0:00:05 > 0:00:10The power of the sun drives the seasons, transforming our planet.
0:00:11 > 0:00:15Vast movements of ocean and air currents
0:00:15 > 0:00:18bring dramatic change throughout the year.
0:00:22 > 0:00:26And in a few special places, these seasonal changes
0:00:26 > 0:00:29create some of the greatest wildlife spectacles on earth.
0:00:34 > 0:00:36Here in the Arctic, each summer,
0:00:36 > 0:00:39the sun begins to melt the winter ice.
0:00:41 > 0:00:46Nearly three million square miles of ice will disappear,
0:00:49 > 0:00:54opening up a narrow window of opportunity for millions of animals.
0:01:04 > 0:01:07For many it's their best chance to feed and breed.
0:01:11 > 0:01:14But for polar bears,
0:01:14 > 0:01:16it's the hardest time of the year.
0:01:16 > 0:01:22They have to survive the greatest seasonal change on the planet.
0:01:43 > 0:01:46Winter in the Arctic.
0:01:46 > 0:01:50The Northern Lights flicker across the sky.
0:01:54 > 0:01:59It's a land of continuous night, where temperatures plummet to -40.
0:02:10 > 0:02:15Polar bears are in their element, hunting for seals on the frozen sea.
0:02:17 > 0:02:20But the long night is coming to an end.
0:02:20 > 0:02:26In February, the sun rises for the first time in four months.
0:02:33 > 0:02:35In the coming weeks,
0:02:35 > 0:02:39the strength of the sun will power an enormous change.
0:02:39 > 0:02:43But for now, its rays offer only a little warmth.
0:02:55 > 0:02:59Despite the sun's return, 6 million square miles
0:02:59 > 0:03:03of the planet's far north is still cloaked in ice.
0:03:06 > 0:03:09Deep snow covers the mountains.
0:03:19 > 0:03:24Even the sea is frozen solid, many metres deep.
0:03:36 > 0:03:40Groups of ringed seals haul out through holes in the ice
0:03:40 > 0:03:42to bask in the weak sunlight.
0:03:44 > 0:03:47But they're vulnerable and have to keep a look-out.
0:03:47 > 0:03:50With the sea still frozen, it's easier for predators to get close,
0:03:50 > 0:03:56and the seals' greatest enemy is the polar bear.
0:04:01 > 0:04:04A mother bear and her four-month-old cub are hunting.
0:04:06 > 0:04:11Seals make up most of the bears' diet, and to find them,
0:04:11 > 0:04:15she must lead her youngster out onto the ice for the first time.
0:04:21 > 0:04:25Scientists looking at how a changing climate is affecting bears
0:04:25 > 0:04:29have fitted the female with a radio collar.
0:04:30 > 0:04:32The ice here is thinner,
0:04:32 > 0:04:36and she must spread her weight to avoid breaking through.
0:04:40 > 0:04:43For her cub, it's all just a game.
0:05:03 > 0:05:07By sticking close to mum, he'll learn how she hunts for seals,
0:05:07 > 0:05:11a critical lesson for his future survival.
0:05:16 > 0:05:20At this time of year, the frozen Arctic is empty of life,
0:05:20 > 0:05:23and only a few hardy residents can survive.
0:05:25 > 0:05:29For a female Arctic fox, the winter has been a time of hunger.
0:05:29 > 0:05:33Food is scarce and she's had to wander far and wide,
0:05:33 > 0:05:36scavenging from the remains of bear kills.
0:05:49 > 0:05:51But her fortunes are changing.
0:05:51 > 0:05:55A dead reindeer is a lucky find.
0:05:56 > 0:06:00If no other scavengers come along, this could sustain her
0:06:00 > 0:06:02for the rest of the winter.
0:06:03 > 0:06:06WIND SWIRLS
0:06:19 > 0:06:23With the sea frozen, polar bears are busy hunting.
0:06:24 > 0:06:27This bear has caught a seal.
0:06:27 > 0:06:30He waited for it to surface through a hole in the ice
0:06:30 > 0:06:33and his patience has been rewarded.
0:06:34 > 0:06:37He must catch a seal about once a week,
0:06:37 > 0:06:39and this is the best hunting season.
0:06:39 > 0:06:42As long as there is sea ice, the bears won't go hungry,
0:06:42 > 0:06:45as walking on the ice is the easiest way
0:06:45 > 0:06:48for a bear to get close to a seal.
0:06:53 > 0:06:59But the ice will soon start to break up, and hunting will get harder.
