1 00:00:04,537 --> 00:00:06,437 Welcome to the Earth Ride: 2 00:00:07,107 --> 00:00:11,100 fasten your seatbelts for a roller-coaster ride around the only planet 3 00:00:11,244 --> 00:00:14,680 in the solar system that supports life. 4 00:00:16,616 --> 00:00:20,882 This is your chance to go to parts of the world you can't normally reach... 5 00:00:24,290 --> 00:00:28,624 You'll travel to extreme places and meet extraordinary animals! 6 00:00:52,952 --> 00:00:55,420 Earth is a planet of immense variety. 7 00:00:55,555 --> 00:01:01,016 but every form of life depends on one thing... water. 8 00:01:07,767 --> 00:01:12,932 And water gets everywhere... whether we like it or not! 9 00:01:21,514 --> 00:01:25,280 The water on Planet Earth is shared between all living things 10 00:01:25,552 --> 00:01:28,783 But there's only a limited amount to go around. 11 00:01:29,355 --> 00:01:33,052 So where's all it been. and where's it all going? 12 00:01:33,526 --> 00:01:37,326 To find out. hop aboard the Earth ride for a blast 13 00:01:37,530 --> 00:01:39,225 through nature's greatest spectacles 14 00:01:39,365 --> 00:01:44,393 as you take the roller-coaster ride of life! 15 00:02:20,039 --> 00:02:21,870 All creatures. including you. 16 00:02:22,008 --> 00:02:25,569 depend on water none can survive without it. 17 00:02:26,513 --> 00:02:28,276 You are two-thirds water... 18 00:02:28,414 --> 00:02:32,976 and the water that's inside your body right now has had a remarkable history. 19 00:02:36,523 --> 00:02:40,653 Some of the water that you'll drink today once filled the deepest oceans. 20 00:02:44,697 --> 00:02:48,497 ln every glass are drops that have been shared with other animals. 21 00:02:53,039 --> 00:02:54,904 Whenever you drink. you join them 22 00:02:55,041 --> 00:02:58,636 as part of the planet's never-ending water cycle. 23 00:03:27,040 --> 00:03:32,171 The Earth ride will follow water's weird and wonderful course around the world. 24 00:03:33,646 --> 00:03:37,241 Sometimes we'll race along at death-defying speeds... 25 00:03:37,517 --> 00:03:39,007 at other moments slow right down 26 00:03:39,152 --> 00:03:43,282 to witness the water cycle's most intricate twists and turns. 27 00:03:44,591 --> 00:03:46,491 Water has shaped every habitat on Earth. 28 00:03:46,593 --> 00:03:50,654 lt's the only substance that can naturally exist in all three forms... 29 00:03:51,030 --> 00:03:53,863 As ice it dominates our winter landscape... 30 00:03:55,735 --> 00:04:00,570 ... as liquid. it supports the richness of the tropical seas. 31 00:04:02,542 --> 00:04:05,909 And water vapour bathes the rainforests in moisture. 32 00:04:06,045 --> 00:04:09,139 creating an explosion of life. 33 00:04:15,555 --> 00:04:18,456 From the driest deserts to the snow-capped peaks. 34 00:04:18,524 --> 00:04:22,187 water is constantly recycled around the planet. 35 00:04:22,829 --> 00:04:23,761 And at any moment. 36 00:04:23,896 --> 00:04:27,229 there are 12 thousand billion tons of it up in the atmosphere. 37 00:04:27,367 --> 00:04:28,595 circling the globe. 38 00:04:29,335 --> 00:04:30,996 But before we start our ride. 39 00:04:31,137 --> 00:04:35,164 there's something we need to know: what is so special about water? 40 00:04:35,508 --> 00:04:39,842 After all. it's got no colour. no smell and no taste. 41 00:04:40,513 --> 00:04:44,506 To find out. let's have a closer look... inside a single drop! 42 00:04:51,557 --> 00:04:58,156 This is a water molecule - H2O two atoms of hydrogen and one of oxygen. 43 00:04:58,865 --> 00:05:00,799 Each molecule is connected to the next 44 00:05:00,933 --> 00:05:04,130 by electrical forces called 'hydrogen bonds'. 45 00:05:04,804 --> 00:05:09,832 And these bonds - as we'll soon discover are what makes water so unique. 46 00:05:12,812 --> 00:05:14,746 Water is now the most common substance 47 00:05:14,881 --> 00:05:17,441 on the planet's surface but once upon a time. 48 00:05:17,517 --> 00:05:19,007 it wasn't here at all. 49 00:05:19,886 --> 00:05:22,446 How did water first appear on earth? 50 00:05:22,689 --> 00:05:23,348 To answer that. 51 00:05:23,489 --> 00:05:25,753 the Earth ride's going to take us back in time 52 00:05:25,892 --> 00:05:28,656 to when the world was very young. 