Bbc planet earth 05 — deserts
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 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. 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 thousand 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 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, 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.