The state of our planet's environment at any given instant is defined by a global system made up of many interacting components that are constantly changing. Some changes take place over days, months or years, others on time scales of centuries and eons. Some occur over small areas of the earth's surface, while others affect large regions or the whole planet.

For most of the earth's history, the principal components of this global system were the air, water, land, and ice that comprise the physical environment of our planet and the plants, animals, and humans living in that environment. Over the last century, however, human activities have had an increasingly important role in global environmental change.

Human impacts on the environment have increased enormously as the world's population has grown and the scale of human activities such as industry, agriculture, and extraction of natural resources has increased. Industrial emissions into the atmosphere can influence climate. Changes in land use, such as clearing forests for agricultural production, can disrupt natural ecosystems and affect the chemistry of the atmosphere.

Human activities affect the environment, and human life and society are affected in many important ways by changes in the global environmental system. Droughts cause crop failures, food shortages, malnutrition and starvation. Persistent drought can turn fertile agricultural land into desert. Shifts in ocean circulation and temperature affect the productivity of fisheries. Melting of polar ice could raise ocean levels enough to threaten coastal cities. Climate variability may bring more severe storms and increased loss of lives and property to some regions and more benign weather to others.

Global environmental change is one of the greatest challenges that humanity faces today. Those who make policy and decisions for our society need better tools to help them face this challenge. Those tools must be forged through improved understanding of the behavior of the global system that defines the environment of our planet and the options that are available for responding to changes in this complex system.