NOVA

NARRATOR

500 miles from the coast of Antarctica, the icebreaker Polarstern has forced her way deep into the ice pack surrounding the continent. Now a team of scientists is leaving the safety of the ship to venture out onto the ice. They're in search of a tiny shrimp-like creature-- seemingly insignificant-- but with a vital role in the food chain.

OSCAR SCHOFIELD

The penguins eat them, the fish eat them, the seals eat them.

NARRATOR

But all this is under threat. We have a decrease of over 50%.

HUGH DUCKLOW

Krill have declined in some areas by 90%.

NARRATOR

Because something unknown is decimating the krill population.

CHRISTINE KLAAS

That means the system is changing dramatically, and we have to understand why... why that is.

NARRATOR

The answer may lie hidden under the ice. Finding it means a dangerous dive into a dark and unexplored world, with no guarantee that they can solve the "Mystery Beneath the Ice." Right now, on NOVA. This is the Antarctic ice pack proin the dead of winter.... Conditions here are so harsh that few ships would dare to enter. But this icebreaker is taking a group of scientists on a mission. Something unknown threatens the most extensive natural ecosystem on Earth-- the food web that sustains all life around the vast continent of Antarctica... A place as remote as it is beautiful.

SCHOFIELD

The early explorers called Antarctica "the Land of the Gods." And the first time you come down here that's pretty much how you feel. It's giant mountain ranges that descend directly down into the ocean. You have it carved up by glaciers. You have icebergs all around. It's just a spectacular, stunning place.

NARRATOR

And the waters here teem with life.

LLOYD PECK

If we look at the seas around the Antarctic Peninsula, we have the most fantastic wildlife. Massive diversity, iconic animals.

NARRATOR

Antarctica is home to tens of millions of penguins, millions of seals, hundreds of thousands of whales. But all these creatures depend, to a greater or lesser extent,

on a single species

Antarctic krill.

DUCKLOW

The Antarctic krill, euphausia superba, is a shrimp-like organism about the size of your little finger. And it's the key link in the Antarctic food chain.

SCHOFIELD

The whales eat them, the penguins eat them, the fish eat them, the seals eat them.

NARRATOR

There are thought to be perhaps 500 trillion krill in the waters around Antarctica. Just as well; one whale can consume a million in a single meal. Krill essentially support all this life we see around us. But there's a threat to life in Antarctica. Evidence suggests that the population of krill has been in decline.

SCHOFIELD

Still the most effective way to do a krill census is to drag a net through the ocean and physically count them. If you compile all those records, the general trend suggests that they are declining. In some areas by 90%. So, yeah, it's pretty significant.

NARRATOR

And it's a potential disaster for wildlife.

SCHOFIELD

Many of the penguins come back to the same location and breed year after year, so if the krill disappear in that location, those penguins are toast.

NARRATOR

Already, some penguin populations on the Antarctic Peninsula have declined by as much as 85%. Wildlife populations on the peninsula, an important breeding ground for many animals, are intensively monitored by scientists, such as those based at the British Antarctic Survey Station at Rothera Point on Adelaide Island.

PECK

Rothera is a phenomenal place to come and work. All stations, all stations, Victor Bravo Bravo on final, runway three six. And it's phenomenal because what you see in the life here you don't see anywhere else. If we look at the seas around the Antarctic Peninsula, where we are now, then the areas on this part of the Antarctic, along the peninsula, are probably the biggest breeding ground for Antarctic krill. That's why we have the most fantastic wildlife in this part of the world.

NARRATOR

Krill are a key link that connects top predators such as penguins and seals with the base of the food chain. Here, as everywhere in the oceans, that consists of algae and other microscopic floating plants, called phytoplankton.

PECK

In the summertime here we have very, very intense phytoplankton blooms. And they're so intense that the visibility for divers is very poor. The sea at the moment is like green soup. That green soup is absolutely the bottom of the food chain that provides the energy for everything else up in the levels of the food chain.

