#rC3 - The big melt: Tipping points in Greenland and Antarctica

preroll music

Herald: It's simple when ice gets above
0°, it melts. But is it really that simple

if we are not talking about a small ice
cube, but a big sheet of ice covering an

entire continent? Is that really the only
factor? And, am I right with my

assessment? I'm looking forward to be
enlightened by Professor Doctor Ricarda

Winkelmann. Ricarda Winkelmann is a
professor of climate science at the

University of Potsdam, and she's also a
researcher for climate impact. She leads

the Ice Dynamics Working Group and Co-
leads PIK Future Lab on Earth Resilience

in the Anthropocene. Her research focuses
on tipping elements from the Earth system.

And today she'll be talking about the
Greenland and Antarctic ice dynamics and

the future sea level rise that are
impacted by them. It appears like she's

surely an expert on all things related to
ice. So please give a warm hand of

applause for Professor Doctor Ricarda
Winkelman with her talk: "The Big Melt:

Tipping Points in Greenland and
Antarctica" Have fun!

[no audio]
in between music

Ricarda Winkelmann: audio not working
Thanks and welcome. Today, we're going to

take a little excursion to the far north
and the far south, to our polar ice sheets

on Greenland and Antarctica. As this year
is coming to a close, I thought we'd take

a brief moment to reflect back. 2020 has
certainly been an exceptional year for all

of us. It was supposed to be a super year
for nature and the environment, as world

leaders put it at the beginning of the
year. It's five years after the Paris

climate accord. It's five years after the
Sustainable Development Goals have been

announced. However, 2020 turned out to be
the year when we've had to face several

global crises, including the ongoing
covid-19 pandemic and also the ongoing

climate crisis. What almost got lost in
the turmoil is that this year also saw

several weather and climate extremes,
which spaned the globe from pole to pole,

with temperatures reaching record highs in
the Arctic and Antarctica with +38°C in

the Arctic and in Siberia. That's the
highest temperature that was ever recorded

north of the Arctic Circle and it's
roughly 18° warmer than the average

maximum daily temperature in June, when
this was recorded. And we also saw +18° at

the Antarctic Peninsula, which is, again,
the highest temperature ever recorded in

Antarctica. And this was followed by
widespread melting on nearby glaciers.

Now, if we're kind of zooming out and
taking a look at the bigger picture, we're

also at a very significant point in
Earth's history. Here you see the global

mean temperature evolution since the last glacial
maximum. So the last ice age until today.

And whenever I look at this graph, I see
two things that still strike me to this

day. One is that the Holocene, the
interglacial or the warm age, in which

human civilizations have developed and
thrived, has been characterized by very

stable climate conditions, by a very
stable global mean temperature. And the

other thing is that the difference between
an ice age, here, 20 000 years ago

roughly, and a warm age, that's roughly
three to four degrees of global average

temperature change. And right now we're on
the verge of achieving the same

temperature difference, but at much, much
faster rates. So here you see several

future temperature projections from the
IPCC. And what you can see is, that in all

of them, the temperature increase, even
the lowest one, the temperature increase

is much faster than it was ever recorded
before. So I think it's safe to say that

we have truly entered the Anthropocene and
that humans have become a geological

force. So in the Anthropocene, humans have
become the single most important driver of

global change affecting the entire Earth
system, including our ice sheets. But it

was kind of the opposite in the past. Like
no other forces on the planet, ice ages

have actually shaped our surroundings and
thereby determined our development as

human civilizations. For instance, we owe
our fertile soils, to the last ice age,

that also carved our current landscapes
that we see all around us, leaving

glaciers behind, rivers and lakes. So even
though the ice sheets on Greenland and

Antarctica might seem far away sometimes,
they're actually crucial also for us here

today. And today, I want to leave you with
an impression why they are so important.

