Whats hidden under the Greenland ice sheet Kristin Poinar
When was I was 21 years old,
I had all this physics homework.
Physics homework requires taking breaks,
and Wikipedia was relatively new,
so I took a lot of breaks there.
I kept going back to the same articles,
reading them again and again,
on glaciers, Antarctica and Greenland.
How cool would it be to visit these places
and what would it take to do so?
Well, here we are
on a repurposed Air Force cargo plane
operated by NASA
flying over the Greenland ice sheet.
There’s a lot to see here,
but there’s more that is hidden,
waiting to be uncovered.
What the Wikipedia articles didn’t tell me
is that there’s liquid water
hidden inside the ice sheet,
because we didn’t know that yet.
I did learn on Wikipedia
that the Greenland ice sheet is huge,
the size of Mexico,
and its ice from top to bottom
is two miles thick.
But it’s not just static.
The ice flows like a river
downhill towards the ocean.
As it flows around bends,
it deforms and cracks.
I get to study these amazing ice dynamics,
which are located in one of the most
remote physical environments
remaining on earth.
To work in glaciology right now
is like getting in on the ground floor
at Facebook in the 2000s.
(Laughter)
Our capability to fly airplanes
and satellites over the ice sheets
is revolutionizing glaciology.
It’s just starting to do for science
what the smartphone
has done for social media.
The satellites are reporting
a wealth of observations
that are revealing new hidden facts
about the ice sheets continuously.
For instance, we have observations
of the size of the Greenland ice sheet
every month going back to 2002.
You can look towards the bottom
of the screen here
to see the month and the year go forward.
You can see that some areas
of the ice sheet melt
or lose ice in the summer.
Other areas experience snowfall
or gain ice back in the winter.
This seasonal cycle, though, is eclipsed
by an overall rate of mass loss
that would have stunned
a glaciologist 50 years ago.
We never thought that an ice sheet could
lose mass into the ocean this quickly.
Since these measurements began in 2002,
the ice sheet has lost so much ice
that if that water were piled up
on our smallest continent,
it would drown Australia knee-deep.
How is this possible?
Well, under the ice lies the bedrock.
We used radar to image the hills,
valleys, mountains and depressions
that the ice flows over.
Hidden under the ice sheet are channels
the size of the Grand Canyon
that funnel ice and water
off of Greenland and into the ocean.
The reason that radar
can reveal the bedrock
is that ice is entirely
transparent to radar.
You can do an experiment.
Go home and put
an ice cube in the microwave.
It won’t melt,
because microwaves, or radar,
pass straight through the ice
without interacting.
If you want to melt your ice cube,
you have to get it wet,
because water heats up easily
in the microwave.
That’s the whole principle
the microwave oven is designed around.
Radar can see water.
And radar has revealed
a vast pool of liquid water
hidden under my colleague Olivia,
seven stories beneath her feet.
Here, she’s used a pump
to bring some of that water
back to the ice sheet’s surface.
Just six years ago, we had no idea
this glacier aquifer existed.
The aquifer formed
when snow melts in the summer sun
and trickles downward.
It puddles up in huge pools.
From there, the snow acts as an igloo,
insulating this water
from the cold and the wind above.
So the water can stay
hidden in the ice sheet
in liquid form year after year.
The question is, what happens next?
Does the water stay there forever?
It could.
Or does it find a way out
to reach the global ocean?
One possible way
for the water to reach the bedrock
and from there the ocean
is a crevasse, or a crack in the ice.
When cracks fill with water,
the weight of the water
forces them deeper and deeper.
This is how fracking works
to extract natural gas
from deep within the earth.
Pressurized fluids fracture rocks.
All it takes is a crack to get started.
Well, we recently discovered
that there are cracks available
in the Greenland ice sheet
near this glacier aquifer.
You can fly over
most of the Greenland ice sheet
and see nothing,
no cracks, no features on the surface,
but as this helicopter
flies towards the coast,
the path that water would take
on its quest to flow downhill,
one crack appears,
then another and another.
Are these cracks filled with liquid water?
And if so, how deep
do they take that water?
Can they take it to the bedrock
and the ocean?
To answer these questions,
we need something
beyond remote sensing data.
We need numeric models.
I write numeric models
that run on supercomputers.
A numeric model
is simply a set of equations
that works together to describe something.
It can be as simple
as the next number in a sequence –
one, three, five, seven –
or it can be a more complex
set of equations
that predict the future
based on known conditions in the present.
In our case, what are
the equations for how ice cracks?
Well, engineers already have
a very good understanding
of how aluminum, steel and plastics
fracture under stress.
It’s an important problem in our society.
And it turns out
that the engineering equations
for how materials fracture
are not that different
from my physics homework.
So I borrowed them, adapted them for ice,
and then I had a numeric model
for how a crevasse can fracture
when filled with water from the aquifer.
This is the power of math.
It can help us understand
real processes in our world.
I’ll show you now
the results of my numeric model,
but first I should point out
that the crevasse is about
a thousand times narrower than it is deep,
so in the main panel here,
we’ve zoomed in to better see the details.
You can look to the smaller
panel on the right
to see the true scale
for how tall and skinny the crevasse is.
As the aquifer water
flows into the crevasse,
some of it refreezes
in the negative 15 degree Celsius ice.
That’s about as cold
as your kitchen freezer.
But this loss can be overcome
if the flow rate in from
the glacier aquifer is high enough.
In our case, it is,
and the aquifer water drives the crevasse
all the way to the base of the ice sheet
a thousand meters below.
From there, it has a clear path
to reach the ocean.
So the aquifer water is a part
of the three millimeters
per year of sea level rise
that we experience as a global society.
But there’s more:
the aquifer water
might be punching above its weight.
The ice flows in complex ways.
In some places, the ice flows very fast.
There tends to be water
at the base of the ice sheet here.
In other places, not so fast.
Usually, there’s not water
present at the base there.
Now that we know the aquifer water
is getting to the base of the ice sheet,
the next question is:
Is it making the ice itself
flow faster into the ocean?
We’re trying to uncover these mysteries
hidden inside the Greenland ice sheet
so that we can better plan
for the sea level rise it holds.
The amount of ice
that Greenland has lost since 2002
is just a small fraction
of what that ice sheet holds.
Ice sheets are immense, powerful machines
that operate on long timescales.
In the next 80 years, global sea levels
will rise at least 20 centimeters,
perhaps as much as one meter,
and maybe more.
Our understanding
of future sea level rise is good,
but our projections have a wide range.
It’s our role as glaciologists
and scientists
to narrow these uncertainties.
How much sea level rise is coming,
and how fast will it get here?
We need to know how much and how fast,
so the world and its communities can
plan for the sea level rise that’s coming.
Thank you.
(Applause)