It's going to be like herding stray cats, and the political leaders who can do it will be remembered as the same kind of geniuses who pulled off the American Revolution.

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.

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.

Even if we stop the growth, we'd still be adding a constant amount of fossil carbon to the atmosphere each year.

Climate change has a very high procrastination penalty that just grows with each passing year of inaction - rather like what happens if you don't pay off your credit card. But for climate, there is no such thing as a fresh start from bankruptcy.

If we don't start thinking big about the CO2 problem, we may miss our opportunity to stop a climate runaway that will trash the habitable parts of the earth.

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