53 00:06:03,529 --> 00:06:06,896 Some of our planet's water came from outer-space. 54 00:06:07,033 --> 00:06:12,130 where massive quantities of hydrogen and oxygen combined to form 'space ice'. 55 00:06:13,005 --> 00:06:16,873 The average comet is made up of more than a billion tonnes of ice. 56 00:06:17,810 --> 00:06:21,746 We and all other forms of life on earth partly owe our existence 57 00:06:21,881 --> 00:06:24,509 to this extra-terrestrial water. 58 00:06:34,627 --> 00:06:39,223 Just think - that makes you part extra-terrestrial too! 59 00:06:42,101 --> 00:06:45,332 The other main source of Earth's water was from hydrogen and oxygen 60 00:06:45,505 --> 00:06:48,133 combining deep within the forming planet. 61 00:06:51,110 --> 00:06:54,170 This produced steam - boiling water vapour 62 00:06:54,313 --> 00:06:56,747 which emerged from ruptures in the Earth's crust 63 00:06:56,883 --> 00:06:59,374 such as geysers and volcanic springs. 64 00:07:05,324 --> 00:07:08,122 Over the next 3 billion years our planet's atmosphere 65 00:07:08,261 --> 00:07:10,752 became saturated with water vapour. 66 00:07:41,961 --> 00:07:43,952 This vapour envelope around the earth 67 00:07:44,096 --> 00:07:47,691 helped to protect the planet from the sun's scorching rays. 68 00:07:51,671 --> 00:07:55,664 lt also trapped the sun's heat. so as each day flashed by. 69 00:07:55,808 --> 00:07:59,005 Earth's atmosphere began to warm. 70 00:08:18,965 --> 00:08:22,423 Without this the original 'Greenhouse Effect' 71 00:08:22,502 --> 00:08:26,871 the average temperature on Earth would still be minus 18oC. 72 00:08:27,006 --> 00:08:30,169 and our world would be a frozen wasteland. 73 00:08:31,744 --> 00:08:34,838 Life as we know it could never have begun. 74 00:08:39,018 --> 00:08:42,954 Eventually. the atmosphere had warmed enough for rain to fall. 75 00:08:44,724 --> 00:08:47,454 Rainwater filled the oceans and energy from the sun 76 00:08:47,527 --> 00:08:51,293 began to drive the great cycle of water around the planet. 77 00:08:51,964 --> 00:08:55,161 And we're all part of the same loop today. 78 00:08:57,003 --> 00:09:02,498 The rain that fell then has by now circled the globe around 8 million times! 79 00:09:06,012 --> 00:09:08,480 The water cycle runs on solar power 80 00:09:08,614 --> 00:09:12,482 the sun's heat evaporates water from the surface of the seas. 81 00:09:13,319 --> 00:09:15,753 During this process. all its salt is lost. 82 00:09:15,888 --> 00:09:19,756 leaving just clean fresh water spread across the globe. 83 00:09:23,462 --> 00:09:26,158 lt's our planet's most precious asset. 84 00:09:26,299 --> 00:09:29,962 But each day some is lost into space from the upper atmosphere. 85 00:09:32,838 --> 00:09:37,434 So does this mean that one day. our supply of water will run out? 86 00:09:38,678 --> 00:09:43,240 To answer that. we're going to the centre of the Earth. 87 00:10:13,179 --> 00:10:14,840 30 kilometres beneath our feet. 88 00:10:14,981 --> 00:10:17,916 temperatures exceed 1000 degrees Celsius. 89 00:10:18,050 --> 00:10:20,018 This is the beginning of the mantle 90 00:10:20,152 --> 00:10:22,746 the layer between the Earth's crust and the core. 91 00:10:23,422 --> 00:10:28,018 Down here it's mostly hot rock but in deeper parts there is also water... 92 00:10:28,227 --> 00:10:31,993 maybe 10 times the volume of all the world's oceans! 93 00:10:33,232 --> 00:10:36,895 This giant reservoir replaces the water lost to outer space 94 00:10:37,136 --> 00:10:41,129 lt's released as steam when volcanoes erupt! 95 00:11:17,510 --> 00:11:21,810 Volcanoes shaped our primitive planet long before any life evolved. 96 00:11:22,314 --> 00:11:25,249 Then. as now. most of them were submerged. 97 00:11:25,384 --> 00:11:30,048 so they erupted largely underwater and released steam straight into the sea. 98 00:12:08,627 --> 00:12:12,893 More water was released into the oceans through ruptures in the sea floor... 99 00:12:14,200 --> 00:12:16,862 Prepare to enter the abyss! 100 00:12:32,618 --> 00:12:34,483 These are 'black smokers'. 101 00:12:35,621 --> 00:12:39,580 natural chimneys on the seabed that spew out superheated water. 102 00:12:39,725 --> 00:12:41,386 black with minerals. 103 00:12:46,532 --> 00:12:51,492 lt's in such deep. dark places Evolution may have first begun. 