Desert Survival

Why the deserts sit where they do

Look at a world map of hot deserts and they are not scattered: the Sahara, the Arabian, the Thar, the Kalahari, the Atacama and the Australian deserts all sit in two bands, roughly 15 to 30 degrees north and south of the equator. That is not coincidence. Intense heating at the equator lifts air which cools as it rises, dumping its moisture as tropical rainfall. That air, now dry, drifts polewards high in the atmosphere and sinks back down at around 30 degrees. Sinking air warms, and warming air can hold more moisture rather than releasing it, so it takes water out of the ground instead of putting it in. High pressure, clear skies, almost no rain. The working definition of a desert is land receiving under 250 mm of rain a year, which is less than London gets in three months. The Sahara covers roughly 9 million square kilometres, about the area of the United States. But the shortage of water is only half the difficulty. With no cloud and almost no water vapour, there is nothing to hold heat in at night, so the ground radiates it straight back to space: daytime air temperatures above 40 degrees Celsius can be followed by nights near freezing, a swing of 30 degrees or more in a single day. Add thin, stony soils with almost no organic matter, and rain that arrives rarely but violently enough to strip those soils away in flash floods, and you have an environment that sounds uninhabitable. It is not. Plants and animals are extraordinarily well adapted to it, and the Thar Desert in India and Pakistan carries around 83 people per square kilometre, making it the most densely populated desert in the world. This module covers both: how life survives here, and what happens when people try to develop it.

Five desert terms with their numbers

The first three describe the environment, the fourth describes how plants beat it, and the fifth is the problem that arrives with the solution.

Order the chain that makes a desert

These six stages explain why the great hot deserts lie where they do. Put them in order, starting at the equator.

• Intense heating near the equator warms the air at the surface
• That air rises, and cools as it climbs
• Cooling forces it to shed its moisture, giving the tropics their heavy rainfall
• The now dry air travels away from the equator high in the atmosphere
• At around thirty degrees of latitude it sinks back towards the surface
• Sinking air warms, so instead of releasing moisture it draws it out of the ground

Why does the temperature swing so far?

A hot desert can be above 40 degrees Celsius in the afternoon and near freezing before dawn. Which answer explains that correctly?

• There is almost nothing in the air or on the ground to store heat. Clear skies and very low humidity mean the sun's energy arrives almost unblocked by day, and with no cloud layer and little water vapour to absorb and re-radiate it, that heat escapes straight back to space after sunset. Dry sand also holds heat poorly compared with moist soil or water, so the surface cools fast, producing ranges above 30 degrees Celsius in a single day.
• The sun is much closer to the desert than to other places.
• Cold winds blow in from the poles every night.
• The sand absorbs the heat and locks it away permanently.

The Thar: four opportunities, four challenges

AQA questions on this element ask for both sides, and the mark scheme punishes a one-sided answer. This is the Thar Desert, spanning north-west India and south-east Pakistan, roughly 200,000 square kilometres and home to more people per square kilometre than any other desert. Note how several rows are the same fact read twice.

Match each living thing to how it copes

Balance, and the trap in the camel

The single commonest way to lose marks on hot deserts is the one-sided answer. A question that says "development opportunities and challenges" wants both, and an answer that gives four opportunities and one thin challenge cannot reach the top level however good the four are. Plan for balance before you start writing: two developed points on each side beats five on one. The strongest structure available to you is the fact that cuts both ways. The Indira Gandhi Canal is the best example on this case study: it is simultaneously the largest single opportunity in the Thar, because it made commercial farming possible where there had been sand, and a serious challenge, because irrigating in that heat has waterlogged and salinised some of the land it created. Writing one fact from both directions shows the examiner judgement rather than recall. Two accuracy points worth having. A camel's hump stores fat, not water; the fat is a food reserve, and keeping it in one lump rather than spread under the skin lets the rest of the body lose heat. And cactus spines are not only defensive: they are modified leaves, and having almost no leaf surface is precisely how the plant avoids losing water. Examiners see the water-filled hump every year. Finally, mind your command words. "Describe the characteristics of a hot desert" wants the features and no reasons. "Explain how plants are adapted" wants the mechanism: not that a cactus has spines, but that spines present almost no surface for water to escape from.

