It’s a familiar argument: we know that putting a price on carbon will impose economic costs, but we can’t be absolutely sure that major climate change will happen. Therefore, we shouldn’t impose a carbon price, or at least we should avoid doing so in a recession, and be very reticent to do so at any point. The argument strikes many as logical and wise.

It is neither. And it won’t help make good policy or make progress towards consensus on what good climate policy should be.

At its core, the argument claims that any uncertainty about climate change means we should either give up, or at least wait indefinitely for better evidence. I call it the “McClellan principle.” Like the Civil War general, proponents of the argument counsel doing nothing until absolutely certain of success. The principle is frequently stated or assumed to be true in climate policy debates, often but not always by professed climate skeptics. To give a few recent examples, Stephen Calabresi states the principle explicitly in a Politico debate last week, while Steve Everley of Newt Gingrich’s American Solutions outfit uses the stealth version of the principle by listing costs of a carbon price while failing to mention climate change at all. But perhaps the most common form of the principle is simply as a concluding statement, thrown in as if its implications were obvious and unworthy of debate. The Wall Street Journal does this when criticizing California’s AB32 cap-and-trade policy in an April editorial:

While almost all of AB32′s benefits are speculative and uncertain, its costs are hitting businesses and residents now. This is one more blow to jobs and growth that California doesn’t need.

The appeal of the McClellan principle may come from the fact that it is cloaked in rational language, but it isn’t a rational approach to policy at all. In fact, it’s the inverse of the familiar “precautionary principle” advanced by many Greens (at least in the precautionary principle’s strong form). The strong precautionary principle would require a policy response even if uncertainty is large. The McClellan principle requires inaction even if uncertainty is small. Both principles are simplistic, and neither leads to good policy decisions.

The reason for this is that there are both costs and uncertainty about those costs associated with climate policy and with doing nothing. Both are real choices with consequences, even if we can’t say with complete precision what those consequences are.

The McClellan principle stresses that the economic costs of climate policy—primarily higher energy prices—are certain, while there is at least some chance that all the climate science models are wrong and that there will be no costs associated with doing nothing. Holding out hope that the Earth will not warm (or that we can do nothing about it) strikes me as absurdly Panglossian, but the basic premise that we can be more confident in estimates of the economic costs of policy—particularly that they will not be zero—is probably right.

The McClellan principle’s conclusion does not follow from this premise, however. Making policy based only on which kind of costs we think are more or less likely to be zero doesn’t make sense. We should instead do the best we can in estimating the two costs, both their magnitude and precision, and make the policy we can based on those estimates. That is of course incredibly difficult in practice. It raises questions about discounting of future costs and benefits, the tensions between national policy and global risks, and distributional impacts, among others. But it has to be the basic framework for making a decision. Both the McClellan and (strong) precautionary principles try to offer shortcuts, but in doing so they obfuscate rather than clarify.

I illustrating this is hard because conversations about climate policy are, unfortunately, so loaded with politics and preconceived ideas. Instead, let’s look at another issue loaded with different politics and preconceived ideas: crime. Imagine you are on a parole board considering release of a prisoner. There is a cost to releasing the felon (he might commit another crime) and a cost to keeping him in prison (prisons are crowded and expensive, and he might contribute to society if released). You know the cost of prison is not zero. The cost of release might be zero, or it might be big. But that doesn’t mean you should release the felon— or even that you should be any more likely to. Setting moral/ethical considerations about the prisoner aside, all we should care about is balancing our best guess about the costs of both options.

Of course, the way that parole boards work in practice—or at least the way most people demand that they work—is that any real chance of repeat offense is regarded as a reason for denying parole. So why is there such a dissonance between the way many people view parole decisions and the way so many view the climate policy debate? Why does the mainstream view on releasing felons appear to be a form of the precautionary principle, while the McClellan principle, if not the mainstream view on climate, is at least a major and usually uncriticized one? Surely a big factor is that the risks of crime are viewed as more personal and visceral, even if the chances of actually being a victim of a re-offender are low. It might be as simple as saying that most of us fear criminals more than we fear the more emotionally and temporally, if not probabilistically distant risks of climate change—and that mainstream positions are defined by what we most fear. That’s unavoidable to some extent, but it’s not a rational approach to making good policy.

Others, most notably Richard Posner, have made a similar analogy between major climate change and asteroid impacts—for which uncertainty is similarly paired with catastrophic downside risk. This analogy is useful because asteroid impacts are completely politically irrelevant—there’s no party line, and little fear—and as a result few people seem to have either a precautionary or McClellan principle-style reaction. A rational approach is the most appealing, though the same lack of fear may cause us to ignore the risk entirely and do nothing.

As these analogies hopefully illustrate, precision is important, but lack of it shouldn’t keep us from acting—on climate or on other problems of risk. Precision is just another factor in estimation of risks and costs. And whether the costs of action or inaction might have a chance of being zero doesn’t provide a shortcut out of the difficult task of balancing the two and making policy. Uncertainty matters, but it does not and cannot do the work alone.

I suspect that many people who advance the McClellan principle as their argument against pricing carbon would still oppose a price even if there were much less uncertainty about climate change risks (or would simply disbelieve claims of certainty). In their case, the McClellan principle may provide cover for less politically-acceptable positions, like an economic or political interest in fossil fuels or a very large discount rate. But many people who state the principle are not being disingenuous just to score rhetorical points. You don’t even have to reject climate science to advance the McClellan principle—you just need to point to the uncertainty within it.

But even for the intellectually honest, the wellspring of the principle’s appeal is, again, fear. Especially in a recession, the downside of pricing carbon sparks greater concern than climate change does, at least for many people. To them, the economic costs of a carbon price are very real, immediate, and personal, while the costs of climate change are distant and abstract. This is to some extent true for everyone, though if you are unemployed and live in a coal state, economic costs are certainly more apparent: a recent study suggests that unemployment and some measures of concern about climate change are negatively correlated. In a democracy, these perspectives cannot and should not be dismissed. They are valuable and should be listened to when considering climate policy, and in particular its distributional impacts.

But the fact that costs are tangible—that they are feared—doesn’t mean the McClellan principle is any more logically sound. Lack of certainty about climate change risks doesn’t justify inaction any more in Ohio than it does in California—or places at great risk from warming, like Bangladesh. The McClellan principle is ultimately based on fear, not reason. Stripping the principle of its thin cloak of rationality might therefore make a difference, however small, in the politics of climate policy. As I mentioned above, I’m certainly not the first person to try to do this, but the principle remains a resilient meme. It’s worth having the counterargument in your pocket.  Next time you hear it, ask its proponent what they would do on a parole board.