We finally know where the red line for climate really is. After the rapid melt of arctic ice in the summer of 2007, our best scientists, led by NASA's Jim Hansen, went back to work and produced a series of papers showing that with more than 350 ppm (parts per million) of carbon dioxide in the atmosphere, we couldn't have a planet "similar to the one on which civilization developed and to which life on earth is adapted."

[Political actions] has to happen on the local and state level; we have to convince our cities to join the growing number of more than 200 American cities who have signed on to the mayor's climate campaign.

One degree turns out to be enough to really unhinge all sorts of earth's ecosystems.

Permafrost in the soil [is melting], in the boreal and arctic areas in the world, and, probably even more alarming in the last six or eight months, the data on what is happening to the ice shelves in Greenland and the west Antarctic has begun to cause people to radically reassess the earlier conviction that those ice shelves were stable on a kind of century-long time scale.

I think fracking for gas will reduce the incentive to turn to renewables, and I think it will do a lot of other damage across the countryside.

Without a movement pressing for change, there's little hope. We've got to work the political system to make this happen fast. The physics and chemistry are daunting. The resources on the other side are very large.

... the constant flow of images undercuts the sense that there's actually something wrong with the world. How can there really be a shortage of whooping cranes when you've seen a thousand images of them - seen ten times more images than there are actually whooping cranes left in the wild?