Carrying on as usual carries enormous risks, condemning today's students to a world of constant insecurity and frequent catastrophes.

Since Serengeti-scale savanna scenes are only one or two million years old, our earliest after-the-apes ancestors didn't move into this scene so much as they evolved with it, as the slower climate changes and uplift produced more grass and less forest.

Our CO2 mixes with everyone else's within a year, then hangs around for centuries like a shroud.

Perhaps we should put up posters in such places reminding people that where they stand was underwater the last time that the Earth ran a 3°F fever.

Carbon-free energy is simply something we have to do. The time for talk is past. If we turn around net carbon emissions by 2020 rather than 2040, we get another 2° of fever rather than 3° - and that's a big difference.

Carrying on as usual carries enormous risks, condemning today's students to a world of constant insecurity and frequent catastrophes.

Stopping emissions growth represents the most minimal of do-something responses.