Bill McKibben

"I remember, after Katrina and after An Inconvenient Truth, 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."

Tom Burke

"It's the line from Eliot, 'This is the way the world ends, not with a bang but a whimper.' You're just looking at the way human beings have done on a global scale what they've done several times before on a regional scale: collapse and disappear civilizations."

Elizabeth Kolbert

"The discussion of climate change has often been inflected by this notion that it matters what you think of it. The fact is, it doesn't matter what you think of gravity or quantum mechanics and it doesn't matter what you think of climate change. They're all physical realities that are beyond the ability of human opinion to influence."

Joe Romm

"The world leaders before Copenhagen said, 'We're not going to have a final agreement. We're going to try to move the ball forward in a two-step process. I don't think there's any question that it was Obama's personal intervention that saved the day. Because otherwise the Chinese would have succeeded in shutting down the process. It was only Obama going directly to the presidents of China and India and Brazil and South Africa and working out text - which is unprecedented, it just never happens."

Jennifer Morgan

"It's the problem from hell. Not only because there are so many sources of the problem - you can't laser in and solve one specific piece and it's done and all the world's rivers are fixed. Rather, you have to go at the cars and the oil and the power plants and the way we farm and which food we eat and whether you fly. I mean, it's personal, it's everywhere. And associated with those sources are huge political and financial stakes."

Brice LaLonde

"It became the first issue for me not only because of the problem; but I also saw it as a way to unite humanity. I was a bit Candide. Perhaps because we have a common threat we can work together and create something, some beautiful common work together."

Philip Shabecoff

"If Copenhagen is any indication, world leaders are saying the right things. Some are even making the right promises. But I see no indication that they will actually be forced to take action. We've heard promises before and maybe I'm wrong - I hope I am - I think it's going to take something else. Perhaps a catastrophe for global consciousness to reach the peak of outrage that is required to force governments and the industry to act, to do something about this overwhelming problem."

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."