A third of the land\non our planet is desert.
These great scars on the face of the earth\nappear to be lifeless.
But surprisingly, none are.
In all of them life manages, somehow,\nto keep a precarious hold.
Not all deserts are hot.
Fifty-mile-an-hour winds blowing in from Siberia
bring snow to the Gobi Desert in Mongolia.
From a summer high of 50 degrees centigrade,
the temperature in mid-winter\ncan drop to minus 40,
making this one of the harshest deserts of all.
Few animals can survive these extreme changes.
Wild Bactrian camels,\none of the rarest mammals on the planet,
and perhaps the hardiest.
Their biggest problem is the lack of water,
particularly now in winter when the little there is\nis locked up as ice.
Surprisingly, snow here never melts.
The air is just too cold and too dry for it to do so.
The sun's rays turn it straight into vapour.
It evaporates.
But it is the only source of water
so Bactrian camels eat it.
Elsewhere in the world,
a camel at a waterhole can drink\nas much as 200 litres during a single visit.
Here, the strategy is to take little and often,\nand with good reason
for filling the stomach with snow could be fatal.
The camels must limit themselves\nto the equivalent of just 1 0 litres a day.
Winter is the time for breeding.
This extraordinary performance\nis a male Bactrian camel's way
of attracting the attention of a passing female.
In summer, the camels can't stray\nfar from waterholes.
But now, with mouthfuls of snow lying everywhere,
they can travel widely in search of mates.
Today, less than 1 ,000 of these desert specialists\nremain in the wild.
The Gobi, hostile though it is,\nis their last stronghold.
There is no other desert quite like the Gobi.
But why is this place a desert?
There is one simple and massive cause.
The Himalayas.
Clouds blowing from the south\nhit this gigantic barrier.
As they're forced upwards,
so they empty their moisture\non the mountain slopes,
leaving little for the land on the other side.
From space, deserts are very conspicuous.
Dunes of sand hundreds of miles long\nstreak their surface.
With no cloak of vegetation to conceal them,
strange formations are exposed in the naked rock.
Africa's Sahara is the largest desert of all.
It's the size of the United States
and the biggest source of sand and dust\nin the entire world.
Sandstorms like these appear without warning
and reduce visibility for days\nover areas the size of Britain.
Dromedaries, single-humped camels,\ntake these storms in their stride.
The heavy sand rises only a few metres\nabove the ground.
But the dust can be blown\n5,000 metres up into the sky.
The ferocious wind, armed with grains of sand,
is the agent that shapes all deserts.
Reptiles have armoured, scaly skins\nthat protect them from the stinging grains.
For insects, the bombardment\ncan be very severe indeed.
The only escape is below the surface.
As the winds rise and fall, swirl and eddy,
so they pile the sand into dunes.
These sand seas can be hundreds of miles across.
In Namibia, the winds have built\nsome of the biggest dunes in the world.
Star dunes like these can be 300 metres high.
Grains swept up the flanks\nare blown off the crests of the ridges
so it's only the tops that are moving.
The main body of these dunes\nmay not have shifted for 5,000 years.
Few rocks can resist the continuous blast\nof the sand-carrying wind.
These outcrops are standing\nin Egypt's White Desert.
But they will not do so for much longer.
They are being inexorably chiselled away\nand turned into more sand.
Now lumps of heavily eroded rocks\nhave been marooned in a sea of sand.
These jagged pyramids, 1 00 metres tall,
were once part of a continuous rocky plateau.
The blasting sand will eventually\neliminate them altogether.
The relentless power of the wind
ensures that the face of a desert\nis continually changing.
But there is one constant presence.
The desert sun.
The sun's heat and the power to evaporate water
has had a profound effect on the bodies\nand habits of everything that lives here.
This sun, potentially, is a killer.
And the red kangaroos must acknowledge that.
Right now, while the sun is low,\nthere's no immediate cause for concern.
But this situation won't last long.
Australia is the world's most arid continent\nwith blistering daytime temperatures.
Every hour, the temperature rises\nby five degrees centigrade.
Soon the heat will reach a critical point.
Any kangaroo out in the open\nis in serious danger of overheating.
In the full sun, the temperature on the ground\nsoars to 70 degrees.
By midday, the radiation is so intense\nthey must take shelter.
In the shade, they are shielded\nfrom much of the sun's energy
but their body temperature can still rise.
So they lick saliva onto their forearms
where there is a network of blood vessels\nclose to the surface of the skin,
and as the saliva evaporates, their blood is cooled.
This thermal image shows\njust how effective the process is.
The blue areas on the body are the cooler parts.
As the saliva dries, it has to be replaced.
And this is a real drain\non the kangaroo's body fluids.
Even in the shade, the earth is baking hot.
So the kangaroos dig away the warmed topsoil\nto get at the cooler ground beneath.
By staying in the shade\nand licking to control their body temperature,
kangaroos manage to get through\nthe hottest part of the day without heatstroke.
