Is it growth, or just smoke and mirrors?

The US economy is back.

That’s what one nitwit on Wall Street declared yesterday after the earliest estimate of the nation’s third-quarter gross domestic product rose at an annualized rate of 3.5 percent.

Look, I don’t want to disparage the optimists out there. Nor do I want to stifle upbeat opinions, no matter how blatantly wrong, self-centered or foolishly naive they may be. But anyone with half a brain and a smidgen of awareness knows that the growth from last quarter was produced not just with smoke and mirrors but with smoke and mirrors paid for in ways that’ll cost us big in the future.

The problem is where this growth is coming from.

All you have to do is look at the GDP press release to see that 1.66 percentage points of the 3.5 percent gain came from car sales, meaning it was driven by this past summer’s “cash for clunkers” program.

What’s more, while this wasn’t specifically mentioned by the government in its report, it turns out that a 0.53 percentage point of the GDP figure came from home sales, which as you know were also subsidized by a tax break of as much as $8,000 for first-time homebuyers.

Welcome to the culture of taxpayer subsidies.

Most of the remaining growth came from inventory replenishment. Companies can’t sell stuff unless they make it. But inventory buildup isn’t a continuous thing, especially in a subpar economy, so the contribution it gave to GDP will probably not repeat itself in the last three months of this year.

What’s wrong with the government having a hand in priming the economic pump? Normally, nothing.

But the world is a different place than it was when FDR came up with the idea. Now, every penny spent by Washington has to be borrowed. And there’s already a mountain of $11 trillion in debt we’re already dealing with.

Growth in the GDP will inevitably cause borrowing costs to rise, which is a drag on the economy. But before that rate drag hits, you want to make sure the growth is real and sustainable.

So the guy who was proclaiming that the economy was “back” really wasn’t doing anyone a favor.