0:00:03 > 0:00:06We're all on an amazing journey.
0:00:06 > 0:00:11A 940 million kilometre voyage through space.
0:00:15 > 0:00:17Even though we can't feel it,
0:00:17 > 0:00:20we're travelling at over 100,000 kilometres an hour,
0:00:20 > 0:00:24circling a star we call the Sun.
0:00:27 > 0:00:30Every year, our planet, the Earth,
0:00:30 > 0:00:35travels around the Sun - and we go with it.
0:00:35 > 0:00:40We're looking at the huge impact this journey has on our world.
0:00:40 > 0:00:42Wow, look at that!
0:00:42 > 0:00:47We'll see how the Earth's tilt gives us our seasons
0:00:47 > 0:00:50and leads to monsoon rains.
0:00:53 > 0:00:56How the planet's spin stirs the atmosphere
0:00:56 > 0:00:58into giant, rotating hurricanes.
0:01:01 > 0:01:06And how small changes in the Earth's movement
0:01:06 > 0:01:09can cover the planet with ice.
0:01:12 > 0:01:14Our yearly orbit around the Sun has created
0:01:14 > 0:01:17and shaped the world we live in.
0:01:36 > 0:01:39We start our circumnavigation of the Sun
0:01:39 > 0:01:42at a very special place,
0:01:42 > 0:01:45and on a very special day.
0:01:45 > 0:01:50It's March the 20th, the spring equinox.
0:01:50 > 0:01:55This is the great pyramid in Chichen Itza -
0:01:55 > 0:01:58an ancient Mayan city in Mexico.
0:02:02 > 0:02:06Built 1,500 years ago,
0:02:06 > 0:02:09the city is one of the world's great archaeological sites.
0:02:09 > 0:02:13And it contains a remarkable insight
0:02:13 > 0:02:16into our journey through space.
0:02:18 > 0:02:21The ancient Maya had developed a deep understanding
0:02:21 > 0:02:23of the Earth's movement around the sun,
0:02:23 > 0:02:25and they built it into the very fabric of this city.
0:02:29 > 0:02:34But it's something that can only be seen at two very precise
0:02:34 > 0:02:38and magical times of the year.
0:02:38 > 0:02:42One of those is today, March the 20th.
0:02:45 > 0:02:47As afternoon approaches, the city fills
0:02:47 > 0:02:49with followers of Mayan beliefs...
0:02:54 > 0:02:57..and those curious to see an ancient wonder.
0:03:04 > 0:03:07There is a unique and particular feature of our planet
0:03:07 > 0:03:10as it orbits the sun,
0:03:10 > 0:03:14and it's encoded in the way that light and stone
0:03:14 > 0:03:17interact at the great pyramid.
0:03:17 > 0:03:19CHEERING
0:03:21 > 0:03:24This is the moment that all these thousands of people
0:03:24 > 0:03:27have been waiting for, they've all stood up
0:03:27 > 0:03:30and there are hands raised to welcome in the sun,
0:03:30 > 0:03:34and it's now aligned perfectly on the edge of the steps here,
0:03:34 > 0:03:40creating this very specific pattern of light and shade which resembles
0:03:40 > 0:03:43the body of a snake. And that's no coincidence
0:03:43 > 0:03:45because it joins up with the carved snake's head
0:03:45 > 0:03:47at the bottom of the pyramid.
0:03:53 > 0:03:59The Maya believed the snake, known as Kukulcan,
0:03:59 > 0:04:03was a messenger between gods and man.
0:04:03 > 0:04:08This is a remarkable display of Mayan architectural design.
0:04:08 > 0:04:12The appearance of this snake isn't an accident,
0:04:12 > 0:04:15they absolutely planned it
0:04:15 > 0:04:18and it happens on the same day every year.
0:04:18 > 0:04:21This is the spring equinox.
0:04:23 > 0:04:27DRUMS BEAT
0:04:27 > 0:04:29So, more than 1000 years ago,
0:04:29 > 0:04:33the Maya recognised the equinox as a pivotal moment in the year.
0:04:47 > 0:04:50Here on Earth, there are a few moments that we all share,
0:04:50 > 0:04:54because we're all on the same journey around the Sun.
0:04:54 > 0:04:56And one of those moments is the equinox,
0:04:56 > 0:04:58when day and night are equal.
0:05:02 > 0:05:06'It's a time of balance we can all experience,
0:05:06 > 0:05:09'wherever we are on the planet.'
0:05:12 > 0:05:15So whether you are here in Britain,
0:05:15 > 0:05:18amongst the fitful showers and overcast skies,
0:05:18 > 0:05:20'or in the bright spring sunshine of Mexico,'
0:05:20 > 0:05:22on the March equinox
0:05:22 > 0:05:26you'll get 12 hours of daylight and 12 hours of night time.
0:05:26 > 0:05:32That's if the sun ever comes through the clouds!
0:05:32 > 0:05:35But it's more than just a time of balance.
0:05:35 > 0:05:38It's also a turning point in our year.
0:05:40 > 0:05:41From the March equinox onwards,
0:05:41 > 0:05:44the days get longer in the northern hemisphere,
0:05:44 > 0:05:48'while in the southern hemisphere, the opposite occurs.
0:05:50 > 0:05:53'This is because of a special feature of our planet
0:05:53 > 0:05:56'as it journeys through space.'
0:05:56 > 0:05:59Let's say this rock is the sun.
0:05:59 > 0:06:02This is going to be our Earth,
0:06:02 > 0:06:06and as the Earth travels around its orbit
0:06:06 > 0:06:10spinning like this, it travels around on a flat plane.
0:06:10 > 0:06:13So you would think that its axis would point upwards
0:06:13 > 0:06:18but it isn't, it's tilted over at 23.4 degrees.
0:06:18 > 0:06:22'This means that the North Pole, the stem of the apple,
0:06:22 > 0:06:26'isn't vertical, it's at an angle.'
0:06:26 > 0:06:29And that tilt stays pointing in the same direction
0:06:29 > 0:06:31as the Earth travels around on its orbit.
0:06:35 > 0:06:38Because of this tilt for part of our orbit,
0:06:38 > 0:06:42the hemisphere north of the equator leans towards the sun.
0:06:45 > 0:06:47This brings with it extra solar energy,
0:06:47 > 0:06:50which fuels spring and then summer.
0:06:57 > 0:07:00Six months later, the situation is reversed.
0:07:00 > 0:07:03The southern hemisphere now leans towards the sun,
0:07:03 > 0:07:07while the northern hemisphere experiences declining energy,
0:07:07 > 0:07:09ushering in winter.
0:07:09 > 0:07:12Tilt creates the Earth's seasons.