0:06:59 > 0:07:01The calories the bear takes in now
0:07:01 > 0:07:03will have to see him through hard times ahead.
0:07:07 > 0:07:11For polar bears, it's the survival of the fattest.
0:07:14 > 0:07:18At this time of year, there are still only five hours of daylight.
0:07:18 > 0:07:23But as the sun rises higher, each day lasts 40 minutes longer.
0:07:31 > 0:07:35It's March, and with increasing sunlight,
0:07:35 > 0:07:39the first in a huge wave of migrants are arriving.
0:07:48 > 0:07:53Millions of seabirds are travelling north to reach the frozen coast.
0:07:59 > 0:08:02Huge flocks of little auks and guillemots
0:08:02 > 0:08:06fly across many miles of ice from the nearest open water.
0:08:15 > 0:08:20Their destination is the still-frozen sea cliffs.
0:08:20 > 0:08:23It's an inhospitable place,
0:08:23 > 0:08:26but in a few weeks, everything here will change.
0:08:35 > 0:08:40It pays to be early, even though the snow has yet to melt.
0:08:40 > 0:08:45The first arrivals get the best nesting ledges and a head start.
0:08:45 > 0:08:51They have just 50 days from laying their eggs to raise their chicks.
0:08:59 > 0:09:03The Arctic's silent wilderness is coming to life.
0:09:15 > 0:09:17It's three months since the sun's return,
0:09:17 > 0:09:20and its power is growing daily.
0:09:21 > 0:09:25The air temperature rises slowly,
0:09:25 > 0:09:32and once above zero, at long last, the melt begins.
0:09:51 > 0:09:56Melting snow feeds freshwater streams which pour off the land.
0:09:58 > 0:10:03In just a few short days, the melt unveils a whole new landscape.
0:10:30 > 0:10:35The coastal cliffs now teem with nesting seabirds,
0:10:35 > 0:10:38and it's not long before they attract unwelcome attention.
0:10:40 > 0:10:43The fox's white winter coat has disappeared with the snow.
0:10:45 > 0:10:50Her new camouflage will allow her to change from scavenger to hunter.
0:10:50 > 0:10:54The short summer will be her best chance to raise a family.
0:11:09 > 0:11:12The snow may be disappearing from the land...
0:11:14 > 0:11:17but the sea ice has yet to melt.
0:11:26 > 0:11:29The coastline is still locked in ice.
0:11:34 > 0:11:38The islands of the Arctic are surrounded by
0:11:38 > 0:11:41thousands of miles of white frozen ocean.
0:11:42 > 0:11:45Because the sea ice is so thick,
0:11:45 > 0:11:48it won't begin to break up until the temperature stays
0:11:48 > 0:11:50above zero for a number of days.
0:11:52 > 0:11:55Until then, few creatures can penetrate this icy barrier.
0:12:03 > 0:12:06The sun reflects from the white frozen sea,
0:12:06 > 0:12:09creating a desert of icy mirage.
0:12:23 > 0:12:27The bear can still walk on the ice to hunt for seals,
0:12:27 > 0:12:29but not for much longer.
0:12:29 > 0:12:31It's getting warmer by the day.
0:12:53 > 0:12:57The bear's world is about to melt away.
0:13:12 > 0:13:17It's now June, and the sun beats down 24 hours a day.
0:13:17 > 0:13:21The temperature remains constant above zero degrees.
0:13:21 > 0:13:24The frozen sea begins to melt.
0:13:26 > 0:13:28Pools form across the surface,
0:13:28 > 0:13:32absorbing more of the sun's heat, speeding the thaw.
0:13:41 > 0:13:44Sunlight penetrates the frozen surface,
0:13:44 > 0:13:48illuminating a strange world beneath the ice.
0:13:55 > 0:13:57The frozen barrier has split.
0:13:57 > 0:14:00EERIE CREAKING
0:14:05 > 0:14:09Mysterious voices echo against the icy ceiling.
0:14:15 > 0:14:20Giant cracks or leads form at weak points,
0:14:20 > 0:14:23creating a pathway for new arrivals.
0:14:36 > 0:14:37Narwhal.
0:14:54 > 0:14:59Known as the Arctic unicorn because of their strange spiral tusks,
0:14:59 > 0:15:02narwhal are some of the most secretive and elusive
0:15:02 > 0:15:04animals in the world's oceans.
0:15:04 > 0:15:06The leads provide passage
0:15:06 > 0:15:10for thousands of these mysterious whales.