104 00:12:53,105 --> 00:12:56,939 And life has been harnessed to water ever since. 105 00:12:58,744 --> 00:12:59,972 Microbes still live here. 106 00:13:00,112 --> 00:13:04,344 2.5 kilometres down surviving under immense pressures . 107 00:13:04,483 --> 00:13:08,180 lnstead of relying on sunlight like most living things. 108 00:13:08,320 --> 00:13:11,847 they exist on expelled minerals and volcanic heat. 109 00:13:14,660 --> 00:13:17,891 Sea water was the perfect place for life to start... 110 00:13:18,297 --> 00:13:19,594 with stable temperatures. 111 00:13:19,732 --> 00:13:23,031 physical support and a ready supply of nutrients. 112 00:13:26,639 --> 00:13:31,770 The stage was set for evolution to explode into the oceans... 113 00:13:43,355 --> 00:13:47,689 Strong currents. powered by the sun. spread life across the globe. 114 00:13:55,734 --> 00:14:01,172 As time went by. the seas became richer and the animals more diverse. 115 00:14:13,485 --> 00:14:15,976 ln the warmest seas. near the equator. 116 00:14:16,121 --> 00:14:20,524 coral reefs developed into the richest marine habitats on earth. 117 00:14:52,958 --> 00:14:58,055 Today the oceans teem with life... in all its shapes and sizes. 118 00:15:23,022 --> 00:15:28,756 This is the biggest fish alive in the seas today - the whale shark. 119 00:15:30,329 --> 00:15:33,230 lt's so enormous. it's like a swimming island. 120 00:15:33,365 --> 00:15:36,960 with entire communities of fish living around its body. 121 00:15:44,777 --> 00:15:48,042 Yet the whale shark feeds on microscopic plankton. 122 00:15:48,180 --> 00:15:53,208 scooped up by a mouth so vast. you could swim through it! 123 00:16:17,876 --> 00:16:20,572 lt's relatively recently that evolution broke free 124 00:16:20,713 --> 00:16:23,443 from the sea and reached dry land. 125 00:16:31,357 --> 00:16:34,520 Without the ocean's stable temperatures and physical support. 126 00:16:34,660 --> 00:16:37,959 creatures' bodies and behaviours had to alter radically... 127 00:16:42,267 --> 00:16:44,758 ... they were like aliens in another world. 128 00:16:56,382 --> 00:17:00,409 The mud-skipper is a modern-day equivalent of those early landlubbers. 129 00:17:00,886 --> 00:17:04,447 lt's a fish out of water that solved the problem by walking on its fins 130 00:17:04,590 --> 00:17:07,252 and having sealed-up gill slits. 131 00:17:13,532 --> 00:17:17,093 Water availability became the crucial key to survival 132 00:17:17,870 --> 00:17:22,637 what land animals needed was new ways to conserve it inside their bodies. 133 00:17:27,913 --> 00:17:30,746 Giant insects evolved water-tight skins. 134 00:17:30,883 --> 00:17:32,908 which helped them spread across the globe... 135 00:17:36,488 --> 00:17:39,150 For a while. they ruled the world. 136 00:17:41,760 --> 00:17:44,194 As more time passed. other animals learned 137 00:17:44,329 --> 00:17:46,661 how to manage without so much water. 138 00:17:49,101 --> 00:17:52,036 Huge reptiles started to lay eggs on land. 139 00:17:52,171 --> 00:17:55,265 with harder shells to stop them dehydrating. 140 00:17:57,676 --> 00:17:59,906 But even today. these crocodile babies 141 00:18:00,045 --> 00:18:02,172 still need contact with the water. 142 00:18:03,615 --> 00:18:06,675 and this caring mother helps them get there. 143 00:18:26,705 --> 00:18:29,538 The reptiles' descendants - the birds 144 00:18:29,808 --> 00:18:32,276 held onto the watertight egg design. 145 00:18:32,678 --> 00:18:34,441 Eventually it helped them colonise 146 00:18:34,580 --> 00:18:37,572 some of the most hostile places on the planet. 147 00:18:39,485 --> 00:18:42,181 Mammals are good at storing water in their bodies. 148 00:18:43,589 --> 00:18:46,353 Some have even adapted to life in the desert. 149 00:18:47,059 --> 00:18:51,723 But even this one can't completely manage without water's life-giving powers. 150 00:18:52,598 --> 00:18:56,364 A camel can drink 120 litres at a single sitting. 151 00:18:56,969 --> 00:19:01,235 And all this water contains molecules already drunk by many other animals. 152 00:19:01,440 --> 00:19:04,807 over hundreds of millions of years of evolution. 