NARRATOR

Like plants on land, phytoplankton harness the sun's energy to make living tissue from carbon dioxide dissolved in the water. They're then eaten by krill and other small marine animals, and the krill then become a key food source for the top predators.

PECK

We've seen krill swarms going through, in massive clouds, all feeding on that massive density of phytoplankton.

NARRATOR

And the reason these waters are so productive in the summer is that the days are so long. In mid-December below the Antarctic Circle, the sun never sets, and phytoplankton can grow 24 hours a day. But as summer turns to winter, the days rapidly shorten until, by April, starved of sunlight, virtually all the phytoplankton in the water column have died off.

PECK

In the wintertime the phytoplankton, the bottom of the food chain, just about disappears. You might have visibility like a 100 meters maybe. It's like flying through the air or it's like diving in gin, because it's so clear. So the differences between seasons here are greater than anywhere else on Earth, and as a scuba diver they absolutely hit you in the face.

NARRATOR

Most krill are spawned when the seas are still full of phytoplankton. But to reach maturity they will have to make it through their first winter, a period when there will be little or no phytoplankton for the young krill, called larvae, to feed on. This is the most vulnerable part of a krill's life cycle, and researchers suspect it is the most important factor determining krill population. But to test their ideas, they need to make observations in Antarctic seas in the depths of winter, and that's cold, difficult, and even dangerous. (wind whipping) It's August. At a port on the tip of South America, 50 scientists embark for Antarctica. These are some of the world's leading experts on krill and the Southern Ocean. Their mission is to try to find krill larvae, observe their winter behavior, and so shed light on why their population has declined so drastically.

MATHIAS TESCHKE

Is it science, or is it adventure? I would say it's both, to be totally honest.

CHRISTINE KLAAS

I don't think that many people have the opportunity to make a winter cruise. These cruises are extremely rare.

ROB KING

The big challenge for krill larvae is surviving the Antarctic winter. But the question is, how do they do it? And if we understand how they can do it, we can understand how changes in the planet might affect their ability to do it.

NARRATOR

Leading the expedition is chief scientist Bettina Meyer from Germany's Alfred Wegener Institute.

BETTINA MEYER

I am pretty sure that we will find krill. The question is how much krill we will find. But I think you always have to be optimistic, and I am an optimistic person. (laughs)

NARRATOR

The scientists are aboard the German research vessel Polarstern. She's a scientific icebreaker, specially adapted for work at high latitudes. The first leg of her nine-week voyage will take her east across the Southern Ocean. During this part of the expedition, the scientists plan to deploy a specialized fishing net called a rectangular midwater trawl. The goal is to gather data on how many krill are to be found in these waters in winter.

BRIAN HUNT

It has multiple nets-- three big ones and three small ones, so it's pretty complicated to string it all together. The big nets are working well. And we still have a small problem with the small nets.

KING

Well, it's never easy to find krill. Because you'll come along on one voyage and you'll sail through a sea of red-- you know, you can have super swarms of krill 100 kilometers long-- and then you can sail through another 100 or, you know, 500 kilometers of ocean with nothing. Krill are a schooling animal, there's no two ways to look at it.

The krill in a school is dazzling

one krill by itself wouldn't last, you know, two seconds in the Southern Ocean. The krill itself is basically a feeding machine in the middle, an optical machine at the front for detecting where other krill and predators are with the big eyes, and then at the back end is the propulsion system, which is five swimming legs, and they're just constantly pushing the krill through the water. Krill have a very fine feeding basket at the front. They have six pairs of thoracic legs, if you like-- they're the front legs-- and they form what's called a filtering basket. And that allows them to sieve out, you know, almost the smallest phytoplankton in the sea. It's almost like humans being able to feed on dust. If we could sieve dust out of the air, we'd be doing what krill can do in the ocean. Be careful.

