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 midwinter can drop to -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.

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 as much as 200 litres during a single visit.

Here, the strategy is to take little and often.

And with good reason,

for filling the stomach with snow could be fatal.

The camels must limit themselves ...

to the equivalent of just 10 litres a day.

Winter is the time for breeding.

This extraordinary performance is a male Bactrian camel's way ...

of attracting the attention of a passing female.

In summer, the camels can't stray far from waterholes.

But now, with mouthfuls of snow lying everywhere,

they can travel widely in search of mates.

Today, less than a 1,000 of these desert specialists remain in the wild.

The Gobi, hostile though it is, is their last stronghold.

There's 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're forced upwards,

so they empty their moisture on the mountain slopes,

leaving little for the land on the other side.

From the 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 heaviest sand rises only a few metres above the ground,

but the dust can be blown 5,000 metres out 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, swallow and eddy,

so they pile the sand into dunes.

These sand scenes 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 300 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're being inexorably chiseled away ...

and turned into more sand.

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

These jagged pyramids a hundred metres tall,

were once part of a continuous rocky plateau.

The blasting sand will eventually eliminate them altogether.

The relentless power of the wind ...

ensures that the face of a desert is continually changing.

But there is one constant presence —

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's no immediate cause for concern.

But this situation won't last long.

Australia is the world's most arid continent with blistering daytime temperatures.

Every hour, the temperature rises by 5 degrees centigrade.

Soon the heat will reach a critical point.

Any kangaroo out in the open is in serious danger of overheating.

In the full sun, the temperature on the ground soars to 70 degrees.

By midday, the radiation is so intense they must take shelter.

In the shade they're shielded from much of the sun's energy,

but their body temperature can still rise.

So they lick saliva on to their forearms ...

where there is a network of blood vessels close to the surface of the skin ...

and, as the saliva evaporates, their blood is cooled.

This thermal image shows just 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 on the kangaroo's body fluids.

Even in the shade the earth is baking hot,

so the kangaroos dig away the warmed topsoil ...

to get at the cooler ground beneath.

By staying in the shade and licking to control their body temperature,

kangaroos manage to get through the hottest part of the day ...

without heat stroke.

But for the majority of desert animals,

this strategy would not be enough for survival.

The extraordinary ears of the fennec foxes of Africa radiate heat,

but the animals have another way of keeping cool.

They spend their days underground ...

and only emerge at sunset.

Darkness brings huge changes.

In the Sahara, the temperature can drop as much as 30 degrees during the night,

so it's cool enough to allow these desert fox cubs to play.

All sorts of creatures now appear ...

including some really unexpected ones.

Toads have permeable skins ...

and would quickly die from desiccation out in the daytime heat.

It's only now that they can leave shelter.

The same is true for scorpions,

even though their shells are actually watertight.

In fact, most small desert creatures are nocturnal —

so it's only now that you can judge ...

just how much life there can be in the desert.

But moisture, lost even at night, has to be replaced sometime somehow,

and that problem dominates the lives of all desert dwellers.

The Atacama in Chile.

This is the driest desert in the world.

Some parts may not see rain for 50 years,

and with such a record ...

you'd expect the place to be completely barren.

These are South America's camels, guanacos.

They're 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 on to 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 the land of almost no rain,

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

Further inland, the air remains so warm that its moisture does not condense,

so this slender strip of desert is virtually the only part of the Atacama ...

where life can exist.

And without the fog,

this 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 ...

and dried the surface of the cacti.

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.

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

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