Absence of certainty is not synonymous with absence of risk.

Bad things happen. Although human society has been remarkably successful in developing techniques to preempt some bad events, the successes have raised expectations. As we have devised ways of reducing some risks, smaller risks seem more significant. Simultaneously, technological solutions to some problems have created new risks of their own. The environmental risks that have resulted, both from growth in human population and technologies, fall into two main categories: 1) the possible consequences of misplaced wastes and 2) the wholesale shift of land area from natural systems that produce many different ecosystem services to human-managed systems which produce, at most, one or two. The significance of these changes to human society is the subject of much debate. The debate has, at times, become so polarized that it is difficult to distinguish what is known from what is supposed. The basic concepts of risk and uncertainty are keys to following this debate. Every possible change or action has risks and benefits. Risks are those consequences that human society finds undesirable, and benefits are the desirable consequences. However, risks and benefits both have three components: 1) how likely is it (probability)? 2) how good or bad is it and how large a spatial and temporal area does it affect (magnitude)? and 3) what is the quality of the information on which estimates of probability and magnitude are based (uncertainty)? Uncertainty is a component of both risks and benefits that results from imperfect knowledge about the probability or magnitude of the consequences of a change. While high uncertainty may obscure both the probability of a risk and the magnitude of harm, uncertainty does not eliminate risk. Unrecognized risks are still risks; uncertain risks are still risks; and denied risks are still risks. For example, before the research of Pasteur, Lister, and Koch in the late 1800s, the concept that germs could cause disease was unrecognized. Surgeons routinely operated without washing their hands. The risk of dying from surgery was recognized and weighed against possible benefits. However, many people died, and others gave up possible benefits of surgery because the risk of infection was unabated. Similarly, in the British Navy, it was observed that lime juice reduced the incidence of scurvy among the crews of sailing ships on long voyages (hence "limeys"). Even so, no move to put lime juice on every ship was made for decades. As we deal with potential risks to …