And one reason why they are so important
is because they're an amazing climate

archive. Here you see an ice core taken
from one of the deepest parts of an ice

sheet. And this is basically like counting
tree rings. You can go back to the past

and you can see what the climate was like
in the deep past, ranging several hundreds

of thousands of years back. And you can
see the conditions, for instance, in the

CO2 change, the temperature change over
this really long timescales. So that's one

of the reasons why the ice sheets are so
important. Another one is their so-called

sea level potential. Greenland and
Antarctica are truly sleeping giants. And

to give you an idea of the sheer size of
these two ice sheets, one way of doing

that is to compute their ice volume in the
so-called sea level equivalent. What this

means is, if we were to melt down the
Greenland ice sheet and distribute that

meltwater around the entire globe, then
this would lead to a global sea level rise

of roughly seven meters. For the West
Antarctic ice sheet, it's about five

meters, and for East Antarctica, the
tenfold. So more than sixty five meters in

total of sea level potential that are
stored in these two ice sheets. Now, over

the past decades, the ice sheets have both
been losing mass and they've been losing

mass at an accelerating pace. In fact,
we're currently on track with the worst

case climate change scenario. Here you see
the observations in gray and you also see

several of the projections from the past
for the ice sheets. And as you can see,

we're tracking this upper branch here. So
we're really on track with the worst case

climate change scenario for the ice
sheets. And what this means is even if we

were to stop global warming today, the ice
sheets would still keep losing mass

because of the inertia in the system. So
sea levels would keep rising for decades

or even centuries to come. Why is that?
Well, there are several processes that we

need to understand in order to keep track
of sea level change and also to understand

the ice sheet's evolution in the past and
in the future. Here, you see sort of an

exemplary cut through an ice shelf system,
where the ice sheet is in contact with the

atmosphere. You have a grounded part and
then in many places, you also have these

extensions, these floating extensions, the
so-called ice shelves that surround

particularly Antarctica. The separation
between the two is the so-called grounding

line. Now, generally ice sheets gain mass
through snowfall just on top of the ice

sheet, which then is compressed into ice
and over time, due to the sheer gravity

and the sheer size of the ice sheets, it's
basically pushing its own mass towards the

ocean. And that's one of the reasons why
there's a constant flow of ice. So ice is

really not only a solid, it's also a
fluid. The ice sheets can also lose mass

through surface melting, but also through
melting at the underside of the floating

ice shelves, where they're in contact with
warmer ocean waters. And then there can,

of course, also be ice shelf calving, so
icebergs that break off at the margins of

the ice sheet. Now, what we see here, this
left hand side, that's a typical situation

for the Greenland ice sheet. The Greenland
ice sheet is generally grounded above sea

level in most parts and it's not only much
smaller than Antarctica, but it's also

located further south, so further away
from the pole. And that means it's

generally warmer in Greenland, leading to
more surface melt for the Greenland ice

sheet. Whereas in Antarctica, it's not
only much colder there, but also the ice

sheet is covered and surrounded by
floating ice shelves almost all around the

coastline. And that means that one of the
most important driving processes for mass

loss in Antarctica is this melting
underneath the ice shelves, so the

subshelf melting in contact with the
warmer ocean waters. Just to give you an

impression of the sheer ice thickness, I
brought this picture here. This is my very

first impression of the Antarctic
coastline, the ice shelf margin. This is

close to the German research station
Neumayer III. And I will never forget the

moment that I first saw the ice shelf
edge. It was in the middle of the night,

but we were there in summer, so we had
twenty four hours of daylight. And I woke

up because it suddenly got dark in our
cabin. So I went up to the bridge to see

what was going on and I saw myself in
front of a wall, like really a cliff of

ice. And knowing that these ice shelves
behave like the ice cubes in the water

glass, so only roughly 10 percent are
visible above the sea level, this means

that in this case, we had an ice shelf
edge that was more than 100 meters thick.

