If you think we know enough to figure out how much of the warming is due to greenhouse gasses that are produced by human activity,
then you should easily be able to cite a peer reviewed publication stating that level, of certainty.
I have read many of the studies, and the estimated ECS is an enormous range.(1.5 to 4.5C)
The most fundamental portion of the AGW concept, the CO2 forcing, turns out to be higher than the measured forcing.
(Estimated forcing 3.71 Wm-2, measured 2.38 Wm-2)
If the forcing estimate is 55% too high, what does that say for the more subjective amplified feedbacks?
http://asl.umbc.edu/pub/chepplew/journals/nature14240_v519_Feldman_CO2.pdf
An analogy would be if you knew a town was 90 miles down the interstate, and the speed limit was 60 mph,
it should take you about 1.5 hours to reach the town, yet when you ask someone how long it actually will take,
they tell you between 2 and 6 hours. This is the range of uncertainty described by our current level of understanding!