Environment

I

Introduction

Environment, thin layer of life and life-supports called the biosphere, including the Earth’s air, soil, water, and living organisms.

II

Constituents of the Environment

The atmosphere that shelters the Earth from excessive amounts of ultraviolet radiation and enables life to exist is a gaseous mixture of nitrogen, oxygen, hydrogen, carbon dioxide, water vapour, other elements and compounds, and dust particles. Heated by the Sun and by radiant energy from the Earth, the atmosphere circulates about the planet and modifies temperature differences. Of the Earth’s water, 97 per cent makes up the oceans, 2 per cent is ice, and 1 per cent is the fresh water in rivers, lakes, groundwater, and atmospheric and soil moisture. The soil is the thin mantle of material that supports terrestrial life. It is the product of climate, parent material such as glacial till and sedimentary rocks, and vegetation. Dependent on all these are the Earth’s living organisms, including human beings. Plants use water, carbon dioxide, and sunlight to convert raw materials into carbohydrates through photosynthesis; animal life, in turn, is dependent on plants, in a sequence of interconnected relationships known as the food web.

Throughout its long history, the Earth has changed slowly. Continental drift (the result of Plate Tectonics) separated land masses, oceans invaded and retreated from the land, and mountains rose and were worn down, depositing sediments along the edges of seas (seeGeology). Climates warmed and cooled, and life forms appeared and disappeared as the environment changed. The most recent major environmental event in the Earth’s history occurred in the Quaternary period, during the Pleistocene epoch (between 2.5 million and 10,000 years ago), also known as the Ice Age. The subtropical climate was destroyed and the face of the Northern hemisphere was reshaped. Ice sheets advanced and retreated four times in North America and three times in Europe, swinging the climate between cold and temperate, influencing vegetation and animal life, and ultimately forming the environment as it exists today. The epoch following the Pleistocene is known variously as the Recent, the Postglacial epoch, and the Holocene.

III

Environmental Problems

The species Homo sapiens—that is, human beings—appeared late in the Earth’s history, but was ultimately able to modify the Earth’s environment by its activities. Although human beings apparently first appeared in Africa, they quickly spread throughout the world. Because of their unique mental and physical capabilities, human beings were able to escape the environmental constraints that limited other species and to change the environment to meet their needs.

Although early human beings undoubtedly lived in some harmony with the environment, as did other animals, their retreat from the wilderness began with the first, prehistoric agricultural revolution. The ability to control and use fire allowed them to modify or eliminate natural vegetation, and the domestication and herding of grazing animals eventually resulted in overgrazing and soil erosion. The domestication of plants also led to the destruction of natural vegetation to make room for crops, and the demand for wood for fuel denuded mountains and depleted forests. Wild animals were slaughtered for food and destroyed as pests and predators.

While human populations remained small and human technology modest, their impact on the environment was localized. As populations increased and technology improved and expanded, however, more significant and widespread problems arose. Rapid technological advances after the Middle Ages culminated in the Industrial Revolution, which involved the discovery, use, and exploitation of fossil fuels, as well as the extensive exploitation of the Earth’s mineral resources. With the Industrial Revolution, humans began in earnest to change the face of the Earth, the nature of its atmosphere, and the quality of its water. Today, unprecedented demands on the environment from a rapidly expanding human population and from advancing technology are causing a continuing and accelerating decline in the quality of the environment and its ability to sustain life.

A

Carbon Dioxide

One impact that the burning of fossil fuels has had on the Earth’s environment has been the increase of carbon dioxide (CO₂) in the Earth’s atmosphere. The amount of atmospheric CO₂ apparently remained stable for millennia, at about 260 ppm (parts per million), but over the past 100 years it has increased to 350 ppm. The significance of this change is its potential for raising the temperature of the Earth through the process known as the greenhouse effect. Carbon dioxide in the atmosphere prevents the escape of outgoing long-wave radiation from the Earth to outer space; as more heat is produced and less escapes, the temperature of the Earth increases.

A significant global warming of the atmosphere would have profound environmental effects. It would speed the melting of polar ice caps, raise sea levels, change the climate regionally and globally, alter natural vegetation, and affect crop production. These changes would, in turn, have an enormous impact on human civilization. Since 1850 there has been a mean rise in global temperature of about 1° C (1.8° F). Most scientists have predicted that rising levels of CO₂ and other “greenhouse gases” will cause temperatures to continue to increase, with estimates ranging from 2° to 6° C (4° to 11° F) by the mid-21st century. However, some scientists who research climate effects and trends dispute the theories of global warming, and attribute the most recent rise to normal temperature fluctuations.