NARRATOR

On deck, they're finally getting the trawl into the water. Trawling is the best way to directly survey life below the surface. So biological oceanographers like Brian Hunt and Evgeny Pakhomov never lose an opportunity to sample the waters. But a single trawl on its own won't tell them much. It was only by combining the results from thousands of trawls like this, taken on hundreds of different cruises, that scientists realized there was something strange happening to the krill population in the Southern Ocean. The man who led the project was British oceanographer Angus Atkinson. He found records dating all the way back to the 1920s.

ANGUS ATKINSON

We realized there was a lot of data lying around in various old notebooks and in data archives that hadn't ever been put together. I was involved in Nature paper led by Angus Atkinson, which allowed us for the first time to estimate distribution of the krill around Antarctic continent. We had about a dozen countries' data that we compiled into this central database that we call KRILLBASE.

NARRATOR

The combined data from nearly 12,000 net hauls revealed patterns that were not immediately apparent from the individual results.

PAKHOMOV

We were able to reconstruct the changes of the density of the krill over past about 25, 30 years, since about mid '70s.

NARRATOR

Strikingly, they found that the krill population varies a lot, shrinking and growing from year to year.

ATKINSON

Krill abundance varies tenfold from one year to the next, so obviously we need to know what is causing that.

NAATOR

But the combined data also revealed a long-term trend superimposed on the annual variations.

PAKHOMOV

Remarkably, what we found is a significant decrease in the krill density.

ATKINSON

It's about an 80% decline over 30 years. So that's a significant decline in the abundance of krill.

NARRATOR

In the ten years since Angus Atkinson published his findings, most krill scientists have come to accept that krill numbers have indeed declined dramatically. But why? There's an important clue hidden in Angus's data-- a clue that now takes Polarstern due south, towards the ice. As she approaches Antarctica and the temperature drops, the sea begins to freeze.

ROB KING

I mean you spend a long time sort of bouncing around the ocean on a voyage to, you know, to the sea ice to do work, and you can't help but get a little bit excited when you start to get in there.

NARRATOR

This is the marginal ice zone-- a wide region where dangerous icebergs lurk, disguised by the ocean swell.

KING

People wouldn't understand how much there is to know about sea ice. They'd just say, "Ice, it's ice." Well, it's not. And it really is quite extraordinary what there is to learn about it. Because it is the structure which this ecosystem is built on. Each winter the Southern Ocean freezes over. And the whole ocean environment changes dramatically.

NARRATOR

This animation of satellite data shows how the ice grew out day by day in the years 2009 and 2010.

HUNT

The sea ice extent goes from four million square kilometers in the summertime to 20 million square kilometers in the wintertime. So it's absolutely massive.

KING

So over a period of six months, the size of Antarctica, if you like, you know, more than doubles. So it's, you know, an enormous change to the environment.

NARRATOR

How far the sea ice extends varies from year to year and, analyzing their data, Angus Atkinson and Evgeny Pakhomov found a link between these annual variations and krill population.

PAKHOMOV

In the Nature paper we tried to find some mechanistic explanation for the decline in the krill abundance. The best correlation has been found with density of krill and with the ice coverage. When there was a large increase in the population size of krill in a particular year, we found that six months before that, the previous winter, there was a heavy sea ice coverage in that sector.

NARRATOR

It looked as if the size of the krill population depended on the extent of the ice.

ATKINSON

But it wasn't clear exactly how it depended on ice. Is it providing the krill with shelter, to hide away from predators, for example? Is it providing a food source? Is it just certain types of ice that are useful? Is all ice the same? And that means actually going into the ice, looking at the actual effects, how ice affects the survival of these krill.

NARRATOR

After 24 hours of careful steaming through the marginal ice zone, Polarstern enters the pack ice. The icebreaker has a double hull and a sharply sloping prow designed not to slice through the icepack, but to ride up over it and crack the ice open with her weight.