Complete the hot desert facts

A desert is land receiving under _____ mm of rain in a year, and the great hot deserts lie in two belts around 15 to 30 degrees from the equator, where air that rose and rained at the tropics sinks again and _____ as it descends. With no cloud to trap heat overnight, the daily temperature range can exceed _____ degrees Celsius. Plants cope by storing water in swollen stems, by reaching it with roots driven tens of metres down, or by reducing leaves to _____ so that almost no surface is exposed. In the Thar, the _____ km Indira Gandhi Canal made commercial farming possible, but irrigating in such heat has left salt behind in some of the soil it watered.

Spot the three accurate desert claims

Select the THREE statements that are true.

• A camel's hump stores fat rather than water, and concentrating that fat in one place rather than spreading it under the skin helps the rest of the body shed heat.
• Hot deserts are not empty: the Thar carries around 83 people per square kilometre, more than any other desert, supported by irrigation, mining, energy and tourism.
• The Indira Gandhi Canal is both an opportunity and a challenge, because the irrigation that made farming possible has also caused waterlogging and salinisation in places.
• Hot deserts receive no rainfall at all.
• Almost nothing lives in a hot desert because the conditions are impossible.
• Hot deserts stay hot through the night because sand releases the heat it stored during the day.

A balanced 9-mark answer, marked

"The challenges of developing a hot desert environment outweigh the opportunities." Use a case study of a hot desert to assess this view. [9 marks] The Thar Desert, covering around 200,000 square kilometres of north-west India and south-east Pakistan, offers real opportunities, which is why it supports about 83 people per square kilometre and is the most densely populated desert in the world. Its clearest advantage is the very thing that makes it hostile: relentless sunshine. Rajasthan now hosts the Bhadla solar park, roughly 2,245 MW across about 56 square kilometres, and the Jaisalmer wind park at around 1,600 MW, generating power on land that has no competing use. Mineral extraction adds to this, with gypsum, phosphorite and the limestone quarried near Jaisalmer, while tourism built around the fort and camel safaris puts money directly into local hands. Against this, the challenges are severe and some of them are worsening. Summer temperatures of 45 to 50 degrees Celsius restrict outdoor work to the beginning and end of the day and actually reduce the efficiency of solar panels, so the heat both creates and limits the opportunity. Water is the harder constraint: with roughly 120 to 240 mm of rain a year, nearly all of it in three monsoon months, groundwater is being extracted faster than it is replenished. The 650 km Indira Gandhi Canal shows the difficulty most clearly, because the irrigation that turned sand into wheat and cotton has also produced waterlogging and salinisation, degrading some of the land it created. Overall I partly agree. The challenges are more fundamental, since water shortage limits every activity while the opportunities are specific to particular sites, but they have not outweighed the opportunities in practice: the population of the Thar is growing, not shrinking. The honest judgement is that development here is possible but conditional, and that whether it lasts depends on whether the water can be managed. What earned the marks: both sides developed with named places and real figures rather than listed; the canal and the sunshine each read from both directions, which demonstrates judgement; and a conclusion that answers "assess this view" with a qualification and a criterion instead of simply agreeing.

Find the two that explain rather than describe

Five sentences about desert plants. Three of them only state a feature. Tap the TWO that explain how the feature actually works.

• A cactus carries spines instead of leaves, which leaves it almost no surface area from which water can escape into the air.
• A saguaro cactus can grow to over 12 metres tall.
• Many desert plants open their pores only after dark, so that the exchange of gases happens when the air is cool and far less water is lost.
• Desert plants often have a thick waxy coating on the outside.
• Vegetation in a hot desert is sparse and widely spaced.

Build the two-sided sentence

Assemble a sentence about the canal that shows judgement rather than listing.

Four decisions in the Thar

Each stage is a genuine development choice in a hot desert. Pick the option that fits the conditions, and notice that every one of them has a cost.

• Your village has grown on the back of canal irrigation, but the water table beneath the fields is rising and white crusts are appearing on the surface of the soil after each hot spell.
• A company wants to build a large solar installation on open desert near your district. The land grows nothing, but the site is remote and summer surface temperatures are extreme.
• Camel safaris out of Jaisalmer are growing fast. Operators are pushing further into the dunes, and there is pressure to build hotels with pools.
• A quarry near Jaisalmer needs to raise output. Afternoon temperatures in June regularly reach the high forties and two workers have already been treated for heat exhaustion.

Assess development in a hot desert

Using a named hot desert, assess the extent to which the opportunities for development outweigh the challenges. [9 marks]

• Shows the examiner you have a real case study
• Solar and wind capacity installed; the length of the canal and what it grows
• Temperature, water supply and accessibility
• The canal that created farmland and then salinised some of it
• Not simply declaring one side the winner