked about
a slow down and not so much about where

this tipping point is. One reason is we
don't know really. We know there is this

tipping point, that is a robust result of
many different studies and model

experiments and theory, but we don't know
how far away we are from this. That is

very typical for these tipping points
because they involve highly nonlinear

dynamics. That means they can depend very
sensitively on the exact conditions, for

example, in this case, the exact salinity
distribution in the Atlantic and the exact

circulation pattern. And models get these
things kind of approximately right, but

not exactly right. And if you have a
situation where the question of where the

tipping point is is very sensitive to the
exact conditions, then you have a large

uncertainty about where the tipping point
is. And so there is discussion in the

literature. I just point out to one study
here in science advances that try to

correct for the inaccuracies in how we can
reproduce the salinity in the Atlantic

waters and found that if you correct for
that, the circulation is actually a lot

more sensitive than in other models. And
maybe that model is more correct. And of

course, it has other weaknesses as well.
We don't know which of the models is

correct, but should we cross this tipping
point then the North Atlantic circulation

system would break down and you get a
temperature pattern like the one shown

here, the cold blob in the Atlantic that
is now only over the ocean. It exists,

right? It's the only part of the world
that has cooled since the beginning of the

20th century, but it hasn't affected any
land areas. But if the circulation would

break down altogether and not only
weakened by 15%, this cold would expand

greatly and affect Great Britain,
Scandinavia, Iceland, as you can see here,

which would then get a much colder
climate, whereas the rest of the globe

continues to have a warmer climate. This
is really distinct from an ice age. And so

this is also really distinct from that
Hollywood movie The Day After Tomorrow,

where the earth goes into a huge ice age,
an instant freeze. That, of course, is

totally unrealistic. And the the
screenwriter and the director, they knew

this. They actually told me that if they
were in the business of making a movie for

a few million viewers, they would stick to
the laws of physics. But since they make

movies for a few hundred million viewers,
they stick to the laws of Hollywood drama.

But you would get a substantial regional
cooling with a major impact on ecosystems,

on human society. Now, let me come to the
third type of tipping point that I want to

discuss today. This is the coral reefs.
Coral reefs, like many ecosystems, do have

critical thresholds. Coral reefs are very
important, even though they only cover a

very small percentage of the Earth's
surface, they support a quarter of all

marine life. 40% coral cover of the world
has already been lost, 100 countries

depend quite substantially on corals.
There's 800 billion total global assets of

coral reefs. So it does have a major
impact on people. Now, corals, when they

are about to die, they bleach. They are
abandoned by their algae that provides

them with nutrition and that's why they
lose their color. And then after a while,

they die. They get covered by other by
seaweed, non symbiotic algae, and they

die. And they do have a temperature
threshold. It's a critical warming

threshold where this bleaching happens.
But an additional factor, not yet the most

important factor, is the acidification of
water. It's a direct chemical effect of

adding carbon dioxide to the atmosphere,
which then goes partly into the oceans and

acidifies the ocean waters. But the main
effect until now is the marine heatwaves,

which cross more and more frequently the
temperature tolerance threshold of coral

reefs. And here you can see that for the
Great Barrier Reef, a huge, fantastic

world wonder that you can see from space.
And you can see here the bleaching in the

year 2016, 2017, 2020, three major
bleaching events which affect it in each

case, the red area here with the most
severe bleaching, you can see that by now

a very large part of the Great Barrier
Reef has bleached in these three events.

And it's very tragic. And you can see
here, for example, the March, the 2016

bleaching event in March, the coral was bleached.
By May, it was already overgrown by seaweed.

And just in 2015 and 2016, we actually had
worldwide coral reef bleaching, not only

at the Great Barrier Reef in Australia,
only the blue ones out of these hundred

reefs that were observed in this study,
only the blue ones escaped bleaching. So

we are actually in the midst of a great
worldwide coral die off event, which is

another prediction of climate science
coming true. If you look at the latest

IPCC report, it states that with two
degrees warming, virtually all coral reefs

will be lost, more than 99%. One point
five degree warming. If we manage to limit

the warming to one point five degrees, we
can save between 10% and 30% of the

corals. That is really depressing. Now,
let me talk briefly about what can we do.