And that really impressed me. I
immediately had to think of this German

expression, "das ewige Eis", the eternal
ice. And I really wondered if this is

maybe the right expression because it
seemed like it was so static and nothing

was moving. However, that's not true
because even in equilibrium, the ice is

constantly moving. It's here just
visualized by these little snowflakes and

you can see how the ice is moving from the
interior towards the coastlines. And we

have a wide range of velocities at the
surface, ranging from almost zero in the

interior of the ice sheet to several
kilometers per year in the larger ice

shelves and also the so-called ice
streams, the faster flowing ice. If I were

able to take a dive underneath the ice
shelves and I could actually take a look

at the grounding line, this would probably
be what what I could see. This is the

triple point basically where solid earth,
the ice and water all come together. And

this grounding line is a very important
role for Antarctic ice dynamics and also

for the future fate of Antarctica. So what
makes the dynamics of the ice sheets and

shelves so particularly difficult to
understand and also to project the future

evolution is that both ice sheets are
subject to several so-called positive, so

self-reinforcing feedback mechanisms. Here
are just some examples with some of the

major ones we know very well. One is the
ice-albedo-feedback and another one is the

so-called melt-elevation-feedback. As I
said, in Greenland we observe a lot of

surface melting. If you've ever flown
across the Greenland ice sheet in summer,

you can really see these rivers forming
and then even lakes forming at the ice

sheet surface. And over the recent decade,
Greenland has been subject to several

extreme melt events, including
particularly the year 2010, 2012 and also

last year. And the reason there's this
extreme melting at the surface is due to a

combination of factors, it has to do with
the duration of the summer, but also even

here in Europe, we observed very warm and
dry summers. And that's also something

that was observed for Greenland. So that,
for instance, in the year 2019 in August,

almost the entire ice sheet surface was
covered with meltwater. Now, why is this

surface melting so important? The reason
is that there is also a self-reinforcing

feedback that could be driven by surface
melting. And we all know this mechanism

from mountain climbing. If you climb down
from the peak of a mountain towards the

valley, it gets warmer around you. And the
same is true also for the ice sheets. So

if there's enough melting, it could
actually lower the surface to a region

where the temperatures are higher, the
surface temperatures are higher, leading

to more melting, which again lowers the
surface elevation, leading to higher

temperatures, leading to more melting and
so on and so on, so that this can trigger

these self-reinforcing dynamics. And
whenever we have such a positive or self-

reinforcing feedback mechanism, we can
also have a tipping point. And here is the

depiction of a very simple way of
computing, where this tipping point might

be for the Greenland ice sheet, where
we've really done this with just

analytical work. So pen and paper, trying
to understand where we go from a stable

Greenland ice sheet into unstable regime,
which would then lead to a meltdown of the

entire ice sheet until basically no ice is
left at the surface. So this is something

that we can understand in theory, but also
something that we find in more complex

numerical ice sheet models. And they find
that this warming threshold that leads to

basically a decay of the entire ice sheet
lies somewhere between 0.8°C and 3.2°C of

warming above pre-industrial levels. And
you can see that between these

temperatures, somewhere there's almost a
step change. This is now the computed sea

level rise. So up here, this means that
Greenland is ice free. So we're going from

an intact Greenland ice sheet to an ice
free Greenland somewhere between these

temperatures. What this looks like can be
visualized with numerical ice sheet

models. And here you see that once this
threshold is exceeded, basically the

eigendynamics lead to a complete meltdown
off the ice sheet, until there's almost no

ice left except for in the highest regions
here in the east where there are some

small ice caps remaining. Now, something
similar, but also different is going on in

Antarctica because, as I said earlier, in
Antarctica it's much colder. So we have

very little surface melt at the moment.
But at the same time, it's surrounded by

the floating ice shelves and they play the
major role in driving sea changes in

Antarctica. Antarctic mass loss has
tripled over the recent years, especially

in the so-called Amundson and
Bellingshausen Sea regions. So these are

these regions here where you see all these
red parts. So this is all ice loss that's

been detected here. And the reason for
this is due to the ice shelf ocean

interactions. So here you now see the
ocean temperatures surrounding Antarctic

ice shelves. And you can see a stark
difference between the temperatures here

around the Amundson and Bellingshausen
regions and the temperatures, for

instance, here in the Weddell Sea or in
the Ross Sea, the temperature difference

being roughly two degrees. So there's
really been a switch from a colder to a

warmer cavity, for instance, here in the
Amundson Sea region. And that drives more

