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sellers. Welcome to Science Talk,

the more or less weekly podcast of Scientific American, posted on March 19th, 2010. I'm Steve

Merski. A team of 30 scientists from across the globe has put together a list of nine

environmental processes that must remain

within specific limits, they say, or what they call the safe operating space within which

humankind can exist on Earth will cease to be safe. John Foley, director of the University of

Minnesota's Institute on the Environment, and one of the group's leaders, has an article on that effort in the April issue of Scientific American.

We talked when we were both at the annual meeting of the American Association for the Advancement of Science in February in San Diego.

You have this article leading off a section in the magazine, and the way you approach

things is through a discussion of tipping points.

Let's go through them and what you mean by them and what the import of them is.

Well, the idea of this article is about planetary boundaries, which is kind of a

fancy way of saying two things. One is that we're approaching some limits in how our environment

works that are like tipping points. They're like a cliff that you walk over and you can't come back again. Those things are sometimes very bad. We don't want to jump off cliffs without seeing where they are. The other kind of limit we're talking about are where you don't fall off a cliff, but you can't go back again. You suddenly end up on a one-way street where you've degraded something in the environment so much, it's effectively irreversible.

It may not be off the cliff, but you aren't ever going to go back to where you were before.

So this broader notion of planetary boundaries is something that a group of scientists around the

world, led by Stockholm University and others, have recently been pulling together to say, wait a minute,

...