Deserts - Short Version Planet Earth

A third of the land on our planet is desert.

These great scars on the face of the Earth appear to be lifeless,

but surprisingly, none are.

In all of them, life manages somehow

to 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 can drop to minus 40,

making this one of the harshest deserts of all.

Few animals can survive these extreme changes.

Wild Bactrian camels, one 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 is locked up as ice.

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

hit this gigantic barrier.

As they are forced upwards,

so they empty their moisture on 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, streak 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 in the entire world.

Sandstorms like these appear without warning

and reduce visibility for days over areas the size of Britain.

Dromedaries, single-humped camels, take these storms in their stride.

The heavy sand rises only a few metres above the ground,

but the dust can be blown five thousand 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 that protect them from the stinging grains.

For insects, the bombardment can 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 some of the biggest dunes in the world.

Star dunes, like these, can be three hundred metres high.

Grains swept up the flanks are blown off the crests of the ridges,

so it's only the tops that are moving.

The main body of these dunes may not have shifted for 5,000 years.

Few rocks can resist the continuous blast of the sand-carrying wind.

These outcrops are standing in Egypt's White Desert,

but they will not do so for much longer.

They are being inexorably chiselled away and turned into more sand.

Now lumps of heavily-eroded rocks have been marooned in a sea of sand.

The desert sun.

The sun's heat and power to evaporate water has had a profound effect

on the bodies and 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,

there is no immediate cause for concern.

But this situation won't last long.

Australia is the world's most arid continent

with blistering daytime temperatures.

The Atacama in Chile.

This is the driest desert in the world.

Some parts may not see rain for fifty years, and with such a record

you would expect the place to be completely barren.

These are South America's camels - guanacos.

They are very good at conserving moisture,

but they nonetheless need 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 from the surface of the land.

Clearly there must be something else that takes the place of rain.

The secret is a cold sea current that runs parallel to the land.

The cold water cools the moist, warm air above it

and that produces banks of fog.

At the same time, wind blowing onto the shore sweeps the fog inland.

Before long, the cacti are dripping with dew.

The fog is so regular that moisture-loving lichens

are able to grow on the cacti and they absorb liquid like a sponge.

In a land of almost no rain,

these precious drops are life-savers for many different creatures.

The Sonoran Desert in Arizona is not quite so dry as the Atacama.

Some rain does fall,

but it is infrequent and when it does arrive,

animals and plants have 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

After a rainstorm, the saguaros' long shallow root system sucks up the water,

and the pleats on its trunk enable it to expand rapidly.

When full, a saguaro stem can store up to five tons of water,

and that's enough to see it through many months of drought.

The trunks of these huge plants provide homes for the Gila woodpecker.

But birds are not the only animals to benefit

from the presence of the cacti.

During four weeks of the summer

the saguaros bloom - at night - to attract visitors.

The pollen and nectar with which these flowers are loaded

attract long-nosed - and long-tongued - bats.

The bats left Mexico a few days earlier to escape the heat of summer

and are on their way north to the southern United States.

To get there, they have to cross the Sonoran Desert.

But the desert is so big

that for most of the year they would be unable to cross it.

Now, with the saguaro in bloom, they can refuel on the way.

So the saguaro's success in developing a way to store water

is now crucial to most of the animals that live or even travel through this land.

The scarcity of rain determined the shape of this icon of the desert,

but water, scarce though it is,

has also, like the wind, shaped the land itself.

In the deserts of Utah,

ancient rivers flowing across sandstone country

steadily widened their canyons

until now the land between them has been reduced

to spires and pinnacles.

Subtitles by Red Bee Media Ltd

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