0:07:13 > 0:07:16But there's a moment, twice a year as we orbit,
0:07:16 > 0:07:19when the sun favours neither hemisphere.
0:07:19 > 0:07:24At this point, both experience 12 hours of daylight and night time.
0:07:26 > 0:07:28This is the equinox.
0:07:33 > 0:07:35If the Earth wasn't tilted,
0:07:35 > 0:07:38every day would be like the equinox,
0:07:38 > 0:07:42with the 24 hours equally split between day and night.
0:07:42 > 0:07:45And that would mean no seasons.
0:07:46 > 0:07:50But the Earth's tilt means as we travel from the equinox,
0:07:50 > 0:07:52seasonal changes do occur.
0:07:57 > 0:07:58On the equinox,
0:07:58 > 0:08:03the Sun's energy is felt most strongly directly on the Equator.
0:08:03 > 0:08:05As we pass through spring,
0:08:05 > 0:08:08this focused energy moves slowly northward.
0:08:10 > 0:08:15All over the northern hemisphere, this solar shift means the Sun
0:08:15 > 0:08:18arcs higher in the sky, and the hours of daylight increase.
0:08:31 > 0:08:34The Earth orbits the Sun not standing up straight,
0:08:34 > 0:08:38but tilted over at an angle of 23.4 degrees.
0:08:47 > 0:08:51Our 23 degree tilt is just right.
0:08:51 > 0:08:54It's enough to provide a relatively benign seasonal shift.
0:08:54 > 0:08:57It makes our planet habitable.
0:08:59 > 0:09:03However, it may have been the result of a cosmic accident,
0:09:03 > 0:09:07and here in America, we can gain an insight into what happened.
0:09:14 > 0:09:18This is the Barringer Crater in Arizona.
0:09:18 > 0:09:2350,000 years ago, a meteorite struck this site
0:09:23 > 0:09:28and just look what it left behind - this enormous hole in the ground.
0:09:32 > 0:09:36'This impact would have thrown debris out
0:09:36 > 0:09:39'over tens of thousands of square kilometres.'
0:09:39 > 0:09:42And all the rock around here, like this,
0:09:42 > 0:09:46is what's left after that explosive event.
0:09:47 > 0:09:52This enormous crater is like a lesson in how size isn't everything,
0:09:52 > 0:09:55because the crater itself is a kilometre across,
0:09:55 > 0:09:59but the thing that caused it was only about 50 metres in diameter,
0:09:59 > 0:10:01which is really quite small.
0:10:01 > 0:10:05And the reason that such a small thing could cause such a big hole
0:10:05 > 0:10:08is because it was travelling so fast.
0:10:10 > 0:10:12'Impacts like these are extremely rare,
0:10:12 > 0:10:16'but in the Earth's past, they were far more common
0:10:16 > 0:10:18'and a lot bigger.'
0:10:22 > 0:10:24Around four and a half billion years ago,
0:10:24 > 0:10:28the solar system was still in the process of formation.
0:10:30 > 0:10:37The Earth was just one of many of protoplanets that orbited the sun.
0:10:37 > 0:10:39Amongst these protoplanets
0:10:39 > 0:10:43was a small Mars-sized planet that's been named Theia.
0:10:45 > 0:10:49Its orbit put it on a collision course with the Earth.
0:10:53 > 0:10:57Theia smashed into the larger Earth and was obliterated.
0:11:01 > 0:11:05The impact very nearly destroyed our planet too.
0:11:05 > 0:11:08The collision knocked the planet over,
0:11:08 > 0:11:11tilting the Earth's axis of rotation.
0:11:14 > 0:11:18This tilted Earth might still be oscillating madly,
0:11:18 > 0:11:23were it not for another consequence of Theia's impact.
0:11:23 > 0:11:27A huge amount of debris was blasted into space.
0:11:27 > 0:11:30Gradually, this debris coalesced,
0:11:30 > 0:11:33captured by the Earth's gravity...
0:11:33 > 0:11:36..and it formed the moon.
0:11:41 > 0:11:45Billions of years later, the gravity of the sun and the moon together
0:11:45 > 0:11:51act as a sort of counterweight, stabilising our tilt.
0:11:51 > 0:11:55It's extraordinary to think that the moon is both evidence
0:11:55 > 0:11:59of what caused Earth's 23 degree tilt
0:11:59 > 0:12:03and the celestial object that helps maintain it.
0:12:03 > 0:12:10Without this stabilising effect, the planet would wobble in space.
0:12:10 > 0:12:15There would be no seasons, and the weather would be chaotic.
0:12:24 > 0:12:29Spring triggers a seasonal transformation on land.
0:12:29 > 0:12:34But the rising temperatures also transform our weather,
0:12:34 > 0:12:38in some places with dramatic effect.
0:12:44 > 0:12:50A tornado is the most volatile of these seasonal weather events.
0:12:50 > 0:12:52They occur most frequently in the spring
0:12:52 > 0:12:56and especially in the Midwest of America -
0:12:56 > 0:12:59a region known as Tornado Alley.
0:13:00 > 0:13:04MAN: 'Did you see that? The whole house came apart!
0:13:04 > 0:13:06'Oh, my God!'
0:13:14 > 0:13:16But despite its violence,
0:13:16 > 0:13:20at the core of a tornado is a very simple process.
0:13:26 > 0:13:27This goes on like a backpack.
0:13:27 > 0:13:31'To experience it, I'm taking to the air,
0:13:31 > 0:13:34'over the Midwestern state of Colorado.'
0:13:34 > 0:13:38One, two, three, go. Run!
0:13:40 > 0:13:42Paragliding pilots like Honza Rejmanek,
0:13:42 > 0:13:45love this time of year.
0:13:54 > 0:13:57Spring provides the perfect conditions for soaring...
0:14:05 > 0:14:09..because the increasing temperatures generate thermals.
0:14:13 > 0:14:15So right now we are in a thermal.
0:14:15 > 0:14:19These are basically almost like invisible smokestacks of rising air.
0:14:19 > 0:14:22Right now we've found one, I'm going to take a turn in it
0:14:22 > 0:14:25and circle around and try to gain height.
0:14:25 > 0:14:30'Thermals form when the sun warms the ground,
0:14:30 > 0:14:33'and the ground, in turn, warms the air above it.'
0:14:33 > 0:14:36What I'm experiencing
0:14:36 > 0:14:39is one of the most fundamental principles of atmospheric physics -
0:14:39 > 0:14:41warmer air rises.
0:14:44 > 0:14:48'When air warms, it expands and becomes less dense.
0:14:48 > 0:14:51'So this air has a lower atmospheric pressure
0:14:51 > 0:14:55'than the cooler air that surrounds it.'
0:14:55 > 0:14:59So it floats upwards, forming this rising thermal column.