But for the majority of desert animals,\nthis strategy would not be enough for survival.
The extraordinary ears\nof the fennec foxes of Africa radiate heat,
but the animals have another way of keeping cool.
They spend their days underground\nand only emerge at sunset.
Darkness brings huge changes.
In the Sahara, the temperature can drop\nas much as 30 degrees during the night.
So it's cool enough\nto allow these desert fox cubs to play.
All sorts of creatures now appear,\nincluding some really unexpected ones.
(CR0AKING)
Toads have permeable skins
and would quickly die from desiccation\nout in the daytime heat.
It's only now that they can leave shelter.
The same is true for scorpions,\neven though their shells are actually watertight.
In fact, most small desert creatures are nocturnal.
So it's only now that you can judge\njust how much life there can be in a desert.
But moisture lost, even at night,
has to be replaced sometime, somehow.
And that problem dominates the lives\nof all desert dwellers.
The Atacama in Chile.\nThis is the driest desert in the world.
Some parts may not see rain for 50 years,\nand with such a record,
you would expect the place\nto be completely barren.
These are South America's camels, guanacos.
They are very good at conserving moisture
but they, nonetheless,\nneed a regular supply of water.
They get it partly from cactus flowers.
But that explanation raises another question.
How do the cacti survive without rain?
Hot winds suck all the moisture\nfrom the surface of the land.
Clearly, there must be something else\nthat takes the place of rain.
The secret is a cold sea current\nthat runs parallel to the land.
The cold water cools the moist, warm air above it\nand that produces banks of fog.
At the same time, wind blowing onto the shore\nsweeps the fog inland.
Before long, the cacti are dripping with dew.
The fog is so regular
that moisture-loving lichens\nare able to grow on the cacti
and they absorb liquid like a sponge.
In the land of almost no rain,
these precious drops are lifesavers\nfor many different creatures.
Further inland, the air remains so warm\nthat its moisture does not condense.
So this slender strip of desert\nis virtually the only part of the Atacama
where life can exist.
And without the fog,\nthis land, too, would be empty.
The guanacos make the most of the dew.
But it will not remain for long.
In an hour or two, the sun will have burnt it off\nand dried the surface of the cacti.
The Sonoran Desert in Arizona\nis not quite so dry as the Atacama.
Some rain does fall.
But it is infrequent,
and when it does arrive, animals and plants\nhave to be ready to make the most of it.
And it's coming.
When the summer monsoon blows in,
the giant saguaros, one of the biggest of all cacti,
are ready to take full advantage of it.
(THUNDER RUMBLING)
After a rainstorm,
the saguaro's long, shallow root system\nsucks up the water
and the pleats on its trunk\nenable it to expand rapidly.
When full, a saguaro stem can store\nup to five tonnes of water,
and that's enough to see it through\nmany months of drought.
The trunks of these huge plants\nprovide homes for the Gila woodpecker.
But birds are not the only animals\nto benefit from the presence of the cacti.
During four weeks of the summer,\nthe saguaros bloom at night to attract visitors.
The pollen and nectar\nwith which these flowers are loaded
attract long-nosed and long-tongued bats.
The bats left Mexico a few days earlier\nto escape the heat of summer
and are on their way north\nto the southern United States.
To get there, they have to cross\nthe Sonoran Desert.
But the desert is so big that for most of the year\nthey would be unable to cross it.
Now, with the saguaro in bloom,\nthey can refuel on the way.
So the saguaro's success\nin developing a way to store water
is now crucial to most of the animals\nthat live or even travel through this land.
The scarcity of rain determined\nthe shape of this icon of the desert.
But water, scarce though it is,\nhas also, like the wind, shaped the land itself.
In the deserts of Utah,\nancient rivers flowing across sandstone country
steadily widened their canyons
until now, the land between them\nhas been reduced to spires and pinnacles.
With little or no soil to retain the water\non the surface of the land,
life here is scarce indeed.
And when resources are limited,\nconflict is never far away.
These are Nubian ibex\nand they are squaring up for a duel.
And when trouble starts, a smart ibex knows\nthat the best thing to do is to gain higher ground.
These are actually subordinate male ibex,\nbut their fights are, nonetheless, serious.
Losing one might mean\nnever getting the chance to breed, ever.
When competitors are evenly matched,\nas they are here,
duels can last for an hour.
In this heat, the effort is truly exhausting.
(PANTING)
But victory here will gain important ranking points\non a male's way to the top.
There's so much at stake that not all play fair.
The battle has produced a winner.
But the ultimate prize is not his yet.
That currently belongs to the dominant male ibex.
His rank earns him the loyalty\nof a harem of females
and they follow him closely
as he travels across this desert\nsearching for food and water.
He doesn't have to waste time looking for mates,\nthey're his for the taking.
So he can concentrate with them\non keeping fit and healthy.
Lizards are desert specialists.
But here, their numbers are extraordinary.
These crevices in South Africa
contain the highest density of lizards in the world.
