[adrianN]

I don't think that a linear relationship between temperature and CO2 level is a safe bet. The climate is pretty chaotic, it seems unlikely that such a simple relationship is true in general. It seems entirely possible that we'll hit some kind of runaway effect at some point that completely breaks the linear relationship.

Anyway, I'm not a climate scientist and there are a number of people who claim that 1000 GtCO2 lead to 2 degree warming, so your 1500 GtCO2 seem only a little optimistic. However, just reducing CO2 production is not enough. We have to completely stop, because CO2 takes basically forever to sequester from the atmosphere, and every degree of warming we cause increases the likelihood that we leave the "safe" linear-relationship territory.

See http://trillionthtonne.org/