0:15:04 > 0:15:07The atmosphere tries to even out differences
0:15:07 > 0:15:09in air temperature and pressure,
0:15:09 > 0:15:13attempting to return to equilibrium.
0:15:13 > 0:15:17So the rising thermal will mix with the cooler air above.
0:15:19 > 0:15:23This basic process of moving towards equilibrium
0:15:23 > 0:15:26lies at the heart of every significant weather event
0:15:26 > 0:15:28on the planet.
0:15:40 > 0:15:44'But in the springtime air over Tornado Alley,
0:15:44 > 0:15:47'particularly powerful storms can develop.
0:15:47 > 0:15:50'This is due to the unusual conditions here
0:15:50 > 0:15:53'that intensify this basic atmospheric process.'
0:15:57 > 0:16:00There's a stable layer of dry air that acts as a barrier
0:16:00 > 0:16:04between the warm air down below and the cooler air higher up.
0:16:04 > 0:16:06So the warm air is trapped,
0:16:06 > 0:16:09and what's more, the ground keeps heating it as the day goes on.
0:16:09 > 0:16:12WIND WHISTLES
0:16:12 > 0:16:14THUNDERCLAP
0:16:14 > 0:16:19The thermals get more and more powerful until, by late afternoon,
0:16:19 > 0:16:24they finally punch through the barrier layer at colossal speed.
0:16:24 > 0:16:28These rapid updraughts of less dense, lower pressure air
0:16:28 > 0:16:33are so strong that they generate huge thunderstorms.
0:16:33 > 0:16:36THUNDER RUMBLES
0:16:38 > 0:16:42It's from these thunderstorms that, in certain conditions,
0:16:42 > 0:16:45tornadoes can emerge.
0:16:53 > 0:16:55'I'm going to investigate how this happens...'
0:16:55 > 0:16:58Not as bad as north of us.
0:17:00 > 0:17:04...with the help of atmospheric scientist, Josh Wurman.
0:17:04 > 0:17:07I don't know what to make of these stringy little features.
0:17:08 > 0:17:12The first step in our quest for a tornado
0:17:12 > 0:17:14is locating a promising storm.
0:17:14 > 0:17:17After a couple of days on the road, we manage to intercept
0:17:17 > 0:17:21one moving north through Colorado.
0:17:23 > 0:17:26So what's happening behind me is the storm is building
0:17:26 > 0:17:28and in the middle of that storm over there,
0:17:28 > 0:17:31there's an updraught with low pressure at the centre of it.
0:17:31 > 0:17:34And all the air around the outside has higher pressure,
0:17:34 > 0:17:37and that high pressure is pushing air into the centre
0:17:37 > 0:17:40and up into the storm, and that's what building the storm.
0:17:45 > 0:17:47The atmosphere tries to even out
0:17:47 > 0:17:49the extreme differences in temperature
0:17:49 > 0:17:51that have been generated.
0:17:51 > 0:17:53So the air movements at the core of the storm
0:17:53 > 0:17:55become exceptionally powerful.
0:17:58 > 0:18:02'Hail is one characteristic product of this atmospheric violence.'
0:18:06 > 0:18:09'The hail formed when an updraught cooled rapidly,
0:18:09 > 0:18:15'so that water condensed out of the air, and turned immediately to ice.'
0:18:15 > 0:18:19SHOUTING: This is what was carried from the south,
0:18:19 > 0:18:22and it was pushed up into the storm
0:18:22 > 0:18:25and it gave the storm its energy.
0:18:25 > 0:18:27And now it's falling back down on me!
0:18:27 > 0:18:30GIGGLES
0:18:30 > 0:18:31Wow!
0:18:31 > 0:18:36CAMERAMAN: That's it. Let's get inside. This is too hard now.
0:18:36 > 0:18:39And even though this is chaotic and messy,
0:18:39 > 0:18:44what this is, is a demonstration that the atmosphere is an unstable place,
0:18:44 > 0:18:48and there are all these differences in temperatures and pressures.
0:18:48 > 0:18:52And this is what happens when the atmosphere moves around
0:18:52 > 0:18:57to even everything out, and make it all the same.
0:18:57 > 0:19:01When tornadoes do form, they are often preceded by hail.
0:19:04 > 0:19:07But this time, there's no twister.
0:19:09 > 0:19:11So we're back on the road,
0:19:11 > 0:19:14still trying to see a storm spawn a tornado.
0:19:17 > 0:19:19Josh's specialist radar detects one
0:19:19 > 0:19:23which shows a revealing swirl of clouds.
0:19:25 > 0:19:27JOSH: Going out ahead, this big dark area's the core.
0:19:27 > 0:19:30So we're basically going to penetrate through the core
0:19:30 > 0:19:33and see what's interesting.
0:19:34 > 0:19:38Tornadoes form when powerful rotating cylinders of air
0:19:38 > 0:19:40within the storm
0:19:40 > 0:19:43get caught by an updraught and are knocked on their side
0:19:43 > 0:19:47by a powerful atmospheric wind called the jet stream.
0:19:49 > 0:19:54Right now, we're kind of in the centre of the coiled part.
0:19:54 > 0:19:59When that column of rotating air touches the ground,
0:19:59 > 0:20:01a tornado is born.
0:20:12 > 0:20:16At the tornado's core is an area of intense low pressure,
0:20:16 > 0:20:21which draws high pressure air towards it.
0:20:21 > 0:20:24The dust and debris picked up by the tornado
0:20:24 > 0:20:27reveal the swirling pattern of winds.
0:20:46 > 0:20:52Just 15 minutes after it first touched down, the tornado dissipates.
0:20:54 > 0:20:57There's still so much that we don't understand about storms.
0:20:57 > 0:21:00We don't understand when they're going to produce hail,
0:21:00 > 0:21:02when they're going to produce rain,
0:21:02 > 0:21:04when they are going to produce tornadoes.
0:21:04 > 0:21:08But what we do understand is that a storm like this
0:21:08 > 0:21:13is a manifestation of something happening round us all the time.
0:21:16 > 0:21:21Our planet's atmosphere is a mosaic of warmer and cooler air masses,
0:21:21 > 0:21:23constantly in motion.
0:21:25 > 0:21:30The air is rising, falling and swirling around
0:21:30 > 0:21:35as it seeks to balance differences in temperature and pressure.
0:21:35 > 0:21:37During April and May,
0:21:37 > 0:21:42the effect of the Earth's tilt is to enhance those differences.
0:21:44 > 0:21:47So all over the northern hemisphere,
0:21:47 > 0:21:50spring is the season for volatile storms.
0:21:50 > 0:21:55Tornadoes are only one consequence.
0:21:57 > 0:22:02The heavy and sudden downpours from storms can result in flash floods,
0:22:02 > 0:22:06like the one that hit the town of Barranquilla in Colombia
0:22:06 > 0:22:09in May 2011.