ULRICH FREIER

Even after 80 years of krill science, it is quite unclear where krill is in wintertime. And I guess that is one of the main topics of this voyage of Polarstern in the Antarctic winter, to just look under the ice. And that means a diving operation.

NARRATOR

The plan is to dive beneath the ice to search for krill and film their behavior. But to make sure they are observing undisturbed animals, they'll have to venture at least half a mile from Polarstern and the noise of its engines. So they need to find an ice floe thick and stable enough to support a dive camp away from the safety of the ship. (motor humming) To help them find the right floe, they will scout the area ahead with a flying instrument called the EM Bird.

The Bird carries two precision instruments

a laser that measures the distance to the surface of the ice floe, and another instrument which measures the distance to the seawater beneath the ice.

THOMAS KRUMPEN

The difference is basically the ice thickness. So, for us, ice thickness means ice plus the snow on top.

NARRATOR

The Bird reveals a strange world of floes of varying thickness, divided by ribbons of open water, which look almost like rivers on land. Thanks to the Bird, team members identify thicker floes to the south, and by the 31st of August-- two and a half weeks into the expedition-- the ship is stationed next to a floe they believe will be safe. Now it's time to find out for sure.

TESCHKE

When we first stepped on the first ice floe, yeah, it was... a feeling maybe a little bit like, you know, the first men on the moon had, you know? You make the first step on a... on a very new piece of land or something. It was exciting. You have to keep in mind every day that you are walking basically on ocean surface. You have 4,000 meters of water under you. It's absolutely astonishing.

NARRATOR

But it's not the surface of the ice that interests the scientists-- it's what lies below. And that means venturing beneath the ice, in winter, in Antarctica.

FREIER

You have an environment which is super icy and has temperatures about -10, -20, -30 degrees. We have to have a big, big dome tent where the divers can operate sheltered away from the wind and from the quite fast-changing environment.

NARRATOR

Only Ulli, the dive leader, has any experience at all of diving beneath sea ice, and an operation on this scale in the middle of the shifting ice pack has never been attempted before. As the drill breaks through the ice, a momentary silence comes over the team.

SVEN KERWATH

All of us sort of looked at this entry hole with sort of mixed emotions. How will I react? Will I be able to do my work down there? Will I be able to function?

NARRATOR

Outside the dive tent, wind chill makes it -58. Inside, the divers prepare to enter the water for the first time.

TESCHKE

To get prepared for a dive needs a lot of time. You have a mixture of feeling. You don't know what is under the ice, you know? It was not very secure, you need to secure yourself. Some claustrophobic feelings were always present.

NARRATOR

Ulli has recruited a team of scientist-divers experienced in marine ecology, which usually means surveying the life on the sea floor in relatively shallow seas.

NOYAN YILMAZ

I have never dived in the ocean before-- not in the ice, not in the ocean. I mean, it's not just the ice, it's also more than 4,000 meters deep where you dive. So it is... different. Of course I was nervous. Because it is a new thing and there's a lot of effort being made around you and you don't want to disappoint anybody. So I think it was excitement and also nervousness. But I guess you have to be nervous when something new like that comes around.

FREIER

It is quite hard in the first stage to find the way to jump in and to feel happy. It is quite a hard psychological moment in the first dives to come over this point. What you see is a black hole, which looks really a little bit like the entry in the underworld. And then you dive down. You need one meter to go down and then you see... then you just see the white of the ice. But then you come under this, and then your eyes are looking in the distance and you have the idea that you can look more or less in all directions without limits. It's stunning.

NARRATOR

The first surprise is how varied the under-ice landscape is.

TESCHKE

The under-ice landscape is full of colors, full of light, full of life. It's like you step into a forest. It's very diverse, and it's beautiful.