153 00:19:12,351 --> 00:19:16,253 Our world is a closed system like this glass 154 00:19:17,055 --> 00:19:19,421 it holds only a limited amount of water. 155 00:19:20,492 --> 00:19:23,620 So what happens to a water droplet when it leaves the sea 156 00:19:23,762 --> 00:19:25,457 and evaporates into the sky? 157 00:19:26,465 --> 00:19:29,298 As the water vapour rises. it expands and cools. 158 00:19:30,169 --> 00:19:34,162 lt condenses around microscopic particles of dust or pollen. 159 00:19:35,574 --> 00:19:37,303 and a cloud is born. 160 00:19:38,477 --> 00:19:42,709 As water droplets in the cloud collide. they swell and fall as rain. 161 00:19:44,483 --> 00:19:48,920 The rain replenishes what was lost and the cycle begins again. 162 00:19:50,789 --> 00:19:53,758 The water-cycle is like a roller-coaster ride 163 00:19:53,892 --> 00:19:58,625 a constant process of evaporation. condensation and precipitation. 164 00:20:00,632 --> 00:20:03,624 ln a cloud water can exist in all three forms: 165 00:20:03,769 --> 00:20:06,761 as liquid. vapour and as particles of ice. 166 00:20:08,140 --> 00:20:11,303 As molecules shift from one form to another. 167 00:20:11,443 --> 00:20:14,344 the hydrogen bonds connecting them break and reconnect. 168 00:20:14,479 --> 00:20:16,743 sucking in or releasing energy. 169 00:20:18,483 --> 00:20:22,852 And it's this rapid transfer of massive amounts of energy inside the cloud 170 00:20:22,988 --> 00:20:26,890 that whips up winds and fuels storms. 171 00:21:22,648 --> 00:21:26,084 But this is only the beginning of a raindrop's epic journey 172 00:21:26,218 --> 00:21:28,277 from the clouds back to the sea... 173 00:21:29,288 --> 00:21:32,985 you're about to take the ultimate white water ride! 174 00:21:33,792 --> 00:21:36,056 Sometimes the roller coaster will slow and 175 00:21:36,194 --> 00:21:39,994 we can experience the effects water has on individual animals. 176 00:21:40,232 --> 00:21:43,224 Then it will speed up again for the next ride. 177 00:21:44,703 --> 00:21:46,728 So how does it all begin? 178 00:21:47,239 --> 00:21:51,573 Bizarrely. on a hot day most rain starts as ice. 179 00:21:53,145 --> 00:21:57,241 Summer storm clouds are mostly made up of super-cooled water particles. 180 00:21:57,382 --> 00:22:01,250 and when one of these collides with an ice crystal floating in the cloud. 181 00:22:01,386 --> 00:22:05,083 it grows into a ball of ice - a hailstone. 182 00:22:06,358 --> 00:22:09,293 Gravity takes over and it plummets to earth. 183 00:22:19,137 --> 00:22:21,605 But many hailstones don't survive the trip 184 00:22:21,740 --> 00:22:23,571 they melt as they pass into the lower. 185 00:22:23,709 --> 00:22:27,372 warmer air and turn into large drops of rain. 186 00:22:29,047 --> 00:22:30,981 Rather than being 'teardrop' shaped. 187 00:22:31,116 --> 00:22:33,846 falling raindrops look like burger buns. 188 00:22:35,320 --> 00:22:38,653 They get distorted in the battle between surface tension 189 00:22:38,790 --> 00:22:40,849 and the air that's rushing past. 190 00:22:47,699 --> 00:22:52,932 Exactly what happens to a water droplet next depends on where it falls. 191 00:22:56,708 --> 00:22:59,370 lt's a projectile packed with energy. 192 00:23:01,513 --> 00:23:06,007 To a small insect. this is equivalent of an artillery bombardment. 193 00:23:10,989 --> 00:23:14,447 ln mountain areas. a storm can mean flash floods. 194 00:23:14,993 --> 00:23:17,723 So how do tiny creatures deal with flooding? 195 00:23:25,737 --> 00:23:29,264 South American Fire ants are powerless to resist. 196 00:23:30,108 --> 00:23:34,169 All they can do is link their legs together and go with the flow! 197 00:23:34,646 --> 00:23:36,705 Who knows where they'll end up! 198 00:23:41,586 --> 00:23:43,816 A torrent like this. pouring off the hills. 199 00:23:43,955 --> 00:23:45,946 can cut through solid granite. 200 00:23:48,660 --> 00:23:50,287 ln the space of 10 kilometres. 201 00:23:50,429 --> 00:23:56,925 it can reduce a granite rock 20cm across to a pebble just 2cm wide. 202 00:23:58,537 --> 00:24:03,531 A further 10km downstream and there's nothing left but sand. 203 00:24:04,342 --> 00:24:09,780 Given enough time. water power can carve up an entire landscape... 204 00:24:31,837 --> 00:24:34,203 Once water loses height and speed. 