The dichotomy of the 20th century is giving way to a new unity: the idea that a sustainable environment and a sustainable economy may be compatible, that people and nature are intertwined, and that success for one involves success for the other. Science and values come to the forefront when we think about what action to take about a perceived environmental problem for which the science is only partially known. This is often the case because all science is preliminary and subject to analysis of new data, ideas, and tests of hypotheses. Even with careful scientific research, it can be difficult, even impossible, to prove with absolute certainty how relationships between human activities and other physical and biological processes lead to local and global environmental problems, such as global warming, depletion of ozone in the upper atmosphere, loss of biodiversity, and declining resources.
For this reason, in 1992 the Rio Earth Summit on Sustainable Development listed as one of its principles what we now call the Precautionary Principle. Basically, it says that when there is a threat of serious, perhaps even irreversible, environmental damage, we should not wait for scientific proof before taking precautionary steps to pre- vent potential harm to the environment. The Precautionary Principle requires critical thinking about a variety of environmental concerns, such as the manufacture and use of chemicals, including pesticides, herbicides, and drugs; the use of fossil fuels and nuclear energy; the conversion of land from one use to another (for example, from rural to urban); and the management of wildlife, fisheries, and forests.
One important question in applying the Precautionary Principle is how much scientific evidence we should have before taking action on a particular environmental problem.