(introduction...)

People know that their actions can have considerable effect on local environments. When a campfire is lit, hundreds of hectares of forest may disappear in flames. Building a dam can flood an extensive low-lying area. Cities affect the local climate, increasing average temperatures and changing other characteristics of the overlying atmosphere. Large lakes can be rendered lifeless by contaminants discharged into them.

On a regional scale, it is not as easy to recognize environmental changes caused by human activities. However, it is now becoming clear that whole regions downwind of large industrial areas are being strongly affected by acid rain, that some species have disappeared from fishing regions, and that overgrazing or deforestation is affecting the regional climates of the Sahel and Amazon.

It is even more difficult to imagine the effects of anthropogenic action on the global environment. The Earth is so large and the atmosphere so extensive that past experience suggests that human activities will never reach the dimension necessary to produce changes on a planetary scale. Today, however, changes are occurring at an almost exponential rate, and many past theories may no longer apply.