Jericho tokoto on the ice sheets in Greenland and Antarctica are the largest
Greenland and Antarctica are the largest
Greenland and Antarctica are the largest sources of long-term hire and sea level
sources of long-term hire and sea level
sources of long-term hire and sea level rise on earth today and the ice sheet in
rise on earth today and the ice sheet in
rise on earth today and the ice sheet in West Antarctica is the largest source of
West Antarctica is the largest source of
West Antarctica is the largest source of uncertainty in the sea level rise
uncertainty in the sea level rise
uncertainty in the sea level rise projection now to look at a map of the
projection now to look at a map of the
projection now to look at a map of the Antarctic you might not think that
Antarctic you might not think that
Antarctic you might not think that that’s the case the ice sheet in the
that’s the case the ice sheet in the
that’s the case the ice sheet in the East is much larger than the ice sheet
East is much larger than the ice sheet
East is much larger than the ice sheet in the West the coastline of the East
in the West the coastline of the East
in the West the coastline of the East Antarctic Ice Sheet along the Southern
Antarctic Ice Sheet along the Southern
Antarctic Ice Sheet along the Southern Ocean is much longer than the coastline
Ocean is much longer than the coastline
Ocean is much longer than the coastline of the West Antarctic so to understand
of the West Antarctic so to understand
of the West Antarctic so to understand why glaciologists like myself are so
why glaciologists like myself are so
why glaciologists like myself are so concerned about what happens next in
concerned about what happens next in
concerned about what happens next in West Antarctica we’ve got to peel back
West Antarctica we’ve got to peel back
West Antarctica we’ve got to peel back the surface and look at the system in
the surface and look at the system in
the surface and look at the system in more detail this is a map of the surface
more detail this is a map of the surface
more detail this is a map of the surface elevation of the ice sheet the warmer
elevation of the ice sheet the warmer
elevation of the ice sheet the warmer colors are higher the cooler colors are
colors are higher the cooler colors are
colors are higher the cooler colors are lower if you think about that shape the
lower if you think about that shape the
lower if you think about that shape the East Antarctic Ice Sheet is kind of flat
East Antarctic Ice Sheet is kind of flat
East Antarctic Ice Sheet is kind of flat in the middle it gets steep on the edges
in the middle it gets steep on the edges
in the middle it gets steep on the edges it flows slowly in the middle it flows
it flows slowly in the middle it flows
it flows slowly in the middle it flows fast on the edges the West Antarctic Ice
fast on the edges the West Antarctic Ice
fast on the edges the West Antarctic Ice Sheet is fundamentally different than
Sheet is fundamentally different than
Sheet is fundamentally different than that rather than a concave down shape it
that rather than a concave down shape it
that rather than a concave down shape it has a concave up shape and if we look at
has a concave up shape and if we look at
has a concave up shape and if we look at what happens in an interior the ice
what happens in an interior the ice
what happens in an interior the ice sheet is high and as we go towards the
sheet is high and as we go towards the
sheet is high and as we go towards the coast it gets very flat and it’s flat
coast it gets very flat and it’s flat
coast it gets very flat and it’s flat because that ice is floating so there’s
because that ice is floating so there’s
because that ice is floating so there’s something different going on in West
something different going on in West
something different going on in West Antarctica and what I want to do is talk
Antarctica and what I want to do is talk
Antarctica and what I want to do is talk with you about why that’s the case and
with you about why that’s the case and
with you about why that’s the case and why it’s so important if we peel back
why it’s so important if we peel back
why it’s so important if we peel back the surface and look at the thickness of
the surface and look at the thickness of
the surface and look at the thickness of the ice around the Antarctic again
the ice around the Antarctic again
the ice around the Antarctic again warmer colors or thicker ice cooler
warmer colors or thicker ice cooler
warmer colors or thicker ice cooler colors in this map or thinner ice we see
colors in this map or thinner ice we see
colors in this map or thinner ice we see that the ice sheet has overrun
that the ice sheet has overrun
that the ice sheet has overrun everything that’s there it’s filled in
everything that’s there it’s filled in
everything that’s there it’s filled in the valleys and those are the thick
the valleys and those are the thick
the valleys and those are the thick places it’s overrun mountaintops and
places it’s overrun mountaintops and
places it’s overrun mountaintops and those are the thinner places before
those are the thinner places before
those are the thinner places before there was an ice sheet in Antarctica
there was an ice sheet in Antarctica
there was an ice sheet in Antarctica there was a continent and underneath the
there was a continent and underneath the
there was a continent and underneath the East Antarctic Ice Sheet it’s like the
East Antarctic Ice Sheet it’s like the
East Antarctic Ice Sheet it’s like the continental crust that we find in North
continental crust that we find in North
continental crust that we find in North America or Northern Europe but
America or Northern Europe but
America or Northern Europe but underneath the West Antarctic Ice Sheet
underneath the West Antarctic Ice Sheet
underneath the West Antarctic Ice Sheet before there was an ice sheet there was
before there was an ice sheet there was
before there was an ice sheet there was an ocean and the the ice as it does it
an ocean and the the ice as it does it
an ocean and the the ice as it does it grew glaciers and ice caps high in the
grew glaciers and ice caps high in the
grew glaciers and ice caps high in the mountains slowly flowed down toward the
mountains slowly flowed down toward the
mountains slowly flowed down toward the sea they displaced the ocean water and
sea they displaced the ocean water and
sea they displaced the ocean water and they covered over the sea floor and so
they covered over the sea floor and so
they covered over the sea floor and so today the base of the ice sheet is way