205 00:24:34,339 --> 00:24:37,797 the raging torrent slows into a gentle cruise ... 206 00:24:40,545 --> 00:24:44,777 Now animals can take advantage of water in amazing ways. 207 00:24:45,717 --> 00:24:50,620 The basilisk lizard is a water walker. but it must run to stay on top. 208 00:24:53,925 --> 00:24:57,258 lt gains extra lift by making air bubbles with its feet. 209 00:25:07,005 --> 00:25:11,066 So did our rafting fire ants manage to ride out the storm? 210 00:25:18,550 --> 00:25:20,814 With queen and pupae safely in their midst. 211 00:25:20,952 --> 00:25:22,442 they can now let the water 212 00:25:22,587 --> 00:25:26,148 that has almost killed them float them gently in to shore. 213 00:25:28,693 --> 00:25:31,491 Surface tension saved the day. 214 00:25:37,202 --> 00:25:40,729 Once it's slowed down. water has a chance to soak away. 215 00:25:41,072 --> 00:25:43,472 and is sucked into living things. 216 00:25:44,109 --> 00:25:49,240 On the Earth ride we can follow water droplets and enter this inner world. 217 00:25:53,652 --> 00:25:57,053 A tree is about two-thirds water same as you 218 00:25:57,355 --> 00:25:59,687 and water travels through it at amazing speeds. 219 00:25:59,824 --> 00:26:02,054 up to 60 metres per hour. 220 00:26:04,329 --> 00:26:06,991 Plants need water for 'photosynthesis' 221 00:26:07,132 --> 00:26:10,966 they mix it with sunlight and carbon dioxide to make food. 222 00:26:11,303 --> 00:26:14,295 Excess water is expelled as vapour. 223 00:26:18,276 --> 00:26:20,471 From a leaf it's just a short ride 224 00:26:20,612 --> 00:26:23,445 for a water droplet to reach other forms of life. 225 00:26:23,548 --> 00:26:25,106 perhaps an animal... 226 00:26:27,352 --> 00:26:29,582 A monkey eats a juicy leaf... 227 00:26:30,288 --> 00:26:33,746 and Earth ride takes another turn! 228 00:26:39,497 --> 00:26:41,590 Trees make so much water vapour that 229 00:26:41,733 --> 00:26:44,759 large forests actually create their own weather. 230 00:26:45,270 --> 00:26:48,137 Half the world's rain falls in just one place 231 00:26:48,273 --> 00:26:50,764 the Amazon basin in South America. 232 00:26:51,409 --> 00:26:55,846 And 70% of that rain is expelled as vapour by the trees below. 233 00:27:00,986 --> 00:27:05,082 Eventually each water droplet makes its way into the mighty Amazon river. 234 00:27:05,457 --> 00:27:08,915 which carries one fifth of the world's running fresh water. 235 00:27:24,976 --> 00:27:30,004 The floodplain is the final fling in a droplet's trip back to the sea. 236 00:27:38,490 --> 00:27:41,015 At last. this water cycle's over and 237 00:27:41,159 --> 00:27:44,890 each droplet finally returns to where it started out... 238 00:27:46,765 --> 00:27:51,134 An average water molecule will stay here for two thousand years or more... 239 00:27:51,369 --> 00:27:56,739 until one day it will evaporate and start its epic ride all over again. 240 00:27:57,575 --> 00:27:58,473 By that time. 241 00:27:58,610 --> 00:28:02,706 ocean currents may have carried it to the other side of the world! 242 00:28:12,457 --> 00:28:15,688 Water covers more than two thirds of Earth's surface... 243 00:28:15,827 --> 00:28:18,421 only 30% is dry land. 244 00:28:20,031 --> 00:28:22,226 And the amount of life that can exist on land 245 00:28:22,367 --> 00:28:25,530 depends on how much water is available. 246 00:28:30,375 --> 00:28:33,344 Plants are the foundation of the food chain 247 00:28:33,712 --> 00:28:36,010 without water they can't grow. 248 00:28:40,652 --> 00:28:44,554 They carry water up their roots by a process called capillary action. 249 00:28:55,734 --> 00:28:59,795 Add a little water and the driest ground can spring to life. 250 00:29:21,726 --> 00:29:24,194 Water provides mechanical support 251 00:29:24,329 --> 00:29:26,957 without it.a young plant soon wilts. 252 00:29:37,475 --> 00:29:39,602 ... but it's incredibly resilient... 253 00:29:39,844 --> 00:29:43,007 give it a little drink and it may live again. 254 00:29:45,483 --> 00:29:46,950 Remarkably.one animal has evolved 255 00:29:47,085 --> 00:29:49,986 to transport water up its body by capillary action. 256 00:29:50,121 --> 00:29:51,452 just like plants. 