There's always the danger that people will simply sign online petitions, the way they used to just mail in checks, and there's the greater possibility we'll just spend our whole lives staring at screens and never get anything done.

The essential thing we need to understand is that the climate crisis is not some future threat, but a very present peril, the biggest one humans have ever encountered. Until we understand that, we'll dawdle.

If there's horrible flooding in Pakistan or a horrible heat wave in Texas, we're no longer able to call it an act of God, or a natural disaster, or something like that, the way we could have through all of human history until 35 or 40 years ago.

I'm probably the wrong person to ask. My partner in much of this work [climate movement], who really came up with the divestment campaign with me, Naomi Klein, I think has written powerfully about this.

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."

I'm far less a leader than a writer.

Ice in the West Antarctic and over Greenland, i.e., ice that's over a rock at the moment, that will raise the level of the sea as it slides into the ocean, putting at risk everyone and everything that lives on the coasts, and that includes an enormous percentage of the world's people.