0:15:12 > 0:15:17Each summer they travel 600 miles north, navigating through the ice,
0:15:17 > 0:15:19to reach rich fishing grounds.
0:15:23 > 0:15:27More whales travel along the edge of the ice where it meets the open sea,
0:15:27 > 0:15:29to search for the openings of leads.
0:15:43 > 0:15:45It's a hazardous journey.
0:15:45 > 0:15:47As mammals, they need air to breathe,
0:15:47 > 0:15:51and if the ice closes above them, they could suffocate.
0:16:21 > 0:16:25They're looking for holes in the ice where they can surface.
0:16:25 > 0:16:29A single breath will last them for 15 minutes.
0:16:30 > 0:16:35The further they travel, the harder it becomes to find holes in the ice.
0:16:44 > 0:16:47They snatch a breath and then travel on.
0:16:54 > 0:16:58But ahead, the ice forms an impenetrable barrier.
0:17:13 > 0:17:17The narwhal use their heads and their long tusks to break the ice
0:17:17 > 0:17:19and enlarge the breathing holes.
0:17:28 > 0:17:31For now, they've reached a dead end, and must wait.
0:17:31 > 0:17:35The ice will need to melt further if they are to continue.
0:17:37 > 0:17:41All across the Arctic, the sea ice is beginning to retreat.
0:17:43 > 0:17:48The leads widen, forming tracts of open water, wind and ocean currents
0:17:48 > 0:17:51shift the loosening ice, breaking it up.
0:17:56 > 0:18:00As the ice melts, thousands of miles of open ocean become accessible,
0:18:00 > 0:18:04providing a bonanza for millions of seabirds.
0:18:04 > 0:18:06SEABIRDS SQUAWK AND CALL
0:18:13 > 0:18:17In just four weeks, a colony of guillemots can devour
0:18:17 > 0:18:21one and a half million tonnes of Arctic fish.
0:18:28 > 0:18:30As the ice melts, their journey to
0:18:30 > 0:18:34their feeding grounds in the open sea shortens by the day.
0:18:40 > 0:18:43They gather at the edge of the retreating ice
0:18:43 > 0:18:46and dive to hunt for Arctic cod and capelin.
0:19:01 > 0:19:05The shoals are found 80 metres down in the murky depths,
0:19:05 > 0:19:09and the guillemots must hold their breath for nearly two minutes.
0:19:20 > 0:19:22With their crops stuffed with fish,
0:19:22 > 0:19:26they return to the colony to feed their hungry youngsters.
0:19:34 > 0:19:39With 24 hours of daylight, they go on fishing around the clock.
0:19:52 > 0:19:56Warmed by the sun and driven by winds and currents,
0:19:56 > 0:19:59the sea ice is now fragmenting.
0:20:03 > 0:20:07That is a big problem if you need the ice to hunt from.
0:20:10 > 0:20:15A polar bear can smell a seal from over a mile away.
0:20:19 > 0:20:21But the prey he depends on is hard to find
0:20:21 > 0:20:24in this constantly moving landscape.
0:20:32 > 0:20:35He must take to the water to navigate
0:20:35 > 0:20:38through the drifting maze of ice.
0:20:38 > 0:20:43Bears are excellent swimmers, but he's no match for a seal.
0:20:44 > 0:20:48As the ice melts, finding seals gets harder.
0:20:48 > 0:20:53And with more open water, it's easier for a seal to escape.
0:21:07 > 0:21:10The melt has caused a shift in power.
0:21:10 > 0:21:13Now it's advantage seal.
0:21:15 > 0:21:18BEAR GRUNTS
0:21:23 > 0:21:28The bear's chance has slipped away, and his hunger grows.
0:21:38 > 0:21:44The ice barrier broken, strange marine visitors begin to arrive.
0:21:47 > 0:21:49SQUEAKING
0:22:02 > 0:22:07They've swum 600 miles to get here, and with the ice gone,
0:22:07 > 0:22:10they can make their way into the shallows of fresh-water estuaries.
0:22:24 > 0:22:26Beluga whales.
0:22:34 > 0:22:38As more and more belugas arrive, a strange annual ritual begins.
0:22:42 > 0:22:44For belugas, this is a very special event.
0:22:55 > 0:22:57The violent thrashing against the river bed
0:22:57 > 0:23:01loosens their year-old skin and rids them of unwelcome parasites
0:23:01 > 0:23:03they may have picked up on their journey.