sub shelf melting, which in turn leads to
a decrease of the so-called buttressing

effect. What this means is, well, first of
all, the ice shelves do not contribute to

sea level rise directly, at least not
significantly. The reason being that they

are like ice cubes in a water glass. And
if that melts down, it also doesn't raise

the water level in the glass. So it's
similar with the ice shelves, but at the

same time they are still attached to the
grounded part of the sheet. So if the ice

shelves melt or there are larger calving
events in the ice shelves, that means that

the flow behind them from the interior of
the ice sheet into the ocean accelerates.

It's almost like pulling a plug. And this
is what is the so-called buttressing

effects, so the backstress at the
grounding line. So if we have enhanced ice

shelf melting, that means that this
buttressing effect, this buffering effect

is reduced and therefore we have
accelerated outflow into the ocean. Now,

the question is, how does this impact the
ice sheet dynamics overall, in particular,

the stability of the West and East
Antarctic ice sheets. You may have come

across some of these headlines in recent
years. My favorite one is still this one

up here from 2014 where the "Holy Shit
Moment of Global Warming" was declared.

And the reason for this were these
observations from the Amundson region in

West Antarctica. So we're now taking sort
of a flight into the Amundson Sea region.

And what was observed over the recent
decades is not only that the glaciers here

have accelerated, so everything that's
shown in red is accelerated ice flow, but

at the same time, the glaciers have also
retreated into the deeper valleys behind.

So you see this browning at the surface
now. So all of these changes where the

glaciers have basically retreated and with
this comes another self reinforcing

feedback, the so-called marine ice-sheet
instability. For the marine ice sheet

instability to occur, we need two
conditions to hold. One, as depicted here,

is that the ice sheet is grounded below
sea level, which is true for many parts of

West Antarctica, but also some parts of
East Antarctica. And also we need to

generally have a retrograde sloping bed.
So that means that the bedrock elevation

decreases towards the interior of the ice
sheet. And when these two conditions hold,

then we can show in two dimensions,
mathematically, we can prove

mathematically that an instability occurs
in this case. The reason is that we have

an feedback between the grounding line
retreat and the ice locks across the

grounding line. If the grounding line
retreats in a case where we have a

retrograde sloping bed and the ice is
ground below sea level, that means that

the ice thickness towards the interior is
larger. And this generally also means that

the ice flux across the grounding line is
larger, leading to further retreat off the

grounding line and so on and so on. So
again, we have a positive feedback

mechanism that could drive self-sustained
ice loss from parts of the West and East

Antarctic ice sheet. And the concern is
now that this marine ice sheet instability

is potentially underway in the Amundson
basin here in West Antarctica. Now, what's

unclear is, how fast this change would
actually occur. So if we have actually

triggered the marine ice sheet instability
in this region, and that means we have a

committed ice loss of roughly one meter
sea level equivalent, then the question is

still, how fast does this occur? And for
this, it really matters how much further

global warming continues. So and at which
rate the temperature will change in the

future. So this is what's happening in
part of the West Antarctic ice sheet. We

were also asking ourselves, weather could
something like this also happen for East

Antarctica and how stable are each of the
different ice basins in Antarctica? So we

did something of a stability check on the
Antarctic ice sheet to assess the risk of

long term sea level rise from these
different regions. What you will see next

is an animation where we're increasing the
global mean temperature, but we're

increasing it very, very slowly, at a much
slower rate than the typical rate of

change in the ice sheet to test for the
stability of these different parts. And

what we see is that at roughly 2°C, we are
losing a large part of the West Antarctic

ice sheet. So there's a first tipping
point around 2°C. And then as the

temperature increases, also the surface
elevation is lowered. And that leads to,

potentially then also triggering these
surface elevation and melt elevation

feedbacks in East Antarctica. So around
6°C to 9°C, there's another major

threshold. And after this, large parts of
the East Antarctic ice sheet could also be