NARRATOR

On the surface, a thick layer of snow makes the floe look fairly uniform. But underneath, the scientists can see that the floe is really a collage of hundreds of pieces of ice of different sizes, shapes, thicknesses, and ages. GTZ: In some areas it's very smooth, and other areas there's over-rafted pieces of ice sticking out. It's very variable and different and something new every time you look. And, of course, the light is quite nice because it changes with the thickness of the ice in intensity and color. It's almost like an out-of-body experience. Down there under this cathedral of ice with all its different colors, and it's almost like psychedelic. Even though it is expected, they are struck by just how clear the water is. It shows that there's very little phytoplankton in the water column. But there are a few krill in the water, though they're hard to see. So the question is, what are they feeding on? (wind whipping)

KING

Larvae are being spawned, if you like, at the summer and end of summer, and so as all the primary production of phytoplankton is finishing up in the Antarctic, that's when they are coming up and looking for their first lunch, if you like. At that time they have to eat something. If they don't feed, don't eat within 15 days, they die. So I'm really intrigued to have a good look at some of the behavior, the things that we can't normally see from a dead sample, to try and understand a bit more about this puzzle of larval feeding and how they get through this period of food scarcity.

NARRATOR

The second day of diving operations. But even before the divers get into the water, they spot something quite remarkable. Thin ice has formed over the dive hole, and it gives them a completely unexpected view of krill on the underside of the ice.

MEYER

When I saw the first larvae under the ice and how they behaved, I was really amazed because you start to understand the system, how it works, so why... why sea ice might be important for the larvae.

PAKHOMOV

They are actively feeding. They're scooping something from the surface of the ice-- it could be bacteria, or it could be microzooplankton.

NARRATOR

The krill appear to be grazing on an invisible film of microscopic living material on the underside of the ice.

KING

We've seen a very thin layer of ice over the dive hole and the larvae orientate themselves upside down to it, working their way across it, scraping that biofilm off. I mean, it's an incredibly tedious way to feed but if there's something there and nothing anywhere else and you're gonna die if you don't do it, you go for it.

NARRATOR

But how well are the larvae doing on this meager diet? To find out, they need to catch some.

FREIER

I saw these larvae, and it was, "Wow!" And we took samples, of course. We had a MASMA pump, which allows us to suck in the larvae in a very smooth way that they just, whoosh, get sucked in and are in perfect physiological conditions for further experiments on board.

MEYER

This is the first time that the diver catch the krill. Oh, komme hier! Oooh. And now we were really surprised that they were there, and so everybody was very excited and, yeah, started with the hunting.

NARRATOR

It's quickly apparent that the behavior they saw under the dive hole is not unique. In every direction the divers find swarms of krill doing the same thing.

TESCHKE

For me as a krill biologist, it was absolutely awesome to see, actually, how krill larvae really live, how they live in their environment-- how they hide during the day in these little caves and how they feed from the ice surface. It was absolutely beautiful.

KERWATH

That was the first time where I really saw a big swarm of krill hanging under the ice. There were just so many of them, all closely attached to the ice.

MEYER

I was really overwhelmed. When you see the animal you work with in their natural environment, with their natural behavior, and this is breathtaking.

NARRATOR

There are millions of square kilometers of sea ice around Antarctica. If this floe is in any way typical, there must be trillions upon trillions of krill larvae feeding on the underside of the ice. And experiments with the larvae they've caught prove the krill are not just surviving; they're actually growing. As krill grow, they molt, throwing off their old skin like a snake. By comparing a molt with the individual that has just discarded it, Rob King can determine the rate of growth.

KING

What we've found for this ice station is that the krill are growing at about five percent per molt. So if they're molting every sort of 20 to 30 days, that's doing all right for a krill, considering it's winter. Typically in summer you can find them growing around seven or eight percent, so these krill that we've got here were doing very well until we got them.

NARRATOR

But what exactly are the krill eating? Scientists have known for some time that there's more to sea ice than just frozen water. As the Polarstern fights its way through thick floes to a second dive location, what's hidden within the ice becomes clearly visible.

HUNT

The floes crack open and turn up and you can see down the section. Some of them are just rich green-brown colored.

NARRATOR

That color is the color of life-- phytoplankton in the form of algae.

