Paul Krugman’s column in today’s NYT laments the lack of a national policy to combat global warming. He writes:

It’s true that scientists don’t know exactly how much world temperatures will rise if we persist with business as usual. But that uncertainty is actually what makes action so urgent. While there’s a chance that we’ll act against global warming only to find that the danger was overstated, there’s also a chance that we’ll fail to act only to find that the results of inaction were catastrophic. Which risk would you rather run?

He then cites the work of Harvard economist Martin Weitzman, who surveyed the results of a number of recent climate models and found that (in Krugman’s words) “they suggest about a 5 percent chance that world temperatures will eventually rise by more than 10 degrees Celsius (that is, world temperatures will rise by 18 degrees Fahrenheit). As Mr. Weitzman points out, that’s enough to ‘effectively destroy planet Earth as we know it.’”

Krugman concludes, “It’s sheer irresponsibility not to do whatever we can to eliminate that threat” and he calls for opprobrium against those who might impede global warming legislation: “The only way we’re going to get action, I’d suggest, is if those who stand in the way of action come to be perceived as not just wrong but immoral.”

There is merit to the argument that society should consider a policy response to the threat of global warming. A small chance of an enormous calamity equals a risk that may deserve mitigation. That’s why people buy insurance, after all.

However, Krugman doesn’t accept that argument — at least, not when applied to other worrisome risks that trouble people whose politics are different than his. Less than two months ago, he wrote this about another future crisis:

[O]n Friday Mr. Obama declared that he would “extend the promise” of Social Security by imposing a payroll-tax surcharge on people making more than $250,000 a year. The Tax Policy Center estimates that this would raise an additional $629 billion over the next decade. But if the revenue from this tax hike really would be reserved for the Social Security trust fund, it wouldn’t be available for current initiatives. Again, one wonders about priorities. Whatever would-be privatizers may say, Social Security isn’t in crisis: the Congressional Budget Office says that the trust fund is good until 2046, and a number of analysts think that even this estimate is overly pessimistic. So is adding to the trust fund the best use a progressive can find for scarce additional revenue?

In Krugman’s view, policies to address Weitzman’s 5 percent risk of ecological disaster by the early 23rd century (Weitzman’s time frame, which Krugman didn’t specify) are responsible and moral, but policies to address the economic crisis of Social Security’s insolvency in less than four decades’ time are unnecessary and overly pessimistic. And Krugman clobbers anyone who suggests otherwise .

Make sense to you? Me neither.

Krugman’s double-standard on risk is not confined to Social Security. He has (rightly, IMO) blasted the Bush administration for going to war in Iraq. But couldn’t the war be justified as mitigating a small risk of a great catastrophe? Was there, perhaps, a one-in-20 risk that Hussein’s Iraq would develop weapons of mass destruction and direct them at the United States (in the next 200 years)?

I write this not to argue that the United States should be unconcerned about global warming, or about rogue states’ possession of super-weapons, or about Social Security’s (and Medicare’s) unsustainability. All are risks, and it is right for us to consider policy responses for each of them. My point is that it makes little sense to say one risk must be addressed while we should dismiss another risk with an expected value that’s probably the same order of magnitude.

Moreover, if this dichotomy is simply the product of Krugman’s political allegiances (“Red team fears are stupid, Blue team fears are heroic”), isn’t he being irresponsible, wrong and immoral?