A major success is, of course, the Paris
accord, the biggest failure of which is

that it hasn't come 20 years earlier.
After all, the world community already in

1992 decided to stop global warming at the
Rio Earth Summit. The nations signed the

United Nations Framework Convention on
Climate Change, and it took a full 25

years of further negotiations to finally
reach the Paris accord. Now, you can see

here that the goal of this is to hold the
increase in the global average temperature

to well below two degrees above pre-
industrial level. So it's not two degrees,

it's well below two degrees. That's a very
important point. Many countries would not

have signed up if it simply had said two
degrees, which was an older goal, but it

has shown to be insufficient and. And to
sorry and to pursue efforts to limit the

temperature increase to one point five
degrees above pre-industrial levels. So

that is a more stringent Paris goal, but
at least the nations have committed to

pursue efforts. So my view is that every
person should ask their own government

what you are doing here. Is this a
credible effort to try and limit warming

to one point five degrees? We might not
make it, but at least we should try to

limit the warming to one point five to
avoid the risk of destabilization of the

Greenland ice sheet, almost complete coral
die off and many further risks. So what

does this entail? That is an important
point. If you want to limit global warming

to some value, whatever it is, one point
five, two, three, whatever you choose, it

means you can only emit a limited amount
of carbon dioxide. That is because the

amount of global warming is to a good
extent proportional to the total amount of

CO2 that we have ever emitted. So to the
cumulative emissions, it's like filling a

bathtub with water. If you want to draw
the line at any level and say no further

than here, you can only add a limited
amount of water. And if you want to limit

global warming to some value, you can only
add a limited amount of CO2 to the

atmosphere. And this is shown here for two
different examples, two different amounts.

This is actually, the numbers here are
emissions from the year 2016. So it's

don't take these numbers from now. We have
already had four more years of emissions.

The solid lines throw show three scenarios
with six hundred billion tons of CO2 and

they all have the same amount of emission.
So they're all three solid lines, get the

same amount of warming. This is about
actually these lines correspond to about a

50 percent chance of ending up at one
point five degrees. And so they will get

you the same amount of warming, but with
different times of when the peak emissions

are reached. So 2016 went past without us
getting over the peak of the emissions.

2020, maybe we still have a chance.
Emissions have dropped a bit in 2020, but

not for structural change and mostly, but
due to Corona. But we still we have a

chance that maybe next year they are lower
still. And what this shows is that the

longer you wait, the steeper your
reductions have to be, not only because

you're starting later, but also because
you have to reach zero earlier at the end.

Notice how all these three lines, the
later you start with reducing, the earlier

you have to reach zero emissions, because
the surface area under these curves is

what counts for the climate goal. The
dashed lines a more generous goal, which

would end at about 1.75 degrees or so,
best estimate. this is kind of the weaker

Paris goal of well below two degrees,
which would allow us to gradually reduce

emissions to zero by 2050. This is not
counting in any negative emissions

afterwards, by the way. This is the net
emissions, if you like. So we have to

reach net zero emissions in 2050. But of
course, if we wait five more years until

the emissions start to decline, then
they'll have to be at zero five years

earlier. So this is why it's so important
to start now. This, by the way, so from an

article by Christiana Figueres et al. in
Nature, published 2017, where I was a

coauthor as well. Now, a final point. Can
tipping points maybe help us? And I'm

talking here about societal tipping
points. And there are also some

interesting studies on that. The basic
idea is that shown in the top right here,

we are in a kind of stable equilibrium
where the red ball is now and we are stuck

there. It's hard to get out of this, but
there is a better equilibrium, a more

stable one further off to the right. And
the question is, how do we get over the

hill into that beneficial equilibrium of a
sustainable global economy, a sustainable

energy system, a stable climate and so on?
Complete decarbonization, that means no

more fossil fuel use. And these this green
addition there that is added there, this

is just some examples of how we can make
this transition earlier, easier and the

hill that we have to get over smaller, so
we can make this current status quo that

we're in a little bit less comfortable by
putting a price on carbon. We can make the

transition easier by subsidizing renewable
energies. There are there is a greening of

values. There is a tipping point in
thinking, in society. There are many co

benefits of this transformation in terms
of avoided air pollution. For example,

millions of people die every year from
outdoor air pollution, which would which

to a large extent go away if we stop
fossil fuel use. And we have seen a

massive movement by the young people
Fridays for future. He is Greta Thunberg

talking to me at our institute. She came
last year to visit us here, here is a

Fridays demonstration in Berlin where I
took this photo. This is really changing

the societies values and it's changing
election results and it could be a tipping

point towards a sustainable global
society. And with that hopeful message, I

want to end and I thank you very much for
your attention. If you want to read more,

there's a couple of books of mine that
have also come out in English. You can

follow me on the blogs and of course, in
social media, preferably Twitter, but also

the scientist for future logo there,
because many thousands of scientists are

engaged there to try and stop the climate
crisis. This is really a matter of

survival of civilization. Thank you very
much for listening. Stick to science and

leave policy to us. Well, we tried that
approach. You didn't want to hear about

the science when it could
have made a difference.