MEINERS

I get always excited when I see this colored ice because sometimes you have ice algae biomass in such high concentration that actually the ice turns brownish. These are the pigments of the algae.

NARRATOR

These algae, frozen into the ice when it formed at the end of summer, will play a vital role in restocking the ocean with phytoplankton when the sun returns.

KING

So when that ice melts in summer and the phytoplankton drop into the ocean, the nutrients are there and off they go-- boom! Off goes the ecosystem.

NARRATOR

So the ice actually preserves the base of the food chain through the long Antarctic winter, ready to bloom again when summer returns. This gigantic phytoplankton popsicle also helps to keep krill larvae alive through the winter, and that may be why krill are struggling. For in their most important spawning ground, the Antarctic Peninsula, the popsicle is changing.

PECK

So we've traveled six or maybe seven miles from the British Antarctic Survey Station at Rothera Point on the Antarctic Peninsula, through fields and fields of sea ice, and we've arrived here. And this is the Sheldon Glacier. When I first came here 20 years ago, the ice that we now see as this Sheldon Glacier behind us stretched more than two miles in that direction. So not this headland but the one beyond it and all the way across the bay was under ice, under a glacier. Maybe 500 feet of ice was here. So somewhere in the region of trillions of tons of ice have disappeared from this bay and we're now sitting in open water And it's absolutely astonishing to be doing that in the time that I've been coming to this station. It's an absolutely stark reminder of the change that we're getting along the Antarctic Peninsula following this warming that we're getting from climate change.

DUCKLOW

This area, in about the last 50 years, has warmed up by seven degrees Celsius. So that's 11 or 12 degrees Fahrenheit. And it's about five times the global average temperature change. And this area is so poised around the freezing point that that much change is just a really profound, fundamental change in this system.

NARRATOR

The biggest change is not how far the ice extends. In fact, the total area of the Southern Ocean that freezes in winter has increased slightly in recent years. Instead, what's changing on the peninsula is how long the ice lasts and when it forms. Every year it's freezing later and melting earlier.

SCHOFIELD

Over the last 30 years since we've had satellite technology, what we've seen is the winter sea ice season, when it grows out, is 90 days shorter. And again if you think of that from your own perspective when you're sitting at home in wintertime and someone is saying, "We're going to change winter by 90 days," that's a huge effect.

NARRATOR

One effect is on the content of the sea ice. If the sea starts to freeze in late summer, the ice will incorporate lots of phytoplankton. But ice that forms later, when the days become short and the phytoplankton has started to die, will be different. It will lack the biomass that krill larvae need to survive to adulthood and complete krill's reproductive cycle.

SCHOFIELD

The krill need the ice to breed, and so if you shut it down you essentially remove that breeding location. And then that has an impact through the rest of the food web.

NARRATOR

So what does the future hold for krill and all the other animals that depend on them? The answer will lie in how well krill can adapt to the shifting ice season. Will their life cycle and behavior evolve fast enough to track a changing climate? It's six weeks into the voyage, and the Polarstern scientists are continuing their investigation of krill at a second dive location 700 miles further east in the Weddell Sea. They're intrigued by a fascinating contrast in krill's daytime and nighttime behavior. It was first spotted by Rob King. Aboard the ship Rob and his colleagues have been experimenting with a different way of collecting krill. They are using an adapted fish pump, normally used in fish farming, to suck krill directly out of the water beneath the ship's keel, at a depth of about 20 feet. And Rob has noticed a very surprising pattern in when he catches krill.

KING

Yes, so we're doing this around the clock and we're finding that there's nothing turning up for much of the day, and then at this specific time, and it was within two minutes of the same time as last night, out comes this flood of larvae.

TESCHKE

I just wanted to show that this is the situation that we found yesterday.

So very exactly at 7

22, the larvae appeared here in the fish pump. And it's a pretty clear signal exactly at sunset. And then with sunrise the signal went off, so no larvae appeared anymore.

