The April issue of Scientific American includes an exclusive excerpt from Bill McKibben's new book, Eaarth: Making a Life on a Tough New Planet, plus an interview that challenges his assumptions. Expanded answers to key interview questions, and additional queries and replies, appear here.

McKibben is a scholar in residence at Middlebury College in Vermont and is a co-founder of the climate action group, He argues that humankind, because of its actions, now lives on a fundamentally different world, which he calls Eaarth. This celestial body can no longer support the economic growth model that has driven society for the past 200 years. To avoid its own collapse, humankind must instead seek to maintain wealth and resources, in large part by shifting to more durable, localized economies—especially in food and energy production.

[A Scientific American interview with McKibben follows.]

You entitled your book Eaarth, because you claim that we have permanently altered the planet. How so? And why should we change our ways now?
Well, gravity still applies. But fundamental characteristics have changed, like the way the seasons progress, how much rain falls, the meteorological tropics—which have expanded about two degrees north and south, making Australia one big fire zone. This is a different world. We underestimated how finely balanced the planet's physical systems are. Few people have come to grips with this. The perception, still, is that this is a future issue. It's not—it's here now.

Is zero growth necessary, or would "very slight" growth be sustainable?
A specific number is not part of the analysis. I'm more interested in trajectories: What happens if we move away from growth as the answer to everything and head in a different direction? We've tried very little else. We can measure society by other means, and when we do, the world can become much more robust and secure. You start having a food supply you can count on, and an energy supply you can count on, and know they aren't undermining the rest of the world. You start building communities that are strong enough to count on, so individual accumulation of wealth becomes less important.

If "growth" should no longer be our mantra, then what should it be?
We need stability. We need systems that don't rip apart. Durability needs to be our mantra. The term "sustainability" means essentially nothing to most people. "Maintenance" is not very flashy. "Maturity" would be the word we really want, but it's been stolen by the AARP. So durability is good; durability is a virtue.

In part, you're advocating a return to local reliance. How small is "local"? And can local reliance work only in certain places?
We'll figure out the sensible size. It could be a town, a region, a state. But to find the answer, we have to get the incredibly distorting subsidies out of our current systems. They send all kinds of bad signals about what we should be doing. In energy we've underwritten fossil fuel for a long time; unbelievable gifts to the "clean coal" industry, and on and on. It's even more egregious in agriculture. Most of the United States's cropland is devoted to growing corn and soybeans--not because there's an unbelievable demand to eat corn and soybeans, but because there are federal subsidies to grow them—written into the law by huge agricultural companies who control certain senators. Once subsidies wither, we can figure out what scale of industry makes sense. It will make sense to grow a lot of things closer to home.

It's plausible to "go local" in, say, your home state of Vermont, where residents have money and are forward-looking—and their basic needs are met. But what about people in poor places; don't they need outside help?
Absolutely. The rich nations have screwed up the climate. It's our absolute responsibility to figure out how to allow poor people to have something approaching a decent life. What happens to the poorest and most vulnerable people in the world? They get dengue fever. The fields they depend on are ruined by drought or flood. The glaciers that feed the Ganges will be gone, yet 400 million people depend on that water. We are not helping the poor by destabilizing the planet's systems. Meantime, what works best for them? Local, labor-intensive, low-input agriculture: It provides jobs, security, stability and food, and helps make local ecological systems robust enough to withstand the damage that's coming.

U.S. debt is rising to insane levels because the country has lived beyond its means, which supports your call to switch from growth to maintenance. But how do countries like the U.S. get out of debt without growing? Do we need a transition period where growth eliminates debt, and then we embrace durability?
My sense is that all of this will flow logically from the physics and chemistry of the world we're moving into, just like the centralized industrial model flowed logically from the physics and chemistry of the fossil-fueled world. The primary political question is: Can we make change happen fast enough to avoid all-out collapses that are plausible, even likely, under the patterns we're operating in now? How do we force global changes that move these transitions more quickly than they want to move? We have an incredibly small amount of time; we have already passed the threshold points in some respects. We best get to work.