0:22:10 > 0:22:16These occur when the rain inundates densely saturated ground.
0:22:16 > 0:22:21The water isn't fully absorbed, but instead flows rapidly downhill
0:22:21 > 0:22:24in a near-instantaneous torrent.
0:22:26 > 0:22:30Thunderstorms can also give birth to an unexpected phenomenon...
0:22:34 > 0:22:38..massive dust storms called haboobs.
0:22:38 > 0:22:41This one blew into Phoenix, Arizona in 2011.
0:22:46 > 0:22:49Haboobs are produced in normally arid regions,
0:22:49 > 0:22:52when the leading edge of a storm collapses,
0:22:52 > 0:22:55generating a super-fast downdraught
0:22:55 > 0:22:59that kicks up a wall of dust and sand in front of it.
0:23:12 > 0:23:15As May turns to June, more solar energy
0:23:15 > 0:23:18is reaching the northern hemisphere,
0:23:18 > 0:23:22and it drives the biggest single weather event on the planet.
0:23:25 > 0:23:28An event centred on the Indian subcontinent.
0:23:32 > 0:23:34TRAFFIC HUMS
0:23:34 > 0:23:36CAR HORNS TOOT
0:23:36 > 0:23:40This is the city of Udaipur in Rajasthan.
0:23:44 > 0:23:48It's in the northwestern corner of India.
0:23:52 > 0:23:55Since March, temperatures here
0:23:55 > 0:23:58have been steadily rising as the Earth's tilt
0:23:58 > 0:24:00has warmed the northern hemisphere.
0:24:00 > 0:24:06But by June, everything is on the brink of an exhilarating change.
0:24:13 > 0:24:18I'm here at the time of an epic weather event of huge importance
0:24:18 > 0:24:21not just to Rajasthan but to the whole subcontinent
0:24:21 > 0:24:25and the over billion people who live here.
0:24:27 > 0:24:33'There's a wonderful place to appreciate the event's significance,
0:24:33 > 0:24:37'on one of the hills that overlook the city,
0:24:37 > 0:24:41'here, at this cliff-top palace.'
0:24:41 > 0:24:43It was built at the end of the 19th century
0:24:43 > 0:24:47by the 72nd Maharana of Udaipur
0:24:47 > 0:24:51and it's known as Sajjan Garh.
0:25:00 > 0:25:05He built this palace to get a pure, unadulterated view of the sky
0:25:05 > 0:25:09and the clouds that start to build at this time of year.
0:25:09 > 0:25:13Sajjan Garh is the monsoon palace.
0:25:17 > 0:25:20When the rains do eventually arrive,
0:25:20 > 0:25:25they'll be an essential relief from the heat of the Indian summer.
0:25:25 > 0:25:29But what's intriguing is that the monsoon is actually a consequence
0:25:29 > 0:25:34of the rising seasonal temperatures that precede it.
0:25:34 > 0:25:39To reveal why this is, we need to travel 2,000 kilometres...
0:25:39 > 0:25:41..south.
0:25:50 > 0:25:54I'm in the coastal state where the monsoon first arrives in India -
0:25:54 > 0:25:56Kerala.
0:26:00 > 0:26:05The key to understanding the monsoon is here, on the beach.
0:26:05 > 0:26:07The monsoon is powered by a simple,
0:26:07 > 0:26:10but incredibly significant difference -
0:26:10 > 0:26:14the difference between land and sea.
0:26:14 > 0:26:20And in particular, the differing ways in which they respond to the sun.
0:26:24 > 0:26:26Take this sand as an example.
0:26:26 > 0:26:29The sun's energy is heating all of this surface,
0:26:29 > 0:26:33but if I dig down just a little way...
0:26:33 > 0:26:36..the sand underneath is quite cool, and that's quite familiar,
0:26:36 > 0:26:38we see that on sunny beaches all the time.
0:26:38 > 0:26:40And here, where it gets really hot,
0:26:40 > 0:26:44the surface can reach 40 degrees Celsius.
0:26:44 > 0:26:46Just 15 centimetres down into the sand,
0:26:46 > 0:26:48it can be only 7 degrees Celsius.
0:26:48 > 0:26:51So, all the sun's energy is going into a really thin surface layer,
0:26:51 > 0:26:56and that layer heats up really, really, quickly.
0:26:56 > 0:26:58The sun is also beating down on the ocean,
0:26:58 > 0:27:01and that responds very, very differently.
0:27:08 > 0:27:11This water is much warmer than the sea at home
0:27:11 > 0:27:13but it's much cooler than the beach,
0:27:13 > 0:27:15and the reason for that
0:27:15 > 0:27:18is that the ocean takes much more of the sun's energy to heat it up.
0:27:18 > 0:27:22So a kilogram of water will take three times as much energy
0:27:22 > 0:27:24as a kilogram of sand to heat by one degree.
0:27:26 > 0:27:29The ocean is also relatively cool because to heat the surface
0:27:29 > 0:27:32you have to heat much more than just a thin layer.
0:27:32 > 0:27:36What happens is that winds that blow across the surface of the ocean
0:27:36 > 0:27:38generate turbulence which mixes that top layer.
0:27:38 > 0:27:41So as soon as some water's been heated at the top,
0:27:41 > 0:27:43it gets mixed down below.
0:27:46 > 0:27:50'This means that, unlike the land, the ocean warms up only very slowly,
0:27:50 > 0:27:53'as the sun's energy is absorbed.
0:27:55 > 0:27:58'So as we enter summer, the land heats up quickly,
0:27:58 > 0:28:01'while the ocean lags further and further behind.'
0:28:03 > 0:28:06This increasing temperature difference is critical,
0:28:06 > 0:28:10because both land and sea heat the air above them.
0:28:13 > 0:28:16As the sun has baked the Indian subcontinent,
0:28:16 > 0:28:19the land has warmed the air above it.
0:28:19 > 0:28:23The warmer air is less dense, so it rises.
0:28:23 > 0:28:26This draws in the cooler air from the ocean.
0:28:26 > 0:28:30Because of India's particular geography,
0:28:30 > 0:28:33this process is magnified.
0:28:33 > 0:28:36It's a triangular peninsula, with wide, hot plains
0:28:36 > 0:28:40and, crucially, a very long coastline.
0:28:40 > 0:28:42This combination sets up a powerful
0:28:42 > 0:28:46and sustained movement of cooler ocean air -
0:28:46 > 0:28:48the monsoon wind.
0:28:51 > 0:28:53Of course when most of us think of a monsoon
0:28:53 > 0:28:55we think not of seasonal winds, but of rain.
