On our planet of extremes, perhaps the most frightening and yet fascinating environments are the deserts. Covering around a third of the earth's land surface, they are the harshest and most barren environments in the world - inhospitable and seemingly incapable of sustaining anything but the most rudimentary plant and animal life. In this feature we focus our attention upon those deserts which lie along the meridian line.
We begin by taking a look at the factors such as climate and landscape that make the deserts distinctive. Following this we consider the geography of deserts - where in the world they are to be found. Other sections explore the plant and animal life that can be found in the deserts along the meridian line, as well as some of the important conservation issues which we are facing in connection with this type of environment.
Hot and dry. These are the terms that characterise the desert climate in most people's minds, but is this an accurate depiction? As a general rule, areas subject to an average rainfall of less than 100mm a year can be classed as deserts. Sometimes a broader classification is used whereby deserts are split into three categories - semi-arid, arid and hyper-arid. Areas of the first kind receive less than 600mm of rainfall a year, areas of the second kind receive less than 200mm, whilst rainfall in hyper-arid desert regions never exceeds 25mm.
But low average annual rainfall is only half the story. Not only do deserts experience very little rain, but to make matters worse, this small amount is highly irregular, both in terms of time and space. Sometimes an area of desert will be without rain for years and then, seemingly for no reason at all, a few large storms will provide enough rain in a short period to bring the average annual rainfall back to normal. Desert storms can be extremely localised, centring upon one area and leaving adjoining parts entirely dry. These are some of the factors that contribute to making deserts some of the most inhospitable environments on the planet.
Water shortage in the desert is not limited to low rainfall. Humidity and consequently cloud cover are also in short supply. This in turn means that the surface, and any living things on it, are continually exposed to direct sunlight, causing intense evaporation of any water faster than the rain can replenish it.
Where deserts are to be found close to the sea, fogs often occur in the early mornings as a result of cold sea water meeting warm air. When the warm air, laden with moisture, comes into contact with the surface of the land, which is still cold from the previous night, it cools and condenses, leaving a film of water. The Namib Desert of southern Africa and Chile's Atacama Desert are both cases in point.
Contrary to popular belief, high temperature is not the most important feature of deserts: lack of water, as we have seen, is of much greater significance. However, it is true that most of the world's deserts are subject to extreme heat for at least part of the year. Many people are surprised by how much temperatures can vary. With cloud cover so scarce, air temperatures in the desert frequently reach 40° C, and in some circumstances they can soar to 50° C. The temperatures of the rocks and sand are even higher, up to 75° C. But these maximum figures can vary by up to 30° C. Seasonal temperature variation can also be considerable in the desert, particularly those furthest from the equator - and in some places, winter nights can even bring frost.
Deserts often experience high winds which, coupled with the sand and dust particles which are typical of most deserts, leads to the formation of the distinctive desert landscape.
The climatic extremes of deserts have created a wide range of landscapes. Try to picture a desert in your mind and the chances are you are imagining a vast featureless sandy plain as far as the eye can see, with only the odd cactus to break the monotony. Although this picture is not actually wrong, it only gives about a quarter of the story. Sand covers just 20 per cent of the world's desert.
Sand is formed by the erosion of rocks into tiny particles. This erosion is the result of a number of processes, the most important of which are the heating and cooling of rocks and the action of the wind and rain. The enormous temperature ranges experienced by most desert regions cause the rocks to expand and contract, eventually cracking and disintegrating in a process known as weathering. The smaller fragments of rock that break off are carried by the wind, and sometimes water, and they in turn erode other rocks. Gradually they become so small that they are merely grains.
The same wind that helps to form sand particles also shapes the landscape in other ways. Given the right conditions, the easily transported grains of sand will accumulate to form sand dunes. These extraordinary features can vary from small heap-like structures of only about a metre in height to enormous sand mountains of 1,000 metres. Some dunes often reach several kilometres in length. They are often found in large groups known as sand seas or ergs. The largest of these, covering around 560,000 square kilometers, is in the Arabian Desert. An important and intriguing characteristic of sand dunes is their tendency to move around. Typical annual movement of sand dunes is between 10 and 20 metres, depending upon size, but in extreme cases small dunes may travel up to 50 metres in a year.
