Found Via The Oil drum:
"Damn! I think we just passed the last exit for the Holocene!"
"I'm sorry, honey, I wasn't looking."
"We have to get off this highway. What's the next exit?"
"It's a long way ahead. Goes to somewhere called Perdition."
(Ragged chorus from the back seat) "Are we there yet, Daddy?"
...
You can do a lot with climate models, but the Earth hasn't actually seen a carbon dioxide concentration as high as 450 ppm since about 35 million years ago. So Hansen and some colleagues went to work on exactly that period, and came back with some bad news. If you leave the world at even 425 ppm for very long, all the ice will probably melt: Greenland, Antarctica, the lot. And the sea level will go up 240-270 feet.
...
Now, there's a big margin of error when you are dealing with 35 million years ago: Plus or minus 75 ppm, in this case. That means that the fatal number when all the ice disappears could be as high as 500 ppm - or it could be as low as 350 ppm. If that is the range within which all the world's ice will eventually melt, and you like living in the Holocene, then you probably should not put all your money on a 450 ppm ceiling for CO2.