Introduction
Human impact on the environment can be explained in two important ways. One is how humans have adapted to and changed their environment to survive and make life more comfortable and convenient.  The other is the effects of these changes and adaptations.

Positive Changes/Adaptations
Humans have made many changes to their geographical situations to better suit their needs and wants.  Most of these changes/adaptations have had a positive impact on the lives of humans, but were not necessarily good for the environment.

Irrigation
Irrigation systems bring water from nearby sources, often rivers, to areas where crops are grown.  Early civilizations such as those in Egypt and China used irrigation systems to grow more food, enabling them to expand and grow.  In modern times, more advanced irrigation systems have been developed to grow crops in areas once thought barren.

Terrace Farming
Terrace farming involves cutting out flat areas (terraces) into near vertical slopes to allow farming.  Terrace farms appears as steps cut into a mountainside.  This adaptation allowed both the early Chinese, and the Inca of Mesoamerica to grow enough food for their large populations.

Roads
The development of roads allowed civilizations to exchange goods and services with other cultures as well as invade and conquer.  The Roman and Incan Empires built elaborate road systems throughout their empires.  These roads allowed them to maintain good communication and establish a strong, centralized government.  Today, the various systems of roads that crisscross most of the continents of the world continue to allow for good communication, trade, and cultural diffusion.

Canals & Dams
Canals are man made waterways used for trade and transportation.  The early Chinese civilization built an extensive canal network that allowed them to trade and communicate with most of their population.  In modern times, canals such as the Suez Canal linking the Red Sea to the Mediterranean Sea, and the Panama Canal linking the Atlantic and Pacific Oceans, have contributed heavily to global trade and communication.
Dams are man made structures that block water from flowing.  Dams can be used on rivers and streams, or as a protective measure against floods. Dams have been used throughout history for a variety of tasks.  An example would be the Aswan High Dam in Egypt.  The building of this dam has allowed the Egyptians to control the flooding of the Nile River, and has resulted in the creation of much new farmland.

Fossil Fuels
Fossil fuels are any carbon bases fuel derived from the decomposed remains of prehistoric plants and animals.  The burning of fossil fuels, such as oil, coal, peat, petroleum, and natural gas have allowed human to develop many different technologies to improve life.  The Industrial Revolution that changed the world was fuel by this resource.  Today, fossil fuels power our cars, heat our homes, and run the factories that manufacture everything we use in our lives.  However, recent science has suggested that the use of fossil fuels has damaged the environment, and many groups are pushing for cleaner forms of energy.  The Middle East is a major producer of fossil fuels, while the industrialized nations in Europe and North America are the major users.

Nuclear Power
Nuclear power is usually electrical power produced from nuclear fusion or fission.  Nuclear power is supposed to be cleaner to produce than the energy created by the burning of fossil fuels.  Whereas fossil fuels will someday run out, the ability to produce nuclear power should not.  However, nuclear power does have dangerous side effects.  Radioactive waste products must be stored somewhere, and the threat of nuclear accidents, like the incident at the Chernobyl power plant in Ukraine.

Effects on the Environment
Pollution
Pollution is the contamination of the environment by human acts.  Pollution is harmful to all living things and can take many forms including, air pollution from the burning of fossil fuels, and water and soil pollution form the dumping of waste products and the use of chemical fertilizers and pesticides. Pollution has caused health problems in humans including respiratory disease and different forms of cancer.  Pollution is also responsible for destroying various animals, plants, and insects as it destroys their natural habitat.

Ozone Layer
The Ozone Layer is the upper portion of the Earth's atmosphere that screens out most of the Sun's harmful ultraviolet radiation. Recent science has suggested that industrial air pollutants, such as chlorofluorocarbon, has damaged the ozone layer by creating a hole in it.  Through this hole, damaging UV radiation penetrates to the Earth's surface. Increased exposure to this radiation can cause skin cancer in humans, damage crops, and destroy the marine ecology.  Many nations around the world have ceased using CFCs in the production of industrial materials.

Deforestation
Deforestation is the widespread destruction of the world's forests.  One of the largest areas of destruction are the tropical rainforests.  These forest are cut down for the hardwood lumber, to clear space for farming, for building settlements, and for grazing animals.
 

The effects of this destruction include a change in weather patterns, continued buildup of CO_2, a greenhouse gas, and extinction of plants and animals, which will result in the destruction of entire ecosystems.  Many worldwide organizations are attempting to stop deforestation, but as most of it occurs in developing nations dependent on the financial revenues from such destruction, stopping it is very hard.

Global Warming
Global Warming is the rising of the average temperature worldwide.  Scientist are concerned about this because of the potential destructive effects of this phenomena.  Global warming, if it continues, will reach a point where the arctic glaciers begin to melt, causing worldwide floods. Scientists believe global warming is caused by the greenhouse effect, which is a build up of warm air in the lower atmosphere.  This occurs from the use of CFCs, the burning of fossil fuels, and deforestation. many countries around the world have been working to limit these destructive forces.

Desertification
Desertification is the process of fertile land being transformed into desert land.  This is generally resulting from human interaction either by deforestation or by the over grazing of farm animals.  As the plant life is destroyed, winds blow the fertile soil away, thus spreading the desert.  The Sahara Desert in Africa is spreading about 50 miles a year due to this process.  In North Africa, where this problem is most prevalent, attempts to halt desertification include reducing the use of the threatened land and improved irrigation systems.

Acid Rain
Acid rain happens when rain is polluted by airborne contaminants such as those left by the burning of fossil fuels, automobile exhaust, and the use of CFCs.  Acid rain is damaging to farmland, bodies of water, and can travel large distances due to strong winds.  Like other environmental problems, steps are being taken in the international community to stop the production of these pollutants.

Nuclear Power
The use of nuclear power and the building of nuclear weapons represents a very great threat to the environment.  The biggest threat comes from nuclear accidents, such as the accident at the Chernobyl Power Plant in Ukraine in 1986.  This accident release large amounts of radiation that not only affected the immediate area, but also was carried on strong winds across many countries in Europe.  The effects of this accident have to date been an increase in cancer victims, numerous birth defects, and the destruction of many acres of good land.
The other problem with nuclear power is the waste products produced.  Storage of this material and the potential for accidents with it are the concern of the major nuclear powers.  The dumping of this material into the sea or burial underground has been outlawed by international treaty, but many of these countries are still searching for safe disposal solutions.  As the use of nuclear power becomes more prevalent, these issues will have to be addressed.