Stewart Brand

"The earth system idea that really was transformative was the Gaia hypothesis: the notion that Earth, as a whole, was a self-moderating, self-healing system... What we’re finding out is that if Gaia heals itself from our greenhouse gas emissions by going to 5 degrees Celsius warmer and stabilizes there – it’s fine for Gaia, but lousy for us. Because that’s a world in which there’s carrying capacity for one and a half billion people, versus seven that we have now. That would be a very tough century."

Jennifer Morgan

"Why is it the problem from hell? It’s the problem from hell not only because there are so many sources of the problem. So you can’t just solve one specific piece and it’s done. You have to go at the cars, and the oil, and the power plants, and the way we farm, and which food we eat. It’s everywhere. And associated with those sources are huge political and financial stakes."

Joe Romm

"On our current emissions path, we could easily hit 800 to 1,000 parts per million. And that is a so-called tripling or quadrupling from pre-industrial levels. The consequences are so dire that most scientists haven’t even studied them. Because they never believed that humanity would be so stupid as to let it happen."

Steven Schneider

"I am technologically optimistic that we can prevent a lot of dangerous outcomes - not all, but many. But I'm politically bleak that that we're going to do it until we have enough tangible damage to tip us over the political tipping point of long-term action. We got close in '88, maybe a little bit in Katrina - and then faded away each time. I don't know, do we have to have a hurricane take out Miami and Shanghai to have everybody else wake up? If that happens in 2025, by then it's going to be too late to prevent melting of Greenland. If it happened next year, it might be possible to still do that. But what a hell of a way to run a planet."

Bill McKibben

"All the polling data showing that Americans understood what the danger was, but still nothing happening in Washington. Not a damn thing. Twenty years without any legislation that would have done anything to deal with the biggest problem that the world's ever faced. I started emailing people to say, 'We're going for a walk.' We left from Robert Frost's old cabin, because we liked that most cliched of all poems about the road not taken. Now we've moved on to do the global version of this: 350.org - in reference to Jim Hansen's number, this red line for the planet."

Paul Hawken

"There’s no question in my mind that, as people who care deeply about the environment, we keep looking for love in all the wrong places. And that’s from our political leaders. If we haven’t learned yet, then we should get it now. This is not going to be top-down. It goes right back to the hundreds of millions of people on Earth who are trying to find and craft and create solutions every single day. Going around, I encountered organizations I had never heard of. That’s when I thought, well gee, how many are there? I started to count and I got to about thirty thousand and I thought, “Okay, this is big. This is bigger than the Catholic Church, you know?” (laughs) There are literally two million organizations in the world that are working on these issues of social justice and the environment -- because they’re inseparable. It’s growing, it’s growing, it’s growing. Because it’s not a movement. It’s in a sense, humanity’s immune response."

Mark Hertsgaard

"Every social movement arrives at a certain point in history, a point it does not choose. But it has the power to make history.  All of us who care about the environment must live in hope – live in the hope that things can change, that history is unpredictable. Nobody thought six months ago that Egypt and Tunisia and other dictatorships in the Middle East would ever fall to popular power. And yet they did. So we in the environmental movement, if you are part of that sensibility, you must keep up that sense of hope. And you do your part. And you leave the results to Gaia or god or what ever you may believe in -- history. But you must do your part."