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