0:28:58 > 0:28:59'By setting up a time-lapse camera,
0:28:59 > 0:29:02'I'm hoping to watch the rain clouds forming.'
0:29:08 > 0:29:11THUNDERCLAP
0:29:13 > 0:29:16Wow!
0:29:25 > 0:29:27There is an enormous process on the go here.
0:29:27 > 0:29:29When the sun shines down on the ocean surface,
0:29:29 > 0:29:32some of the water at the surface will evaporate,
0:29:32 > 0:29:35so water and energy are carried up into the atmosphere.
0:29:35 > 0:29:37And as the monsoon winds come inland
0:29:37 > 0:29:39and they carry that water vapour with them,
0:29:39 > 0:29:43the heated land makes that moist air rise,
0:29:43 > 0:29:46goes up into the clouds and there droplets condense -
0:29:46 > 0:29:49the water condenses out, becomes visible, we see clouds.
0:29:49 > 0:29:53When those droplets join together to form droplets which are large enough,
0:29:53 > 0:29:55we get rain like this.
0:29:55 > 0:29:58And it's really raining hard now!
0:30:01 > 0:30:04None of this would be happening if it wasn't for the Earth's tilt.
0:30:06 > 0:30:09It's the seasonal heating is what widens the gap in temperature
0:30:09 > 0:30:12between the land and the sea, and this drives everything.
0:30:12 > 0:30:16And this massive system of rain and wind rushes inland
0:30:16 > 0:30:18and that's the monsoon.
0:30:22 > 0:30:2780% of all India's rains arrive in this seasonal deluge.
0:30:29 > 0:30:33It's not just the volume of the monsoon rains which is impressive.
0:30:33 > 0:30:35It's the distance they travel.
0:30:38 > 0:30:40As summer progresses in India,
0:30:40 > 0:30:45the difference in temperature between land and ocean actually increases.
0:30:45 > 0:30:51This makes the whole monsoon system more powerful,
0:30:51 > 0:30:55drawing this moisture-laden air further and further inland.
0:30:59 > 0:31:03From when the monsoon first arrives on the Kerala coast
0:31:03 > 0:31:04around June the 1st,
0:31:04 > 0:31:07it spreads more than 2,000 kilometres
0:31:07 > 0:31:12until it eventually reaches the far north of the country.
0:31:21 > 0:31:24It's now early September.
0:31:24 > 0:31:26Although the summer is almost at an end,
0:31:26 > 0:31:29in the northern hemisphere, it has a sting in its tail.
0:31:30 > 0:31:32Because this is hurricane season.
0:31:37 > 0:31:40The development of a hurricane is a wonderful example
0:31:40 > 0:31:43of how the Earth's spin controls the weather.
0:31:44 > 0:31:47I'm hoping to see one in action.
0:31:47 > 0:31:50Tropical Storm Nate. Now, that one looks like it's got potential.
0:31:50 > 0:31:54It's trapped in the Gulf, due to grow into a hurricane by tomorrow
0:31:54 > 0:31:57and it looks as though it's almost certain to get to the Mexican coast.
0:32:03 > 0:32:0724 hours later, I'm in eastern Mexico,
0:32:07 > 0:32:11heading towards the Gulf of Mexico and the oncoming storm.
0:32:17 > 0:32:20The winds are building up and the normal sunny skies
0:32:20 > 0:32:26are replaced with cloud and rain.
0:32:26 > 0:32:29At this time of year, the Gulf of Mexico
0:32:29 > 0:32:33has the perfect ingredients to make a hurricane.
0:32:33 > 0:32:37The sea is relatively shallow and close to the equator,
0:32:37 > 0:32:40so the water gets particularly hot.
0:32:40 > 0:32:43This water is warm, really warm
0:32:43 > 0:32:46and the reason for that is that the ocean out there
0:32:46 > 0:32:48has been absorbing the sun's energy, storing it up.
0:32:48 > 0:32:52And now, it's that energy which can build tropical storms.
0:32:58 > 0:33:01The way the storm is built is that the warm ocean
0:33:01 > 0:33:02heats the air above it.
0:33:02 > 0:33:05And once the air is warm, it expands and rises.
0:33:05 > 0:33:08THUNDER RUMBLING
0:33:08 > 0:33:11As the warm air rises, the pressure drops,
0:33:11 > 0:33:15sucking in even more moist air, creating powerful winds.
0:33:20 > 0:33:24But there's one final ingredient needed to create a hurricane.
0:33:25 > 0:33:27It needs to start turning.
0:33:27 > 0:33:32And that rotation comes from the spin of the Earth
0:33:32 > 0:33:37through a phenomenon known as the Coriolis effect.
0:33:37 > 0:33:41Now, let's say this is our planet, the northern hemisphere
0:33:41 > 0:33:43and that's the North Pole.
0:33:43 > 0:33:45Now, this planet isn't spinning,
0:33:45 > 0:33:50so when I throw a ball in a straight line...it travels in a straight line.
0:33:52 > 0:33:55But we live on a rotating world.
0:33:57 > 0:34:01So, let's take our planet and make it spin,
0:34:01 > 0:34:04round anticlockwise, like in the northern hemisphere.
0:34:09 > 0:34:12So, now I'm on a spinning planet, things look quite different.
0:34:12 > 0:34:15When I try and throw a ball in a straight line,
0:34:15 > 0:34:17it bends around to the right.
0:34:24 > 0:34:28From my point of view, this ball is always curving to the right,
0:34:28 > 0:34:31even though I'm trying really hard to throw it in a straight line.
0:34:34 > 0:34:37Now, the reason that this matters
0:34:37 > 0:34:40is that this ball represents winds on Earth
0:34:40 > 0:34:44and when the wind blows in the northern hemisphere,
0:34:44 > 0:34:46the wind is also moved to the right.
0:34:47 > 0:34:50In the southern hemisphere, the effect is reversed
0:34:50 > 0:34:52and the winds bend to the left.
0:34:53 > 0:34:57And that is all the Coriolis effect is.
0:35:03 > 0:35:07A hurricane shows the Coriolis effect in action.
0:35:08 > 0:35:12Winds are drawn inwards towards the low pressure
0:35:12 > 0:35:14at the centre of the hurricane.
0:35:15 > 0:35:18But as they head towards the centre,
0:35:18 > 0:35:21the Coriolis effect makes them turn to the right.
0:35:24 > 0:35:28This creates the hurricane's characteristic
0:35:28 > 0:35:31circular swirl of wind.
0:35:31 > 0:35:35It also means that the wind never reaches the centre of the storm.
0:35:36 > 0:35:40So the eye of the hurricane remains calm.
0:35:47 > 0:35:50Out at sea, Nate has the characteristic rotating,
0:35:50 > 0:35:52swirling clouds of a hurricane.