today the base of the ice sheet is way
today the base of the ice sheet is way below sea level
below sea level
below sea level and that fundamentally creates different
and that fundamentally creates different
and that fundamentally creates different flow and different behavior for the ice
flow and different behavior for the ice
flow and different behavior for the ice sheet to explain this or to understand
sheet to explain this or to understand
sheet to explain this or to understand it we’re going to need a little bit of
it we’re going to need a little bit of
it we’re going to need a little bit of physics we’ll talk about that physics
physics we’ll talk about that physics
physics we’ll talk about that physics using a cartoon and then we’ll look at
using a cartoon and then we’ll look at
using a cartoon and then we’ll look at the real system so this is sort of a
the real system so this is sort of a
the real system so this is sort of a slice through the West Antarctic Ice
slice through the West Antarctic Ice
slice through the West Antarctic Ice Sheet it’s thick in the interior it’s
Sheet it’s thick in the interior it’s
Sheet it’s thick in the interior it’s thin on the edge snow falls on the
thin on the edge snow falls on the
thin on the edge snow falls on the surface it densifies it becomes glacier
surface it densifies it becomes glacier
surface it densifies it becomes glacier ice as that ice gets thick and has some
ice as that ice gets thick and has some
ice as that ice gets thick and has some surface slope it starts to flow it flows
surface slope it starts to flow it flows
surface slope it starts to flow it flows down toward the coast it gets it speeds
down toward the coast it gets it speeds
down toward the coast it gets it speeds up and gets thinner as it goes
up and gets thinner as it goes
up and gets thinner as it goes eventually it gets thin enough to go
eventually it gets thin enough to go
eventually it gets thin enough to go afloat on the ocean surface at that
afloat on the ocean surface at that
afloat on the ocean surface at that point we don’t call it an ice sheet
point we don’t call it an ice sheet
point we don’t call it an ice sheet anymore we call it an ice shelf and the
anymore we call it an ice shelf and the
anymore we call it an ice shelf and the boundary between those two or the
boundary between those two or the
boundary between those two or the transition between the I’ve grounded ice
transition between the I’ve grounded ice
transition between the I’ve grounded ice sheet and the floating ice shelf we call
sheet and the floating ice shelf we call
sheet and the floating ice shelf we call the grounding line and if we want to
the grounding line and if we want to
the grounding line and if we want to understand change in West Antarctica we
understand change in West Antarctica we
understand change in West Antarctica we need to understand what’s happening with
need to understand what’s happening with
need to understand what’s happening with that grounding line it’s the place where
that grounding line it’s the place where
that grounding line it’s the place where ice that has been stored in the ice
ice that has been stored in the ice
ice that has been stored in the ice sheet returns to the sea so when we snow
sheet returns to the sea so when we snow
sheet returns to the sea so when we snow on the surface we’re taking water out of
on the surface we’re taking water out of
on the surface we’re taking water out of the ocean and sea level might go down
the ocean and sea level might go down
the ocean and sea level might go down when the ice flows back through the
when the ice flows back through the
when the ice flows back through the grounding line into the floating ice
grounding line into the floating ice
grounding line into the floating ice shelf the water is returning to the sea
shelf the water is returning to the sea
shelf the water is returning to the sea if we do more of that than we do
if we do more of that than we do
if we do more of that than we do snowfall then the sea level go back up
snowfall then the sea level go back up
snowfall then the sea level go back up to understand where the grounding line
to understand where the grounding line
to understand where the grounding line is we need to understand something about
is we need to understand something about
is we need to understand something about the physics of the system so we’ve got
the physics of the system so we’ve got
the physics of the system so we’ve got gravity is the force that causes the ice
gravity is the force that causes the ice
gravity is the force that causes the ice to flow it flows under its own weight
to flow it flows under its own weight
to flow it flows under its own weight and then a bunch of other forces in the
and then a bunch of other forces in the
and then a bunch of other forces in the system that balance that gravitational
system that balance that gravitational
system that balance that gravitational we call it a driving force and just
we call it a driving force and just
we call it a driving force and just start in the interior and we think about
start in the interior and we think about
start in the interior and we think about what the ice has got to flow over
what the ice has got to flow over
what the ice has got to flow over there’s some kind of bed underneath the
there’s some kind of bed underneath the
there’s some kind of bed underneath the ice sheet right the land surface and the
ice sheet right the land surface and the
ice sheet right the land surface and the ice has to push over that land surface
ice has to push over that land surface
ice has to push over that land surface to make its way to the sea if you think
to make its way to the sea if you think
to make its way to the sea if you think of a deck of cards sitting on a table
of a deck of cards sitting on a table
of a deck of cards sitting on a table and you push sort of down and across
and you push sort of down and across
and you push sort of down and across that deck of cards the table resists
that deck of cards the table resists
that deck of cards the table resists that that motion and it resists it the
that that motion and it resists it the
that that motion and it resists it the most where the cards are close to the
most where the cards are close to the
most where the cards are close to the table and the least where the cards are
table and the least where the cards are
table and the least where the cards are far away from the table and you get some
far away from the table and you get some
far away from the table and you get some shear in that deck of cards the same