Herald: Thank you so much Stefan for your
talk. Now we have some questions from the

Internets. Let's see the first question
Question: Which additional tipping points

will be triggered at two degrees, three
degrees and so on?

Stefan: That is actually a difficult
question to answer because of the

uncertainty that I mentioned in my talk
about where these tipping points are.

There is one in Antarctica, the Wilkes
basin, that is a part of the Antarctic ice

sheet that that could be triggered, say,
below three degrees. There are others like

the ocean circulation where you probably
at least we hope you have to go beyond

three degrees to really trigger a collapse
of the Gulf Stream system. But the truth

is that they are very large uncertainty
ranges. And the main fact is that with

every bit of extra warming, we increase
the risk of crossing more tipping points.

Herald: And are there some of these
tipping points that are interrelated or

correlated? For instance, could we save
some tipping points if we are able to save

others, for instance, the collapse of the
Gulf Stream?

S: Yes, there are these interconnections.
For example, if the Gulf Stream system

collapses, it will affect the atmospheric
circulation. The monsoon systems then can

shift the tropical rainfall balance. This
is not just theoretical. We see that in

paleoclimate where we have seen these
collapses of the North Atlantic

circulation and the paleo climatic proxy
data show that it comes with shifts in the

tropical rainfall belts that could then in
this way trigger a major drought in the

Amazon region if the Gulf Stream system
collapses. And so it would be very wise to

prevent these tipping points, especially
when it comes to the ocean circulation or

atmospheric circulation, because it's
really going to mess up the weather

patterns in a major way.
Herald: How long have we known about

human caused climate change?
S: Well, in principle, in the 19th

century, Alexander von Humboldt, actually,
wrote in 1843, if I remember correctly,

that humans are changing the climate by
cutting down forests and emitting large

amounts of gases at the centers of
industry. That's almost a little literal

quote by Alexander von Humboldt. We've
known about how sensitive the climate is

to a change in CO2 since the Swedish Nobel
laureate Svante Arrhenius, remotely

related to Greta Thunberg by the way, in
India studied the effect of CO2 doubling.

He wasn't worried by that because he
thought global warming would be great.

Bring it on. It just died, now it's back.
You can see my picture so?

Herald: yeah
A: and so he suggested, you know, burning

a lot of coal to enhance global warming. I
guess he came from Sweden and thought cold

is bad without thinking it through
properly. But the first real expert

reports warning the US government, Lyndon
B. Johnson, of the coming global warming

due to fossil fuel use was a rebel report
in nineteen sixty five, exactly 50 years,

half a century before finally the Paris
agreement was reached.

Herald: Will you be publishing your
slides from the talk?

S: Yes, I will. Uploading the slides.
Herald: What is or what should be the

ultimate goal of the climate change
mitigation? For instance, is it saving

lives, saving other species?
S: Well, I think the the ultimate goal is,

of course, preserving human civilization,
as we know it, but because I think if we

let this run, we will not only destroy a
lot of ecosystems and biodiversity, but we

will probably cause major hunger crisis,
which with big droughts like the one in

Syria before the unrest in Syria started
in 2011, the country went through the

biggest drought in history. And according
to settlement data from the eastern

Mediterranean, it was the worst drought in
at least nine hundred years. And then I

think especially in some unstable,
conflicted countries, this can really turn

them into failed states. That is what
happened in Syria. And it's what a German

report for the German government actually
warned in 2009. It was called climate

change as a security risk, I was actually
one of the coauthors of that report

because I was in the German government's
advisory panel on global change at the

time. And I think we will see increasing
hunger crisis, failed states and all the

effects that that has on international
politics if we cannot keep global warming

below two degrees.
Herald: And finally, is there a specific

call to action for the chaos community? Is
there anything that we can do with our

mindset and our skills?
S: That's a good question that I haven't

thought about, but maybe you can know
yourself the best thing, what you can do,

I think the key is really to keep up the
pressure on the political world, like

Fridays for future has been doing: Go on
the streets, protest, vote with climate as

a priority. I think these are the key
things that everyone should be doing and

specifically in whatever profession they
are. They will see some ways of how you

can help to reduce emissions in your
company, put sustainability at the top of

the agenda and so on.
Herald: Stefan, thanks so much for taking

the time to join us today.
Stefan: It's a great pleasure and honor.

Herald: Always welcome. And now the news.

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