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Kai Ryssdal: Here's one from the Marketplace Desk of What If. What if people just disappeared? All six-something billion of us, gone tomorrow. What happens to everything we've left behind?

It's a thought experiment that Alan Weisman indulges in his new book, "The World Without Us." Mr. Weisman, good to have you with us.

Alan Weisman: Thanks, Kai.

Ryssdal: Let's pick up with the instant that the human race disappears from this planet, and then play out for me the consequences. Do years and decades and centuries of human economic development and society start being overcome immediately, within hours or days? Or are we talking a matter of geologic time here?

Weisman: Almost instantly, because things are already going on, anybody who's a homeowner — and I have a chapter in here about how your own home would be dismantled by nature rather efficiently if you weren't there maintaining it — turns out that the same forces that would tear apart a house would tear apart New York, for instance.

I descended into the subways with engineers, who showed me that if it wasn't for the fact that they were pumping ground water away constantly with about 800 pumps, the subway tunnels would flood. Then the columns that hold up the streets would start to corrode. And within 15, 20 years, some of them would start to collapse, streets would start caving in, Lexington Avenue would start being a river again.

Ryssdal: Are we spending money now sort of in spite of ourselves trying to maintain our presence on this planet? Have we made, in our economic developments over the past century or so, life harder for ourselves in terms of maintaining our presence?

Weisman: I think we have in some ways. Let's think about agriculture. We have developed crops that produce much more per acre, but the cost of that has been chemical inputs that we've all heard an awful lot about that may be backfiring on us. That stuff's gonna stay in the soil for a long time.

And we now make our food travel, or let our food travel. And that's really problematic for two reasons — one, the incredible amount of energy expenditure. And the other is a lot of the ecosystem that is being scraped away in places so we can grow export crops.

Ryssdal: Other than the obvious thing, which is nuclear waste, is there stuff on this planet today that just won't die ever, no matter how long we wait?

Weisman: Plastic's an element, or I would say material, that showed up in our ecosystem on a large scale just after World War II. There's nothing out there right now that knows how to biodegrade it.

Ryssdal: But we're told all the time that we've got biodegradable shopping bags now and all that stuff.

Weisman: There are some biodegradable shopping bags, and I'm told that there's some that are actually . . . they're made out of organic polymers that will dissolve. But the majority of them — and I talked to scientists who have tested them — found out that they really have matrixes made out of materials that will make them sort of shred into little particles, and they'll kind of vanish, but the particles of plastic continues to exist.

Ryssdal: If humanity went away tomorrow, how long would it take for carbon dioxide levels in the atmosphere to get back to pre-human levels of 10 [thousand], 15 [thousand], 20,000 years ago?

Weisman: Well, carbon dioxide has . . . the amount of it in the atmosphere has varied in the past, depending on what was going on geologically. Now, that said, there are three ways that carbon get absorbed, and the ocean is one of the most significant.

The ocean turns over every thousand years, so in the first thousand years, if the ocean couldn't get it all, it would get a good percentage — perhaps up to 80 percent of the carbon that we've overloaded into the atmosphere. And in the first few centuries, a lot of that would go. But to get back to where we were before we started tinkering with the atmosphere, think 100,000 years.

Ryssdal: The book by Alan Weisman is called "The World Without Us." Mr. Weisman, thanks a lot for your time.

Weisman: Thank you, Kai.

Follow Kai Ryssdal at @kairyssdal