shear in that deck of cards the same
shear in that deck of cards the same thing happens in the ice and the
thing happens in the ice and the
thing happens in the ice and the Antarctic ice sheet as the ice gets
Antarctic ice sheet as the ice gets
Antarctic ice sheet as the ice gets toward the coast the material underneath
toward the coast the material underneath
toward the coast the material underneath the ice changes so if you imagine now
the ice changes so if you imagine now
the ice changes so if you imagine now rather than
rather than
rather than your table and the deck of cards have
your table and the deck of cards have
your table and the deck of cards have got some water in between the cards and
got some water in between the cards and
got some water in between the cards and the table that water will help the cards
the table that water will help the cards
the table that water will help the cards as a whole stack shift across the table
as a whole stack shift across the table
as a whole stack shift across the table and you’ll you’ll start to transition
and you’ll you’ll start to transition
and you’ll you’ll start to transition from a place where a shearing is the
from a place where a shearing is the
from a place where a shearing is the most important thing to where the whole
most important thing to where the whole
most important thing to where the whole stack is sliding together and when we go
stack is sliding together and when we go
stack is sliding together and when we go afloat on the ocean surface the water is
afloat on the ocean surface the water is
afloat on the ocean surface the water is really weak compared to the ice and
really weak compared to the ice and
really weak compared to the ice and there’s no shearing at all it’s all just
there’s no shearing at all it’s all just
there’s no shearing at all it’s all just stretching out and in each of those
stretching out and in each of those
stretching out and in each of those transitions the ice flows faster than it
transitions the ice flows faster than it
transitions the ice flows faster than it did before so this is sort of at the
did before so this is sort of at the
did before so this is sort of at the heart of understanding how the West
heart of understanding how the West
heart of understanding how the West Antarctic Ice Sheet behaves it’s going
Antarctic Ice Sheet behaves it’s going
Antarctic Ice Sheet behaves it’s going to be the balance of all those forces
to be the balance of all those forces
to be the balance of all those forces together that place the grounding line
together that place the grounding line
together that place the grounding line wherever it happens to be and what we
wherever it happens to be and what we
wherever it happens to be and what we worry about in West Antarctica is that
worry about in West Antarctica is that
worry about in West Antarctica is that if we can do something to change the
if we can do something to change the
if we can do something to change the forces on the floating side of the ice
forces on the floating side of the ice
forces on the floating side of the ice shelf the grounded part of the ice sheet
shelf the grounded part of the ice sheet
shelf the grounded part of the ice sheet will respond as well and I’ll explain
will respond as well and I’ll explain
will respond as well and I’ll explain this in a sequence of steps if we
this in a sequence of steps if we
this in a sequence of steps if we increase the melting underneath the
increase the melting underneath the
increase the melting underneath the floating ice because the ocean is
floating ice because the ocean is
floating ice because the ocean is warming up because the atmosphere is
warming up because the atmosphere is
warming up because the atmosphere is warming up we can cause a little bit of
warming up we can cause a little bit of
warming up we can cause a little bit of ice right at the grounding line to go
ice right at the grounding line to go
ice right at the grounding line to go afloat when it goes afloat it doesn’t
afloat when it goes afloat it doesn’t
afloat when it goes afloat it doesn’t feel as much resistance as it did before
feel as much resistance as it did before
feel as much resistance as it did before so it flows faster than it did before
so it flows faster than it did before
so it flows faster than it did before when it starts to flow faster it
when it starts to flow faster it
when it starts to flow faster it stretches more it thins when the ice at
stretches more it thins when the ice at
stretches more it thins when the ice at the grounding line thins more ice goes
the grounding line thins more ice goes
the grounding line thins more ice goes afloat it feels a different traction on
afloat it feels a different traction on
afloat it feels a different traction on the underside it starts to stretch more
the underside it starts to stretch more
the underside it starts to stretch more it fins more the grounding line retreats
it fins more the grounding line retreats
it fins more the grounding line retreats when the grounding line retreats some
when the grounding line retreats some
when the grounding line retreats some more ice goes afloat it starts to
more ice goes afloat it starts to
more ice goes afloat it starts to stretch faster then some more ice goes
stretch faster then some more ice goes
stretch faster then some more ice goes afloat and if we can push the system
afloat and if we can push the system
afloat and if we can push the system fast enough we can actually generate
fast enough we can actually generate
fast enough we can actually generate kind of a runaway retreat in the system
kind of a runaway retreat in the system
kind of a runaway retreat in the system where the amount of the push doesn’t
where the amount of the push doesn’t
where the amount of the push doesn’t matter anymore in all that’s going on is
matter anymore in all that’s going on is
matter anymore in all that’s going on is that internal physics and the grounding
that internal physics and the grounding
that internal physics and the grounding line retreats simply because the
line retreats simply because the
line retreats simply because the grounding line is retreating we call
grounding line is retreating we call
grounding line is retreating we call this a marine I should retreat and
this a marine I should retreat and
this a marine I should retreat and physics we’d call it an instability so
physics we’d call it an instability so
physics we’d call it an instability so what we want to know when we study the
what we want to know when we study the
what we want to know when we study the West Antarctic Ice Sheet is we