257 00:29:53,024 --> 00:29:56,255 ln the Australian desert. this lizard - the thorny devil 258 00:29:56,394 --> 00:29:59,329 can drink from a puddle just by standing in it 259 00:29:59,898 --> 00:30:03,231 thanks to the bonds that hold the water molecules together. 260 00:30:08,873 --> 00:30:12,741 The bonds are strong enough for water molecules to be pulled upwards. 261 00:30:12,877 --> 00:30:14,606 against gravity... 262 00:30:14,979 --> 00:30:18,210 they travel through canals in the thorny devil's skin. 263 00:30:18,349 --> 00:30:20,544 until they reach its mouth. 264 00:30:26,691 --> 00:30:28,556 The amount of rainfall each part of the Earth 265 00:30:28,693 --> 00:30:31,457 receives limits what can survive there. 266 00:30:31,896 --> 00:30:32,863 The drier it gets. 267 00:30:32,997 --> 00:30:37,127 the more the water cycle directly controls animals' lives. 268 00:30:39,938 --> 00:30:43,430 Next. the Earth ride takes you to the places on the globe 269 00:30:43,575 --> 00:30:46,738 where the water cycle runs along a knife-edge... 270 00:30:46,878 --> 00:30:49,506 the driest spots on Earth. 271 00:31:00,825 --> 00:31:02,918 Here on the East African Savannah. 272 00:31:03,061 --> 00:31:05,359 the water roller coaster is in trouble. 273 00:31:06,231 --> 00:31:08,358 There are just two short rainy seasons. 274 00:31:08,499 --> 00:31:11,491 producing only about a metre of rainfall each year. 275 00:31:16,875 --> 00:31:19,173 lt's too little for trees to grow. 276 00:31:19,310 --> 00:31:23,007 instead this is a landscape dominated by grass. 277 00:31:43,968 --> 00:31:47,927 And when the rains do come. it's a deluge! 278 00:32:12,363 --> 00:32:13,125 On the face of it. 279 00:32:13,264 --> 00:32:17,064 a rainy season should be a good time for predators like this cheetah. 280 00:32:17,735 --> 00:32:21,364 ln a storm gazelles turn to face away from rain. 281 00:32:21,739 --> 00:32:24,333 Their vision is blurred and their hearing muted. 282 00:32:24,509 --> 00:32:26,704 so they can't spot danger coming. 283 00:32:35,019 --> 00:32:39,149 But in reality. rain causes problems for the hunter too... 284 00:32:50,034 --> 00:32:53,697 A sprinter. like this cheetah. likes it dry underfoot. 285 00:33:24,102 --> 00:33:26,935 lt's hard to corner on a slippery surface... 286 00:33:27,572 --> 00:33:30,132 This meal slips away. 287 00:33:39,584 --> 00:33:40,608 ln drier areas. 288 00:33:40,752 --> 00:33:42,310 it's sudden unexpected rainfall 289 00:33:42,520 --> 00:33:45,512 that brings far flung desert dwellers together. 290 00:33:46,157 --> 00:33:47,749 A desert is defined as any place 291 00:33:47,892 --> 00:33:50,861 where less than half a metre of rain falls each year. 292 00:33:51,662 --> 00:33:53,653 Precious puddles don't last long 293 00:33:54,032 --> 00:33:56,227 and these African sand grouse may have travelled up 294 00:33:56,367 --> 00:33:59,928 to 60km from their nests to find a drink. 295 00:34:02,240 --> 00:34:03,298 But they have a dilemma... 296 00:34:03,908 --> 00:34:05,102 the adults drink their fill. 297 00:34:05,243 --> 00:34:08,007 but what about their chicks. who yet can't fly? 298 00:34:12,150 --> 00:34:17,611 The male grouse has an elegant solution and he keeps it close to his chest. 299 00:34:18,222 --> 00:34:22,591 His breast feathers are four times more absorbent than blotting paper 300 00:34:22,727 --> 00:34:25,491 and he uses them to carry water home. 301 00:34:43,314 --> 00:34:44,747 Back at the nest. 302 00:34:44,882 --> 00:34:47,976 his thirsty chicks strip his feathers dry. 303 00:35:06,370 --> 00:35:08,736 Deserts are constantly shifting in size. 304 00:35:08,873 --> 00:35:11,569 according to subtle changes in the climate. 305 00:35:12,343 --> 00:35:13,833 But over the past century. 306 00:35:13,978 --> 00:35:16,845 desertification has gone into overdrive. 307 00:35:17,582 --> 00:35:21,450 Deserts now cover one fifth of Earth's land surface. 308 00:35:31,562 --> 00:35:35,089 Chile's Atacama Desert is as dry as it gets. 309 00:35:35,500 --> 00:35:38,628 There are places here where it has never rained. 310 00:35:42,340 --> 00:35:46,743 For now. there's just no water left to ride. 311 00:36:13,704 --> 00:36:17,196 These rolling dunes may be how we imagine deserts. 312 00:36:17,341 --> 00:36:21,835 but they're not always so hot. and not always made of sand. 313 00:36:27,585 --> 00:36:31,681 ln fact. the coldest place on earth is largely desert. 