Humans evolved on earth, with its atmosphere, land and water systems, and types of climate, in a way that permits us to cope reasonably well with this particular environment. Being intelligent and inquisitive, humans have not only investigated the environment extensively, but have done many things to change it. Other living things also change their environment; the roots of trees can crack rocks, and the herds of elephants in present-day African game parks are uprooting the trees and turning forests into grasslands. We are beginning to understand that microorganisms play a major role in determining the nature of our environment, not only through their actions on organic material, but also in systems traditionally regarded as strictly inorganic—and not just at the earth's surface, but also at considerable depths. However, no other living things can change their environment in so many ways or as rapidly as humans. The possibility exists that we can change our environment into one that we cannot live in, just as the African elephant may be doing on a smaller scale. The elephants cannot learn to stop uprooting the trees they need for survival, but we should be capable of learning about our environment and about the problems we ourselves create.

One vital problem (among many) that we must solve is how to continue our technologically based civilization without at the same time irreversibly damaging the environment in which we evolved. This environment, which supports our life, is complex; the interrelations of its component parts are subtle and sometimes unexpected, and stress in one area may have far-reaching effects. This environment is also finite (hence the expression "spaceship earth"). That being so, there is necessarily a limit before significant environmental changes take place. Our modern technological society, coupled with largely uncontrolled population growth, places extreme stress on the environment, and many environmental problems come from prior failure to understand the nature of these stresses and to identify the limits. Many problems also come from refusal to accept that there are limits. Even for a component of our environment that is so large that for all practical purposes it can be taken as infinite, such as the water in the oceans, there are limits. We cannot destroy a significant amount of ocean water, but we can contaminate it.

Many environmental problems and processes are chemical, and to understand them, understanding the basic chemistry involved is necessary (but not necessarily sufficient). This is certainly true for those who wish to solve environmental problems, but also for those who need to make more general decisions that may have environmental consequences. It is important as well for those who would like to have some basis for understanding or evaluating the many, often contradictory, claims made regarding environmental problems. While the basic science underlying most environmental processes is well understood, many details are not. Too often, this lack of knowledge of the details of complex interactions is taken as an excuse to deny the validity of sound general conclusions, particularly when they run counter to political or religious dogma.

In this book, we shall use the term "environment" to refer to the atmosphere-water-earth surroundings that make life possible; basically this is a physicochemical system. Our total environment consists also of cultural and aesthetic components, which we will not consider here. Neither shall we deal extensively with ecology—that is, the interaction of living things with the environment—although some effects of the physicochemical system on life, and conversely, of living things on the physicochemical environment, will be included. Indeed, organisms play a major role in both organic and inorganic processes in the environment, and living things have made our environment what it is. Most obviously, the biological process of photosynthesis created the oxygen atmosphere in which we live.

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