They're called flat lizards, for obvious reasons,
and they flaunt their multicoloured bellies\nin territorial disputes.
He's made his point\nand now it's time to find some food.
As the day warms up,\nthe lizards move away from their cracks
and head down to the bottom of the gorge.
Their goal is the river.
There is no food at the edge,\nbut this desert river holds a secret.
Each day, black fly rise\nfrom turbulent stretches of the river.
This is what the lizards have come for.
The black fly never land
so the lizards have to leap for their food.
In one day, each of these acrobatic little lizards\nmay catch 50 flies.
There are plenty of flies to go around,\neven with hundreds of lizards competing for them.
Away from these rapids,
flat lizard populations\nare found in much smaller numbers.
But here, one unusual abundance\nhas produced another.
Deserts are created by the lack of water
but what actually kills animals here\nis not heat or thirst, but lack of food.
So how on earth does a plant-eater this size
survive in a place apparently\ntotally devoid of vegetation?
Elephants in Namibia are the toughest in Africa.
And they need to be.
What little food exists is so dispersed
that these elephants walk up to 50 miles a day
as they travel up the dry river channels\nsearching for something to eat.
At times, the task looks truly hopeless.
Elephants may seem out of place\nin this landscape,
but they're not the only ones.
Amazingly, lions live here, too.
In savannah country, huge herds of game\nsupport prides containing 20 lions or more.
But to live here,\nlions have had to change their habits.
Prides are much smaller\nand their home ranges are very much bigger.
And there's an added problem.
Their food is always on the move.
Like the elephants,
the lions must travel great distances\nto find enough to live on.
But lions can't go everywhere.
They won't attempt to cross\nthis field of sand dunes,
and the oryx know it.
The lions must wait for the oryx\nto leave the safety of the dunes,
which eventually they must to find food and water,
and then the lions will ambush them.
The elephants have found\nsome of their favourite food.
Grasses are the staple diet of all elephants,
but this herd concentrates\non digging up the roots,
which have more nutrition and moisture\nthan the stems.
It's the sort of behaviour
that can make all the difference\nin a place of serious shortages.
Yet all this can change in an instant.
The fortunes of many deserts\nare ruled by distant rains.
This water fell as rain\nin mountains more than 1 00 miles away.
It's known as a flash flood
and called that because the water may run\nfor just a single day.
It's an event that only happens\nonce or twice a year at the most.
The sandy riverbed acts like\na giant strip of blotting paper,
sucking up the water as soon as it appears.
But every square metre of soil\nmoistened by this river
will increase the chances of survival\nfor those that live here.
Waterholes are filled temporarily.
Elsewhere in Africa elephants drink everyday
but the lack of water here means that\ndesert elephants can only refill their tanks
once every four or five days.
Within a week, the flash flood has produced\na flush of green,
more than enough to draw the oryx\nout of the dunes.
It's a rare chance for them\nto build up their food reserves.
The flood has made life easier for the lions, too.
The flesh of this oryx will keep the family going\nfor a week at the most.
But for a while, the hunting will be easier\nnow that the river channel has turned green.
The good times for lions and oryx are brief.
But these are the short moments
that make it possible\nto live in deserts the year round.
Death Valley is the hottest place on Earth.
Yet even this furnace\ncan be transformed by water.
A single shower can enable seeds\nthat have lain dormant for 30 years or more
to burst into life.
And there hasn't been a bloom like this one\nfor a century.
The periods of boom in Death Valley are short
but they're just frequent enough\nto keep life ticking over.
A sudden flush of vegetation\nis what every desert dweller waits for
and when it happens,\nthey must make the most of it.
There is no other species on the planet
that responds as quickly and as dramatically\nto the good times
as the desert locust.
Eggs that have remained in the ground\nfor 20 years begin to hatch.
The young locusts are known as hoppers,\nfor at this stage they're flightless.
They find new feeding grounds\nby following the smell of sprouting grass.
Normally it takes four weeks\nfor hoppers to become adults.
But when the conditions are right, as now,
their development switches to the fast track.
As the vegetation in one place begins to run out,
the winged adults release pheromones,\nscent messages,
which tell others in the group\nthat they must move on.
And when groups merge, they form a swarm.
An adult locust eats\nits entire body weight every day.
And a whole swarm can consume\nliterally hundreds of tonnes of vegetation.
They have to keep on moving.
The swarm travels with the wind.
It's the most energy-saving way of flying.
Following the flow of wind
means that they're always heading\ntoward areas of low pressure.
Places where wind meets rain\nand vegetation starts to grow.
As they fly, swarms join up with other swarms\nto form gigantic plagues
several billion strong\nand as much as 40 miles wide.
They will consume every edible thing\nthat lies in their path.
This is one of planet Earth's greatest spectacles.
It's rarely seen on this scale and it won't last long.
0nce the food has gone,
the steady roar of a billion beating locust wings
will once again be replaced by nothing more\nthan the sound of the desert wind.