committed to long term sea level rise. At
about 10°C, the Antarctic ice sheet could

potentially become ice free on the long
term. And, this is really important. What

we're seeing here are not projections, but
what we're seeing here is a stability

check. So we're not looking at something
that's happening within the next century

or so, but rather we're interested in
understanding, at which temperatures the

Antarctic ice sheet could still survive on
the long term. We also wanted to see if

some of these changes are reversible. And
what we find is a so-called hysteresis

behavior of the Antarctic ice sheet. That
means, as we're losing the ice and we'll

then cool the temperatures back down, the
ice sheet does not regrow back to its

initial state, but it takes much, much
colder temperatures to regrow the same ice

sheet volume that we are currently having
at present day temperature levels. So

there's a significant difference between
this retreat and the regrowth path. And

this can be up to 20 meters of sea level
equivalent in the difference between these

two paths. What this looks like
regionally, you can see here. So again, we

have the retreat and the regrowth path at
2°C of global warming, and 4°C of global

warming. So these are the long term
effects at these temperature levels. And

you can see that, for instance, for 4°C
large parts of East Antarctic and also of

the West Antarctic ice sheet do not regrow
at the same temperature level. So we

clearly observe this hysteresis behavior.
That's another sign that the Antarctic ice

sheet is the tipping element in the
climate system. So both Greenland and

Antarctica are tipping elements in the
climate system. There are a number more

candidates for tipping elements, including
some of the larger biosphere components,

for instance, the Amazon rainforest, the
tropical coral reefs, and also the boreal

forests, as well as some of the large
scale circulations. So, for instance, the

Atlantic thermohaline circulation, what we
often term the Gulf Stream, and the Indian

summer monsoon are tipping candidates in
the climate system. Now, if we go back to

our temperature evolution since last
glacial maximum, and we now insert what we

know about the tipping thresholds of these
different components in the Earth system,

then this is what we get. And we see, that
there are basically three clusters of

tipping elements in comparison to the
global mean temperature here. And you see

in these burning ember diagrams that some
of these tipping elements are at risk of

switching into a different state, even
within the Paris range of 1.5 - 2°C of

warming. And among these most vulnerable
tipping elements are the West Antarctic

ice sheet and the Greenland ice sheet and
in general, the cryosphere elements which

seem to react to global warming and
climate change much faster and therefore

belong to the most vulnerable parts of the
Earth system. So, if there's one thing

that I would like you to take away from
this talk, it is that ice matters. I've

presented you with three reasons why.
First of all, polar ice acts as a climate

archive. It also acts as an early warning
system. Secondly, glaciers and ice sheets

are important contributors already to
current sea level rise, but they will

become even more important in the future
as the global mean temperature keeps

rising. And thirdly, both Greenland and
Antarctica are tipping elements in the

Earth system. And one of the next things
we need to understand is how these tipping

elements interact with one another.
Because we have a very good understanding

by now of the different mechanisms behind
these tipping elements and of the

individual temperature thresholds. But one
of the, I think, most important questions

we need to ask ourselves, is how the
interaction of the tipping elements

changes the stability of the Earth system
as a whole and if there could be something

like domino effects in the Earth system.
And with this, thank you so much for your

attention. And I'm very much looking
forward to questions.

Herald: Yeah, OK, fine, good, läuft, könnt
ihr mich also hör'n, und ihr müsst mir

also sagen, wann ich wieder drauf bin.
Off: Du bist live.

H: Hallo, wilkommen zurück! Thanks for
this awesome talk, Ricarda, and we are now

going to have a Q&A. And if you have any
questions regarding this awesome talk,

then please post them to the signal
angels. They are following on Twitter and

the Fediverse here, using the hashtag
#rc3one, because this is rc1. And you can

also post your questions to the IRC. You
know, I already have a first question. I

don't know, Ricarda, if you can hear me,
but is there anything that this specific the CCC

community of nerds and hackers can do more
than anyone else to help with this issue?