0:23:17 > 0:23:20In fresh water, warmed by the strengthening sun,
0:23:20 > 0:23:23and with the smooth pebbles in the shallows to rub against,
0:23:23 > 0:23:26the whales whistle with pleasure.
0:23:26 > 0:23:29WHISTLING AND SNORTING
0:23:50 > 0:23:53But the ice-free summer will be short.
0:23:53 > 0:23:59The whales can only enjoy their Arctic spa for two weeks.
0:23:59 > 0:24:03Soon the belugas must leave the shallows and make the most of
0:24:03 > 0:24:07what's left of the summer to hunt the shoals of Arctic fish.
0:24:15 > 0:24:18It's early July, and at the bird cliff,
0:24:18 > 0:24:20the fox has caught a nesting fulmar.
0:24:25 > 0:24:27She has a family now,
0:24:27 > 0:24:32eight tiny mouths to feed, and one bird is not enough to go round.
0:24:43 > 0:24:46The cubs bicker over their dinner.
0:24:46 > 0:24:49They're only three weeks old, and in just another two weeks,
0:24:49 > 0:24:51they'll have to be ready to fend for themselves.
0:24:56 > 0:25:01The winter ahead will be so harsh that only two of these eight cubs
0:25:01 > 0:25:06are likely to gain enough weight to survive.
0:25:06 > 0:25:09Their lives depend on every mouthful.
0:25:13 > 0:25:18When food is scarce, the most dominant cub will feed.
0:25:18 > 0:25:21And the smallest will go hungry.
0:25:31 > 0:25:33The sea birds have been working round the clock
0:25:33 > 0:25:35to keep their chicks fed,
0:25:35 > 0:25:39and to prepare them for the first big challenge of their lives.
0:25:39 > 0:25:43If they're to escape the winter, they'll have to leave the cliffs
0:25:43 > 0:25:46right away, and get far out to sea.
0:25:46 > 0:25:50But it's only been 20 days since they hatched,
0:25:50 > 0:25:54and their wings are still too weak for them to fly.
0:25:54 > 0:25:57So far their feet have never left the ground,
0:25:57 > 0:26:00and there's only one way down to the safety of the sea -
0:26:00 > 0:26:04300 metres below.
0:26:18 > 0:26:24Reluctantly, the chicks take a few nervous steps towards the edge.
0:26:32 > 0:26:35Encouraged by a gentle nudge,
0:26:36 > 0:26:39he leaps into the unknown.
0:26:50 > 0:26:55Dad follows right behind him, reassuringly calling to his chick.
0:27:07 > 0:27:09Made it.
0:27:32 > 0:27:35The landings may not be stylish, but at least they're on target.
0:27:50 > 0:27:56It's easy to misjudge the distance and some fall short of the water,
0:27:56 > 0:27:59but even now, Dad sticks close by.
0:28:03 > 0:28:07He encourages his chick to take the last few steps
0:28:07 > 0:28:10towards the safety of the waves.
0:28:10 > 0:28:14But some chicks land a long way from the water...
0:28:21 > 0:28:25They're sturdy enough to survive the fall,
0:28:25 > 0:28:28but this is no place to be left alone.
0:28:39 > 0:28:44For the mother fox, it's easy pickings.
0:28:54 > 0:28:57With so many free meals falling from the sky,
0:28:57 > 0:28:59she runs from one kill to the next.
0:29:16 > 0:29:20The chicks will be jumping for only three days,
0:29:20 > 0:29:22so she must make the most of this bonanza.
0:29:22 > 0:29:26The survival of her own family depends on it.
0:29:55 > 0:29:59With more chicks than she can possibly carry,
0:29:59 > 0:30:01the mother fox has to be clever.
0:30:05 > 0:30:09What her family can't eat now, she buries.
0:30:09 > 0:30:12For the days ahead, she'll have a well-stocked larder,
0:30:12 > 0:30:14enough for all eight of her cubs.
0:30:24 > 0:30:28Their bellies now full, at last, the cubs can relax in the sun.
0:30:41 > 0:30:44For the guillemot chicks that survive,
0:30:44 > 0:30:47the ordeal has only just begun.
0:30:47 > 0:30:52They must retreat south 600 miles, beyond the reach of winter.
0:30:52 > 0:30:56But the youngsters can't fly yet - their wings aren't strong enough.
0:30:56 > 0:31:00So led by their parents, they'll have to swim.
0:31:14 > 0:31:15At the height of summer,
0:31:15 > 0:31:20even the permanent ice caps are touched by the power of the sun.
0:31:25 > 0:31:27Meltwater channelled down from high on these
0:31:27 > 0:31:32ice caps pours over a precipice where the ice meets the ocean.
