*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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