West Antarctic Ice Sheet is we
West Antarctic Ice Sheet is we understand these physics theoretically
understand these physics theoretically
understand these physics theoretically this is certainly what goes on but does
this is certainly what goes on but does
this is certainly what goes on but does it happen in the real world as well we
it happen in the real world as well we
it happen in the real world as well we have a couple of ways we can try to
have a couple of ways we can try to
have a couple of ways we can try to answer that question one we can look to
answer that question one we can look to
answer that question one we can look to the geologic record of the past and say
the geologic record of the past and say
the geologic record of the past and say has this happened before
has this happened before
has this happened before the other thing we can do if we want to
the other thing we can do if we want to
the other thing we can do if we want to think about the future is build a
think about the future is build a
think about the future is build a computer model that simulates this kind
computer model that simulates this kind
computer model that simulates this kind of
of
of behavior and ask is the warming we
behavior and ask is the warming we
behavior and ask is the warming we expect in the future
expect in the future
expect in the future enough to generate that kind of response
enough to generate that kind of response
enough to generate that kind of response so we’ll talk about the geologic record
so we’ll talk about the geologic record
so we’ll talk about the geologic record first and we’ll compare what’s happening
first and we’ll compare what’s happening
first and we’ll compare what’s happening today with what’s happened in past warm
today with what’s happened in past warm
today with what’s happened in past warm worlds so in these plots I’ll show you
worlds so in these plots I’ll show you
worlds so in these plots I’ll show you atmospheric co2 relative to the
atmospheric co2 relative to the
atmospheric co2 relative to the pre-industrial condition so before
pre-industrial condition so before
pre-industrial condition so before people started burning fossil carbon the
people started burning fossil carbon the
people started burning fossil carbon the global mean temperature relative to the
global mean temperature relative to the
global mean temperature relative to the pre-industrial condition and then sea
pre-industrial condition and then sea
pre-industrial condition and then sea level relative to where we are today so
level relative to where we are today so
level relative to where we are today so the most recent warm time is about a
the most recent warm time is about a
the most recent warm time is about a hundred and twenty five thousand years
hundred and twenty five thousand years
hundred and twenty five thousand years ago it’s a time called the last
ago it’s a time called the last
ago it’s a time called the last interglacial when there was relatively
interglacial when there was relatively
interglacial when there was relatively little ice in what in the northern
little ice in what in the northern
little ice in what in the northern hemisphere compared to today the ice
hemisphere compared to today the ice
hemisphere compared to today the ice sheets in Greenland and Antarctica were
sheets in Greenland and Antarctica were
sheets in Greenland and Antarctica were a little bit different than they are
a little bit different than they are
a little bit different than they are today
today
today and at that time the world was about a
and at that time the world was about a
and at that time the world was about a degree warmer than the pre-industrial
degree warmer than the pre-industrial
degree warmer than the pre-industrial state and sea level was a few meters
state and sea level was a few meters
state and sea level was a few meters higher than it is today
higher than it is today
higher than it is today so we know no matter what happens next
so we know no matter what happens next
so we know no matter what happens next in Antarctica we’ve got some sea level
in Antarctica we’ve got some sea level
in Antarctica we’ve got some sea level rise left to do and that’s for two
rise left to do and that’s for two
rise left to do and that’s for two reasons one glaciers at night and the
reasons one glaciers at night and the
reasons one glaciers at night and the Greenland ice sheet are melting the
Greenland ice sheet are melting the
Greenland ice sheet are melting the other thing is that the ocean water is
other thing is that the ocean water is
other thing is that the ocean water is warming up and it’s expanding if we want
warming up and it’s expanding if we want
warming up and it’s expanding if we want to find a time when atmospheric co2 was
to find a time when atmospheric co2 was
to find a time when atmospheric co2 was as high as it is today we have to go
as high as it is today we have to go
as high as it is today we have to go back three million years to a time that
back three million years to a time that
back three million years to a time that geologists call the Pliocene and in that
geologists call the Pliocene and in that
geologists call the Pliocene and in that world and the global average was several
world and the global average was several
world and the global average was several degrees warmer than today and sea level
degrees warmer than today and sea level
degrees warmer than today and sea level was anywhere between six and many tens
was anywhere between six and many tens
was anywhere between six and many tens of meters the estimates vary higher than
of meters the estimates vary higher than
of meters the estimates vary higher than today so the geologic record tells us
today so the geologic record tells us
today so the geologic record tells us that as we continue to warm the world by
that as we continue to warm the world by
that as we continue to warm the world by increasing atmospheric co2 the ice
increasing atmospheric co2 the ice
increasing atmospheric co2 the ice sheets are going to continue to respond
sheets are going to continue to respond
sheets are going to continue to respond what we want to know next is how fast
what we want to know next is how fast
what we want to know next is how fast does that happen what are the sea level
does that happen what are the sea level
does that happen what are the sea level rise rates likely to be to answer that
rise rates likely to be to answer that
rise rates likely to be to answer that kind of question we need a computer
kind of question we need a computer
kind of question we need a computer model so scientists are interested in
