0:00:38 > 0:00:42A third of the land on our planet is desert.
0:00:51 > 0:00:56These great scars on the face of the Earth appear to be lifeless,
0:00:56 > 0:00:59but surprisingly, none are.
0:01:05 > 0:01:08In all of them, life manages somehow
0:01:08 > 0:01:11to keep a precarious hold.
0:01:31 > 0:01:34Not all deserts are hot.
0:01:34 > 0:01:36Fifty-mile-an-hour winds,
0:01:36 > 0:01:39blowing in from Siberia,
0:01:39 > 0:01:41bring snow to the Gobi Desert in Mongolia.
0:02:06 > 0:02:10From a summer high of 50 degrees centigrade,
0:02:10 > 0:02:14the temperature in mid-winter can drop to minus 40,
0:02:14 > 0:02:18making this one of the harshest deserts of all.
0:02:26 > 0:02:31Few animals can survive these extreme changes.
0:02:37 > 0:02:43Wild Bactrian camels, one of the rarest mammals on the planet,
0:02:43 > 0:02:45and perhaps the hardiest.
0:02:46 > 0:02:51Their biggest problem is the lack of water, particularly now in winter
0:02:51 > 0:02:54when the little there is is locked up as ice.
0:03:01 > 0:03:04There is no other desert quite like the Gobi.
0:03:04 > 0:03:07But why is this place a desert?
0:03:08 > 0:03:14There is one simple and massive cause - the Himalayas.
0:03:17 > 0:03:19Clouds blowing from the south
0:03:19 > 0:03:22hit this gigantic barrier.
0:03:22 > 0:03:23As they are forced upwards,
0:03:23 > 0:03:27so they empty their moisture on the mountain slopes,
0:03:27 > 0:03:30leaving little for the land on the other side.
0:03:34 > 0:03:38From space, deserts are very conspicuous.
0:03:41 > 0:03:45Dunes of sand, hundreds of miles long, streak their surface.
0:03:50 > 0:03:54With no cloak of vegetation to conceal them,
0:03:54 > 0:03:55strange formations
0:03:55 > 0:03:57are exposed in the naked rock.
0:04:04 > 0:04:07Africa's Sahara is the largest desert of all.
0:04:07 > 0:04:10It's the size of the United States
0:04:10 > 0:04:14and the biggest source of sand and dust in the entire world.
0:04:53 > 0:04:56Sandstorms like these appear without warning
0:04:56 > 0:05:01and reduce visibility for days over areas the size of Britain.
0:05:09 > 0:05:14Dromedaries, single-humped camels, take these storms in their stride.
0:05:18 > 0:05:23The heavy sand rises only a few metres above the ground,
0:05:23 > 0:05:28but the dust can be blown five thousand metres up into the sky.
0:05:33 > 0:05:36The ferocious wind, armed with grains of sand,
0:05:36 > 0:05:40is the agent that shapes all deserts.
0:05:49 > 0:05:55Reptiles have armoured scaly skins that protect them from the stinging grains.
0:05:57 > 0:06:02For insects, the bombardment can be very severe indeed.
0:06:04 > 0:06:08The only escape is below the surface.
0:06:24 > 0:06:31As the winds rise and fall, swirl and eddy, so they pile the sand into dunes.
0:06:35 > 0:06:39These sand seas can be hundreds of miles across.
0:06:43 > 0:06:44In Namibia,
0:06:44 > 0:06:48the winds have built some of the biggest dunes in the world.
0:06:55 > 0:06:59Star dunes, like these, can be three hundred metres high.
0:06:59 > 0:07:04Grains swept up the flanks are blown off the crests of the ridges,
0:07:04 > 0:07:06so it's only the tops that are moving.
0:07:13 > 0:07:18The main body of these dunes may not have shifted for 5,000 years.
0:07:30 > 0:07:36Few rocks can resist the continuous blast of the sand-carrying wind.
0:07:36 > 0:07:41These outcrops are standing in Egypt's White Desert,
0:07:41 > 0:07:44but they will not do so for much longer.
0:07:46 > 0:07:51They are being inexorably chiselled away and turned into more sand.
0:08:29 > 0:08:35Now lumps of heavily-eroded rocks have been marooned in a sea of sand.
0:08:45 > 0:08:46The desert sun.
0:09:01 > 0:09:06The sun's heat and power to evaporate water has had a profound effect
0:09:06 > 0:09:10on the bodies and habits of everything that lives here.
0:09:14 > 0:09:17This sun, potentially, is a killer,
0:09:18 > 0:09:22and the red kangaroos must acknowledge that.
