The secret landscape buried under the Antarctic ice sheet
[Music]
what if i told you
that the shape of the bedrock buried
under thousands of meters of ice was the
key to future sea level rise
what you see in this photo is a glacial
fjord it’s a fjord that has been carved
by a glacier over the past tens of
thousands of years and is now exposed
because the ice has retreated in this
region and this is the sort of landscape
that we expect to see under the rest of
the aishi but it’s just very difficult
to see through the ice and see what the
standscape looks like
so today i will tell you how we
accidentally found a way to unveil this
landscape and how we found the deepest
point on land
so how did we get there i’m a
glaciologist i make computer models
in order to make predictions of sea
level rise and how the ice sheets are
affected by climate change and have been
doing this for about 13 years
while it is clear that sea level is
rising today we can measure it
we don’t know to this day whether the
ice sheets are going to
catastrophically collapse over the next
100 200 years it’s just very difficult
to do there is a lot of uncertainty in
sea level predictions and it remains too
uncertain for policy makers to make
informed decisions about how to cope
with rising seas
to understand how the ice sheets respond
to climate change we must understand why
they are there in the first place
and it is so cold in greenland and
antarctica that
most of the snow that falls every year
on the ice sheet doesn’t melt away and
so year after year this no this snow
slowly becomes solid ice we may think of
the ice sheets and glaciers as
big pieces of ice cubes that don’t move
but they actually move they flow it
deforms under its own weight like a very
viscous fluid like a thick syrup in a
way so what you see in this video
is a compilation of photos that were
taken a few hours apart and you see two
years of that compilation so you can see
that the ice is flowing downhill like a
river
so eventually this ice that is formed by
the transformation of snow into ice
is transported to the coast in the form
of fast rivers of ice that we call ice
streams and eventually this ice melts or
there’s the formation of icebergs that
end up melting in the ocean
so if we want sea level not to change if
you if we don’t want sea level to change
we need the perfect balance between new
eyes formed by the slow transformation
of snow into ice and the loss of ice
through icebergs and melting if these
two processes balance one another then
we don’t see sea level change
but what we’ve observed over the past
two three decades is that
our glaciers are accelerating they’re
putting more icebergs into the ocean and
also in the arctic in greenland we have
warmer summers longer summers a lot more
melt on the surface so overall the
balance now we have about the same
amount of snow as before or maybe a
little less and a lot more mass loss so
that’s why the ice sheets are shrinking
now there is an important point that i
want to make
the rate at which the ice sheets are
losing mass depends to a large extent on
the shape of the bedrock and there is
two reasons for that one that’s fairly
simple is
one reasons for which that explains why
our ice sheets has been have been
accelerating like this is that there is
warm water in the ocean that used to
stay away from the ice it used to
to not interact with it and now because
of climate change it’s we have warm
water intrusion into fjords under the
ice where we have flowing ice and it’s
melting it from below
and so you can imagine that if the bed
is shallow if we have big ridges
mountain wrenches that may block this
warm water that is at depth that is
under 300 400 meters it may block this
warm water so the glaciers that have a
big bump a big mountain range in front
of them may be protected
but more importantly the shape of the
bedrock
can make a difference between a slow
gradual retreat
or a fast unstoppable retreat
and the reason for that is that the ice
sheets
are they they weigh you know a lot it’s
a lot of mass and so over time it has
pushed the land below and in the
interior of the ice sheet many times
it’s it’s the bed is below sea level and
you may have that configuration that we
call retrograde bed slope where the bed
gets deeper as we move inland and that’s
a configuration that we know isn’t
stable if a glacier starts to retreat in
a region where the bed gets deeper and
deeper
there is no stopping it it’s going to
continue to do so even if we stop
burning fossil fuel
so we need to know where the bed is
retrograde we need to know where we have
bumps and ridges that may stop that
retreat otherwise if we don’t have a
good representation of the bed there is
no way we can make accurate predictions
of sea level rise
but it’s incredibly difficult to see
through thick eyes it’s just very
difficult to know what it looks like but
we need that information i’m lying a
little bit there is one way to see
through the eyes that’s pretty powerful
and it’s basically based on radar so you
mount a big radar under the wings of an
airplane and
it emits a signal that will penetrate
the ice and when it comes back we can
interpret this as some sort of x-ray so
here um in that image you can see the
air you can see how deep the ice is we
have incredible details in the bedrock
so it’s really a wonderful method
but we only get information directly
underneath the aircraft there is no
information about the sites
what you’re seeing here
all these colorful lines are all the
lines for which we have measurements of
the bed
so this represents 50 years of
international campaigns very costly
international campaigns to try to
determine how deep the ice is
and you may wonder why is he complaining
we have so much data but
this is austria
we have to remember that these ice
sheets are huge and there are many
places where we don’t have any
measurements for within a radius of tens
hundreds of kilometers
and that’s making our life as modelers
difficult
i’m going to talk to you a little bit
about models because as i told you
earlier i’m a modeler and when i was a