What do you think that
we can do to help this?

R: Yeah, thank you so much. Great
question. Let me start by saying I'm a

nerd and hacker myself. I'm a developer,
or code developer, of the parallel ice

sheet model. That's one of the ice sheet
models for Greenland and Antarctica that's

being used around the globe with many
different applications. So, yeah, as a

fellow nerd and hacker, I can say there's
lots we can do, in particular towards

understanding even better the different
dynamics of the Greenland and the

Antarctic ice sheet, but also beyond that,
for the Earth system as a whole. I think

we're now at a point where we understand
the individual components of the Earth

system better and better. We also have
better and better observations, satellite

observations, but also observations at the
ground to further understand the different

processes. But what we need now is to
combine this with our knowledge in the

modeling community and also with some of
the approaches from big data, machine

learning and so on, to really put this
together, all the different puzzle pieces

to understand what this means for the
Earth system as a whole. And what I mean

by that is, we now understand that there
are several individual tipping points in

the Earth system. And we also know that as
global warming continues, we're at higher

risks of transgressing individual tipping
points. But what we still need to

understand is what does this mean for the
overall stability of our planet Earth?

H: Thank you for this extended answer to
this question. I have another one. I would

like to know, I mean, you showed a slide
where you showed the browning of the ice

surface and then explained that this
speeds up the process of melting as well.

But, can we just paint it white or with a
reflective paint on it? Has this been

simulated? Is this of interest to you
scientists?

R: Yeah, very good question. So basically
what you're addressing here is the

question of the so-called ice albedo
feedback. We all know this. As we're

wearing black clothes in summer, it's
warmer than when we're wearing white

clothes. And the same is basically true
for our planet as well. So the ice sheets

and also the sea ice in the Arctic and
Antarctica, they contribute considerably

to a net cooling still of the planet. So
if we didn't have these ice landscapes,

that would mean that the planet would warm
even faster and even further than it

already is today. So currently, the ice
albedo feedback is still helping us with

keeping the temperatures at lower levels
than they would be without the ice

landscapes. And, yeah, therefore, it is
definitely of interest to further

understand what would this mean for, for
instance, the global mean temperature, but

also regional changes, if we were to lose
our ice cover completely? And also the

reverse question, of course, if we were to
whiten parts of the planet, then how would

this affect temperature? One thing that we
found out is that if we were to lose the

ice sheets and the sea ice in terms of the
ice albedo feedback alone entirely, then

this could already lead to an additional
global warming of roughly 0.2°C. Now, that

may not seem very much, but it certainly
is important in the grand scheme of

things. As we're thinking of, for
instance, the Paris range of 1.5°C to 2°C

of warming, every tenth of a degree
matters. So, yeah, very interesting

question. And this is something that has
been done with numerical models, just to

understand what kind of an effect these
kind of what-if-scenarios would have also

in terms of the albedo.
H: Very interesting. So should we now

start to develop drones
who can spray paint?

R: laughs That's a good question. I
don't think that's the solution. I think

we have a much better solution. And that
is we know that we need to to mitigate

climate change and reduce greenhouse gas
emissions. And that is one that would work

for sure. Whereas these questions of,
well, should we spray paint all of our

buildings at the at the top white? That is
something that cannot be done at such a

large scale as we would need it in order
to reverse global warming. And another

thing to keep in mind is that even if we
were able to reduce the global signal,

this still doesn't mean that we could also
reverse the regional scale changes. We're

already experiencing a large increase in
extreme weather and climate events. And

that is certainly something that I haven't
seen so far, that this could also be

reversed just by reversing the global mean
temperature change as a whole.