0:35:54 > 0:35:58but frustratingly, Nate begins to lose power.
0:36:00 > 0:36:03Before it can make landfall, the winds die away.
0:36:10 > 0:36:14Instead, the 2011 hurricane season
0:36:14 > 0:36:17became famous for a different storm.
0:36:17 > 0:36:20Hurricane Irene.
0:36:20 > 0:36:22Unusually for a hurricane,
0:36:22 > 0:36:25it travelled far enough up the east coast of the USA
0:36:25 > 0:36:27to flood parts of New York city.
0:36:29 > 0:36:34It caused billions of dollars worth of damage.
0:36:34 > 0:36:37And all this because our planet spins.
0:36:43 > 0:36:47In January, the northern hemisphere is locked in winter.
0:36:47 > 0:36:52And yet there is a paradox about our winter,
0:36:52 > 0:36:56because in January, winter is still getting colder,
0:36:56 > 0:37:00even though the northern hemisphere is receiving more energy from the sun.
0:37:10 > 0:37:12I've come to Northern Canada,
0:37:12 > 0:37:17to the best - or perhaps the worst - place to explore this paradox.
0:37:20 > 0:37:23Whoo!
0:37:23 > 0:37:27Cor! This...
0:37:27 > 0:37:30..is Yellowknife.
0:37:30 > 0:37:33It has the dubious distinction
0:37:33 > 0:37:36of being the coldest city
0:37:36 > 0:37:39in the whole of North America.
0:37:41 > 0:37:44Today is January the 19th.
0:37:44 > 0:37:49On average, this is the coldest day of the year across the northern hemisphere.
0:37:49 > 0:37:55It's minus 35 degrees Celsius, which certainly qualifies as cold to me.
0:37:57 > 0:38:01When you breathe, it hurts.
0:38:01 > 0:38:04It kind of gets you at the back of the throat.
0:38:04 > 0:38:07Your nose feels like it's permanently frozen solid.
0:38:07 > 0:38:11And despite the fact that I've got the feathers of about 25 geese
0:38:11 > 0:38:14stuffed into this jacket, and more thermal underwear
0:38:14 > 0:38:17than I thought possible to wear at exactly the same time,
0:38:17 > 0:38:20I still feel cold.
0:38:20 > 0:38:26In these conditions, even familiar things behave in unfamiliar ways.
0:38:26 > 0:38:29You can take a lovely, hot, steaming cup of coffee,
0:38:29 > 0:38:35throw it in the air, and the steam from that coffee will freeze instantly.
0:38:35 > 0:38:37Well, you've got to give it a go, haven't you?
0:38:39 > 0:38:41Right...
0:38:41 > 0:38:42Here goes.
0:38:46 > 0:38:47Wow!
0:38:48 > 0:38:51That is amazing!
0:38:58 > 0:39:04There's something curious about the way winter peaks towards the end of January.
0:39:04 > 0:39:07The winter solstice falls on December the 21st
0:39:07 > 0:39:10and this marks the day when the northern hemisphere
0:39:10 > 0:39:14receives the least amount of solar energy from the sun.
0:39:14 > 0:39:20So you might expect the December solstice to be the coldest day of the year.
0:39:20 > 0:39:21But it's not.
0:39:22 > 0:39:26On average, temperatures on the 19th of January are colder
0:39:26 > 0:39:29than they are in mid-December.
0:39:29 > 0:39:32But, you say, the days are getting longer.
0:39:32 > 0:39:35The northern hemisphere is getting more sun.
0:39:35 > 0:39:38It should be warming up.
0:39:41 > 0:39:44In Yellowknife, there are people
0:39:44 > 0:39:48whose livelihoods depend on the way winter's peak is delayed.
0:39:49 > 0:39:52In the driving seat is Blair Weatherby.
0:39:52 > 0:39:55His family have been driving through the bitter cold of this region
0:39:55 > 0:39:57for three generations.
0:39:57 > 0:40:03He's not an ordinary trucker. He's an ice road trucker.
0:40:03 > 0:40:05And this is his highway.
0:40:24 > 0:40:27In the summer, what happens here?
0:40:27 > 0:40:28We'd be in a boat!
0:40:30 > 0:40:35That's because we're not driving on land, but on a frozen lake.
0:40:40 > 0:40:44So really to appreciate Yellowknife's splendid isolation, you have to look at a map.
0:40:44 > 0:40:49And here it is, right on Great Slave Lake.
0:40:49 > 0:40:52At this time of year, of course, it freezes.
0:40:52 > 0:40:57So what time of year can you start driving on the lake,
0:40:57 > 0:40:59as opposed to boating on the lake?
0:40:59 > 0:41:03The season starts towards the end of January.
0:41:03 > 0:41:09It's about 30 inches thick at this point. It just keeps getting thicker and thicker.
0:41:12 > 0:41:16So whilst the northern hemisphere's coldest day is the 19th of January,
0:41:16 > 0:41:20here in Yellowknife, it's still bitterly cold for many weeks to come.
0:41:20 > 0:41:25For the truckers, this delayed winter means their work season
0:41:25 > 0:41:28runs from late January well into March.
0:41:34 > 0:41:38So why is the worst of winter delayed so long
0:41:38 > 0:41:41after the solstice on December the 21st?
0:41:42 > 0:41:44It's all about the balance
0:41:44 > 0:41:48between the heat coming in and the heat going out.
0:41:50 > 0:41:53Throughout early winter, the northern hemisphere
0:41:53 > 0:41:57receives declining amounts of the sun's energy,
0:41:57 > 0:42:00so it starts to cool down.
0:42:00 > 0:42:02But there's a lag in this cooling,
0:42:02 > 0:42:06because the Earth's surface loses heat relatively slowly.
0:42:06 > 0:42:11So well into January, the Earth's surface is still losing heat,
0:42:11 > 0:42:16even though solar energy is slowly increasing.
0:42:16 > 0:42:22It isn't until around the 19th of January that a tipping point is reached.
0:42:22 > 0:42:26From this day onwards, the increase in solar radiation
0:42:26 > 0:42:29will overwhelm the effects of the heat loss
0:42:29 > 0:42:32and the northern hemisphere will begin to warm up.
0:42:32 > 0:42:35But it'll still be a few more weeks yet
0:42:35 > 0:42:41before the ice here is too thin to support the weight of the trucks.
0:42:46 > 0:42:51We've seen how the Earth's journey through space is critical for life
0:42:51 > 0:42:56and how the Earth's angle of tilt defines our seasons.
0:42:56 > 0:43:01But you only really understand just how important our orbit is for our planet
0:43:01 > 0:43:04when you look into the Earth's past.
0:43:07 > 0:43:11There's evidence in the most unexpected places.