314 00:36:38,329 --> 00:36:43,198 There is water in the Antarctic. but it's all locked up as ice. 315 00:36:57,882 --> 00:37:00,976 So where does all this frozen water come from? 316 00:37:01,852 --> 00:37:05,413 When conditions are right. ice crystals form in the clouds. 317 00:37:05,790 --> 00:37:09,624 They grow until they are heavy enough to fall as snowflakes. 318 00:37:23,107 --> 00:37:24,597 All snowflakes are six-sided 319 00:37:24,742 --> 00:37:28,303 as every water molecule bonds to five others. 320 00:37:31,749 --> 00:37:33,876 They come in a marvellous variety of shapes. 321 00:37:34,018 --> 00:37:37,454 but each one has the same basic hexagonal symmetry. 322 00:37:57,208 --> 00:37:59,472 When blizzards fall on mountaintops. 323 00:37:59,610 --> 00:38:03,239 the snow compacts and forms the river of compressed ice 324 00:38:03,447 --> 00:38:05,244 that will become a glacier. 325 00:38:17,695 --> 00:38:20,596 lt takes thousands of years for a single ice crystal 326 00:38:20,731 --> 00:38:23,199 to travel the length of a large glacier. 327 00:38:47,491 --> 00:38:49,459 But one day. it will be released 328 00:38:49,593 --> 00:38:54,326 either as melt-water or as part of an iceberg. 329 00:39:04,508 --> 00:39:08,945 Today about a tenth of all land on Earth is blanketed by ice. 330 00:39:23,260 --> 00:39:26,752 And ice. like liquid water. is unique. 331 00:39:27,098 --> 00:39:32,536 lt's strong enough to carve up bedrock. yet so light. it floats in water. 332 00:39:35,606 --> 00:39:40,771 ln fact. water is the only known substance to have a solid lighter than its liquid! 333 00:39:51,355 --> 00:39:55,348 And if it didn't. what would happen to the penguins? 334 00:40:04,568 --> 00:40:06,763 And never mind the penguins. 335 00:40:07,204 --> 00:40:11,766 if ice sank like other solids do. we wouldn't be here either! 336 00:40:14,111 --> 00:40:17,308 Life on Earth could never have evolved. 337 00:40:17,915 --> 00:40:20,406 During the winter. seas would freeze solid... 338 00:40:20,651 --> 00:40:23,620 ice would reflect the sun's heat back into space 339 00:40:23,754 --> 00:40:26,450 and the planet would never have thawed out. 340 00:40:51,449 --> 00:40:56,910 But why does ice float. and why does that prevent our seas from freezing? 341 00:40:59,990 --> 00:41:02,618 When frozen. water molecules form a rigid. 342 00:41:02,760 --> 00:41:06,218 six-sided structure that's less dense than liquid water. 343 00:41:06,764 --> 00:41:09,665 There are gaps between the molecules that make them buoyant. 344 00:41:18,576 --> 00:41:22,137 Water is most dense at around 4 degrees Celsius. 345 00:41:22,513 --> 00:41:24,481 the temperature underneath the ice. 346 00:41:34,525 --> 00:41:36,993 So the sea remains unfrozen... 347 00:41:37,228 --> 00:41:41,358 and it's home to a spectacular array of life. 348 00:41:50,541 --> 00:41:53,601 But at these extreme outposts of the water ride. 349 00:41:53,744 --> 00:41:56,941 penguins and the other animals are particularly vulnerable. 350 00:41:57,615 --> 00:41:59,742 lf the roller coaster switches direction. 351 00:41:59,884 --> 00:42:03,752 the effects will be most dramatic at the poles. 352 00:42:10,561 --> 00:42:15,191 And at no time in our history has the water cycle changed as rapidly 353 00:42:15,332 --> 00:42:19,029 and as dramatically as it is changing right now. 354 00:42:20,004 --> 00:42:25,340 What's round the next bend in the Earth ride. no one knows! 355 00:42:26,510 --> 00:42:29,673 ln recent years we humans have diverted vast amounts of water 356 00:42:29,813 --> 00:42:32,111 for our cities and our industry. 357 00:42:37,521 --> 00:42:40,615 We've built some of our greatest cities close to water. 358 00:42:41,125 --> 00:42:44,891 We use it for decoration and because we like its sound. 359 00:42:49,900 --> 00:42:50,867 Water's fun! 360 00:42:56,540 --> 00:43:00,533 lt helps us exercise and to keep clean! 361 00:43:03,247 --> 00:43:04,805 And to stay hydrated. 362 00:43:04,949 --> 00:43:08,077 we should drink at least two litres of it every day. 363 00:43:11,555 --> 00:43:15,252 Since it started out as cosmic ice drifting in outer space. 