0:31:34 > 0:31:36Along this 200-mile wall of ice,
0:31:36 > 0:31:40A thousand fresh water cascades plummet into the sea.
0:31:58 > 0:32:03As July draws on, the great melt reaches its peak.
0:32:10 > 0:32:14The greatest seasonal change on the planet has taken place.
0:32:28 > 0:32:33The sea ice that once extended all the way to the horizon
0:32:33 > 0:32:34is now open ocean.
0:32:36 > 0:32:42In just three months, the sun has won its battle with the ice.
0:32:45 > 0:32:49Over 2.5 million square miles of ice has melted away,
0:32:49 > 0:32:54uncovering thousands of islands surrounded by open ocean.
0:32:54 > 0:32:59But in recent years, the scale of this melt has been growing.
0:33:00 > 0:33:04And for one animal, this is a critical issue.
0:33:21 > 0:33:27A mother bear and her adolescent cub rest on a fragment of sea ice.
0:33:27 > 0:33:32With the melt, they're forced to swim ever greater distances
0:33:32 > 0:33:33to hunt for seals.
0:33:37 > 0:33:42Their Arctic home is increasingly vulnerable to a changing climate,
0:33:42 > 0:33:46and this year, there has been even less ice than normal.
0:33:53 > 0:33:56If future melts are as extreme as this one,
0:33:56 > 0:34:00bears like these may starve or drown, lost at sea.
0:34:07 > 0:34:12This is one of the last pieces of ice now adrift in the open ocean.
0:34:12 > 0:34:17The polar bears' icy world has melted away.
0:34:28 > 0:34:29For many others,
0:34:29 > 0:34:34the open water provides the greatest feast of the year.
0:34:34 > 0:34:37The narwhal have made it through the ice.
0:34:37 > 0:34:41They gather in bays where they can hunt for Arctic cod and squid.
0:34:55 > 0:34:58Here, they are joined by other ocean migrants.
0:35:02 > 0:35:05Bowhead whales.
0:35:05 > 0:35:10These 100-tonne giants feed on millions of tiny plankton
0:35:10 > 0:35:13that flourish in the sunlit waters.
0:35:24 > 0:35:27Belugas have come for the rich fishing,
0:35:27 > 0:35:30and rest on the surface between dives.
0:35:38 > 0:35:42Even on the sea bed, there is plenty on the menu.
0:35:52 > 0:35:57Two tonnes of pulsating blubber forages for clams in the sediment.
0:35:57 > 0:36:02Walrus are year-round residents of the Arctic,
0:36:02 > 0:36:05following the ebb and flow of the ice.
0:36:06 > 0:36:09In the open summer waters, they can reach huge areas of the ocean floor,
0:36:09 > 0:36:13rich feeding grounds for these giant seals.
0:36:15 > 0:36:19They hoover up clams with their rubbery lips.
0:36:19 > 0:36:24A single walrus can eat up to 4,000 clams in one ten-minute dive.
0:36:33 > 0:36:36Bellies full, they come up for air.
0:36:50 > 0:36:55With the sea ice gone, the walrus haul out on dry land to rest.
0:36:56 > 0:36:58WALRUS GROWLS
0:36:58 > 0:37:02They're used to huddling together to keep warm, and even now
0:37:02 > 0:37:07that it's 12 degrees above freezing, they prefer to stick close together.
0:37:09 > 0:37:13In the warmth of the sun the walrus begin to shed their old skin,
0:37:13 > 0:37:15and they spend hours scratching.
0:37:17 > 0:37:21Some places are harder to reach than others.
0:37:25 > 0:37:28These irritations make them bad-tempered,
0:37:28 > 0:37:30and arguments often break out.
0:37:30 > 0:37:33GROWLING AND SNARLING
0:37:43 > 0:37:48Spitting, stabbing and bellowing iron out any disagreements.
0:37:53 > 0:37:56All disputes settled,
0:37:56 > 0:37:59a peace of sorts returns once more.
0:38:06 > 0:38:10Digesting a bellyful of clams generates a lot of wind,
0:38:10 > 0:38:14making walrus colonies very fragrant places.
0:38:14 > 0:38:17GAS BUBBLES AND HISSES
0:38:33 > 0:38:36Fed by the abundant supply of guillemot chicks,
0:38:36 > 0:38:40all eight fox cubs seem to have boundless energy.
0:38:41 > 0:38:45Soon they will be ready to face the world without their mother.