model so scientists are interested in
model so scientists are interested in this kind of question use the geologic
this kind of question use the geologic
this kind of question use the geologic record to build models that can
record to build models that can
record to build models that can reproduce all those variations over
reproduce all those variations over
reproduce all those variations over climate cycles they drive the model up
climate cycles they drive the model up
climate cycles they drive the model up to the present day and then head off
to the present day and then head off
to the present day and then head off somewhere in the future and we don’t
somewhere in the future and we don’t
somewhere in the future and we don’t know what the future is likely yet
know what the future is likely yet
know what the future is likely yet because it hasn’t happened but we can
because it hasn’t happened but we can
because it hasn’t happened but we can make estimates and these estimates and
make estimates and these estimates and
make estimates and these estimates and that the Intergovernmental Panel on
that the Intergovernmental Panel on
that the Intergovernmental Panel on Climate Change uses are called
Climate Change uses are called
Climate Change uses are called representative
representative
representative concentration pathways which is a
concentration pathways which is a
concentration pathways which is a complicated term but essentially what it
complicated term but essentially what it
complicated term but essentially what it means is that we think about what
means is that we think about what
means is that we think about what decisions global communities are going
decisions global communities are going
decisions global communities are going to make about fossil fuel-burning and
to make about fossil fuel-burning and
to make about fossil fuel-burning and then estimate what the atmospheric co2
then estimate what the atmospheric co2
then estimate what the atmospheric co2 concentration will be like for different
concentration will be like for different
concentration will be like for different kinds of policy and consumption
kinds of policy and consumption
kinds of policy and consumption scenarios and I’ll show you models that
scenarios and I’ll show you models that
scenarios and I’ll show you models that are driven with two different scenarios
are driven with two different scenarios
are driven with two different scenarios one in which we sort of globally get our
one in which we sort of globally get our
one in which we sort of globally get our act together and limit the amount of
act together and limit the amount of
act together and limit the amount of fossil fuel burning that we do in the
fossil fuel burning that we do in the
fossil fuel burning that we do in the future and keep atmospheric co2
future and keep atmospheric co2
future and keep atmospheric co2 relatively low and then another scenario
relatively low and then another scenario
relatively low and then another scenario that’s often called the
that’s often called the
that’s often called the business-as-usual scenario where we just
business-as-usual scenario where we just
business-as-usual scenario where we just keep doing more or less what we’re doing
keep doing more or less what we’re doing
keep doing more or less what we’re doing right now
right now
right now so here’s five hundred years into the
so here’s five hundred years into the
so here’s five hundred years into the future for that first scenario the the
future for that first scenario the the
future for that first scenario the the world in which we’re very aggressive in
world in which we’re very aggressive in
world in which we’re very aggressive in our carbon emissions policies and we
our carbon emissions policies and we
our carbon emissions policies and we greatly reduce the emissions and in this
greatly reduce the emissions and in this
greatly reduce the emissions and in this world not much happens in Antarctica the
world not much happens in Antarctica the
world not much happens in Antarctica the ice sheet doesn’t change very much again
ice sheet doesn’t change very much again
ice sheet doesn’t change very much again I’m showing you a thickness map of the
I’m showing you a thickness map of the
I’m showing you a thickness map of the ice all around the Antarctic if on the
ice all around the Antarctic if on the
ice all around the Antarctic if on the other hand we continue on the path were
other hand we continue on the path were
other hand we continue on the path were on right now five hundred years from now
on right now five hundred years from now
on right now five hundred years from now the West Antarctic Ice Sheet is gone and
the West Antarctic Ice Sheet is gone and
the West Antarctic Ice Sheet is gone and we’ve lost significant parts the marine
we’ve lost significant parts the marine
we’ve lost significant parts the marine parts of the East Antarctic Ice Sheet as
parts of the East Antarctic Ice Sheet as
parts of the East Antarctic Ice Sheet as well and the global mean sea level rise
well and the global mean sea level rise
well and the global mean sea level rise that we get out of Antarctica alone is
that we get out of Antarctica alone is
that we get out of Antarctica alone is about 15 meters that’s a dramatic number
about 15 meters that’s a dramatic number
about 15 meters that’s a dramatic number it doesn’t happen all at once it happens
it doesn’t happen all at once it happens
it doesn’t happen all at once it happens over time so we should look at what how
over time so we should look at what how
over time so we should look at what how how the sequence of events unfolds
how the sequence of events unfolds
how the sequence of events unfolds that’s what this plot shows you the
that’s what this plot shows you the
that’s what this plot shows you the black line is that low emission scenario
black line is that low emission scenario
black line is that low emission scenario the red lines are the high emission
the red lines are the high emission
the red lines are the high emission scenario with two slightly different
scenario with two slightly different
scenario with two slightly different ways that the ice sheet and it’s the
ways that the ice sheet and it’s the
ways that the ice sheet and it’s the overlying atmosphere interact the
overlying atmosphere interact the
overlying atmosphere interact the important thing to look at here is that
important thing to look at here is that
important thing to look at here is that by the middle of the next century the
by the middle of the next century the
by the middle of the next century the rate of sea level rise year-on-year is
