Both Matt and Ezra have commented on this question, quoted approvingly by Jonah Goldberg from a reader email:

If Al Gore were to be convinced that global warming WAS a natural phenomena, would he be so worked up about it?  I don’t think so, yet the consequences would be the same.

Let’s address this in three ways.

1. Would it make a practical difference if global warming were natural? Would it change our response? Of course. I don’t know how to put it any more simply than I did in this post:

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If it’s the case that human activity is driving rapid global warming, then obviously scaling back GHG emissions should be our first priority. If it’s the case that human activity isn’t driving global warming — that warming is part of a natural cycle — then reducing GHG emissions isn’t a priority at all.

This point seems thuddingly obvious, but this "what if it were natural?" question seems to obsess some conservatives, so there you have it. If global warming is anthropogenic, we need to both stop exacerbating it and start adapting to the effects that are already inevitable. If it is natural, then we only need to adapt, since apparently nothing we do can affect the natural course of climate changes. OK?

2. Would Al Gore, and environmentalists generally, be so worked up about global warming if it were natural?

There are some semantic issues here. If there were an asteroid approaching earth on a collision course, presumably everyone would be concerned. Would that be an "environmental" problem? Would people concerned about it be "environmentalists"? I don’t know. But I feel pretty confident saying this: Al Gore is passionate about global warming because it stands to visit incredible suffering on the planet’s most vulnerable. If it were natural, it would still stand to visit that suffering, so Al Gore (and anybody else who isn’t a deluded jackass) would still be worked up about it, yes.

Or to more straightforwardly address the common conservative point: The notion that global warming activism is animated by hatred of capitalism and America is paranoid fiction of the sort that seems to substitute for analysis in far-right circles these days.

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3. In the real world, global warming is a huge problem, and it is driven by human activity, and so who cares what activists would do in some fanciful counterfactual situation? As Matt says:

Faced with a large-scale environmental problem and the question of what to do about it, they’re fixating on the fact that they don’t like environmentalists

Matt and Ezra both profess bafflement over this, but I think they’re just being coy. It’s not that big a mystery. Over the past few decades, the animating conservative philosophy has rotted away. Bush’s catastrophic presidency has put the final nail in it. What’s left? Partisanship. Jostling for power and influence. Pure political warfare.

Partisan hacks like Goldberg approach every question through the lens of his team vs. the other team. To him, that’s all there is. Naturally, he projects this partisanship on his opponents as well. He assumes everyone sees the world this way. So when environmentalists — or worse, Democrats — raise alarm over global warming, the only analysis available to Goldberg is partisan analysis: What’s their game? What are their motives? What advantage are they trying to gain? It simply doesn’t occur to him that people might be genuinely concerned about the fate of everyone on the planet, including Republicans.