But sand is just one of a number of landscape types to be found in the desert. Rocks and stones feature quite prominently in most of the world's arid places, often with vast plains covered in gravel, or large towering cliffs, eroded into extraordinary shapes by the action of wind and water. Elsewhere, the evaporation of ancient lakes has left enormous areas known as salt flats, which represent one of the greatest obstacles to plant and animal life to be found on the planet. Other areas are covered with clay or mud which has been dried hard by the heat of the searing desert sun. One feature that is common to many desert areas is the scarcity of soil, and consequently vegetation.
Deserts provide us with some of the most spectacular and stunning scenery on the planet. From the dramatic rock formations of Death Valley in the US, to the sand dunes of the Sahara, the world's arid zones constitute a natural gift that should be neither overlooked nor taken for granted.
Deserts occur in five of the world's seven continents. North America's Sonora and Chihuaua deserts, situated in the south-west corner of the continent, extend into Mexico, while the Great Basin, covering most of the states of Utah and Nevada, is home to the infamous Death Valley, and the Great Salt Lake Desert. Further south, in the states of Arizona and California, lie the Mojave or High Desert and the Colorado or Low Desert. The North American deserts are renowned for their spectacular landscapes and searing heat.
Between the Andean Mountains and the Pacific Ocean lie the coastal deserts of Peru and Chile on the South American continent. These are the Sechura and the Atacama respectively. Also in Peru is the Altiplano desert, which extends into Bolivia. This desert is known for its dry salt basins, which were left when ancient lakes evaporated. Patagonia in Argentina boasts large areas of cold semi-desert.
Chief among the African deserts, and probably the most famous of them all, is the Sahara. Extending from the Atlantic Ocean in the West, to the Red Sea in the East, and with an area of over nine million square kilometres - around the same size as the United States - the Sahara is the largest desert on Earth. In east Africa the Danakil, Ogaden, Nubian, Chalbi and Didi Galgalu deserts extend throughout Kenya, Sudan, Ethiopia, Eritrea and Somalia. Southern Africa is home to the Kalahari and Namib deserts, which together occupy large portions of Namibia, Botswana and South Africa.
The Gobi Desert in Mongolia and China is the fifth largest in the world. Tibet has substantial desert cover despite its very low temperatures, and many central Asian countries are subject to desert conditions. India, Pakistan, Iran and the Arabian Peninsula all contain large desert areas, including the Thar or Great Indian desert and the Arabian Desert.
Australia is the most arid continent on Earth. Its principal desert areas include the Simpson Desert, the Tanami Desert, the Great Victoria Desert, the Great Sandy Desert, and the Gibson Desert, and these are situated mainly in the centre of the country - the Outback.
The status of the vast continent of Antarctica is a matter of controversy. In many respects it resembles the deserts of other continents - particularly in its low annual rainfall - but many scientists feel dubious about including it in the same class as these.
The only true desert to be found along the 0° meridian line also happens to be world's largest. The Sahara covers most of north Africa, more than a third of the continent, and an area around the same size as the United States. It is a desert of extraordinary variety. Temperatures in excess of 55° C have been recorded in parts of Libya, while in some places frost can be seen during the winter. All the standard desert landscape types are present in the Sahara, from great fields of shifting sand dunes or ergs, to vast plains filled with rocks, known as reg.
Rainfall in most parts of the Sahara is scant and erratic - some areas endure several years without even a hint of a shower. In common with other desert regions, storms in the Sahara can be extremely localised, often affecting an area as small as 20 square kilometres. Strong, unpredictable winds are typical of the Saharan weather systems, and these have come to be known by names such as khamsin, sirocco, shahali, and simoom. These winds can blow for days on end, bringing with them vast amounts of dust and sand, which cover everything in their path and reduce visibility close to zero. From time to time, particularly powerful sandstorms can be extremely unpleasant and dangerous for anyone caught in them. Dust devils, which are like whirlwinds, also occur, hurling sand, dust, small animals and plants into the air.
