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Thanks to the climate crisis,
Antarctica’s glaciers are losing ice at an unprecedented rate.
The polar regions have warmed faster than anywhere else on Earth.
Especially worrying is Antarctica, where a lot of ice is held up on land.
If that ice melts, all that water would flow into the ocean,
dramatically raising Earth’s sea level.
We know these glaciers are already on the way out,
so we need to find out whether we can bulk them back up again.
For that, we turn to the glaciers’ past – and a lot of penguin bones.
Scientists are especially worried about two glaciers in West Antarctica:
the Thwaites and Pine Island glaciers.
These glaciers flow out into the Amundsen Sea.
They rest on a slope, such that the part of the glacier
at the bottom supports the rest of it.
Which means if the sections at the bottom of the slope melt completely,
there’s nothing to hold up the rest of the glaciers.
Scientists are worried that if that happens, the glaciers could collapse rapidly.
And if they do that,
they could take out the entire West Antarctic Ice Sheet with them.
This ice sheet has enough water in it to raise global sea level
by 3.4 meters over the next century or two.
That’s about as high as two refrigerators, stacked on top of each other.
Now, we know that the glaciers are losing ice,
because we can directly observe them melting.
But climate researchers wonder whether these glaciers had ever lost ice in the past,
before human-caused climate change.
If the glaciers had melted before and then bounced back to their pre-industrial levels,
that would give us some hope that these glaciers could rebound in the future
instead of continuing to collapse.
This theory, that the glaciers could possibly rebound, has been discussed before.
But it took some time to figure out how to actually find some evidence for it.
A 2022 study solved this using penguin bones.
Antarctica has been covered in ice for a very long time,
but about 20,000 years ago during the last ice age,
Antarctica’s ice sheets were much thicker than they are today.
The ice back then was so heavy that it actually pushed down the land beneath it.
You might think of Earth as being solid, but
the Earth’s crust rests on a layer of springy mantle.
So resting anything heavy on the Earth’s surface
can push that section of crust down into the mantle,
sort of like how when you press your hand into a mattress,
you can see the impression your hand leaves behind.
And just like how when you remove your hand,
the mattress slowly springs back to normal, Earth’s crust bounces
back when the weight is removed.
But that process doesn’t happen quickly, so if any of Antarctica’s beaches
were at sea level during the last ice age, they’ve gradually risen in height.
Many of these older beaches are now a few meters above the ocean and still rising.
If the glaciers had ever gotten thicker between the last ice and today,
their weight would have temporarily pushed the beaches down again.
So the beaches move whenever the land bobs up and down again.
This means sea level at those beaches depends
more on the weight and thickness of the glaciers than trends in global sea level.
So by looking at the history of sea level changes along those beaches,
scientists can map out the history of the glaciers.
To do that, the researchers scoured the beaches for biological material
that they could carbon date.
Carbon dating involves looking at the amount of carbon-14 in organic material.
Carbon-14 is a slightly heavier version of carbon that’s radioactive,
which means it decays with time at a constant rate.
Living things all have roughly the same amount of carbon-14 in their bodies.
That’s because we constantly get new infusions of it
from the air we breathe and the food we eat.
Don’t be alarmed – it’s not radioactive enough to be harmful.
It’s just kind of everywhere.
But after an organism dies, its stored carbon-14 breaks down without being replaced.
That means by measuring carbon-14 from the remains of living things,
researchers can get an idea of when an organism died.
Along the Antarctic beaches, the most useful material scientists found
were bits of seashells and penguin bones.
The seashells were likely deposited when the beach formed,
because they came from mollusks that lived in shallow water, below the surface.
Meanwhile, the penguin bones likely came from birds
that nested on the beaches once they stabilized,
and were regularly exposed to air.
So by comparing the ages of the newest seashells found on a beach with the oldest penguin bones,
researchers can basically bookend when the beach was in contact with the water.
It had to have been sometime after the last of the seashells,
but just before the first penguin bones.
When they did this analysis for the beaches around the glaciers,
they found that these two glaciers have been remarkably stable.
What they saw was that sea level has pretty much only dropped.
And pretty consistently, receding 3 to 4 millimeters every year over 5500 years.
Since it’s so steady, that likely means that the land
is still rebounding from the last ice age,
and there haven’t been any surprise increases in the thickness of the ice.
That means there’s no support for the idea that glaciers can rebound after shrinking.
That doesn’t mean that the glaciers can’t rebound,
but it does mean that they haven’t done so at any point we can see.
This result might not sound surprising, but it’s good science to always test our assumptions.
Even things that might seem obvious, like
“glaciers that are melting will continue to melt,
might hide surprising truths.
For example, the stability of the glaciers in their history
is very different from what’s currently happening,
where the glaciers are losing ice quickly enough
to raise global sea levels by 1.5 millimeters per year.
That’s one more argument to fight the climate crisis now,
because we can’t count on being able to save the glaciers once it’s too late.
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