graduate student
i developed with two colleagues a new
ice sheet model that was supposed to be
better than anything that had been done
before more accurate with better physics
and to put it in a nutshell a model a
numerical model is based on fundamental
physical principles like the
conservation of mass conservation of
energy
and
basically the conservation of mass if
there is one bad summer where there is a
lot of melts the conservation of mass
tells us by how much we should lower the
surface topography of the ice sheet so
in the model
we we were ready to run it it was a
beautiful model we were ready to run at
a higher resolution than ever before
and we ran it forward but greenland was
gaining mass when clearly when you look
at every single observation it’s
supposed to lose mass
so we were really
like what is going on the model is wrong
is it a bug in the code is it the
physics is it are we missing an
important physical process are we you
know what can it be and it’s very
difficult to narrow down
why the model is not behaving the way it
should
so we spent months
trying one by one every single single
hypothesis hypothesis
and we had to come to um the idea that
it was the poor representation of the
bad topography especially where we did
not have measurements we what we had was
not good enough was blocking the model
from flushing that ice towards the ocean
and as a result the ice was gaining mass
so we were like okay what do we do we’re
not going to spend thousands of
airplanes um and measure the whole of
the ice sheet within a resolution of you
know a few hundred meters it’s not going
to be possible so what do we do
and
by accident a little bit we developed
that method basically in
in in the mass conservation approach
what we do is we look at bi speed we can
get that from from satellite it’s very
easy to know how fast the ice is flowing
we combine that with ice depth and that
gives us
the thinning rates so by how much the
eyes get thicker or thinner and we have
that eureka moment we thought okay
we know that mass conservation is good i
mean it’s a physical principle that’s
always verified we have high speed we
have the thinning rates from satellite
interferometry we know where the ice is
thickening or thinning so why don’t we
move these terms around and say let’s
use the same physics but
our unknown instead of computing the
thinning rates we now compute the ice
depth and so we were able to use that
method to fill in the gaps where we did
not have data we where we did not have
any measurements but based on that
physical principle and on combining
other data sets that were not used in
the past to try to map the bed
so after
years of sweat and many sleepless nights
we had i managed to map the whole ice
sheet of greenland and antarctica at an
unprecedented level of resolution
so i’m going to show you here a few
results
this animation shows you the west coast
of greenland
so
to your right i believe is the ice
velocity from satellite and then to the
left we peel off the ice and you can see
the bed so everything that is blue means
that the bed is below sea level but
there is no ocean there remember that
it’s it’s a fjord that’s full of ice
and what you see is we have these fjords
these valleys that we did not know about
before for many of them
extend sometimes for tens or hundreds of
kilometers
and so what this means is as the ice
retreats
because the bed the these valleys are so
deep they will still be in contact with
that warm water that is at depth
under antarctica
the landscape is pretty different
we don’t have that many narrow entrance
valleys what we have in many places are
these very wide valleys big basins that
are below sea level you see how
everything here is blue so we have a
pile of ice on top it’s grounded ice
but the bed is below sea level and this
is a glacier we’re particularly worried
about to its glacier because it’s been
thinning and retreating and accelerating
over the past decades
and you see how upstream the bed gets
deeper retrograde bed slopes
if it starts getting into that region
there is no stopping it
in east antarctica a lot of the bed is
above sea level but we found a few
a few troughs a few valleys that we
didn’t know about this one is nina’s
glacier but there is another glacier
denman glacier
that people scientists knew it was deep
what we never knew we’ve never been able
to figure out how deep it was the radar
that people were using
could not detect how deep the bed was
because it was just so deep
and so by combining these data sets
together the conservation of mass we
were able to map for the first time this
deep canyon that you’re seeing here and
it’s more than 3.5 kilometers below sea
level
so there are places that are deeper
under the ocean but this is considered a
continent a continent this is land ice
so that’s the deepest point on land that
has ever been mapped
so
overall we
uncovered the landscape hidden beneath
the eyes by combining
data sets together by looking at physics
by looking at remote sensing from
satellite interferometry by using
modeling
we combine disciplines together
and when it comes to climate change
we’re facing many challenges and we have
to be creative we have to have more
collaborations more cooperation across
disciplines in order to overcome the
challenges we’re facing
we discovered major features we
discovered canyons mountain ranges
valleys that we did not know were there
and they have major implications on
future sea level rise but if the shape
of the bed preconditions their stability
now we know which region may be more
vulnerable to climate change which one
may be more protected we have to
remember that it is our actions today
that will determine the fate of these
ice sheets they’re not melting because
of the bed they’re melting because of
the concentration of carbon dioxide in
the atmosphere
so yes our future lies underneath the
eyes but we should do everything in our
power to keep the ice there so let’s try
to keep this landscape hidden for as
long as we can
and reduce future sea level rise for the
next generations thank you
you