0:38:55 > 0:38:58In just five weeks she's raised her cubs,
0:38:58 > 0:39:01making the most of the short summer.
0:39:07 > 0:39:11The mother fox has won her race against time.
0:39:16 > 0:39:21Between June and July, the Arctic is the land of the midnight sun.
0:39:21 > 0:39:25An empty wilderness has been transformed.
0:39:50 > 0:39:54The once-frozen ocean is now bursting with life,
0:39:54 > 0:39:57as all the animals enjoy the summer feast.
0:40:29 > 0:40:32But polar bears aren't so fortunate.
0:40:34 > 0:40:39With no sea ice to hunt on, they're now trapped on dry land.
0:40:42 > 0:40:46A hungry bear will eat any food it can get its paws on.
0:40:46 > 0:40:51But for a meat eater, a few scraps of dry lichen won't go far.
0:40:57 > 0:41:01It may have been four months since his last kill,
0:41:01 > 0:41:03and he won't find any seals here.
0:41:06 > 0:41:10If the yearly increase in the scale of the melt continues,
0:41:10 > 0:41:13more bears will starve.
0:41:13 > 0:41:18Two thirds of the world's polar bears could vanish by 2050.
0:41:20 > 0:41:23The great melt has always been difficult for bears,
0:41:23 > 0:41:28but soon, surviving the summer may become impossible.
0:41:31 > 0:41:36The scale of the summer melt has changed over the last 30 years.
0:41:36 > 0:41:392007's melt broke all records.
0:41:41 > 0:41:45400,000 extra square miles of ice disappeared -
0:41:45 > 0:41:48the greatest melt ever recorded.
0:41:48 > 0:41:54The latest predictions suggest that the Arctic may be entirely ice-free
0:41:54 > 0:41:57in summertime within 20 to 40 years.
0:42:03 > 0:42:05By September, the sun's power begins to ebb.
0:42:05 > 0:42:10The summer comes to an end and the Arctic empties of life.
0:42:11 > 0:42:14The last seabirds begin their long journey south,
0:42:14 > 0:42:18leaving only a few hardy residents behind.
0:42:23 > 0:42:27The fox cubs now face the changing season alone.
0:42:34 > 0:42:37Already, the first snow is beginning to fall.
0:42:37 > 0:42:41The full Arctic winter is just six weeks away.
0:42:53 > 0:42:58For hungry bears, six weeks may be too long to wait.
0:42:59 > 0:43:05The sea ice will not freeze properly till it reaches -2 degrees.
0:43:05 > 0:43:06For now, it's still too warm.
0:43:09 > 0:43:11This ice is no use to the bears.
0:43:11 > 0:43:14They can't walk on it to hunt.
0:43:24 > 0:43:29The normally unsocial bears gather in groups, trapped on the shoreline.
0:43:33 > 0:43:38This is the time of year the male bears spar.
0:43:41 > 0:43:45Only the biggest bears have the energy reserves to fight.
0:43:45 > 0:43:49After a summer without food, the bears' systems are in low gear.
0:43:49 > 0:43:51These aerobics help warm them up,
0:43:51 > 0:43:54in readiness for the winter hunting ahead.
0:44:45 > 0:44:49It's late October, but still the sea hasn't frozen.
0:44:49 > 0:44:53For every degree rise in the average temperature,
0:44:53 > 0:44:56the summer melt is extended by a whole week.
0:44:56 > 0:44:59That's more bad news for polar bears.
0:45:09 > 0:45:14Smaller, younger bears don't have the energy of the big males.
0:45:14 > 0:45:18Each day they are without food, they lose nearly a kilo.
0:45:18 > 0:45:20Some have lost half of their body weight.
0:45:23 > 0:45:28These hungry bears must now rest in the snow and conserve energy.
0:45:28 > 0:45:31All they can do is wait.
0:45:44 > 0:45:50At last the wind changes, blowing from the cold north across the sea.
0:45:50 > 0:45:53The air temperature plummets to 20 degrees below.
0:45:56 > 0:45:59It's now cold enough for ice crystals to form in the ocean.
0:45:59 > 0:46:04They knit together, forming a greasy layer of surface ice.
0:46:05 > 0:46:08This thickens into plates which bump and collide in the swell,
0:46:08 > 0:46:11forming pancake ice.
0:46:18 > 0:46:22These pancakes lock together to form a continuous surface.
0:46:29 > 0:46:32At long last, the sea has frozen.
0:46:37 > 0:46:42Only now can the bears head out onto the sea ice to hunt for seals.