rate of sea level rise year-on-year is
rate of sea level rise year-on-year is between four and six centimeters per
between four and six centimeters per
between four and six centimeters per year in these simulations those are huge
year in these simulations those are huge
year in these simulations those are huge numbers other modeling groups who use
numbers other modeling groups who use
numbers other modeling groups who use the same kind of approach will generate
the same kind of approach will generate
the same kind of approach will generate slightly different rates of change
slightly different rates of change
slightly different rates of change slightly different timelines for the
slightly different timelines for the
slightly different timelines for the magnitude of this change but the end
magnitude of this change but the end
magnitude of this change but the end result is always the same and it doesn’t
result is always the same and it doesn’t
result is always the same and it doesn’t matter too much if you live in a coastal
matter too much if you live in a coastal
matter too much if you live in a coastal community the difference between three
community the difference between three
community the difference between three centimeters
centimeters
centimeters year and six centimeters per year of
year and six centimeters per year of
year and six centimeters per year of sea-level rise is not the thing you’re
sea-level rise is not the thing you’re
sea-level rise is not the thing you’re worrying about those are huge numbers
worrying about those are huge numbers
worrying about those are huge numbers either way what I want to talk about
either way what I want to talk about
either way what I want to talk about next is weather once we step on to that
next is weather once we step on to that
next is weather once we step on to that red line if that’s if we’re committed if
red line if that’s if we’re committed if
red line if that’s if we’re committed if that’s the direction we’re going to go
that’s the direction we’re going to go
that’s the direction we’re going to go is that sort of a fait accompli that if
is that sort of a fait accompli that if
is that sort of a fait accompli that if we don’t act now is there nothing else
we don’t act now is there nothing else
we don’t act now is there nothing else we can do to answer that kind of
we can do to answer that kind of
we can do to answer that kind of question we need to look in more detail
question we need to look in more detail
question we need to look in more detail at specific parts of the Antarctic Ice
at specific parts of the Antarctic Ice
at specific parts of the Antarctic Ice Sheet rather than just thinking of the
Sheet rather than just thinking of the
Sheet rather than just thinking of the system as a whole so me to talk about
system as a whole so me to talk about
system as a whole so me to talk about some work that I’m doing with Scott
some work that I’m doing with Scott
some work that I’m doing with Scott Weibull and Charles Jackson in which we
Weibull and Charles Jackson in which we
Weibull and Charles Jackson in which we start out thinking about the whole ice
start out thinking about the whole ice
start out thinking about the whole ice sheet then focusing just on West
sheet then focusing just on West
sheet then focusing just on West Antarctica and then focusing more
Antarctica and then focusing more
Antarctica and then focusing more specifically on one glacier in West
specifically on one glacier in West
specifically on one glacier in West Antarctica called the Thwaites glacier
Antarctica called the Thwaites glacier
Antarctica called the Thwaites glacier it’s a place that is changing right now
it’s a place that is changing right now
it’s a place that is changing right now quite rapidly this is a map of ice flow
quite rapidly this is a map of ice flow
quite rapidly this is a map of ice flow all around the Antarctic and warm colors
all around the Antarctic and warm colors
all around the Antarctic and warm colors are fast flow the cool colors are slower
are fast flow the cool colors are slower
are fast flow the cool colors are slower flow you can see there’s sort of a
flow you can see there’s sort of a
flow you can see there’s sort of a network of tributaries that feed into
network of tributaries that feed into
network of tributaries that feed into the Thwaites glacier as it makes its way
the Thwaites glacier as it makes its way
the Thwaites glacier as it makes its way to the sea and when we set up this model
to the sea and when we set up this model
to the sea and when we set up this model what we do is simulate the present-day
what we do is simulate the present-day
what we do is simulate the present-day situation so we’d create a model that
situation so we’d create a model that
situation so we’d create a model that represents this flow exactly and that
represents this flow exactly and that
represents this flow exactly and that kind of embodies the geology as we know
kind of embodies the geology as we know
kind of embodies the geology as we know it in the climate as we know it in the
it in the climate as we know it in the
it in the climate as we know it in the system this is a map of the bed
system this is a map of the bed
system this is a map of the bed elevation underneath the ice sheet and
elevation underneath the ice sheet and
elevation underneath the ice sheet and as advertised you can see that it gets
as advertised you can see that it gets
as advertised you can see that it gets deeper the seafloor gets deeper as we go
deeper the seafloor gets deeper as we go
deeper the seafloor gets deeper as we go into the interior the cool colors here
into the interior the cool colors here
into the interior the cool colors here the light blue through the magenta our
the light blue through the magenta our
the light blue through the magenta our elevations below sea level the warm
elevations below sea level the warm
elevations below sea level the warm colors green and above our elevations
colors green and above our elevations
colors green and above our elevations above sea level so you can see the bed
above sea level so you can see the bed
above sea level so you can see the bed gets deeper and deeper and deeper as we
gets deeper and deeper and deeper as we
gets deeper and deeper and deeper as we go into the interior and that runaway
go into the interior and that runaway
go into the interior and that runaway effect if we can get it to start would
effect if we can get it to start would
effect if we can get it to start would indeed continue in this system so I’ll
indeed continue in this system so I’ll
