Their computer models are all that predict any harmful warming. I wouldn't be too worried about that because they don't reflect the real world very well just fyi. Most likely the sensitivity to CO2 warming is less than the computer models assume. The world has been warming for 300 years but for some reason the alarmists want to attribute all of the warming of the last century to humans. The reason is actually obvious. They only get power and money if this is a crisis and they can blame humans. Its politics. To answer your question, the climate is too complex to know for sure.
This guy answered it.. Translated, that means we contributed about a 1/3 of the increase in the last centurry or less than a quarter of a degree. That isn't taken very well in the alarmist community where the better the news is for the world the worse it is for them and that is not enough to worry about.
"...Professor Lindzen comments that this failure of observation to match prediction cannot be so easily explained, since the transient response would be likely to exceed the equilibrium response. He concludes that no more than about a third of the observed trend at the surface is likely to be due to greenhouse warming, and adds: “This is about as close as one ever gets to proof in climate physics."
http://scienceandpublicpolicy.org/monckton_papers/greenhouse_warming_what_greenhouse_warming_.html