- Aridity-state of land being deficient of moisture leading to little or no vegetation.
- Desertification-process in which desert like conditions slowly and steadily encroach on formerly productive agricultural land.

A Dry Arid Area (Image: Courtesy)

Natural Causes

• Low and unreliable rainfall below 250mm per annum causing little or no vegetation and absence of animal and biological life causing soil forming processes to be incomplete.
• High temperatures which cause high rates of evaporation which exceed evaporation or low temperatures which reduces air capacity to hold moisture causing a place to receive little or no rain.
• Where a place is washed by ocean currents causing moist onshore winds to cool and then drop moisture over the sea and reach the land as dry winds e.g. Kalahari when onshore westerlies cross the cold Benguela Current.
• Where relief barriers such as hills or mountains cause some areas to lie on the rain shadow hence rain winds drop most of their moisture on the windward side and they drop on the leeward side, are warmed and hold onto moisture causing dry conditions e.g.
Kalahari and Namib on the rain shadow of Drakensberg mountains.
• Location of some places very far from the sea causing them to be far removed from wet onshore winds e.g. Gobi Desert.
• Where hot dry winds blow over a region causing drying effect on land e.g. Harmattan over West Africa.
• Where cool air descends causing no rain because cool air has to rise before condensation takes place.

Human Activities

• When people clear forests which causes run-off to exceed infiltration which interferes with the water cycle.
• Keeping large number of animals which exceed the carrying capacity of land they eat vegetation leaving the land bear exposing the land to soil erosion.
• Poor agricultural practices such as over-cultivation, monoculture and slashing and burning which lead to soil erosion.
• Industrialisation which releases green house gases such as co2 to the atmosphere which absorb more heat making the earths temperature to rise.
• Reclamation of water logged areas which lowers the water table causing arid conditions to set in plants when plants can’t access ground water.
• Poor irrigation methods when evaporation takes place and salt from below are brought to the surface and are deposited on the top soil making the soil salty and hence unable to support plants

• Infertile soils which support little or no vegetation.
• Low agricultural production due to insufficient rainfall leading to famine.
• Shortage of water for domestic and industrial use which may also lead to shutting down of my sons.
• Migration of people from areas affected by aridity and desertification leading to population pressure and eventually conflicts.
• Destruction of vegetation which exposes land to soil erosion.
• Can lead to extinction of some plants and animal species causing loss of biodiversity.

1. Afforestation and reafforestation because trees protect soil from erosion, increase run off and release moisture to the atmosphere leading to increased rainfall.
2. Adopting soil conservation measures such as terracing, contour ploughing, planting cover crops etc.
3. Rearing a number of animals which is proportional to the carrying capacity of land.
4. Irrigating dry lands.
5. Introduction of energy saving stoves to reduce demand for wood fuel which will reduce deforestation.
6. Use of alternative sources of energy which don’t pollute the environment e.g. solar and water.
7. Introducing drought resistant crops in the arid areas.
8. Controlling industrialisation by setting laws governing pollution.

Natural Causes
1. Variations in the Earths Orbital Characteristics
- Changing or earths orbital characteristics within 1000 years from elliptical (aphelion) to nearly circular (perihelion) when the earth is nearest to the sun and receives maximum solar energy and back to elliptical when the earth is farthest from the sun and receives least solar energy.
2. Variation in the Atmospheric Carbon Dioxide
- When natural rise in temperature cause carbon dioxide held up in cold ocean waters to be released to the atmosphere after oceans warmed.
3. Volcanic Eruptions
- When large quantities of volcanic ash and dust thrown out of the ground block some of the solar insolation from reaching the earths surface causing temperatures on the earths surface to drop for a short period.
- When sulphur dioxide given off during volcanic eruptions reacts with water vapour forming a bright layer within stratosphere reducing the amount of solar radiation reaching the surface by reflecting some of it back which also lowers temperatures on the surface.
4. Variation in Solar Output
- Changes in the amount of solar energy given off by the sun whereby at times its less causing drop of temperature on the earths surface and at others its gives off more causing rise in temperature on the surface.

Human Causes

1. Burning of fossil fuels in industries, transportation, electricity generation etc. which contributes 65% of additional co2 in the atmosphere which is the main green house gas.
2. Burning of vegetation e.g. in shifting cultivation and forest fires which also adds co2 in the atmosphere.
3. Clearing large tracts of forests foe agriculture, settlement etc. which reduces the main disposal system for CO² from the atmosphere by photosynthesis.
4. Industrial developments which add gases like methane, nitrous oxide and those containing chlorine and chlorofluorocarbons which damages ozone layer which filters a greater percentage of ultra violet radiation given off by the sun which causes the average temperatures on the earth to rise.

Consequences of Climate Change

1. Global warming due to green house effect by gases added in to the atmosphere and destruction of ozone layer.
2. Increased rainfall as a result of high temperatures causing high rates of evaporation causing wet areas to become wetter and dry areas to become drier.
3. Effect on agriculture by causing crop growing areas to shift to cooler altitudes and latitudes e.g. wheat growing areas of Canada shifting to the poles and causing dropping or failure of crop yields in area where temperatures have increased.
4. Water shortage when climate becomes drier causing less water to infiltrate underground and hence less water to feed rivers.
5. Submergence of coastal areas causing flooding when Antarctic and Arctic glaciers melt and water is added to the oceans.
6. Heat waves due to increased temperature which leads to death of people.
7. Receding and disappearance of ice caps on mountains e.g. Mt. Ruwenzori.
8. Abnormal growth of plants due to increased amounts of co2 causing increased rate of photosynthesis which may lead to increased yields of major crops, poor soils due to soils having to sustain high rates of plant growth.
9. increased levels of ultra violet radiation which causes human diseases such as skin cancer, lowering crop production by slowing photosynthesis and germination, lowering fish population by damaging plankton which fish eats and degradation of paint and plastics.

Solution to Climate Change

• Afforestation and reafforestation.
• Use of energy saving stoves to reduce the rate of deforestation.
• Use of alternative sources of energy which are environmentally friendly e.g. solar and water instead of fossil fuels.
• Proper maintenance of vehicle to reduce emissions from their exhausts.
• Use of public transport to reduce the amount of fossil fuel used and hence the amount of co2 added into the atmosphere.