Whether to apply the “precautionary principle” in an actual situation depends on the size of the potential disaster, the probability that the disaster will come about if no action is taken, and the costs – including the opportunity costs – of taking the action. So at the end of the day there’s not much “principle” left, just a heuristic for doing decision analysis.
But one point ought to be clear: greater uncertainty argues for more caution – more willingness to accept certain current losses to avoid possible large future losses – not less. That’s because it’s easier to adjust to small changes than it is to large ones, so damage is likely to increase more-than-proportionally as the size of the change increases.
Assume some climate model predicts that, under some set of assumptions, average global temperature would rise 3° C by 2100. If the model were very accurate and precise, that might be 3°± 1°. If the mechanisms involved remain obscure and the data unclear – as is the case today – that might be 3°± 5°: that is, the best guess would be a 3° increase, but the actual outcomes might range between an 8° increase and a 2° decrease.
Given how bad a 3° increase would likely be, if we knew for sure that would be the outcome in the face of inaction there would be a strong agument for making big and expensive policy changes to prevent it from happening. And if we knew that for sure, it would be very hard politically to argue against doing something about the problem.
By contrast, 3°± 5° means that proponents of inaction get to say “We’re not even sure there’s any problem at all.” That makes the political case for action much weaker. But it makes the logical case for action much stronger.
The world – especially the much richer world of our great-great-grandchildren in 2100 – could adjust to a 3°, or even a 4°, increase in global average temperature, though at great cost in species extinctions, land area lost to rising sea levels (and therefore the forced migration of some large populations), and more extreme weather. That hotter planet would be, on average, a less pleasant place to live. But it would still be habitable.
But an 8° C average temperature increase is a completely different proposition, rendering much of the tropics virtually uninhabitable and, quite plausibly, hitting various triggers for positive-feedback effects such as the melting of the polar ice caps, which would reduce the amount of solar energy reflected back into space, and the melting of the Siberian permafrost, which would release a huge amount of methane, a potent greenhouse gas. (Most systems are more stable in the face of small changes than they are in the face of large changes.) Thus a primary increase of 8° might really mean an increase even larger than that: an increase that might not be reversible even if greenhouse-gas emissions were then sharply curtailed.
That would be the kind of disaster to which some version of the precautionary principle reasonably applies: if the risk of its happening is more than merely speculative, it’s not a risk to take, even if avoiding that risk requires major dislocations. It’s the same principle that tells you not to deal with a tighter family budget by canceling your homeowner’s insurance, even though the annual risk of a major house fire – unless you’re the kind of drunk likely to pass out in bed while smoking a cigarette – is way below 1%.
Ordinarily, it is the proponents of action who bear the burden of persuasion. But in this case political inaction means, in effect, licensing a massive gamble, though no individual chooses to make it. Rather, the gamble would be the outcome of billions of uncoordinated self-interested decisions: precisely the sort of process that, in the absence of external costs, leads to efficient outcomes. But none of the arguments for the freedom of economic activity applies to activities with huge, indirect, deferred, and diffuse external costs: by contrast with Adam Smith’s baker, there is simply no “invisible hand” mechanism that directs private action in such a situation in the direction of the public interest.
The willingness of some politicians and pundits to bet the planet on the claim that climate scientists are talking through their hats, which involves the larger claim that they have managed to assemble an enormous conspiracy to perpetrate a hoax, calls either their intelligence or their morals into serious question. And that goes for the journalists and media moguls who treat their ravings as if they represented simply one side in a reasonable argument.
If anyone tries to tell you that uncertainty about climate change is a reason for inaction, he’s either a fool or a scoundrel. Probably a bit of both.