A progressive interview with Bill McKibben.

• Position: Interview

Bill McKibben is author of "The End of Nature" and many other works, including his latest, "Eaarth: Making a Life on a Tough New Planet." McKibben is also the founder of the activist group 350.org.

Q: What's with the extra a in Eaarth ?

Bill McKibben: The planet we're living on is shifting beneath us. It's already changed enough. Global warming is no longer a future threat but a present reality. In certain ways, we're living in quite a different place.

The atmosphere holds about 5 percent more water vapor than it did forty years ago. That means we get deluge and downpour in unprecedented fashion. It's the hundred-year flood every three of four years. The ocean is 30 percent more acid. Last winter was among the warmest ever. It's a different planet already, and different planets require different habits if we're going to live on it.

Q: People used to say we've got to deal with global warming for our grandchildren.

McKibben: Forget about grandchildren, this is us! If our parents had done what they should have done, we'd be in a somewhat better place now.

Q. Why have things gone downhill faster than predicted?

McKibben: Twenty years ago, we knew that humans were putting a lot of carbon in the atmosphere because we were burning a lot of coal and gas and oil, and we knew that things were going to heat up. The only question we didn't know was how quickly. And being human, we wanted to believe there was more time.

But the Earth turned out to be more finely balanced than we expected. Things like the very rapid melting of Arctic Sea ice in the summer of 2007 shocked a lot of scientists. I got calls from people I've known for many years. Before they were worried. Now they were panicked. Pretty much any physical feature of the planet you can name is undergoing the same kind of chaotic change.

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When you look around, the ground really is shifting beneath our feet. It can get a lot worse. Just because we've crossed this threshold doesn't mean there aren't other thresholds to cross. We still have enormous work to do to slow down the amount of carbon we pour into the atmosphere. But we better start figuring out how we're going to live on a degraded Earth.

Q: Is this the tipping point we've crossed?

McKibben: There are going to be many tipping points along the way. There are some systems we've dearly perturbed too far to recover. The Arctic Sea ice is one of them. We don't want to go further down this road and deeply damage the monsoon...