0:46:42 > 0:46:46But the new ice has a surprise in store.
0:47:02 > 0:47:05This season, for the first time ever recorded,
0:47:05 > 0:47:08even the winter ice is thinner.
0:47:13 > 0:47:17The ice here is too thin to climb out on,
0:47:17 > 0:47:20and he struggles to free himself.
0:48:02 > 0:48:07At last, safely on the firmer ice, he rolls in the snow to dry his fur.
0:48:22 > 0:48:26In November, the sun sets over the frozen north.
0:48:26 > 0:48:30It will not appear again for four months.
0:48:35 > 0:48:37The summer melt provides opportunities
0:48:37 > 0:48:38for millions of animals,
0:48:38 > 0:48:41but has now become a threat
0:48:41 > 0:48:44to the polar bear's very survival.
0:48:46 > 0:48:51This season, the bear has survived the greatest melt yet recorded,
0:48:51 > 0:48:55and made it through to the Arctic winter.
0:49:18 > 0:49:22The frozen Arctic Sea is one of the most demanding of all
0:49:22 > 0:49:27environments, and the location for a rarely seen spectacle.
0:49:27 > 0:49:29The Nature's Great Events team
0:49:29 > 0:49:35wanted to film the annual migration of the elusive Arctic narwhal.
0:49:35 > 0:49:40To do so, they would have to live on, dive beneath and fly over
0:49:40 > 0:49:44the ice, during the climax of the great melt.
0:49:54 > 0:49:58The quest for the narwhal started at the height of the melt, in July,
0:49:58 > 0:50:01when the sea ice was at its most dangerous.
0:50:01 > 0:50:05This made the task of first finding the whales a tough prospect.
0:50:11 > 0:50:14This is rough ice!
0:50:14 > 0:50:15And we're stuck!
0:50:15 > 0:50:17OK!
0:50:17 > 0:50:21Narwhal are so elusive, and the conditions on the ice so difficult,
0:50:21 > 0:50:24that the crew allowed a month to track them down.
0:50:26 > 0:50:28The plan was to get to the edge of the ice
0:50:28 > 0:50:32in the hope of finding the whales at the beginning of their migration.
0:50:37 > 0:50:41The melt was in full swing, and the team were in the thick of it.
0:50:42 > 0:50:47With 24-hour sunlight, the leads along which the narwhal migrate
0:50:47 > 0:50:50were getting wider every day.
0:50:59 > 0:51:00After three weeks of searching,
0:51:00 > 0:51:04the whales were living up to their secretive reputation.
0:51:13 > 0:51:15The crew couldn't see the narwhal,
0:51:15 > 0:51:18but there was evidence that they were not far away.
0:51:18 > 0:51:22A sensitive hydrophone enabled them to eavesdrop on the whales,
0:51:22 > 0:51:23under the ice.
0:51:24 > 0:51:26CLICKING
0:51:28 > 0:51:33I am hearing what could be whistles and clicks, so it could be narwhal.
0:51:38 > 0:51:41It's very distant, and sound travels a long way under water,
0:51:41 > 0:51:44so they are probably still far away, but it's a good sign.
0:51:44 > 0:51:49They could hear the whales, but would they be able to see them?
0:51:50 > 0:51:53By now the cracks had opened wide enough
0:51:53 > 0:51:55for them to dive to search for them.
0:51:59 > 0:52:01Ice diving is dangerous at the best of times,
0:52:01 > 0:52:05but in such a remote location, days away from the nearest help,
0:52:05 > 0:52:07they had to be especially careful.
0:52:11 > 0:52:14Even though it was the height of the summer,
0:52:14 > 0:52:18the icy sea water was still a chilly -1.6 degrees.
0:52:18 > 0:52:21So the team needed not only specialist equipment,
0:52:21 > 0:52:23but specialist attitude.
0:52:25 > 0:52:26You're very quiet there, Tom.
0:52:26 > 0:52:30I was just thinking we don't really want the ice to close up
0:52:30 > 0:52:31while we're underneath it.
0:52:33 > 0:52:36- No, closing up would be a bad thing. - It would be a bummer!- Yes!
0:52:37 > 0:52:42And Tom, from Florida, was taking no chances against the cold.
0:52:44 > 0:52:46Ah, that feels really good.
0:52:48 > 0:52:51- Sam, we gotta do this more often. - Wehey!
0:53:03 > 0:53:06Now, they were in the whales' realm.
0:53:07 > 0:53:13A vast underworld of frigid water enclosed beneath a ceiling of ice.