The Sahara is crossed by the Nile and Niger rivers, which together support most of the desert's human population. Even so, it still presents a great obstacle to animal and plant life. In later sections we consider some of the animals and plants which inhabit the Sahara, and we look at how they have adapted so that they can live there.
Water is essential for all plants, so survival in arid environments is a real challenge. However, some of the ways in which desert flora have adapted are ingenious. Generally, they either avoid or endure periods of low rainfall. Ephemeral or annual plants survive periods where water is in short supply because their seeds germinate only after heavy rain, grow rapidly, and live their whole life-cycle in just a few days. These plants often produce large, brightly coloured flowers to attract the insects that are essential to their pollination.
Perennial plants take the alternative route, enduring the dry periods and making the most of the scarce water supplies. These plants utilise a number of interesting methods to survive.
Grasses
Grasses are extremely hardy plants and are found in most of the world's environments. They have extremely large and complex root systems that enable them to collect water over a wide area. During extremely hot and dry periods the parts of the plants that are above the surface may wither and die, but the root systems remain alive. They reproduce by growing new stems. The extensive root systems of grasses play an important role in keeping the sparse desert soils together.
Geophytes
These plants survive the harsh desert conditions by remaining underground for most of the time, often as bulbs. When rain does come they quickly produce stems and flowers.
Succulents
These plants take in large amounts of water during times of plenty and store it for use during times of drought. They have a low surface area in comparison with their volume, which reduces water loss due to evaporation. Succulents typically have vast, shallow root systems in order to make the most of any water reaching the ground.
Like most other plants, succulents use photosynthesis to convert carbon dioxide into organic compounds by means of the energy of sunlight. The best-known succulents are cacti. These are found almost exclusively in North and South America, and are often spectacular in shape and size. The most celebrated are the saguaro, which can grow up to 15 metres over many years. The African equivalents of the cacti are the euphorbia, which resemble their American counterparts in many ways.
Shrubs and stunted trees
Unlike succulents, these plants do not differ greatly from those found in other regions, and use more conventional methods of surviving drought. They are small trees or shrubs, normally with very small leaves that are often shed during the hottest part of the year. Their root systems are extensive, and can often penetrate as far as 50 metres into the ground in search of water. Sometimes they have short thick trunks that act as reservoirs for excess water taken up during wet seasons. Examples of this kind of plant are the tamarisk, the creosote bush, the acacia, the mesquite, and some species of eucalyptus. Like succulents, they grow slowly.
Desert plants often protect themselves against being eaten by animals by growing spikes or other deterrents.
If the desert is an inhospitable environment for plants, then it is even more so for animals. Shortage of water, lack of food, and extremes of temperature are just some of the obstacles that animals must overcome if they are to survive. However, all types of animal life are found in the world's deserts, from the smallest protozoa right up to large carnivores. Even fish are found in some parts of the North American desert!
Where maximum daily temperatures are consistently high, animals are at risk from overheating and water loss. To avoid this problem, animals will either spend most daylight hours in the shade of rocks or vegetation, or they will burrow into the sand where the temperature remains almost constant. The other alternative is to adapt to the high temperatures.
Did you know that:
• deserts cover a third of the earth's surface
• 13 per cent of the world's population live in deserts
• evaporation rates in deserts are often 20 times the annual precipitation
• a temperature of 58° C (136.4° F) has been recorded in the shade at Azizia in Libya
• no rain fell for over 40 years in the Atacama desert in Chile
• night temperatures in some deserts can fall below freezing
• sand dunes can reach heights of up to 300m
• sand covers less than 20 per cent of the world's desert areas
• dust from the Sahara has occasionally been carried as far afield as the UK and Germany
• the Sahara Desert accounts for around 8 per cent of the world's land area
• there are an estimated 1,200 species of plant to be found in the Sahara
• in the last 50 years the Sahara has spread south to cover an extra 65 million hectares
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