Even after conducting a successful experiment to test a hypothesis, scientists may still have an incomplete picture of a natural phenomenon. For example, the scientists at the Duke experimental forest proposed that higher levels of atmospheric CO₂ would increase growth of forest vegetation. Indeed, they found that root growth increased in their experimental plots. Case closed, right? Not exactly. While one well-conducted experiment can provide scientific evidence in support of or against a hypothesis, uncertainty will remain in regard to its generality. For example, would an increase in atmospheric CO₂ result in increased root growth at other forests? With additional research using different types of experiments under different conditions, the information supporting a particular hypothesis may increase.

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At some point in the history of the investigation of a phenomenon, scientists may develop a theory (Figure 1.10). A theory is a general explanation of a phenomenon that has been supported by repeated observation, experimentation, and modeling of specific hypotheses. Some of the most famous and well-established theories have names such as the theory of evolution by natural selection or the germ theory of disease, both of which have been developed over the course of testing hypotheses about a wide variety of species and diseases. More recently, when we talk about human-caused climate change as a scientific theory, we mean that the overwhelming majority of evidence supports the view that Earth’s climate is changing due to human activity.

There are many sources of scientific uncertainty. Looking back at the story of the discovery of the ozone hole, we can see how science gradually reduced uncertainty about this phenomenon. The first source of scientific uncertainty is simply how much we already know about a system. In building their model of CFCs, the chemists Molina and Rowland acknowledged a number of areas of uncertainty, including a lack of knowledge of the rates at which molecules moved through the stratosphere and the rate at which chlorine produced in the stratosphere would be removed. Because of these uncertainties and others, Molina and Rowland could not predict when environmental problems due to depletion of stratospheric ozone might begin.

A second potential source of uncertainty is measurement error, as a result of either human or instrument error. For instance, when the British Antarctic Survey first began recording low levels of stratospheric ozone with their Dobsonmeter (Figure 1.11), they considered the possibility that the machine was malfunctioning. To eliminate this possibility and reduce uncertainty, the team sent for a backup Dobsonmeter, which confirmed the first set of results.

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The level of certainty can also be limited by the methods used in a scientific study. The original report on ozone depletion was limited to the area of the atmosphere that could be sampled by a Dobsonmeter from the Antarctic base. Consequently, the extent of the atmosphere that was depleted of ozone was uncertain until NASA conducted satellite-based measurements the following year (Figure 1.11). Because of the nature of scientific inquiry, there will always be some degree of uncertainty in our understanding of natural phenomena.

Because uncertainty is characteristic of science, scientists must cultivate a critical view of their disciplines, their hypotheses, prevailing theories, and scientific understanding in general. This skepticism, however, is a double-edged sword in environmental science. While it drives the process of science, it can also slow action on global environmental problems. Ozone depletion, for instance, is a situation in which there is a significant probability of environmental damage. Though uncertainties remained, particularly in the early days of ozone depletion research, would it have been wise to take no action?

Science cannot eliminate uncertainty about nature entirely, which is why we often fall back on the precautionary principle. At a 1998 conference at the Wingspread Conference Center in Racine, Wisconsin, 32 environmental scientists expressed the principle as follows: “Where there are threats of harm to human health or the environment, precautionary measures should be taken even if some cause and effect relationships are not fully established scientifically.” You may have heard the precautionary principle as a piece of folk wisdom: “Better safe than sorry.”

The precautionary principle shifts the burden of proof from those supporting environmental protection to those recommending actions that may be harmful or may result in harm if no action is taken. However, because such a position may hurt businesses or other stakeholders, it often receives significant opposition. For instance, measures to control ozone-depleting CFCs were initially met with strong resistance from those in the industry.