From the text:
[A global temp. rise of] 6 degrees [C; 12F] is something of a worst case scenario — that’s what happens if we stay on a business-as-usual trajectory for the rest of the century. Scientists will tell you that this is implausible. But as I’ve argued before, they say so because they’re sure we’ll do something about it — that business-as-usual won’t continue.
After decades of tepid, notional progress in international climate negotiations, however, the idea that we’d be so super-duper stupid is not as ludicrous as it once seemed.
(and a John Kerry speech on the climate challenge comes highly recommended by Chris Mooney (I haven't watched it yet) - On Eve of Rio+20, An Honest Assessment of Climate Change Challenge.
For the record, I'm not so hot on Mooney's "Republicans are..." theme, which no doubt sells lots of books to PO'd Democrats but also increases the polarization (and gives a black-and-white representation of a shades-of-gray issue). Better to reach out than to seal off, and also to look inward, since we all be sinners.)
(updated ~3pm Wed, and 8am Thurs (added quote).)