New York Times reporter Andy Revkin’s innovative Dot Earth blog is one of the best assemblies of climate and population data on the Web. I’m a big fan, and comment there from time to time. Revkin spoke recently to the Woodrow Wilson Center on the impact of population growth on climate change. He suggested, as a thought experiment, financially rewarding families who have fewer children.

Rush Limbaugh, never one to debate the merits of an issue when he can make a personal attack instead, called Revkin an “environmental wacko” and a “jihad guy.”

This guy from The New York Times, if he really thinks that humanity is destroying the planet, humanity is destroying the climate, that human beings in their natural existence are going to cause the extinction of life on Earth — Andrew Revkin. Mr. Revkin, why don’t you just go kill yourself and help the planet by dying?

Limbaugh’s attack on Revkin is a classic example of the ad hominem fallacy.

The ad hominem fallacy was described – probably not for the first time – by Aristotle. It’s a logical fallacy. Essentially, instead of attacking or debating the issue, the speaker attacks the person raising the issue. It’s an attack on the messenger instead of the message.

For example, if I were to say in response to Limbaugh’s statement that you should ignore him because he is a ex-junkie, a pompous, pill-popping, obese idiot, I’d be engaging the in ad hominem fallacy myself. It’s undeniably entertaining, and the statements may even be true, but it is irrelevant to the issues under discussion. It’s a change of focus that abandons the primary topic. It’s admittedly entertaining, and since Limbaugh is in the business of being entertaining, it suits him well. But don’t mistake it for debate. Or intelligence. Or logic. Or useful political discourse.

Revkin’s correct response should be to point to the fallacy. In a lot of ways, a debater who resorts to the ad hominem fallacy is admitting he or she has no logical response to the primary argument of his or her opponent. The planet is undeniably badly over-populated. By all science tells us, the earth cannot sustain 9 billion people for long. Limbaugh couldn’t rebut the point. So he attacked Revkin instead.

It’s always tempting to respond to an ad hominem attack by defending the speaker. The personal attacks are almost always unfair. I believe Revkin to be about as fair and balanced a newsreporter as there is. And certainly much better informed than Limbaugh. But if you defend the individual, you are falling for the fallacy. Interestingly, NPR makes this mistake. The speaker is not the issue; the issue is the issue. The best response is always to drag the focus back to the topic, away from the speaker.

So long as we let the Glen Beck’s and Rush Limbaugh’s of the world get away with the fallacy, they will use it. And political discourse will continue to suffer as a result. We have to call it out each time it happens.