H: I have another question. I think that's
quite interesting. How old is the oldest

ice in Antarctica? Are you aware of that?
And how long would it take a minimum to

lose that entirely?
R: Yeah, very good question. So the oldest

ice, there's actually an ongoing search
for the oldest ice in Antarctica. So to

say, we know that Antarctica was ice free
for the last time, roughly 34 million

years ago. So when we're talking about
these scenarios that eventually Antarctica

could become ice free with, of course,
very strong global warming scenarios of

about 10°C of global warming, then we need
to keep in mind that this was the case for

the last time, about 34 million
years ago. Now, as we're speaking, there

is an ongoing project, an international
collaboration to find and and also drill

for the oldest ice so that we can really
understand our Earth's history better and

better. And so this is a very exciting
project because, as I said, the ice cores

are kind of like tree rings and we can
count back in time and really understand

what our global climate was like several,
hundreds of thousands of years ago. So,

yeah, with that being said, I think it's
important to keep in mind that this is

something that humans certainly have never
experienced and that's therefore

unprecedented in our world.
H: ...for this very elaborate answer to

this question, I know it is not the core
of your research, but someone from the

internet asked, if it's possible for old
viruses and all the bacteria from back

when Antarctica was like beginning to
freeze over or from like

millions of years ago, is it possible for
them to thaw out again? Is that a danger

for us?
R: Oh, that's also a very interesting

question. So I'm no expert on this, but I
could imagine that at the temperatures

that we have, Antarctica, especially the
core ice body, there we have temperatures

that go down to, well, I think the coldest
temperature was something like -90°C that

was recorded there. But in any case, it's
very cold there. So there might be some

bacteria that can survive these
conditions. And I've read about bacteria

like that, but I wouldn't know that there
are many bacterial species or specimen

that could survive these kinds of
conditions. So to be honest, I would have

to read up on that. That's a very
interesting question.

H: Yeah. Thank you for this answer. I
remember that you watched, that you showed

an animation and a graph for a simulated
ice decline to find the tipping points in

Antarctica. And on the x axis of that, I
couldn't see a time scale. And now someone

asked on the internet, what are the
timescales between reaching a tipping

point? And most of the ice being melted?
Is that years, decades, centuries,

millennia? What's kind of the scale there?
R: Yes, very important point. So it's

important to note that we're here showing
this over the global mean temperature

change. And the reason for this is that
the way these kind of hysteresis

experiments are run is that you have a
very slow temperature increase. So slow,

in fact, that it's much slower than the
sort of internal time scales of the ice

itself. And in this case, for instance, we
had a temperature increase of

10^-4°C/year. And the reason for this is
because this is the way you're approaching

the actual hysteresis curve that we were
interested in. So this should not be

mistaken for sea level projections of any
sort. So what we find here are the actual,

so to say, tipping points, the actual
critical thresholds, that parts of the

Antarctic ice sheet cannot survive.
Nonetheless, of course, we're also working

towards sea level projections and trying
to understand what kind of sea level

change we can expect from the ice sheets
over the next decades to centuries to

millennia. And one important thing there
is that most of the ice loss that could be

triggered now, would actually happen after
the end of this century. So very often,

when we see these sea level curves, we're
looking until the year 2100. So for the

next decades, how does the sea level
respond to changes in temperature? But

because we have so much inertia in the
system, that means that even if the global

warming signal was stopped right now, we
would still see continued sea level rise

for several decades to centuries. And that
is something important to keep in mind. So

I think we really need to start thinking
of sea level rise in terms of commitment

rather than these short term predictions.
That being said, another important

question and factor is the rate of sea
level change, because this is actually

what we need to adapt to as civilizations.
When we think of building dams, there are

two questions we need to answer. One is
the magnitude of sea level rise and and

also in its upper scale and upper limit to
that. And the other question is the rate

at which this changes. And what we find is
that on the long term, there is something

like 2.3m/°C of sea level change. So this
is sort of a number to keep in mind when

we think of sea level projections. And
yeah, I think it's really important to

consider longer timescales than the one to
the year 2100 when we talk about sea level

rise.
H: Thank you for this answer, very

interesting and we are out of time now, so
thanks for all the questions and thank

you, Ricarda, for this amazing talk. The
next talk on this stage will be about a

related topic, measuring CO2 indoors, but
also in the atmosphere in general. But

before that, we have a Herald News Show
for your prepared. So enjoy!

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