364 00:43:15,392 --> 00:43:18,555 this water's travelled a long way to get to you! 365 00:43:20,698 --> 00:43:23,599 Strange then that we should take it so for granted! 366 00:43:28,906 --> 00:43:33,138 But can we - or should we control the water roller coaster. 367 00:43:33,277 --> 00:43:35,302 or are we on track for trouble? 368 00:43:36,080 --> 00:43:40,073 Our manipulation of the water cycle can have surprising consequences. 369 00:43:40,217 --> 00:43:41,741 By building giant dams. 370 00:43:41,885 --> 00:43:46,015 we can now store reserves of polar melt-water at lower latitudes. 371 00:43:46,924 --> 00:43:48,892 But moving millions of tons of water 372 00:43:49,026 --> 00:43:52,189 across the world has had an astonishing effect... 373 00:43:52,696 --> 00:43:55,995 we've actually slowed down the Earth's rotation 374 00:43:56,400 --> 00:44:01,963 a day is now measurably longer than it was just half a century ago. 375 00:44:05,943 --> 00:44:09,003 All this consumption raises one big question: 376 00:44:09,146 --> 00:44:14,641 are we taking more than our fair share of Earth's limited water supply and if so. 377 00:44:15,586 --> 00:44:17,349 where will it all end? 378 00:44:20,658 --> 00:44:24,355 Many factors could affect the water cycle over the next century or so. 379 00:44:24,495 --> 00:44:29,296 but scientists predict the largest impact will come from the giant climate shift 380 00:44:29,500 --> 00:44:31,661 we know as global warming. 381 00:44:37,141 --> 00:44:40,133 1998 was the warmest year on record. 382 00:44:40,277 --> 00:44:45,840 and in 2001 melt-water was discovered at the North Pole for the very first time. 383 00:44:46,784 --> 00:44:47,876 Around the South pole. 384 00:44:48,018 --> 00:44:51,476 vast Antarctic ice shelves are also shrinking. 385 00:44:57,861 --> 00:45:01,888 The impact of this climate change on both the poles is not yet clear. 386 00:45:02,833 --> 00:45:05,427 But sea ice does seem to be disappearing. 387 00:45:05,969 --> 00:45:08,199 And that could spell trouble for some animals. 388 00:45:08,338 --> 00:45:13,002 including the largest carnivore to walk the Earth today. the polar bear. 389 00:45:14,044 --> 00:45:15,705 The polar bear relies on sea ice 390 00:45:15,846 --> 00:45:19,338 as a platform to hunt its favourite quarry. seals. 391 00:45:19,850 --> 00:45:24,480 lf sea ice disappears. then probably. so too will polar bears. 392 00:45:29,760 --> 00:45:33,457 The bear is already skating on thin ice. 393 00:45:33,530 --> 00:45:36,294 and soon there might be no ice left at all. 394 00:46:19,543 --> 00:46:21,477 At this point. no one can be sure 395 00:46:21,612 --> 00:46:25,139 which way the water roller coaster will go next. 396 00:46:29,486 --> 00:46:31,477 Will there be more intense droughts. 397 00:46:31,622 --> 00:46:34,955 more flooding. more devastating storms? 398 00:46:35,292 --> 00:46:39,490 lf there are. it's likely it will happen in our lifetime. 399 00:46:39,997 --> 00:46:43,694 These changes to the water cycle could cause lasting damage: 400 00:46:44,601 --> 00:46:48,731 the sands of time really are running out. 401 00:47:25,676 --> 00:47:30,613 The outcome is uncertain and the Earth ride isn't derailed yet. 402 00:47:32,482 --> 00:47:33,744 And one thing's for sure. 403 00:47:33,884 --> 00:47:36,546 we'll be staying on board to the end of the line. 404 00:47:38,188 --> 00:47:42,887 This is one ride you can't stop. slow down or get off! 405 00:47:47,097 --> 00:47:48,359 Every time we have a drink. 406 00:47:48,498 --> 00:47:52,798 we become the next stage in this never-ending roller coaster ride! 407 00:47:59,743 --> 00:48:03,042 We're linked to everywhere that water has already been 408 00:48:03,180 --> 00:48:07,412 to far-flung places and extraordinary plants and animals. 409 00:48:08,518 --> 00:48:13,114 Water is unique and it drives every form of life on planet Earth. 410 00:48:13,457 --> 00:48:16,790 lt has allowed life to exist where there was none. 411 00:48:36,179 --> 00:48:39,012 Water remains our most precious resource! 412 00:48:39,149 --> 00:48:44,781 Without it. there would be no planet. as we know it. and no magical. 413 00:48:44,922 --> 00:48:48,483 unending roller coaster ride of life. 414 00:48:55,165 --> 00:48:59,067 Thank you for taking the Earth ride. goodbye.