indeed continue in this system so I’ll show you two different simulations
show you two different simulations
show you two different simulations remember our goal is to try to find
remember our goal is to try to find
remember our goal is to try to find thresholds in the system and understand
thresholds in the system and understand
thresholds in the system and understand how vulnerable is the West Antarctic Ice
how vulnerable is the West Antarctic Ice
how vulnerable is the West Antarctic Ice Sheet to this marine kind of runaway
Sheet to this marine kind of runaway
Sheet to this marine kind of runaway retreat effect so we’ll do two things
retreat effect so we’ll do two things
retreat effect so we’ll do two things we’ll take our model that’s built to
we’ll take our model that’s built to
we’ll take our model that’s built to represent today quite well and we’ll
represent today quite well and we’ll
represent today quite well and we’ll start to slowly push it as slowly as we
start to slowly push it as slowly as we
start to slowly push it as slowly as we possibly can now what does global
possibly can now what does global
possibly can now what does global warming do it increases the atmosphere
warming do it increases the atmosphere
warming do it increases the atmosphere temperature that increases the ocean
temperature that increases the ocean
temperature that increases the ocean temperature increasing the ocean
temperature increasing the ocean
temperature increasing the ocean temperature generates more melting
temperature generates more melting
temperature generates more melting underneath the floating ice so the way
underneath the floating ice so the way
underneath the floating ice so the way that we will invoke this change is by
that we will invoke this change is by
that we will invoke this change is by doing melting on the floating side of
doing melting on the floating side of
doing melting on the floating side of the ice sheet and we’ll just and of
the ice sheet and we’ll just and of
the ice sheet and we’ll just and of course you know now from our discussion
course you know now from our discussion
course you know now from our discussion of the physics that when we start to
of the physics that when we start to
of the physics that when we start to melt on the underside of the ice shelf
melt on the underside of the ice shelf
melt on the underside of the ice shelf that the grounding line will retreat and
that the grounding line will retreat and
that the grounding line will retreat and what we’re looking for is not the melt
what we’re looking for is not the melt
what we’re looking for is not the melt induced retreat we’re looking for the
induced retreat we’re looking for the
induced retreat we’re looking for the time when the grounding line keeps
time when the grounding line keeps
time when the grounding line keeps retreating even if we turn the melting
retreating even if we turn the melting
retreating even if we turn the melting off that’s when we’ve actually generated
off that’s when we’ve actually generated
off that’s when we’ve actually generated a real instability in a real runaway
a real instability in a real runaway
a real instability in a real runaway situation in the ice sheet so we’ll
situation in the ice sheet so we’ll
situation in the ice sheet so we’ll slowly slowly slowly drive toward the
slowly slowly slowly drive toward the
slowly slowly slowly drive toward the future when we find that moment when the
future when we find that moment when the
future when we find that moment when the retreat rate no longer depends on what
retreat rate no longer depends on what
retreat rate no longer depends on what the climate is doing we’ll shut off the
the climate is doing we’ll shut off the
the climate is doing we’ll shut off the melting and we’ll watch what happens
melting and we’ll watch what happens
melting and we’ll watch what happens next then we’ll go back to that time and
next then we’ll go back to that time and
next then we’ll go back to that time and we’ll keep pushing a little bit more and
we’ll keep pushing a little bit more and
we’ll keep pushing a little bit more and then we’ll turn it off and we’ll compare
then we’ll turn it off and we’ll compare
then we’ll turn it off and we’ll compare those two scenarios in a set of slices
those two scenarios in a set of slices
those two scenarios in a set of slices through the ice sheet that look just
through the ice sheet that look just
through the ice sheet that look just like my cartoon slices except now we
like my cartoon slices except now we
like my cartoon slices except now we have the real shape of the seafloor
have the real shape of the seafloor
have the real shape of the seafloor underneath and the real shape of the ice
underneath and the real shape of the ice
underneath and the real shape of the ice sheet on top so here’s the state for the
sheet on top so here’s the state for the
sheet on top so here’s the state for the slightly shorter slightly smaller
slightly shorter slightly smaller
slightly shorter slightly smaller warming and the slightly longer slightly
warming and the slightly longer slightly
warming and the slightly longer slightly larger melting underneath the ice shelf
larger melting underneath the ice shelf
larger melting underneath the ice shelf if we go 20 years into the future indeed
if we go 20 years into the future indeed
if we go 20 years into the future indeed we see both grounding lines are
we see both grounding lines are
we see both grounding lines are retreating so we have in fact crossed
retreating so we have in fact crossed
retreating so we have in fact crossed this threshold into a runaway retreat
this threshold into a runaway retreat
this threshold into a runaway retreat kind of scenario if we keep going
kind of scenario if we keep going
kind of scenario if we keep going forward in time however we see that the
forward in time however we see that the
forward in time however we see that the scenario that we pushed a little bit
scenario that we pushed a little bit
scenario that we pushed a little bit harder is retreating faster than the one
harder is retreating faster than the one
harder is retreating faster than the one that we gave a more gentle push if we
that we gave a more gentle push if we
that we gave a more gentle push if we keep going into the future the
keep going into the future the
keep going into the future the simulations will continue to diverge and
simulations will continue to diverge and
simulations will continue to diverge and so what this tells us is that there’s
so what this tells us is that there’s