0:09:24 > 0:09:26Right now, while the sun is low,
0:09:26 > 0:09:28there is no immediate cause for concern.
0:09:30 > 0:09:33But this situation won't last long.
0:09:33 > 0:09:36Australia is the world's most arid continent
0:09:36 > 0:09:39with blistering daytime temperatures.
0:09:46 > 0:09:48The Atacama in Chile.
0:09:48 > 0:09:52This is the driest desert in the world.
0:09:53 > 0:09:58Some parts may not see rain for fifty years, and with such a record
0:09:58 > 0:10:01you would expect the place to be completely barren.
0:10:06 > 0:10:10These are South America's camels - guanacos.
0:10:10 > 0:10:14They are very good at conserving moisture,
0:10:14 > 0:10:17but they nonetheless need a regular supply of water.
0:10:30 > 0:10:33They get it partly from cactus flowers,
0:10:33 > 0:10:37but that explanation raises another question.
0:10:39 > 0:10:43How do the cacti survive without rain?
0:10:49 > 0:10:53Hot winds suck all the moisture from the surface of the land.
0:10:55 > 0:11:00Clearly there must be something else that takes the place of rain.
0:11:01 > 0:11:07The secret is a cold sea current that runs parallel to the land.
0:11:24 > 0:11:27The cold water cools the moist, warm air above it
0:11:27 > 0:11:30and that produces banks of fog.
0:11:52 > 0:11:58At the same time, wind blowing onto the shore sweeps the fog inland.
0:12:13 > 0:12:17Before long, the cacti are dripping with dew.
0:12:26 > 0:12:30The fog is so regular that moisture-loving lichens
0:12:30 > 0:12:35are able to grow on the cacti and they absorb liquid like a sponge.
0:12:37 > 0:12:39In a land of almost no rain,
0:12:39 > 0:12:44these precious drops are life-savers for many different creatures.
0:12:55 > 0:13:01The Sonoran Desert in Arizona is not quite so dry as the Atacama.
0:13:01 > 0:13:02Some rain does fall,
0:13:02 > 0:13:06but it is infrequent and when it does arrive,
0:13:06 > 0:13:10animals and plants have to be ready to make the most of it.
0:13:12 > 0:13:13And it's coming.
0:13:21 > 0:13:25When the summer monsoon blows in, the giant saguaros,
0:13:25 > 0:13:30one of the biggest of all cacti, are ready to take full advantage of it.
0:13:32 > 0:13:34THUNDER
0:14:02 > 0:14:09After a rainstorm, the saguaros' long shallow root system sucks up the water,
0:14:09 > 0:14:14and the pleats on its trunk enable it to expand rapidly.
0:14:21 > 0:14:27When full, a saguaro stem can store up to five tons of water,
0:14:27 > 0:14:31and that's enough to see it through many months of drought.
0:14:35 > 0:14:40The trunks of these huge plants provide homes for the Gila woodpecker.
0:14:43 > 0:14:46But birds are not the only animals to benefit
0:14:46 > 0:14:49from the presence of the cacti.
0:15:04 > 0:15:07During four weeks of the summer
0:15:07 > 0:15:12the saguaros bloom - at night - to attract visitors.
0:15:15 > 0:15:19The pollen and nectar with which these flowers are loaded
0:15:19 > 0:15:23attract long-nosed - and long-tongued - bats.
0:15:37 > 0:15:42The bats left Mexico a few days earlier to escape the heat of summer
0:15:42 > 0:15:46and are on their way north to the southern United States.
0:15:48 > 0:15:52To get there, they have to cross the Sonoran Desert.
0:15:58 > 0:16:00But the desert is so big
0:16:00 > 0:16:05that for most of the year they would be unable to cross it.
0:16:05 > 0:16:08Now, with the saguaro in bloom, they can refuel on the way.
0:16:18 > 0:16:22So the saguaro's success in developing a way to store water
0:16:22 > 0:16:29is now crucial to most of the animals that live or even travel through this land.
0:17:06 > 0:17:11The scarcity of rain determined the shape of this icon of the desert,
0:17:11 > 0:17:13but water, scarce though it is,
0:17:13 > 0:17:17has also, like the wind, shaped the land itself.
0:17:33 > 0:17:35In the deserts of Utah,
0:17:35 > 0:17:39ancient rivers flowing across sandstone country
0:17:39 > 0:17:42steadily widened their canyons
0:17:42 > 0:17:45until now the land between them has been reduced
0:17:45 > 0:17:47to spires and pinnacles.
0:18:46 > 0:18:48Subtitles by Red Bee Media Ltd
0:18:48 > 0:18:51E-mail subtitling@bbc.co.uk