0:43:13 > 0:43:17A few miles out there is one of the most spectacular wonders of the world,
0:43:17 > 0:43:20but I can't see it from here because it's underwater.
0:43:20 > 0:43:22I'm in Belize in Central America
0:43:22 > 0:43:26and what I'm going to see is known as the Blue Hole.
0:43:34 > 0:43:37It's not often that nature produces something
0:43:37 > 0:43:39as beautifully symmetrical as this.
0:43:39 > 0:43:41It's almost a perfect circle.
0:43:45 > 0:43:49But it's more than just a stunning piece of natural architecture,
0:43:49 > 0:43:51because deep down there are clues
0:43:51 > 0:43:54to some of the most dramatic events in Earth's history.
0:44:06 > 0:44:09The bottom here is 120 metres down,
0:44:09 > 0:44:13and I'm just dropping into the abyss.
0:44:20 > 0:44:23Finally, I've reached my goal.
0:44:28 > 0:44:31So down here at 40 metres...
0:44:32 > 0:44:34..it's really eerie.
0:44:34 > 0:44:35Gloomy.
0:44:37 > 0:44:40And this is what I've come to see.
0:44:41 > 0:44:42And they're stalactites.
0:44:48 > 0:44:53But there's only one way I know of for stalactites to form.
0:44:53 > 0:44:57And it isn't down here, in 40 metres of water,
0:44:57 > 0:44:59with sharks swimming about nearby.
0:45:04 > 0:45:10Stalactites are created when mineral-rich water drips from the roof of a cave,
0:45:10 > 0:45:13over hundreds or even thousands of years,
0:45:13 > 0:45:16leaving behind mineral deposits.
0:45:16 > 0:45:20In other words, they didn't form in the ocean.
0:45:24 > 0:45:28That means that when these grew,
0:45:28 > 0:45:32the sea level was much, much lower than it is today.
0:45:35 > 0:45:40Scientists have precisely dated stalactites from the Blue Hole
0:45:40 > 0:45:45and, by comparing these and other sea level indicators from around the world,
0:45:45 > 0:45:49they've built up a picture of changing sea levels
0:45:49 > 0:45:52dating back hundreds of thousands of years.
0:45:53 > 0:45:56It reveals a striking pattern.
0:45:56 > 0:46:01Sea levels across the world have risen and fallen over time.
0:46:08 > 0:46:1220,00 years ago, the entire surface of the world's oceans
0:46:12 > 0:46:16was 120 metres below where it is today.
0:46:16 > 0:46:21And that means if I was standing here 20,000 years ago,
0:46:21 > 0:46:25all of this, including the Blue Hole cave system, would be dry land.
0:46:28 > 0:46:30So where did the ocean go?
0:46:32 > 0:46:35The answer is that it was on land.
0:46:35 > 0:46:38But it wasn't liquid water, it was ice,
0:46:38 > 0:46:42because 20,000 years ago, our planet was in the middle of an ice age.
0:46:45 > 0:46:48The Earth has experienced regular ice ages
0:46:48 > 0:46:52in a cycle going back several million years.
0:46:54 > 0:46:58These dramatic changes to the state of our planet
0:46:58 > 0:47:02are triggered by small changes in the Earth's orbit.
0:47:10 > 0:47:12I've travelled back to Britain
0:47:12 > 0:47:16to uncover the relationship between the Earth's orbit and an ice age.
0:47:19 > 0:47:24Snowdonia's peaks and valleys were carved out in the last ice age.
0:47:26 > 0:47:31It's in mountainous locations like this that an ice age would have begun
0:47:31 > 0:47:34as snow gradually built up.
0:47:40 > 0:47:44When we think of ice ages, we think of extreme cold during the winter.
0:47:44 > 0:47:46But, counterintuitively,
0:47:46 > 0:47:50it's summer temperatures which are important in starting ice ages.
0:47:52 > 0:47:56And the reason for that is, now, ice will build up here during the winter,
0:47:56 > 0:47:58but it will all melt away in the summer.
0:47:58 > 0:48:03But if the summer is a little bit cooler, a layer of ice will be left behind.
0:48:03 > 0:48:04And a series of cool summers
0:48:04 > 0:48:08will leave layer after layer, one on top of the other, building up.
0:48:08 > 0:48:11And here, the ice could have been hundreds of metres high.
0:48:13 > 0:48:16Ice ages always start in the northern hemisphere
0:48:16 > 0:48:20because there's so much more land surface on which ice can build up.
0:48:24 > 0:48:29So the question is, what causes cooler summers in the northern hemisphere?
0:48:30 > 0:48:34The answer comes from small changes in the Earth's orbit,
0:48:34 > 0:48:38caused by the gravitational pull of other planets.
0:48:40 > 0:48:43Our orbit isn't exactly the same every time.
0:48:43 > 0:48:48Aspects of it change just slightly, in cycles lasting thousands of years.
0:48:48 > 0:48:52And when all of those cycles reach their most extreme point
0:48:52 > 0:48:54all at the same time,
0:48:54 > 0:48:59that can change our summer temperatures just enough to tip us into an ice age.
0:49:01 > 0:49:04There are three cycles to do with the Earth's orbit
0:49:04 > 0:49:07that must all coincide to trigger an ice age.
0:49:10 > 0:49:12The first of these cyclical changes
0:49:12 > 0:49:15affects the time of year when perihelion occurs.
0:49:15 > 0:49:19This is the day when the Earth is closest to the sun.
0:49:21 > 0:49:25Today, perihelion is in January,
0:49:25 > 0:49:29but over thousands of years, the date of perihelion changes.
0:49:29 > 0:49:33When perihelion occurs in the northern hemisphere summer,
0:49:33 > 0:49:36it makes summers particularly hot.
0:49:37 > 0:49:40But when it occurs in winter, as it does today,
0:49:40 > 0:49:44then northern hemisphere summers are cooler.
0:49:45 > 0:49:52So at the moment, the perihelion cycle is at the right point to generate an ice age.
0:49:53 > 0:49:58But two other cycles are not in an ice age phase.
0:49:58 > 0:50:00The first is the angle of the Earth's tilt.
0:50:05 > 0:50:10The Earth's tilt is currently at an angle to the vertical of 23.4 degrees.
0:50:10 > 0:50:16But that angle changes between 22 and 24.5 degrees.
0:50:16 > 0:50:21It's only when the angle is at its shallowest - 22 degrees -
0:50:21 > 0:50:24that the seasons become less extreme and the summers cooler.
0:50:27 > 0:50:31Today, the angle of tilt is too great for an ice age.
0:50:35 > 0:50:40The final cycle affecting an ice age is the shape of the Earth's orbit.
0:50:42 > 0:50:44The Earth's orbit is an ellipse,
0:50:44 > 0:50:49but over time, it becomes slightly more, and then slightly less, elliptical.
