Category Geography

People have made many changes to the world they live in and some of these have had harmful consequences. As the world population increases, there is more need for bigger cities, new towns and farming land. Land has to be cleared, destroying the habitats of many animals and plants. Some of these are now extinct and others are in danger of becoming so. Hunting has also put animals in danger.

There is now nearly three times as much desert in the world as there was 100 years ago. This is partly due to farmers over-grazing their animals on the meagre grass and shrubs and to the clearance of woodland trees for firewood.

Industrialization is a major threat to the environment. Pollution is just one example. Factories discharge harmful chemicals into rivers and seas, killing fish and plants.

The production of chlorofluorocarbons (CFCs) is reckoned to be responsible for damaging the ozone layer, the protective layer in the Earth’s atmosphere that prevents harmful rays from the Sun reaching the surface. CFCs are synthetic chemicals used in aerosol sprays and refrigerators. When they are released into the atmosphere, they destroy the ozone. There is now a hole in the ozone layer above Antarctica.

Air pollution is caused by fumes and smoke from vehicles, factory chimneys and power stations. Poisonous gases, such as sulphur dioxide, mix with rain and mist to produce acids. When the acid rain falls, it kills plants over a wide area. Burning fuels also causes a build-up of carbon dioxide.

Carbon dioxide and CFCs are both greenhouse gases. In the right amounts, greenhouse gases in the atmosphere trap heat from the Sun so the Earth is not too hot or too cold. But if the greenhouse gases build up, too much heat is trapped and the Earth becomes warmer. This change in climate, known as global warming, could have disastrous effects. The ice in the Polar Regions may melt, causing severe flooding to low-lying areas. Changes in the climatic pattern worldwide could lead to violent storms and long droughts.

Some nations in the world are rich, while others are very poor. The gap between them seems to grow ever wider. Measured by the average income earned by people in the richest and poorest fifth of the world’s nations, the gap has grown from 30 to 1 in 1960 to nearly 80 to 1 today. Within both rich and poor countries, there are also great contrasts in wealth.

The wealthiest countries are those that have developed industries and services which can supply their own populations with all their needs. They do this either by producing these products and services themselves, or by importing them from other countries, paying for them by exporting goods. In poorer developing countries, people may produce only enough food to feed their families. Disease and climatic disaster may prevent even this.

The population of developing nations has grown greatly in recent years. Their high birth rates means even more mouths to feed.

South America has some of the fastest-growing cities in the world. They include the Brazilian super cities of Sao Paulo and Rio de Janeiro, both with populations of more than 10 million. People from the countryside flock to these cities to find work but there is nowhere for them to live. They build their own shantytowns (locally known as favelas) just outside the city by building shelters and shacks from any material that comes to hand. People who live in shantytowns cannot find work easily and so they are forced to work for very low wages.

The wealth of a nation can be measured by its gross domestic product (GDP).This is defined as the value of all the goods and services produced there, including those produced by foreign-owned firms. The Group of Seven (G7) are the seven leading industrial nations of the world. These nations – the United States, Japan, Germany, France, the United Kingdom, Italy and Canada – account for more than 60% of the world’s GDP. The country with the highest GDP per person in 1996 was Switzerland ($46,000). In the same year, the figure for Rwanda in Africa was $100. More than 1.3 billion people around the world live on less than one dollar a day.

Wealthy nations such as the G7 lend money to developing countries. However, the developing countries often find that, because of their low GDP, they cannot repay the loans and a big debt burden builds up. The largest foreign aid donor in 1996 was Japan with aid amounting to more than $15,000 million.

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Bridges have been used since ancient times to span deep ravines, rivers and other stretches of water. Early bridges were made of wood which rotted easily and could not span great distances. In some parts of the world, people made bridges from wood and rope. They had walkways made of wooden slats and rope handrails. Bridges made of stone may last for centuries. However, stone is heavy to transport and long bridges need to have many sturdy supports.

During the Industrial Revolution, which began in the 18th century, engineers began to build bridges made from iron. These were much more durable than wooden bridges and could span longer distances than stone ones. The first iron bridge was built at Coalbrookdale, England, in 1779.

Modern bridges are built using steel and concrete. They may carry roads or railways over rivers, wide estuaries or high valleys, or above other roads and railways. There are a number of different kinds of bridge design available, each of which may be used in a modern bridge according to the type of crossing required.

A beam bridge is one of the simplest and oldest designs. The beam is supported at each end. The earliest bridges were tree trunks or stone slabs laid across a stream supported by the banks on either side. A clapper bridge is a type of Beam Bridge which is supported from beneath by several columns in the river bed or ground. The deck may be flat or made from a hollow girder (truss) containing the road or railway.

An arch bridge—a very strong type—has a deck supported on an arch fixed to the banks. Some arch bridges are too low for river traffic to pass underneath so a part of their span may be a bascule or lift bridge.

This can be raised in the middle like a drawbridge so that ships can pass through.

A cantilever is a beam or structure that is fixed at one end only. Cantilever bridges are built in two halves, with each beam cantilevered out from the bank and each half balanced on a support in the river. There may be a short central span where the two halves meet.

Suspension bridges can span long distances. The towers of the bridge are built first and long steel cables are suspended from the towers. Lengths of cable called hangers are fixed to the suspending cables. The deck of the bridge is lifted into position and attached to the hangers. Suspension bridges are ideal for long, high spans as they do not require a row of supporting columns that may interfere with river transport. One of the most famous examples of a suspension bridge is the Golden Gate Bridge (1280 metres), near San Francisco in the USA, which was completed in 1937.

A cable-stayed bridge has diagonal steel cables running from high supporting masts at either end connected to the deck of the bridge. The weight of the deck is supported by the masts.

Picture Credit : Google