0:53:17 > 0:53:20This is a world we rarely see -
0:53:20 > 0:53:23sinister but at the same time, uniquely beautiful.
0:53:30 > 0:53:32As the melting ice runs into the open cracks,
0:53:32 > 0:53:36it creates an underwater mirage as the layer of fresh water
0:53:36 > 0:53:40mixes with the denser salt water beneath it.
0:53:46 > 0:53:50If the ice were to close in on them now, they could get trapped.
0:53:52 > 0:53:55As they ventured deeper, the crew carried a lifeline
0:53:55 > 0:53:59to the surface, to guide them back to their opening in the ice.
0:54:21 > 0:54:24Sadly, there were no narwhal to be seen,
0:54:24 > 0:54:28but it was a unique glimpse of the whales' world.
0:54:28 > 0:54:32Sometimes it's a little bit spooky, to have a ceiling above your head.
0:54:32 > 0:54:35And then when the tank starts breaking through a little bit,
0:54:35 > 0:54:37once in a while you think,
0:54:37 > 0:54:39is this really an intelligent thing to be doing?
0:54:39 > 0:54:44But, er... The shot looked cool so I think it was worth it!
0:54:48 > 0:54:51They had spent over a month on and under the sea ice.
0:54:51 > 0:54:56But with still no shots of narwhal, time - and ice - were running out.
0:54:58 > 0:55:03The edge of the ice was now only 15 cm thick, barely enough to support
0:55:03 > 0:55:06the weight of a man, let alone a camera team.
0:55:09 > 0:55:13You can feel the swell just coming up underneath this ice, it's trippy.
0:55:13 > 0:55:17It's just a gentle undulation all around us.
0:55:17 > 0:55:19This won't be here for long.
0:55:24 > 0:55:29But just as the ice was getting too thin, their effort was rewarded.
0:55:29 > 0:55:32What do you see, Sam? Narwhal.
0:55:32 > 0:55:35See where the five guillemots are, out there on the water?
0:55:35 > 0:55:37Six guillemots...
0:55:52 > 0:55:55Tom and his team had found the narwhal.
0:55:55 > 0:55:58The whales had arrived at long last.
0:55:58 > 0:56:02But sadly, the ice was now too dangerous to film from.
0:56:04 > 0:56:08If they were to stand any chance of continuing to film,
0:56:08 > 0:56:10it was time to call for reinforcements.
0:56:19 > 0:56:22After being stranded for seven days due to bad weather,
0:56:22 > 0:56:27the helicopter crew finally arrived, just in time.
0:56:30 > 0:56:35There was now one last chance to film the narwhal - from the air.
0:56:36 > 0:56:39Armed with an advanced aerial camera system,
0:56:39 > 0:56:43they could zoom in on the action from a long distance away.
0:56:43 > 0:56:45This would enable them to find the whales,
0:56:45 > 0:56:48and even get close-up shots without disturbing them.
0:56:54 > 0:56:56As the ice team headed home,
0:56:56 > 0:56:58the helicopter team took over.
0:57:02 > 0:57:04The helicopter could only carry enough fuel
0:57:04 > 0:57:06for a few hours' flying,
0:57:06 > 0:57:11and finding the whales in this vast landscape was a daunting task.
0:57:11 > 0:57:16But at last they caught up with the magical Arctic unicorn.
0:57:16 > 0:57:20Yeah, there they are. They're, kind of, ten o'clock.
0:57:20 > 0:57:21Just coming under us now.
0:57:36 > 0:57:39Nice. That's an amazing image.
0:57:39 > 0:57:40Look at that. Beautiful.
0:57:48 > 0:57:50Unaware of the helicopter flying high above,
0:57:50 > 0:57:53the whales carried on their journey through the ice.
0:57:55 > 0:58:01This is the first time the narwhal migration has been filmed
0:58:01 > 0:58:04from this incredible aerial perspective.
0:58:07 > 0:58:11Hey! That's magic.
0:58:11 > 0:58:13The plan had worked.
0:58:13 > 0:58:17It had taken over six weeks, but the team had managed to film the amazing
0:58:17 > 0:58:22journey of the narwhal in this most harsh and testing of landscapes.
0:58:22 > 0:58:27It was a rare and magical insight into the life of one of
0:58:27 > 0:58:29our planet's most mysterious creatures.
0:59:04 > 0:59:07Subtitles by Red Bee Media Ltd
0:59:07 > 0:59:10E-mail subtitling@bbc.co.uk