0:50:49 > 0:50:54When the orbit is at its most elliptical, the result is lower summer temperatures.
0:50:57 > 0:51:01At the moment, the Earth is midway through this cycle,
0:51:01 > 0:51:04so again, it's not in an ice age phase.
0:51:09 > 0:51:14It's only when all three of these changes to the Earth's cycle line up together
0:51:14 > 0:51:17that they produce the really cool summers
0:51:17 > 0:51:20in the northern hemisphere that result in ice ages.
0:51:27 > 0:51:31It'll be around 60,000 years before the cycles line up again
0:51:31 > 0:51:34and the next ice age starts.
0:51:38 > 0:51:42In our journey around the sun, it's now the beginning of March
0:51:42 > 0:51:46and the shackles of winter are being loosened in Britain
0:51:46 > 0:51:49as we move into spring.
0:51:49 > 0:51:55The land starts greening as longer days bring more energy from the sun.
0:51:55 > 0:51:57But in some parts of the world,
0:51:57 > 0:52:00the effect of that warming has yet to be felt.
0:52:08 > 0:52:13I've come to Greenland, where there's definitely not much sign of spring yet.
0:52:15 > 0:52:21This is Kulusuk. It's a tiny settlement of just 355 people
0:52:21 > 0:52:24perched on the edge of an island in eastern Greenland.
0:52:24 > 0:52:28To the north of here is the Arctic Circle and the Greenland ice cap.
0:52:32 > 0:52:36Kulusuk is surrounded by the Arctic Ocean.
0:52:36 > 0:52:42At this time of year, it's frozen, covered in a thick layer of sea ice.
0:52:47 > 0:52:50Each year, the extent of the sea ice is different.
0:52:50 > 0:52:53To see how far it reaches this year,
0:52:53 > 0:52:57I need to travel right to the edge of the sea ice.
0:53:03 > 0:53:05- Want me to bring this? - Yeah.
0:53:06 > 0:53:12'I meet up with my guide, local hunter Gio Utuaq.
0:53:12 > 0:53:16'His hunting grounds lie right at the edge of the sea ice.
0:53:17 > 0:53:20She's so keen!
0:53:21 > 0:53:26How far do we have to go to get to the hunting grounds?
0:53:26 > 0:53:2920, maybe 25 kilometres.
0:53:42 > 0:53:48After two hours, we reach a huge expanse of sea ice.
0:53:49 > 0:53:54It's impossible to comprehend that the snow we're travelling across sits on ice,
0:53:54 > 0:53:56which sits on the ocean.
0:53:56 > 0:53:59We're travelling across a frozen sea. And look at this!
0:53:59 > 0:54:03This is an iceberg actually trapped within the sea ice.
0:54:03 > 0:54:09It's the most astonishing landscape, or seascape or ice-scape...
0:54:09 > 0:54:12What do you call it?! ..that I've ever seen!
0:54:12 > 0:54:14It's like another world.
0:54:28 > 0:54:32And then, surprisingly quickly, the edge of the ice comes into view
0:54:32 > 0:54:36and I can see the Arctic Ocean.
0:54:39 > 0:54:44For obvious reasons, we make the last stretch of the journey on foot.
0:54:47 > 0:54:50- Are you sure? - SHE CHUCKLES
0:54:50 > 0:54:54There is something
0:54:54 > 0:54:57very disconcerting
0:54:57 > 0:55:00about walking on sea ice
0:55:00 > 0:55:04when the open sea is so close.
0:55:06 > 0:55:10Yeah, it looks pretty solid. How thick is the ice?
0:55:10 > 0:55:12Like this thick?
0:55:16 > 0:55:21It seems strange to be walking across a frozen sea here in Greenland
0:55:21 > 0:55:25when back at home, the daffodils are beginning to come up.
0:55:25 > 0:55:26But what's even stranger
0:55:26 > 0:55:30is that measurements of the sea ice over the last 50 years
0:55:30 > 0:55:36show that it only reaches its full extent now, in early March.
0:55:36 > 0:55:40So clearly there's a lag between the arrival of the warmth of the sun
0:55:40 > 0:55:43and the melting of the ice. But why?
0:55:45 > 0:55:48It comes down to the properties of water.
0:55:48 > 0:55:52We've already seen that, well into January,
0:55:52 > 0:55:55land continues to lose more heat than it gains.
0:55:55 > 0:56:00Because water radiates heat even more effectively than land,
0:56:00 > 0:56:04the oceans take even longer to start warming up.
0:56:04 > 0:56:08So although the land has been warming since January the 19th,
0:56:08 > 0:56:14the sea is still losing heat and the ice continues to grow.
0:56:14 > 0:56:18Greenland sea ice is at its maximum extent at this time of year, in March.
0:56:18 > 0:56:23But over the next few weeks, the tilt of the Earth towards the sun as it orbits it
0:56:23 > 0:56:27will allow the northern hemisphere to get an increasing amount of solar energy.
0:56:27 > 0:56:30The days will get longer and warmer
0:56:30 > 0:56:33and the sea ice will begin to break up and recede.
0:56:33 > 0:56:35Then the hunting season will be over.
0:56:45 > 0:56:49The existence of the sea ice here in Greenland
0:56:49 > 0:56:54is testament to the complex response our planet has to the sun,
0:56:54 > 0:56:56in whose orbit we travel.
0:56:56 > 0:56:59But it's a very delicate balance
0:56:59 > 0:57:04and no-one is more acutely aware of that than the people who live here.
0:57:07 > 0:57:09Gio tells me that this year,
0:57:09 > 0:57:12there was less ice than in previous years.
0:57:12 > 0:57:16It's part of a trend over the whole of the Arctic.
0:57:16 > 0:57:22The area covered by sea ice has shrunk significantly in the last 20 years.
0:57:22 > 0:57:23A series of warm winters
0:57:23 > 0:57:27have meant that the seas haven't cooled down as much as normal
0:57:27 > 0:57:32so not as much ice has been able to form.
0:57:32 > 0:57:36And there are many scientists who argue
0:57:36 > 0:57:39that the cause of the warmer winters is us.
0:57:41 > 0:57:47Global warming can feel like a myth when, back in the UK,
0:57:47 > 0:57:51we've endured a string of very cold winters.
0:57:51 > 0:57:56But here on the front line, it's a reality.
0:58:01 > 0:58:05Most predictions suggest that the Arctic will continue to warm rapidly
0:58:05 > 0:58:08over the course of this century.
0:58:09 > 0:58:16It could be that we may well prove capable of generating the kind of climate change
0:58:16 > 0:58:20that in the past has been created by changes in the Earth's orbit.
0:58:51 > 0:58:54Subtitles by Red Bee Media Ltd