…for Jim Manzi. An interesting interview over at the Economist:

The current UN IPCC consensus forecast is that, under fairly reasonable assumptions for world population and economic growth, anthropogenic global warming (AGW) is expected to cause economic costs of about 3% global GDP in a much wealthier world more than 100 years from now. This is pretty far from the rhetoric of imminent global destruction.

Because (not “though”) the science is uncertain, the rational concern is that impacts could be worse than expected. This has been the subject of intense scientific research for decades, and the IPCC has published probability distributions for various levels of projected warming over the next century. There is no such projected level of warming with materially non-zero probability for typical economic scenarios that would justify what I would estimate to be the actual costs of an emissions mitigation regime, and there is certainly no odds-adjusted case (ie, in which we handicap the odds of more and less severe possible impacts) which could justify such costs.

The only real argument for rapid, aggressive emissions abatement boils down to the point that you can’t prove a negative. If it turns out that even the outer edge of the probability distribution of our predictions for global-warming impacts is enormously conservative, and disaster looms if we don’t change our ways radically and this instant, then we really should start shutting down power plants and confiscating cars tomorrow morning. We have no good evidence that such a disaster scenario is imminent, but nobody can conceivably prove it to be impossible. Once you get past the table-pounding, any rationale for rapid emissions abatement that confronts the facts in evidence is really a more or less sophisticated restatement of the precautionary principle: the somewhat grandiosely named idea that the downside possibilities are so bad that we should pay almost any price to avoid almost any chance of their occurrence.

But if you want to use this rationale to justify large economic costs, what non-arbitrary stopping condition will you choose for how much we should limit emissions? Assume for the moment that we could have a perfectly implemented global carbon tax. If we introduced a tax high enough to keep atmospheric carbon concentration to no more than 1.5x its current level (assuming we could get the whole world to go along), we would expect to spend about $17 trillion more than the benefits that we would achieve in the expected case. That’s a heck of an insurance premium for an event so low-probability that it is literally outside of a probability distribution. Of course, I can find scientists who say that level of atmospheric carbon dioxide is too dangerous. Al Gore has a more aggressive proposal that if implemented through an optimal carbon tax (again, assuming we can get the whole word to go along) would cost more like $23 trillion in excess of benefits in the expected case. Of course, this wouldn’t eliminate all uncertainty, and I can find scientists who say we need to reduce emissions even faster. Once we leave the world of odds and handicapping and enter the world of the precautionary principle, there is really no principled stopping point. We would be chasing an endlessly receding horizon of zero risk.

To put a fine point on it, replace “global warming” in your question with “planet-killing asteroid impact”. Earth-impact asteroids are a non-imaginary threat, and there is already significant government expenditure devoted to this problem. They hold the potential to all but exterminate the human species. By the logic of your question, why would you not invest, say, 2% of global GDP per year into perpetuity (roughly equal to about $1 trillion, or the total annual collections from the US income tax), to develop and deploy an interdiction system for earth-impact asteroids? If not, how do you distinguish between your fear of climate change impacts beyond the consensus scientific forecast, and a fear of asteroids?

In fact, we face lots of other unquantifiable threats of at least comparable realism and severity. In addition to asteroids, a regional nuclear war in Central Asia, a global pandemic triggered by a modified version of HIV, or a rogue state weaponising genetic-engineering technology all come immediately to mind. Any of these could kill hundreds of millions of people. In the face of massive uncertainty on multiple fronts the best strategy is almost always to hedge your bets and keep your options open. Wealth and technology are raw materials for options. The loss of economic and technological development that would be required to eliminate all hypothetical climate change risk would cripple our ability to deal with virtually every other foreseeable and unforeseeable risk, not to mention our ability to lead productive and interesting lives in the meantime. The precautionary principle is a bottomless well of anxieties, but our resources are finite—it’s possible to buy so much flood insurance that you can’t afford fire insurance.

Bold emphasis mine. It’s crazy to pauperize ourselves now for potential economic benefits decades from now. Particularly when the actions don’t even address the problem (e.g., the current cap’n’tax bill that passed the House).

This is really an issue that cries out for a rational, regret analysis.