I low should nations think about worst-case scenarios? If people's intuitions misfire, and if private and public decisions go badly wrong, can we devise a framework that might help? The Precautionary Principle, now used in many international documents, is often said to provide an answer.1 Consider a few examples:

(1) The closing Ministerial Declaration from the United Nations Economic Conference for Europe in 1990 asserts, "Where there are threats of serious or irreversible damage, lack of full scientific certainty should not be used as a reason for postponing measures to prevent environmental degradation."2

(2) The 1992 Rio Declaration proclaims, "Where there are threats of serious or irreversible damage, lack of full scientific certainty shall not be used as a reason for postponing cost-effective measures to prevent environmental degradation."3

(3) The United Nations Framework Convention on Climate Change declares, "Where there are threats of serious or irreversible damage, lack of full scientific certainty should not be used as a reason for postponing [regulatory] measures, taking into account that policies and measures to deal with climate change should be cost-effective so as to ensure global benefits at the lowest possible cost."4

(4) The Final Declaration of the First European "Seas At Risk" Conference states that if "the 'worst-case scenario' for a certain activity is serious enough then even a small amount of doubt as to the safety of that activity is sufficient to stop it taking place."5

Whatever the precise wording of the Precautionary Principle, worst-case scenarios, and the threat of catastrophic harm, lie at the heart of countless discussions about how to deal with hazards to safety, health, and the environment. For terrorism, hurricanes, war, ozone depletion, avian flu, and climate change, potential catastrophe plays a large role in private and public behavior. Many people urge a kind of One Percent Doctrine for all of these risks. They want to know: If there is a 1 percent risk that a military squadron will be destroyed in some battle, shouldn't we take extra steps to protect our soldiers? If there is a small chance of a devastating hurricane in a major city in the United States, shouldn't the government do a lot to protect its citizens? If we cannot exclude the possibility of a serious outbreak of the avian flu in Europe, shouldn't European governments be acting, right now, to eliminate that possibility?

If we focus on the risk of catastrophe, a distinctive version of the Precautionary Principle is possible: When risks have catastrophic worst-case scenarios, it makes sense to take special measures to eliminate those risks, even when existing information does not enable regulators to make a reliable judgment about the probability that the worst-case scenarios will occur. I shall call this the Catastrophic Harm Precautionary Principle.

This principle is at best a start, if only because it is lamentably vague. It does not define "special precautionary measures," and everything turns on exactly how special they are. Nor does the principle specify answers to three key issues: the threshold information that would trigger the principle; the role of costs; and how regulators should incorporate whatever information exists about the probability of catastrophe. We would not want to take infinitely burdensome steps to prevent worst-case scenarios, even when they are potentially catastrophic. After the attacks of 9/11, no one seriously suggested a one-year moratorium on all air travel within the United States, even though the Catastrophic Harm Precautionary Principle might seem to require that step. No one thinks we should respond to the risk of avian flu by banning international air travel. After the outbreak of AIDS, many people took precautions, but most of them did not cease sexual activity altogether.

My goal here is to explore various versions of the Catastrophic Harm Precautionary Principle, to show how they might be defended, and to qualify them. As a first approximation, we want to identify both the probability and the severity of worst-case scenarios; and we want to compare the expected value of precautions against their expected costs. Both expected value and expected costs should be measured in terms of well-being, not in terms of money. What matters is how lives are actually affected, not how much cash is gained or lost.

Of course, the idea of well-being is deeply contested, and so too is the idea of welfare (which I shall use as a synonym). Suppose that climate change is expected to result in significant numbers of extinctions and millions of deaths of animals. How exactly should that influence our assessment of the effects of climate change on well-being? To say the least, people disagree about how to answer that question. Or suppose that certain restrictions on greenhouse gases will result in a 1 percent loss in Gross Domestic Product, producing identifiable increases in unemployment and poverty. If we focus on welfare, is that 1 percent loss acceptable when the result is to reduce climate change by a specified amount? Some people would like to answer such questions by reference to the effects on human capabilities, understood in terms of what people are able to be and to do.6 Those who endorse the "capabilities approach" contest many other understandings of well-being.

For present purposes, I shall use the idea of well-being (or welfare) without specifying its meaning; the idea should be taken as simply a placeholder for the best account. My hope is that for many problems, people who have different understandings of "well-being," or who are uncertain about the right understanding, can agree on the proper course of action notwithstanding their disagreements or their uncertainty. In this domain, as in so many others, we should be able to obtain incompletely theorized agreements on what to do—agreements on the right practice amidst disagreements or confusions about the specific theory that best justifies that practice.7

Whatever the right account of well-being, everyone should agree that if worst-case scenarios are exceedingly unlikely to come to fruition, then there are clear limits on how much we should do to eliminate them. Suppose that precautionary steps would impose huge burdens or that those very steps would subject millions of people to high probabilities of very-bad-case scenarios. If so, then doing everything we can to avoid the worst case is unlikely to be sensible. It is both necessary and possible, in short, to explore what is gained and what is lost by eliminating worst-case scenarios.

As we shall see, an analysis of the expected value of precautions makes sense, and is feasible, when probabilities can be assigned to the various outcomes, including those that qualify as catastrophic. Suppose, for example, that catastrophic harm is most unlikely to occur (say, below the threshold specified by any One Percent Doctrine) and that the cost of eliminating the relevant risk is very high

(say, because it would significantly increase unemployment and poverty). In those circumstances, precautions may be more trouble than they are worth. An analysis of the expected value of precautions should be relevant even if margins of safety are appropriate and even if that analysis is not conclusive—and even if distributional considerations greatly matter, so that regulators should consider whether the harm is likely to be visited on the most vulnerable people or those least likely to be in a position to protect themselves.

The most modest approaches to catastrophic harm focus on both the probability and the magnitude ofthe harm, and appreciate the fact that the expected value ofcatastrophes is often much worse than we anticipate. Such approaches can provide a helpful response to some of the problems explored in Chapter 1. Less modest versions offer an extra layer ofprecaution—a form ofregulatory insurance, designed to protect against the worst-case scenarios. The size of that extra layer depends on what we gain and what we lose from it—including other worst-case scenarios that the extra layer may itself create. For most problems, then, the Catastrophic Harm Precautionary Principle says, very simply, that both ordinary people and regulators should proceed by assessing the expected value of the options, including an appreciation ofthe distinctive harms associated with genuine catastrophes, and adding a margin of safety whose magnitude depends on its own expected value.

The question becomes far more difficult when we cannot assign probabilities to the various outcomes. Suppose, for example, that we have no idea how likely it is that climate change will produce catastrophic harm. My discussion of this problem is detailed and somewhat technical, but I shall suggest that even when we cannot assign probabilities, we can still identify a domain for a Catastrophic Harm Precautionary Principle, by attending to what is gained and what is lost by eliminating the most catastrophic outcome. If the worst-case scenario associated with one course of action is much worse than the worst-case scenario associated with a second course of action, and if we do not lose a great deal by following the second course of action, then the second course of action is the one to choose. The simplest lesson is this: Instead of ignoring worst-case scenarios, or automatically devoting extensive resources to their prevention, we should begin by asking exactly how bad they are, and how much is required to prevent them.

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