so what this tells us is that there’s always an opportunity to change the
always an opportunity to change the
always an opportunity to change the pathway that we’re on the larger the
pathway that we’re on the larger the
pathway that we’re on the larger the push the larger the response but anytime
push the larger the response but anytime
push the larger the response but anytime that we stop we then lock in that that
that we stop we then lock in that that
that we stop we then lock in that that rate of retreat right so the retreat is
rate of retreat right so the retreat is
rate of retreat right so the retreat is inevitable once we’ve crossed this
inevitable once we’ve crossed this
inevitable once we’ve crossed this threshold we’re going to get massive
threshold we’re going to get massive
threshold we’re going to get massive change in West Antarctica no matter what
change in West Antarctica no matter what
change in West Antarctica no matter what we do but we can limit how fast that
we do but we can limit how fast that
we do but we can limit how fast that happens all right we can through our own
happens all right we can through our own
happens all right we can through our own choices that we make as global
choices that we make as global
choices that we make as global communities say
communities say
communities say we can tolerate some amount and the
we can tolerate some amount and the
we can tolerate some amount and the sooner we decide to do that the sooner
sooner we decide to do that the sooner
sooner we decide to do that the sooner we decide to limit the warming the
we decide to limit the warming the
we decide to limit the warming the slower this is going to unfold now I’ve
slower this is going to unfold now I’ve
slower this is going to unfold now I’ve spent all of my career trying to
spent all of my career trying to
spent all of my career trying to understand what’s going on in the
understand what’s going on in the
understand what’s going on in the systems in West Antarctica when I first
systems in West Antarctica when I first
systems in West Antarctica when I first started doing field work in the interior
started doing field work in the interior
started doing field work in the interior of the West Antarctic Ice Sheet we
of the West Antarctic Ice Sheet we
of the West Antarctic Ice Sheet we didn’t even really know what was
didn’t even really know what was
didn’t even really know what was underneath the ice sheet we didn’t know
underneath the ice sheet we didn’t know
underneath the ice sheet we didn’t know how important the sublation condition
how important the sublation condition
how important the sublation condition was going to be for the rate of flow are
was going to be for the rate of flow are
was going to be for the rate of flow are there questions that we still need to
there questions that we still need to
there questions that we still need to ask certainly is there more that we need
ask certainly is there more that we need
ask certainly is there more that we need to know of course there is but we know a
to know of course there is but we know a
to know of course there is but we know a lot we understand the basic physics we
lot we understand the basic physics we
lot we understand the basic physics we understand how to write the equations
understand how to write the equations
understand how to write the equations that describe the way the ice sheet
that describe the way the ice sheet
that describe the way the ice sheet interacts with the ocean and interacts
interacts with the ocean and interacts
interacts with the ocean and interacts with the atmosphere we can ask the
with the atmosphere we can ask the
with the atmosphere we can ask the geologic record really tough questions
geologic record really tough questions
geologic record really tough questions and get meaningful answers we can ask
and get meaningful answers we can ask
and get meaningful answers we can ask these models to show us different
these models to show us different
these models to show us different pathways into the future and we can have
pathways into the future and we can have
pathways into the future and we can have confidence that the models are showing
confidence that the models are showing
confidence that the models are showing us indeed what the future holds what we
us indeed what the future holds what we
us indeed what the future holds what we can’t do is pick which of those futures
can’t do is pick which of those futures
can’t do is pick which of those futures is the right one that’s a decision that
is the right one that’s a decision that
is the right one that’s a decision that all of us collectively all around the
all of us collectively all around the
all of us collectively all around the world have to make together it’s like
world have to make together it’s like
world have to make together it’s like we’re sitting at the top of a hill in a
we’re sitting at the top of a hill in a
we’re sitting at the top of a hill in a car with our kids well with everybody’s
car with our kids well with everybody’s
car with our kids well with everybody’s kids and maybe we’ve been pushed over
kids and maybe we’ve been pushed over
kids and maybe we’ve been pushed over the edge we’re not really sure maybe we
the edge we’re not really sure maybe we
the edge we’re not really sure maybe we haven’t but we know one way or another
haven’t but we know one way or another
haven’t but we know one way or another the sooner we put on the brakes the
the sooner we put on the brakes the
the sooner we put on the brakes the gentler the crash at the bottom of the
gentler the crash at the bottom of the
gentler the crash at the bottom of the hill is going to be now maybe we can
hill is going to be now maybe we can
hill is going to be now maybe we can still be maybe we’re still up at the top
still be maybe we’re still up at the top
still be maybe we’re still up at the top and we can put on the brakes right now
and we can put on the brakes right now
and we can put on the brakes right now and nothing dramatic will happen in West
and nothing dramatic will happen in West
and nothing dramatic will happen in West Antarctica as well maybe we’ve already
Antarctica as well maybe we’ve already
Antarctica as well maybe we’ve already passed that threshold but certainly the
passed that threshold but certainly the
passed that threshold but certainly the sooner we put on the brakes the gentler
sooner we put on the brakes the gentler
sooner we put on the brakes the gentler the impact down at the end thank you
the impact down at the end thank you
the impact down at the end thank you [Applause]
”
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