KING

At sunset we have a peak in catch rate, and then it falls off to a sort of one-third level. And then just before sunrise we see another peak, that's the other peak. And then it drops right off to maybe only catching one or no larvae an hour, compared to the high points, which are like a hundred larvae an hour. So these are big differences.

NARRATOR

So what's going on? Out on the ice, the divers suggest an answer.

KERWATH

Krill only seem to be associated with the ice during daylight, and it seems that once it starts to get dark, then they start to pack their bags and go back into the water column.

NARRATOR

The divers see signs of a daily migration into and out of the water column, and hence the reach of the fish pump. GTZ: Having observed that, we wanted to quantify that with these transects. And, therefore, we dived during the day and also during the night doing the exact same transects, where we follow a line that has been mounted under the ice beforehand.

KERWATH

You can see the different footages. During midday the krill are really sitting on the ice and the little larvae feeding on the ice.

00 and 5

00, towards sunset, they start sort of dispersing. And in all our night dives we have never seen krill on the ice-- in none of the areas, none of the dives. So that's a pretty straightforward result. In the daytime hours they are probably sticking very close to the ice and not moving away from it very much so that they can't be taken by predators very easily. And then at nighttime they can come out of those areas, because it's harder for predators to see them, and they can feed in the open water as well.

NARRATOR

Under the ice, light levels vary a lot, depending not just on the time of day, but on the thickness of the ice and even the weather. But the daily migration of krill from the ice to the water column is strikingly punctual.

ROB KING

The numbers we get each night are different, but we see this clear start. And the question we're interested in is, is there a clock running in these animals?

JARMAN

The circadian clock is the clock that controls sleeping and waking and those really sort of basic behaviors in humans. But it also works in other animals as well.

NARRATOR

It looks as if kri's daily feeding patterns are controlled by an internal circadian clock that is regulated by exposure to light. But everywhere on Earth, except near the equator, the length of the day varies with the seasons. So in many species the circadian clock has a second,

equally vital function

it controls seasonal behavior.

TESCHKE

Because if you see what the circadian clock basically is doing, it's measuring day length. So if over the season day length is changing, the circadian clock can perceive that, can measure that.

NARRATOR

But obviously day length is not something that's affected by climate change. So if krill's seasonal breeding behavior is controlled by day length, rather than a climatic variable like temperature, that could make it much more difficult for them to track the shifting ice season.

DUCKLOW

All the organisms in this ecosystem have their life cycles synchronized and attuned and adapted to the rhythm of the sea ice. And what's happening now with climate change is that that's changing-- the duration of sea ice is getting less over time. So it advances later and it retreats earlier, and, of course, the organisms are pretty hardwired to that original timing. And so as the sea ice changes, literally the habitat of the krill and other organisms is melting away.

NARRATOR

Polarstern and her scientists have spent six weeks in the winter pack ice. It's time to head north again. There's one last series of dives, to observe krill in the marginal ice zone. The scientists have learned a lot about the importance of sea ice to krill and how that has made them vulnerable to the climate change caused mainly by our burning of fossil fuels.

SCHOFIELD

It took the Earth tens of millions of years to make oil, and humans will have burned it in a few hundred years. That experiment has happened, and now we're going to see where that experiment's gonna take us.

NARRATOR

Krill will find it hard to change their behavior. So the question is, can we change ours?

KING

I think the really important thing to get hold of is that we are at a point now where we can understand our ecosystem like we've never understood it before, and we really need to pay attention to what we're learning from it.

FREIER

That was stunning. Hundreds of millions of larvae hanging around under the ice, all in caves and sheltered areas. Swarms and swarms full of larvae sheltered away. Super, super cool dive. Really one of the best I did. Just stunning. To order this program on DVD, visit ShopPBS or call 1-800-PLAY-PBS. Episodes of "NOVA" are available with Passport. "NOVA" is also available on Amazon Prime Video.