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Despite all the water involved, much of the Arctic and Antarctic
get so little precipitation they’re literally deserts.
And like their much warmer siblings, these deserts have their own version of an oasis.
They’re called polynyas, gaps in the sea ice.
And they can be big gaps.
I’m talking up to thousands of square kilometers,
with one back in the 1970s getting as large as New Zealand!
Polynyas are a critical part of the polar ecosystem,
and they have a larger role to play, too…
like affecting how ocean currents churn around the globe.
But the climate crisis is changing the conditions that form them,
and that could have a big domino effect.
Let’s start with the basics.
There are two kinds of polynyas, and each tends to form in a different way.
As their name suggests, coastal polynyas are found along the coast,
and they usually form when winds coming from the land push ice away from the shore,
exposing a bit of the ocean to the air.
New ice starts forming in this gap, but the winds continue to push that stuff away, too.
And so on, and so forth.
This can happen at any time of year,
but things ramp up when temperatures drop in the winter.
During those colder months, coastal polynyas basically become ice factories.
And as this seawater freezes, most of the salt that was dissolved in that water
gets left behind, creating a bunch of extra salty,
and extra dense, liquid water that sinks down to the bottom of the ocean.
This sinking saltwater has enormous implications for our planet’s climate,
but more on that later.
The second type of polynya forms in the sea ice that’s floating on the open ocean.
This is called, very creatively, an open ocean polynya.
These typically form not because of wind, but because of water.
Specifically, when warm ocean water rises up from the depths
and either melts a hole in ice from below, or prevents ice from forming in the first place.
Whenever it does melt a bunch of ice, it releases cold fresh water.
Even though it’s less salty, this cold water is dense enough to sink
and mix with the warm water coming up from below.
That mixing brings even more warmer water to the surface,
keeping the ice melting and the whole cycle going.
But whether they form on the coast or in the middle of seemingly nowhere,
a polynya can be an oasis for an entire polar ecosystem.
Ecosystems, and in particular their food webs, tend to build from the bottom up.
And the bases of most open-ocean food webs
are the microscopic plants called phytoplankton.
Like most plants, they need sunlight to make their food.
Which is a bit hard to come by when you’re floating underneath a thick layer of ice.
But if a polynya forms, there’s no ice to block the light.
So when the Sun comes back after months of polar winter darkness,
polynyas can host the first phytoplankton blooms of the year.
This surge in life quickly travels up the food chain,
attracting zooplankton, fish, and larger animals like whales, seals,
and in the Antarctic, adorable penguins with nightmare mouths.
In fact, polynyas are essential gateways for creatures that breed on land,
but hunt in the water.
For example, according to one study from 2022, 96% of the Antarctic elephant seals
that the research team tracked used polynyas to hunt.
Meanwhile, other research has shown that the more phytoplankton
you’ve got growing in a polynya, the larger the size of a nearby Adélie penguin colony.
And the distance between the colony and polynya is important.
In both Emperor and Adélie penguin colonies,
the male and female penguins take turns getting food or looking after their egg or chick.
The catch is whoever stays behind doesn’t eat.
So if there’s a convenient hole in the ice nearby,
the time the penguins need to fast is shorter.
Researchers have observed that if a nearby polynya doesn’t form, male Adélie penguins
fast for longer, lose more body weight during nesting, and fewer eggs end up hatching.
Over on the other side of the planet, polynyas play a crucial role in the lives
of the humans that call the Arctic desert home.
Pikialasorsuaq, also called the North Water Polynya (apologies for my pronunciation),
opens every year in the ice between Northern Canada and Greenland.
It’s the largest polynya in the Arctic,
and relies on both coastal and open ocean effects to keep it free of ice.
Inuit communities have relied on this polynya for thousands of years.
Its productive ecosystem makes it a perfect hunting ground,
and its open waters have long provided both a migration and two-way trade route.
But like many parts of the polar climate,
polynyas and the conditions that form them are rapidly changing.
According to a study published in 2021,
Pikialasorsuaq shrank between roughly 2,500 and 1,000 years ago.
This meant less phytoplankton growth and fewer birds for people to hunt.
And a few hundred years into this period,
humans abandoned their settlements in Greenland.
Now, the team couldn't prove that was only because the polynya was smaller,
or if it was due to larger climate effects, but they do predict the modern climate crisis
could produce a similar decline in this Arctic ecosystem.
More generally, having fewer polynyas form could have global effects.
As I mentioned earlier, coastal polynyas create a bunch of dense water
that sinks toward the ocean floor, and pushes other water out of the way.
But this isn’t just localized churning.
Polynyas, particularly in the Antarctic, help drive currents that encircle the globe,
and play a critical role in moving colder and warmer water where it wouldn’t normally go.
For example, the Gulf Stream is a current that carries heat from the tropics towards Europe,
making the continent warmer than it should be, given how far north it is.
Without polynyas, entire continents might be a lot less pleasant to call home.
But that’s not even all they do!
By providing a haven for phytoplankton,
which use carbon dioxide as a part of their food-making process,
polynyas can pull that notorious greenhouse gas out of the atmosphere.
When they die, the phytoplankton sink to the seafloor,
storing carbon inside their tiny plant corpses.
Scientists aren’t exactly sure how the climate crisis will impact how well polynyas work
as carbon storage systems, or as global water circulators.
But polynyas can be pretty tricky to study,
since it’s often difficult to get to the poles during the winter when they’re more active.
A lot more research is needed to figure out what polynyas can do…
not just for the animals and people who live near them,
but for every bit of life that calls Earth home.
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